Pipeline Coatings Inspector ICorr Level 2
Pipeline Coatings Inspector ICorr Level 2
Part A
TABLE OF CONTENTS
PART A
...................................................... ........... ............................................................. POLYOLEFINS & OTHER PLASTIC COATINGS ..................................... ENAMEL COATINGS
FUSION BONDED EPOXY
ELASTOMERIC COATINGS
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MULTI-COMPONENT LIQUIDS MCL'S
WRAPPING TAPES Hot applied tapes . ... . . . . .. . . . . . . . .. . . . .. . . .. . . . . . .. ... .. ... .. . . .. ... ... .. ....... ..........
.. ... .. .. . .. .. .. . . . . ... . .. . . . . . .... ... .. . .. . .. . . .. .......... . . . Grease based tapes . . .... . ... . . .. . . . . . . . . . . . . . . . .. . .. . . . . . .. . ....... . ... .. . . .. ........ ..... Self adhesive overwrap tapes . . . .. .. . .. .. .. . . .. . . .. . . .... . ... . . . . ... ... . .. . . .. .. ... .... . ... BRUSHING MASTICS .................................................................. Cold applied laminate tapes
.................................................. .......................................................... TESTING OF COATINGSIWRAPPINGS ...............................................
FILLERS (MASTICS AND PUTTIES) INTERNAL PIPE COATINGS
Factoryllaboratory based tests .. .... .... . . .. . . . .. . . . .. .. .. . ... .. .. . . . . . .. . . . . . . .. .. . . . . . . . . Tests performed by inspection personnel (factory and site) ... . . .. . .. . .. . . . .. .. . . . . .. . .. .. . . Awareness of other tests .. ............ .... .. ... . .. .. ... . ....... . ... . . . . .. .. . . .. . . . .... .. . ..
INSPECTION
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Duties of a pipeline coating inspector . .. .. ... .. . .. .... . . . . ... . . . . .. . . . . . .. . . . . . .. .. ... . .. . . Reports and records .. ................. ..... . . ... ....... ... . ..... .. .. .. . .. . . .. . . . . ... . . . . .. Examples of possible contractor malpractice . ........ .... . ...... ......... . ... .... . . . . .. ...
. HANDLING, TRANSPORT AND STORAGE OF COATED PIPE ....................... DITCHING AND BACKFILLING .....:. ................................................ PEARSON SURVEY ....................................................................
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When using the term enamels in relation to pipe coatings we are referring to coal tar or bitumen (asphalt) based coatings which are applied as a hot liquid. Coal tar and bitumen enamel coatings for full pipe lengths have largely been superseded by FBE and polyolefin coatings, however, many pipeline users do still specify the use of enamels. Pipeline coating inspectors are also likely to encounter these materials on maintenance work. Bitumen and coal tar enamels must not, in any circumstances, be mixed or applied on top of one another, a loss of properties and poor adhesion respectively will be the result. Both materials are black and are very similar in texture; the following simple test can be used to distinguish between the two materials:
Many other solvent types will also give the desired result when carrvina - out an enamel identijication test.
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Take a very small ball. about 3 mm in diameter, of the unknown enamel and place onto a sheet of white blotting paper or filter paper. Put two or three drops of xylene or toluene solvent onto the ball. The ring of liquid running from the enamel and soaking into the paper will show yellow for coal tar and brown for bitumen enamels.
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Enamel coatings are normally reinforced with two layers of fibreglass to provide a rigid durable coating.
Constituents Coal tar enamels Plasticised coal tar pitch from coal, distilled in coke ovens, is the main constituent to which is added various amounts of coal oil (plasticiser), which modifies the material for viscosity properties and temperature tolerance. Fillers are added, usually in the form of talcum powder or slate flour, in proportions of 25% to 35%, these are classed as inert mineral fillers.
Bitumen enamels Bitumen (asphalt) is a heavy residue from the distillation of crude oil. Inert mineral fillers are added, usually in the form of talcum powder or slate flour as for coal tar. The viscosity of bitumen can be reduced by adding certain waxes and increasing refinement. Herbicides are also added to bitumen enamels to discourage the formation of root growth.
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Advantages and limitations Advantages 1.
Low cost system - low material cost and pipe is not heated.
2. 3.
Relatively simple system. Widely known system - users have a good knowledge of application and performance properties.
Limitations 1.
Overall - poor performance properties when compared to more modern systems; higher CP currents required.
2.
Labour intensive at least three operatives will be required for site applications
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3. 4.
For site work bulky melting pots are required. Coating needs reinforcement.
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Safety hazards - (1) toxic fumes from liquid state; (2) danger of burns from hot enamels especially during field application on welded butt joints; (3) flammablz constituents.
6.
Thermoplastic nature of coating can lead to coating damage during handling, storage and ditching.
7.
Susceptible to breakdown in UV light.
8.
Susceptible to coating damage during soil stressing.
9.
Easy disbonding of coating with impact.
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10. Bitumens need herbicides incorporated. 11. Limitations on bending due to brittle nature. 12. Enamels of both types are not suitable for use over plastics of any type - no adhesion.
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Surface preparation Factory Surface preparation in a factory is normally carried out by dry abrasive blast cleaning using totally enclosed centrifugal blast cleaning units. Chemical cleaning, usually using the Footner process, is an alternative. Typical requirements (blast cleaning) Abrasive used - steel grit or grit and shot mix. Profile requirements - Sa21/2, medium profile or 50-75 p. Inspect blasted areas for surface laminations (slivers); if any exist then remove with a grinder, check for correct contour and wall thickness then reblast the area.
Site Full pipe lengths along with the welded joints may also be prepared and coated on site with puTose built equipment to allow for a continuous operation.
Welded joints are blast cleaned using pressure blasting equipment and dry expendable abrasive. Typical requirements (welded joints) Remove solar protective coating and all extraneous material from the existing coating (150 mm is typical requirement). Degrease only if necessary. Chamfer coating by 50 mm or 100 mm as specified, using a blow torch and scraper. Blast clean using dry expendable abrasive -profile requirements - Sa21/2, medium profile or 50-75 pm. Inspect blasted areas for surface laminations (slivers) - if any exist: remove with a grinder, check for correct contour and wall thickness then reblast area. Check for disbonded coating; if any exists then remove and reblast the area.
Coating application Factory In a pipe coating factory (mill) the enamel coating process is often done as a continual process by using spacer collars placed between the pipes, so that in effect, one long continuous pipe length is being coated. Some coating factories will treat each pipe separately. Q Ruane & T P O'Neill Issue 1 :28/04/97
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The sequence of events for coating (showing typical requirements) are as follows: 1.
Residual dust from the abrasive blast cleaning operation is removed from the pipe by soft brush andlor clean dry compressed air or by vacuum suction.
2.
As the pipe is rotating and moving forward, an overhead spray applies the appropriate primer. For coal tar enamel, a fast drying synthetic (chlorinated rubber) type B would be used and for bitumen enamels, a synthetic or bitumen based primer would be used. In both cases the DFT required would be in the region of 15-25 pm. The primers are low volume solids materials and dry in approximately 5-15 minutes, sometimes facilitated by blowing warm air through the pipe.
3.
When the primer is dry, a flood coating of hot enamel is applied using afiood box. The enamel should be constantly agitated and, as a general rule, should not L_ exceed 205"C, especially when there is a delay in application. Maximum application temperature for coal tar enamel: 250°C; discard enamel if temperature exceeds 260°C. Maximum application temperature for bitumen enamel: 230°C; discard enamel if temperature exceeds 240°C.
4.
Simultaneously, two reinforcements are applied in a spiral fashion, using a 150 mm wide strip of glass fibre inner wrap and a 150 mm to 230 mm wide strip of impregnated glass fibre outer wrap. The spiral overlap on each wrap should be at least 25 mm. The inner wrap should be embedded approximately half way into the enamel and should not be within 1 mm of the pipe surface. The outer wrap should be visible on the surface of the pipe coating.
5.
The wrapping (enamel plus reinforcement) should be trimmed back 150 mm from each end of the pipe and possibly bevelled, e.g. by at least 25 rnm.
6.
A solar protective coating should be applied whilst the pipe is still warm. White wash for coal tar enamels; blue wash for bitumen enamels. The washes should terminate 300 mm from each end of the wrapping.
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Typical thickness requirement: 4-7 mm.
Site application (welded joints) In the field, enamels can only be used on welded butt joints when the adjoining pipes are enamel coated.
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The sequence of events is essentially a manual reproduction of the factory system except now the pipes cannot be rotated or moved in any way. Sequence of operations
1. 2. 3.
Remove dust. Apply primer by brush.
4.
When primer is dry, apply the 1st flood coat of enamel. This is poured from a bucket and the coating is smoothed off (preventing icicles) by using a sling which is pulled back and forth around the joint by two operatives.
5.
Apply 2nd flood coat of enamel simultaneously with the application of a fibreglass inner wrap overlapping the existing coating by at least 75 mm.
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Blast clean to Sa2Y2.
Apply a 3rd flood coat of enamel and apply an enamel impregnated outer wrap. 6 Typical thickness requirement: minimum 4 mm.
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Safety When using coal tar enamels, due regard should be paid to the hazardous and irritant fumes. Barrier creams should be applied to all areas of bare skin and ideally masks worn. Contact with coal tar can cause warts. Medical advice should be sought at the first suspicion. It is not advisable to use enclosures during inclement weather because of the fumes. Coal tar and bitumen enamels should normally be used at temperatures below their spontaneous ignition temperature but even so, the vapour space is frequently within the flammable range, therefore smoking or naked flames should not be allowed in the vicinity of a tank or drum containing hot material and no source of ignition should be put into a tank or drum which has contained this material until the tank or drum is gas free.
1 Testing I 40
Tests on the primer 1.
Viscosity measurement using a flow cup at a specified temperature.
2.
Film thickness measurement.
Tests on the enamel material
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1.
Enamel temperature measurements.
2.
Softening point (ring and ball).
3.
Penetration.
4.
Viscosity measurement using a Zahn flow cup at 230°C.
Tests on the enamel coating 1.
Preliminary adhesion.
2.
Tapping tn check for laminations.
3.
Holiday detection.
4.
Bond strength test.
5.
Thickness of wrapping.
6.
Visual check for uniformity of coating contour, e.g. no icicles, ensure outer wrap is not disbonded and bleed through has taken place and ensure no coking exists.
7.
Coupons taken to ensure inner wrap is correctly positioned in enamel. Laminations may also be evident.
Determination of filler fineness, filler content, enamel specific gravity, peel resistance, sag resistance, flexibility (bend tests), low temperature disbondmentlcracking, impact resistance and cathodic disbondment, are other tests carried out at less frequent intervals, e.g. once a year or at the start of a project. The inner and outer wraps must also meet specified requirements for various tests.
Repairs Repairs to enamel coatings can be carried out by various methods depending on the nature and size of the coating fault. Pinholes are sometimes repaired using a hot knife whereas extensive damage would normally require full circumferential removal of the wrapping over the affected area. The bare area may then be repaired with the same enamel (normally incorporating reinforcement), heat shrinkable material, hot applied Q Ruane & T P O'Neill Issue 1 :28/04/97
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tape, cold applied laminate tape or a grease based t a F c . u m ~ ~ y ;.:& .3 2 & adhesive overwrap tape as specified. Multi-component hquids <~,.J:S d s o b : The method employed should be governed by specit~canon. patch QJS repairs s m tapes are not advisable; if tapes are specified then full circumferential wraps \
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(
1 Powder coatings (not epoxy) can be made from thermoplastic materials.
Fusion bonded epoxy (FBE) is a common coating material used on pipelines throughout the world. It is applied in factories on full pipe lengths and on fittings. In the field it is used for coating welded joints but only when the adjacent pipeslfittings are coated with the same material. These coatings are sometimes referred to as resin powder coatings (RPC) and are thermosetting, which means when heat is applied to a cured coating it will not return to a liquid state; they therefore chemically cure by cross-linking. The material is applied as a powder to components which are heated to a specified temperature governed by the powder type being used; the application temperature range will normally exceed 215"C, e.g. 218°C to 246°C for 3M's powder Scotchkote 206N. When the powder makes contact with the hot component it melts and forms a coating typically between 350 pm to 650 pm thick which can become fully cured in approximately three minutes.
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There are four main stages of transformation from a powder to a cured coating: 1.
Flow time:
powder to semi-liquid.
2.
Wetting time: powder to liquid.
3.
Gel time:
powder to gel (to the start of solidification).
4.
Cure time:
powder to completion of cross-linking.
Constituents of coating material In common with other coating materials, several components are used to give the required finished product, these typically include: a. b. c.
Lower softening points require 50 lower storage temperatures.
50% - 60% binder (base resin and curing reagent); 35% - 50% pigment and extenders (fillers); 2% - 4% other additives such as anti-foaming agents, wetting agents and flow control agents.
The base resin and curing reagent are essentially high melting point resins which are solids at normal ambient temperatures. Technically they should have a softening point of above 70°C but generally are in the range of 85" to 90°C. When the materials are melted 6( they have a molecularfreedom, the molecules can move about and cross-link. The warmer the material, the more molecular freedom (lower viscosity). AS the mixture is cooled and eventually solidifies, the curing reaction is stopped.
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During powder manufacture, all the required components are heated together and as soon as possible, when liquid, they are mixed into a homogenous mixture and then immediately frozen to halt any reaction. This material is then micronized into a powder. Powder particle sizes are typically in the range of 25-75 pm and each particle contains the required components to effect a cure when returned to a liquid state.
Advantages and limitations Advantages 1.
NO solvent - no global warming potential (GWP); no ozone depletion potential (ODP).
2.
Low health risk. Low fire risk - although loose powder particles in the air are highly flammable.
3. 4.
Low processing time.
5.
Good film properties - especially chemical resistance.
6.
Low dust pick up (-3 minutes to full cure).
7.
Good edge coverage (no shrinkage therefore good on sharp corners).
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Clean to use.
Limitations 1.
Heating of components can be costly.
2.
Achieving thin films, e.g. <25 pm, can be difficult to achieve if ever requested.
3.
Different powders c a n ~ l dbe illixecl w i h each otlier, e.g. powders with different colours, from different generic types or from different manufacturers.
4.
Prone to chalking when exposed to UV.
5.
Prone to chipping damage due to brittle nature.
6.
High storage temperatures can cause sintering.
Surface preparation Factory The phosphoric acid wash andl or the chromating is done to passivute the surjface byforming a relutively stable strongly adherent corrosion inhibiting layer and to provide a sufuce tu which coatings will readily adhere.
Surface preparation in a factory is normally carried out by dry abrasive blast cleaning followed by the application of a conversion coating (phosphoric acid wash and/or chromating). Sometimes the conversion coating is omitted. It is essential to remove any oil or grease prior blast cleaning. The client's specification or contractor's procedures should clarify this but a typical clause would state 'before surface preparation commences, any oilfgrease shall be removed using suitable solvent'. Typical requirements
Abrasive used - steel grit or ,git and shot mix. Profile requirements - Sa2%, medium profile or 50-75 pm. No more than X hours to elapse before powder application after the conversion coating has been applied.
Site Welded joints are blast cleaned using pressure blasting equipment and dry expendable abrasive. Conversion coatings are not used.
In a factory, long pipes can be manoeuvred and spun in front of blast nozzles or centrifugal blast units as required. Once the pipes are welded together to form a string in the field a different situation arises. The heat generated during welding often causes a certain amount of damage to the edge of the factory coating and scorches and disbonds the coating. These damaged areas need to be removed. Typical requirements
Remove all extraneous matcrial and any loose or blistered coating. Degrease the welded area and onto the existing coating either side of the joint using a suitable solvent. Abrasive used - dry expendable abrasive. Profile requirements - Sa2Y2, medium profile or 50-75 pm. Overlap blast pattern onto the existing coating by 30 mrn - only roughen the coating. Inspect blast cleaned areas for surface laminations (slivers) - if any exist then remove with a grinder, check for correct contour and wall thickness then reblast area. Check for disbonded coating; if any exists then remove and reblast the area.
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Coating application Factory application Tllc G U I I I ~ U I I ~ I I ~LoS be coaled are heabd belore applicalion o l lhe powder. The method of heating can be either by thc usc of induction coils or gas flames. Coils are usual for pipe lengths whereas flame systems can be used for awkward shapes, i.e. bends and other fittings. Coils may also be used inside fittings.
The components are usually heated to a temperature slightly above the maximum application temperature for the powder to allow for heat loss during transit from the heating area to the powder application area. When the components are within the application temperature range the powder is applied either byjlock spray or, as is usual, by electrostatic methods. By increasing or decreasing the voltage, the thickness of the coating can be controlled. A typical voltagefor a system of this type is 65 kV. When running in well controlled conditions these systems are quoted as being 98% eficient. A typical output rate would be 6-12 kg of powder per hour.
When using electrostatic spray methods, charging of the particles is usually achieved by corona discharge or ion bombardment, this is achieved in the spray gun head where the discharge electrode is situated. The powder becomes charged (+ve) when it passes through the ionised air surrounding the electrode. The component to be coated is earthed (into the same circuit) and becomes negatively charged thus attracting the charged particles. Earthing is achieved on full pipe lengths through metal wheels making contact with the pipe ends which are not coated. During powder application the pipes normally move past many spray guns whilst rotating on the wheels. On impact with the hot surface, the particles melt and start to cure. When sufficient material has been attracted to an area, the coating acts as an insulation and the charged particles are more strongly attracted to other areas. Complex shapes, e.g. fittings or bends, may be coated in jluidised beds. This is a system whereby low pressure air is passed upwards through vats of powder. As the air is passed upwards through the powder particles, it allows the particles to move about giving the properties of a fluid and so allowing total immersion of a component. When fluidised beds are operating, the surface of the agitated powder has a rippling effect giving the appearance of a liquid. To allow for handling and inspection, pipes which have just been coated are often quenched with cold water. If carried out too soon, water quenching can stop the cure reaction, therefore careful control should be exercised. Typical DFT requirement: 350-650 pm for pipes. Fluidised bed application on fittings can produce thicknesses up to 1500 pm.
Site application Field application of powder onto welded pipe butts is usually done as the third stage of a continual operation carried out from one vehicle: (1) blast clean, (2) heat, (3) apply powder. A flat back trailer or similar carries a compressor which provides air for grit blasting and powder application, a generator for providing power, an induction heater, a powder application control unit, a small blast pot and the consumables needed. In the first part of the operation the grit blasting operative prepares the butt as previously described, he then moves to the next butt whilst the heating coil moves onto the butt just prepared.
Heating The coils can be built to cater for various bun joint widths.
The induction coil is clipped into position and the generator supplies a 110 V alternating current. The induction coil itself does not touch the pipe and only gets warm from radiated heat from the pipe. The coil sets up a field which causes the metal particles to excite, the resulting friction causes the steel to heat up. Heating time can Q Ruane & T P O'Neill h u e 1 :28/04/97
vary from 1-8 minutes with an average time of 4-5 minutes depending upon starting temperatures, induction heat procedures, wall thickness and pipe diameters. When the specified temperature is reached (checked with touch pyrometer or temperature indicating crayons), the heating is stopped, the coil disconnected and moved to the next butt. The specified temperature would normally be slightly higher than the maximum powder application temperature.
If temperature
indicating crayons are used, then the affected area should be wire brushed prior to powder cipplication.
It is usual and necessary to state a temperature over which the pipe must not be heated, e.g. 300°C. Over the stated temperature the steel may lose certain mechanical properties and blueing of the steel is likely; a cut out is likely to be requested if it occurs. Powder application
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Powder application commences using a spraying technique when the metal temperature drops down to the specified powder application temperature range, e.g. 218°C to 246°C. The specification may state the regions where the temperature has to be checked. The equipment used for coating, which is usually semi-automatic, normally has two coating heads which rotate around the welds at a uniform rate constantly spraying the epoxy powder. When the powder comes into contact with the hot metal surface, it melts to form a liquid film which solidifies and cures within approximately three minutes. To reduce differential curing of the epoxy resin, the required coating thickness is applied in as few passes as practicable, e.g. six. However, the use of only two or three passes can be detrimental to film formation and adhesion characteristics. The fluidised bed which contains the powder should be frequently checked by the contractor to ensure there is enough powder to complete an application process. Typical DFT requirement: minimum 400 pn.
Powder storage 60
Epoxy powder is usually supplied in polyethylene lined cardboard boxes and should be kept dry at all times but should not be left in direct sunlight or stored overnight in the application container, otherwise the curing of the coating may be affected. Batches should be used in strict rotation and faulty batches should be placed in quarantine until written authority is gained to use or destroy.
Safety requirements Any sources of ignition should be kept away from the area where epoxy powders are being used. When the powder is in the air it becomes highly flammable. Masks and goggles should also be worn.
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Factorynaboratory tests Tests on the powder a. Infrared scan - gives a fingerprint of the powder for comparison purposes. b. Gel time. c. Particle size analysis - optimum size depends on method of powder application. Test sieves are used complying to BS 410. d. Density.
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Stability - age for 120 days at 25 +l°C in a sealed container; no significant change in properties should occur.
Tests on the detached film a. b. c. d. e. f.
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Micro-sectioning - for homogeneity. Tensile strength - for maximum strength and strength at break. Elongation. Dielectric strength - quoted in kV1mm. Water permeability. Water absorption - after three months immersion at 20°C.
Tests on the attached cured coating Cissing and pinholing. Blistering and appearance test. Sagging test. Thermal analysis - DSC is used. Flexibility test. Impact resistance test. Adhesion test. Hardness test.
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Environmental tests on the attached cured coating Cathodic disbondment test. Strainlpolarization cracking test. Water immersion test. Hydrostatic pressure expansion test (rare). Thermal stability test. Natural weathering test. Humidity resistance test. Salt spray resistance. Artificial weathering test.
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Site testing
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a. b. c. d. e.
Gel time test (rare on site). Dry film thickness. Holiday detection. Adhesion test. Cure test - DSC (sample taken and despatched to laboratory).
Repairs Minor damaged areas, pinholes and areas found with low DFT are marked for repair by using an indelible material by drawing a circle around the unacceptable area. In factories, repairs are not normally permitted within 200 mm of the end of pipes because heating in the field prior to joint coating would destroy the repaired areas.
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Repair materials used are usually two pack solvent free compounds (epoxy or urethane) and should be mixed and applied according to manufacturer's instructions. It is a rare occurrence for a pipe, fitting or a welded joint to be fully stripped of an FBE coating in order to have a powder reapplied. A typical repair procedure on a small unacceptable area would be: 1.
Abrade the affected area using coarse grade emery, e.g. 100 grit.
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3. 4.
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Remove dust with a clean lint free cloth. Repair using specified ?-pack material - using spatula or blade. Check thickness if required and holiday detect when the coating is hard dry.
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Specifications applicable to field application will usually allow small field repairs to be carried out using materials which harden quickly, e.g. blister pack urethanes or fast cure epoxies supplied in two tubes. A typical repair procedure on a large unacceptable area using more conventional 2-pack products would be:
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1.
Blast clean - to original specified requirements.
2.
Remove dust. Repair using specified material - spray, brush and/or trowel. Check thickness and holiday detect when the coating is hard dry.
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Ruane & T P 0WeiI1 The term plastic, when used with reference to a material, is a loose term but generally means a synthetic polymeric material that is usually pliable and can be formed into a usable shape by moulding, extruding, heating or by using a similar process. The term plastic can also include materials which may also be referred to as resins. There are many types of plastic and many types of coating systems employed which use plastics. This Unit deals with the following broad categories of plastic coatings: 1.
Extruded plastic coatings - mainly polyolefins (polyethylene and polypropylene).
2.
Spray applied plastics.
3.
Heat shrinkable plastics.
Plastic coatings are commonly used on pipelines nowadays and can be applied in both the factory and on site. The thickness of plastic coatings vary widely but are typically between 1 rnm and 6 mm.
Materials Plastic types There are a large number of plastics which may be used as anti-corrosion coatings. Some plastic materials are listed below, but not all of them are currently used to coat pipe or fittings used on pipelines: 1.
Polyolefins (alkenes) - low density polyethylene (LDPE - 0.916-0.934 g/cm3), medium density- polyethylene (MDPE - 0.935-0.940 g/cm3), high density - polyethylene (HDPE - ,07941 g/cm3) and polypropylene (PP). PP coafings are more rigid and harder than PE coatings. There are two main groupings for polyolefins (1) aliphahc and (5aromatiTand within each group there are many types of material other than PE and PP. LDPE is widely used although HDPE and PP may be chosen because they can withstand higher service temperatures. Carbon black is usually added to the polyolefin to protect against UV.
2.
Polyvinyl chloride (PVC).
3.
Thermoplastic polyester.
4.
Polyamide (nylon).
5.
Ethylene vinyl alcohol copolymer (EVA).
6.
Huoroplastics.
The physical form of the raw materials will usually be in pellet, granular or powder form depending on the specific requirements for application. NOTE: For composition of heat shrinkable plastic materials see page C3-4.
Adhesives There are three main groupings of adhesives used: 1.
Mastics.
2.
Copolymers - various types but usually based on the polyolefin type in the top coat; in the form of powder for spraying and pellets for extrusion.
3.
Copolymers of the grafted (interactive) type; in the form of powder for spraying and pellets for extrusion.
Fusion bonded epoxy Some coating systems use 2-pack liquid epoxy rather than FBE powder.
FBE may be used with 3-layer coating systems involving polyolefins. There are various types of FBE with different windows of reactivity and different application temperatures. Q Ruane & T P O'Neill Issue 1 : 28/04/97
Ruane & T P 0weill The FBE is applied to the steel substrate in all cases where FBE is used with a polyolefin coating system. The FBE layer is usually thinner than FBE coatings used as stand alone systems.
Coating systems There are various coating systems used which involve plastic materials, the main systems involve polyolefins and are listed below:
Factory
3-layer coating systems combine the excellent chemical resistance properties of epoxy with the excellent mecluznical properties of the polyolejins.
1.
Fused single layer PE.
2.
2-layer PE with soft adhesive (usually termed a mastic adhesive).
3.
2-layer PE with hard adhesive.
4.
3-layer PE with hard adhesive of either the normal copolymer type or the newer grafted copolymer type. E'BE is applied as the first layer.
5.
3-layer PP with hard adhesive of either the normal copolymer type or the newer grafted copolymer type. FBE is applied as the first layer.
Site 1.
Heat shrinkable plastics.
2.
Flame spray applied PP.
3.
3-layer PP with hard adhesive of the grafted copolymer type followed by the application of a co-extruded sheet of PP. Epoxy is applied as the first layer.
4.
Injection moulded PP.
Advantages and limitations I
Advantages 1.
Polyethylene is non-oxidising.
2.
Resistant to penetration.
3.
Good elongation properties.
4.
Good ageing properties.
5.
Extremely impermeable.
6.
High electrical resistance.
7.
Excellent resistance to rnicro-organisms.
8.
Damage to coating is limited during handling and pipeline operations.
9.
Not susceptible to damage during soil stressing.
10. Multi-layer coatings give excellent service performance results. 11. Multi-layer coatings reduce the cost of cathodic protection.
Limitations 1.
Polypropylene needs additives to retard oxidation.
2.
Cost of heating for application (as with FBE).
3.
Multi-layer processes are complicated and require careful control
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Surface preparation Factory The surface preparation requirement on the steel will obviously be governed by the type of initial coating. Dry abrasive blast cleaning to Sa2% is the most likely; amplitude requirements vary but the range specified would normally fall within 40 pm to 100 pm.
Site As for factory application for most systems, except an expendable abrasive is likely to be used. However, some specifications may allow heat shrinkable plastics to be applied onto surfaces prepared to St3 in certain circumstances. It is important to be aware that the surface preparation procedures may not be as straight fonvard as for other coatings. Limitations on methods used to roughen the existing coating and additional surface preparation requirements will exist especially for multi-layer coatings.
Coating application Factory application 'Annular extrusion' may also he referred to as 'cross-head die extrusion' or 'ring extrusion'. 'Side extrusion' uses 'slot dies' and may also he referred to as 'lateral extrusion:
Polyolefins are applied in factories using either side extrusion (winding) or annular extrusion (sleeving) methods. Side extrusion is used on large diameter pipes and annular extrusion is used on pipes of diameters up to -500 mm. In both cases, the raw material (in powder, granular or pellet form) is melted using temperatures which will vary depending on the specific type of material used, the screw type in the extruder and the extrusion rate (200-220°C is typical). The powder form is only melted when it hits the pipe (like FBE). The granular or pellet forms of material are initially fed into a hopper and past the heating units and as it melts it is fed towards the nozzle(s) on the extruder by a rotating screw (or screws). Pressure is usually applied to the coating shortly after it has been applied in order to improve adhesion and force out any entrapped air. With side extrusion a non-sticking pressure roller may be used, whereas with annular extrusion a vacuum system plus a roller system may be employed. There are a large number of coating systems involving plastics and also many variations in the materials and application methods used within some of these systems. Typical options are shown below for a 3-layer coating procedure; this should demonstrate some basic principles which will also apply to other systems.
1.
Blast clean - remove dust.
2.
Inspect.
3.
Phosphoric acid treatment andlor chromate wash (not always applied).
4.
Heat pipe using either induction coil or gas fired oven to specified temperature. The temperature range used depends on the type of epoxy used but is likely to fall within the 160°C to 250°C range.
5.
Apply epoxy by using a method recommended for the material - electrostatic spray, twin feed hot airless spray or conventional airless spray (depending also on
Q Ruane & T P O'Neill Isue 1 :28/04/97
Ruane & T P O'Nejl the form of the material). Thickness requirements vary but usually come u'ithic 50- 100 pm.
6.
Apply the adhesive (tie-coat) by either extrusion or by powder spray (extrusion usually gives superior properties). 300-400 pm is a typical thickness requirement.
7.
Apply PE or PP by extrusion. The thickness requirement is determined by the pipe diameter and service conditions.
8.
Cool pipe by water spray (after sufficient time has elapsed for polymerisation and crosslinking of the epoxy first coat).
9.
Inspect.
10
'
Site application 3-layer PP The welded joints connecting pipes coated with a 3-layer PP system can be coated using a similar system to that adopted in the factory. This system will require a procedure similar to the following: 1.
Cut back existing PP, using bevelling machine, to reveal existing FBE (25 mm or as otherwise specified).
2.
Mask off existing FBE and blast clean joint area to specified standard, e.g. Sa2% at 50-100 pm. Remove masking and abrade existing FBE using emery cloth.
3.
Heat joint area using an induction coil to the temperature specified, e.g. 240-250°C.
4.
Apply FEE powder when the joint is at the specified application temperature and introduce PP adhesive (copolymer) powder during final passes to produce what is essentially a 2-layered sintered FBEPP coating which will cross-link at the interface. The PP adhesive must be applied within the gel time of the FBE. Typical thickness requirements: 300 pm for the FBE and 200 pn for the PP adhesive (tolerances will apply).
5.
Apply preheated co-extruded PP sheet, which is normally the same thickness as the existing coating, around the joint and apply hinged clamp in position around the coating during the cool down period. The sheet should butt up to the existing PP - overlap onto the existing PP is not desirable. Remove clamp when temperature has dropped below 120°C or as otherwise specified.
6.
After cleaning away any adhesive material, weld the circumferential seams of the PP sheet by using a special welding extruder gun which may be attached to a machine which travels circumferentially around the joint. The plastic welding filler material should be the same material brand as the PP sheet.
7.
Finally, the longitudinal seam is welded with an extruder gun by using a manual technique.
40~
60
A modification to the above procedure which may be encountered is to apply the PP layer by injection moulding in lieu of using a wrap around PP sheet.
Heat shrinkable plastics
Hot melt type adhesives have a higher lap shear strength than mastic type adhesives and would be preferable for certain applicaions such ar thrust boring.
Heat shrinkable plastic materials may be supplied in many pre-expanded forms, some of which will fit tees, flanges and similar components. The common types encountered on pipelines for use on welded joints are supplied either as short continuous sleeves, slightly larger than the pipe diameter, or in bandage type form to wrap around the joint and overlap onto itself. Heat shrinkable tapes also exist. Heat shrinkable products have an adhesive layer composed of either a pressure sensitive mastic or a thermoplastic hot melt type adhesive.
0 Ruane & T P O'Neill h u e 1 :28/04/97
Materials are available in standard and heavy duty forms and are therefore available in different thicknesses. The plastic material is a radiation cross-linked polyolefin which is elastic in nature. Once it is shrunk onto the area to be coated by the application of heat, it will not return to an expanded state on the re-application of heat. 10
Primers are not recommended for mastic systems, but at the other end of the scale these coatings inay be used as part of a 3-layer syslerxl where Lhe firs1 layer is usually a 2-pack epoxy. The heat shrinkable inaterial is applied over the epoxy wheen it is still tacky. When the heat shrinkable material is in position, heat is applied, usually by using a long yellow flame from a propane torch, working from the centre of the wrapping outwards. Induction heating may also be used in some circumstances. Note: Manufacturers of these products issue very detailed step by step instructions on how they should be applied. These instructions should always be followed unless the specification, procedures or instructions issued state otherwise.
I
Testing Tests on polyolefins a. b. c. d. e. f. g. h. i. j. k. 1. m. n. o. p. q.
I
Density. Melt index. Elongation at break. Maximum moisture content. Softening point. Impact resistance. Indentation resistance. Electrical insulation resistance. Resistance to UV energy. Thermal stability. Cathodic disbondment. Flexibility. Peel strength. Thickness. Holiday detection. Visual inspection - blisters, general contours. Weld integrity - coupons cut from seam welds on site applied systems involving wrap around PP sheets.
Repairs The repair materials and procedures used would have to be approved prior to use as applicable to any other coating system. Most standards and draft standards which apply to polyolefin coatings do not identify specific materials or detailed procedures to use. They do however state the obvious, that is to say that any repair material and procedure used shall be compatible with the existing coating and satisfy service conditions.
;k
90
Repairs in the factory or on site can be effected using copolymer PE or PP which, after surface preparation, is melted into the problem area using a special extruder gun or by . Heat shrinkable plastics in patch or bandage form may also be used using a melt - especially on site. When repairs to plastic coatings are necessary on site and on the lower integrity plastic systems, cold applied laminated tapes may be permitted.
0 Ruane & T P O'Neill h e l :28/04/97
Elastomeric pipeline coatings are very flcxible and have good insulation properties over a range of temperatures and can be applied in substantial build thicknesses. They -elongate under stress and return to their original relaxed condition without any adverse results.
1
Elastomer types EPDM EPDM (ethylene propylene diene monomer) is an elastomer which is not widely used nowadays due to required additions of carbon black (pigment). The addition of carbon black improves the material's abrasion and impact resistance properties, although too much adversely affects CP current which is conducted away from the coated structure. EPDM is essentially a mixture of ethylene and propylene. When polymerised, ethylene C,H,(E) gives E-E-E-E in varying numbers of ethylene molecules, hence the different grades, e.g. LDPE, MDPE etc.. Propylene C,H, (P), when polymerised, gives P-P-P-P. A mixture of ethylene and propylene produces a material which is rubber-like in appearance and properties, very unlike the original materials.
' Polychloroprene (Neoprene) - Dupont Manufactured by the chlorination of isoprene, the material looks like rubber. It is supplied in pellet form compounded with fillers for reinforcing, e.g, silica and carbon black, to improve impact and abrasion resistance. Neoprene has the property of being fire retardent because of the chlorine content (Cl,). Conductivity problems do not occur (compare EPDM) and the material has excellent abrasion resistance.
Bituseal - Shell Bituseal is a polymer bitumen and a trade narnc of a Shell product. This material is thermoplastic and has excellent low temperature flexibility. The softening point is much lower than conventional bitumen (85°C compared to 120-130" for ccnventional materials), therefore as the limitations for usage are at the upper end of the temperature scale, it is not recommended for use above 85°C but has excellent properties for use on land at temperatures of less than -20°C. Conventional asphalt and bitumen are brittle at ambient temperatures but bituseal can be cold bent at temperatures in the range of -20°C.
Elastomeric polyurethane The reaction between a polyol and an isocyanate produces polyurethane. Different reactions and formulations give materials with different properties, which vary from elastic to rigid solids. All polyurethanes are thermoset materials and do not soften with the application of heat. Syntactic polyurethanes are of a similar structure to cake, with a tiny closed cell structure and are formed in situ. The closed cell structure significantly improves the normally poor insulation properties. Glass microsphercs are sometimes used as $fillers to improve abrasion resistance with this material.
O Ruane 61 T P O'Neill Issue2 : OY10l/!J8
I 1
Advantages and limitations Advantages 1.
Excellent insulation properties.
2.
Thick coatings can be applied in a single layer.
3.
Flexible, e.g, materials can be used on pipes to be laid by reel barge.
4.
Excellent abrasion resistance.
5.
Good impact resistance.
6.
Good heat stability.
7.
EPDM has relatively high operating temperatures, e.g. 125°C.
Disadvantages 1.
Crosslinking polyurethanes use toxic isocyanates.
2.
Polychloroprenes require vulcanising (heat treatment).
3.
Some elastomeric coating systems are not feasible for site application.
4.
EPDM needs carbon black as a constituent (adverse affect for CP).
5.
EPDM is difficult to stick to anything, especially itself.
Surface preparation Factory Dry abrasive blast cleaning to Sa2% is the most likely. Amplitude requirements vary but the range specified would normally fall within 40-100 pm.
Site Rarely applicable. Syntactic polyurethanes may be cast on-site but over a primed area.
Application Application methods vary from extruded spiral layering over an anti-corrosive (FBE) layer, to interlayering with closed cell pvc foam for extra insulation. In most cases a primer or a bonding agent (adhesive) would be used prior to application of the elastomeric system.
EPDM and Neoprene These materials are applied at up to 180°C and extruded over the primed surface in a continuous sheath. Neoprene is sometimes extruded in a thin strip and spiral wound onto the pipe. The whole pipe is then wrapped with fibreglass or nylon tape and placed into an autoclave for approximately two hours at 140°C for vulcanisation. After vulcanising the support tape is removed, leaving an indented replica pattern. The ends are trimmed back and any required tests are done at this stage.
O Ruane & T P O'Neill Issue 2 : 05110lYX
I 10
Elastomeric polyurethanes Two part polyurethanes can be applied by casting or by controlled rotational drip and pour (sometimes referred to as controlled rotational casting). Casting on pipe lengths is done by placing the pipe central in a female mould, tilting to an angle of about 30-4.5" and filling the cavity through a filling point at the bottom end uf tllc ~lluulcl. A v c 1 1 ~u~ ~ l l cup US~llc~illcdp i p alluws air LU escape bcfurc LIIC cxccss product. Demoulding time is approximately twenty minutes. Controlled rotational casting is done by ejecting a stream of the mixed components onto a rotating pipe; this is done from a position in line with dead centre of the pipe bore and the 12 o'clock position. The rotation and feed speed are adjusted so that no overlap or voids occur, leaving a slightly corrugated surface. The material quickly achieves a degree of cure which allows another layer to be applied following behind at approximately 1 m away.
Bituseal 30
Bituseal is applied hot at approximately 190°C by extrusion, usually over an anti-corrosive primer. Brittle bitumen and coal tar enamels need temperatures in excess of 200°C but have correspondingly higher softening points of 120-130°C. Unlike the conventional bitumen enamels, the elastomeric bituseal does not have an inner layer of reinforcement. A typical thickness range is 4-6 mm with usually an impregnated outer wrap for abrasion resistance.
Thickness 50
Magnetic or electromagnetic induction.
Cure BitusealIEPDM - hardness (shore, barcol, needle penetration). Neoprene - hardness (shore), cure (rheometer). 60
Polyurethane - hardness.
Adhesion Peel adhesion (on pipe ends).
Disbondment Ultrasonic methods. Tapping (sounding).
Pinholes High voltage holiday detection.
O Ruane & T P O'Neill Issue 2 : USllOl98
Multi-component liquid (MCL) is not standard terminology but it may be used to describe any coating products which are supplied in two packs or more. MCL's used are usually solvent free, two pack materials using an epoxy or urethane base. Depending on the particular product, MCL's may be spray applied, brush/trowel applied or applied with a palette knife or similar to repair areas. An MCL may be used to coat pipe lengths but they are more likely used to coat fittings, welded joint areas and to repair defects in certain coating materials.
Constituents Epoxies Base resin in one container with m i n e or amide reagents supplied separately, either in tubes or cans.
Urethanes Urethane, or urethane and pitch, supplied in drums or cans with modified isocyanate or di-isocyanate curing agent in another can or drum. Sometimes provided in blister packs for small repair jobs.
Advantages and limitations Advantages Epoxies 1.
Supplied in various pack sizes to avoid waste.
2.
User friendly.
3.
No solvent - DFT same as WFT; no significant compatibility problems.
4.
Very good film properties.
5.
Fast drying rates (typically 7 days to full cure).
6.
No toxic hazards.
Urethanes 1.
Supplied in various pack sizes to avoid waste.
2.
No solvent - DFT same as WFT; no significant compatibility problems.
3.
Excellent film properties - especially abrasion resistance..
4.
Fast drying rates (typically 7-10 days to full cure).
Disadvantages Epoxies 1.
Short pot life.
2.
Very poor adhesion to plastics.
Urethanes 1.
Short pot life.
2.
Very poor adhesion to plastics.
3.
Toxic components.
4.
Some types are very moisture sensitive.
O Ruane & T P O'Neill Issue 2 : 21/04/98
I
Surface preparation Factory
lo
Only fittings such as bends and tees are likely to be prepared and coated with MCL's in a factory environment. Surface preparation is normally carried out by dry abrasive blast cleaning.
I
Typical requirements (blast cleaning)
20
Abrasive used - steel grit or grit and shot mix. Profile requirements - Sa2?h, medium profile or 50-75 w. Inspect blasted areas for surface laminations (slivers); if any exist then remove with a grinder, check for correct contour and wall thickness then reblast the area.
Amplitude requirement may be up to 100 ,urnfor some urethanes.
30
Site Welded joints are blast cleaned using pressure blasting equipment and dry expendable abrasive. Typical requirements (welded joints) Remove any solar protective coating and all extraneous material from the existing coating (150 rnm is typical requirement). Degrease only if necessary. If existing pipe coating is thick, e.g. if coated with an enamel, chamfer coating by 50 mm or 100 mm as specified, using a blow torch and scraper. Blast clean using dry expendable abrasive. Profile requirements: Sa21/2, medium profile or 50-75 pm. Inspect blasted areas for surface laminations (slivers); if any exist then remove with a grinder, check for correct contour and wall thickness then reblast the area. Check for disbonded coating, if any exists then remove and reblast area.
60
-
Coating application Factory Urethane MCL's are sometimes used for coating pipeline fittings and special fabrications for subsequent delivery to site. Because urethanes (two pack) are isocyanate cured, application by spray can only be carried out in closely controlled conditions. When using moisture sensitive material, the maximum RH% requirement during application and for part of the cure period may be as low as 70%. The minimum airlsteel temperature requirement during application may be 10°C in some instances. It is therefore a common requirement that heaters and dehumidifiers have to be used. Multi-component spray grade urethanes are usually applied using a twin component hot airless spray system. These use twin airless pumps, skid mounted and specially set to dispense pre-set amounts of base and activator. The components are heated, usually by coils around the container, and the pre-set flow rate measures out the correct ratio of base and activator into a mixing chamber. The mixing chamber is usually tube shaped and contains a series of baffles through which the two components pass and so the mixing is accomplished. There is also a solvent feed line entering the chamber. Because the chemical reaction rate increases as the temperature rises, the pot life is considerably shortened, in some cases to a few minutes. Spray lines are usually kept to a limited length. Tip blockages or hold ups of any kind are critical because the material O Ruane & T P O'Neill h u e 2 : 21/04/98
may gel in the lines and gun resulting in loss of equipment - usually costing over £1000. If there is any such occurrence, the . pump - lines are immediately closed, the solvent line is opened and the mixing chamber, spray line and gun are flushed out with solvent. On resumption of work, the solvent has to be completely flushed out of the system with produce before coating recommences, otherwise pinholing and blistering may occur. Typical thickness requirements: for urethane MCL coatings 1 mm or 1.5 mm minimum; for epoxy MCL coatings 500 ptn minimum.
Site 20
MCL's are likely to be brush andlor trowel applied on site. The material will be supplied in cans in any situation other than for minor damage and pinhole repair where epoxy or urethane are likely to be supplied in tubes or blister packs respectively. Becanse of the short pot !ife consideratio:: and the requirement that b ~ t hco=pone::t~ be thoroughly mixed in the correct ratio, mechanical mixing is desirable when components are supplied in cans. For urethanes, ambient conditions may be more stringent than for epoxies. Application should be in accordance with the manufacturer's instructions and for urethane, will usually require application in two layers, the second layer being applied at right angles to the first after a specified time lapse.
1
Safety When using MCL's, especially urethanes, it is advisable to wear protective clothing with a positive air supply face mast for urethanes (isocyanates). If welding of coated components is required, a suitable uncoated margin should be left, e.g. 150 mm. Heat should not be used to remove urethanes because various cyanide gases will be released. Removal of faulty coatings should be by prolonged blasting only.
I
Factoryflaboratory tests Tests on the unmixed components a. Total non-volatile content. b. Viscosity. c. Specific gravity. d. Mixing ratio. e. Pot life. f. Flash point. g. Stability
I
Tests on the detached film a. Micro-sectioning - for homogeneity. b. Tensile strength - for maximum strength and strength at break. c. Elongation. d. Dielectric strength - quoted in kVImm. e. Water permeability. Water absorption - after three months immersion at 20°C. f. Tests on the attached cured coating a. Cissing and pinholing. b. Blistering and appearance test.
Q Ruane & T P O'Neill
Issue 2 : 21/04/98
c. d. e. f. g.
Sagging test. Flexibility test. Impact resistance test. Adhesion test. Hardness and cure test.
R u 6 I)~ DC?Z
Environmental tests on the attached cured coating Cathodic disbondment test. Strainlpolarizationcracking test. Water immersion test. Ejdrostatic pressure expansion test. Thermal stability test Natural weathering test. Humidity resistance test. Salt spray resistance. Artificial weathering test.
Site testing 30
a. b. c. d.
Dry film thickness. Holiday detection. Adhesion test. Hardnesslcure test.
Repairs Repairs to MCL coatings are usually made using the same material as the original coating or, especially on site, by using a fast drying product with the same base material and from the same manufacturer. A typical repair procedure on a small unacceptable area would be:
,,
1.
Abrade the affected area using coarse grade emery, e.g. 100 grit.
2.
Remove dust with a clean lint free cloth.
3.
Repair using specified 2-pack material - using spatula or blade.
4.
Check thickness if required and holiday detect when coating is hard.
Specifications applicable to field application will usually allow small field repairs to be carried out using materials which harden quickly, e.g. blister pack urethanes or fast cure epoxies supplied in two tubes. 70
,,
A typical repair procedure on a large unacceptable area using more conventional 2-pack products would be:
1.
Blast clean - to original specified requirements.
2.
Remove dust.
3.
Repair using specified material - brush and/or trowel.
4.
Check thickness and holiday detect when coating is hard dry.
Q Ruane & T P O'Neill Issue 2 : 21/04/98
Ruane & T P 0'Neil/ Several types of wrapping tape are used in the pipeline industry, the main categories are as follows: 1.
Hot applied tapes.
2.
Cold applied laminate tapes.
3.
Grease based tapes.
4.
Self adhesive overwrap tapes.
5.
Heat shrinkable tapes - see Unit C3.
Whenever tapes are used on risers and similar, care should be taken to ensure spiralling and terminations create a water shed, otherwise disbonding may occur. When spiralling a tape around a pipe the following should be observed: a. b. c. d.
Correct tensioning of the tape Correct overlap distance onto itself - typically 55% of tape width (referred to as double wrap)or 25 mm for thicker tapes. Correct overlap distance onto existing coatings - 75 mm minimum is typical. The tape should face downwards at the start and finish.
Specifications do not normally specify DFT requirements when tapes are used because the thickness of the tape and the overlap requirement will be known.
Hot applied tapes Hot applied tapes are not often used nowadays. Their main application is for welded joints when the existing pipe coating is an enamel, i.e. coal tar or bitumen. They are used in conjunction with a primer.
Constituents A synthetic fibre bandage, e.g. woven nylon, coated with plasticised coal tar or bitumen. The coal tar or bitumen is sometimes applied to one side of the bandage only. In this instance the coated side is placed against the pipe.
Advantages and limitations 1 Advantages 1.
High resistance to mechanical damage.
2.
Available in several widths.
Limitations 1.
Needs a blowtorch or similar for application.
2.
Toxic fumes.
3.
Unpleasant working environment.
4.
Limited use due to compatibility problems with existing coatings (used with enamels only).
5.
Application requires a minimum of two operators.
6.
Susceptible to soil stressing.
Surface preparation 90
Factory Not normally applied in factories.
Q Ruane & T P O'Neill k u e 1 : 28/04/97
1
Site Welded joints are blast cleaned using pressure blasting equipment and dry expendable abrasive. Typical requirements (welded joints): Remove solar protective coating and all extraneous material from the existing coaling (150 rnrn is typical rcquircmcnt). Degrease only if necessary. Chamfer coating by 50 mm or 100 mm as specified, using a blow torch and scraper. Blast clean using dry expendable abrasive -profile requirements - Sa2%, medium profile or 50-75 pm. Inspect blasted areas for surface laminations (slivers); if any exist then remove with a grinder, check for correct contour and wall thickness then reblast the area. Check for disbonded coating, if any exists then remove and reblast the area.
Coating application (site) Hot applied tape based on coal tar should only be used when existing pipes are coated with coal tar enamel. Hot applied tape based on bitumen (asphalt) should only be used when existing pipes are coated with bitumen (asphalt) enamel. Although mainly used for the wrapping of welded butt joints, hot applied tape may be used for wrapping complete pipe lengths. Appropriate primers are considered essential and are usually applied to an area which is slightly larger than the area to be wrapped. When the primer is dry, Operator 1 applies the correct tension and positions the tape to give a spiral wrap with a neat consistent ovenvrap of usually 55% (this ensures double tape thickness over the wrapped area). Operator 2 applies the blowtorch to the contact areas of the tape and any existing enamel, melting both materials and allowing them to fuse together to form one homogenous layer. Problems arise with over and under heating. Overheating results in combustion and/or coking; under heating results in lack of fusion, kinks and voids.
,,
Areas which may consistently give cause for extra vigilance are 12 o'clock and 6 o'clock positions, especially on larger pipes where a hand over to a second crew may be required.
Safety requirements Generally as required for site application of enamels.
I 80
Testing Tests on the primer 1.
Viscosity measurement using a flow cup at a specified temperature.
2.
Visual check for coverage.
3.
Measure D m of primer if requested.
Tests on the final wrapping
I ,,
Tapping to check for laminations.
1 2.
Holiday detection.
3.
Bond strength test.
4.
Visual check for uniformity of coating contour and ensure no coking exists.
O Ruane & T P O'Neill h u e 1 : 28/04/97
8
I
Repairs Repairs to hot applied tape wrappings can be carried out by various methods depending on the nature and size of the coating fault. Pinholes are sometimes repaired using a hot knife whereas extensive damage would normally require full circumferential removal of the wrapping over the affected area. The bare area may then be repaired with either hot applied tape, heat shrinkable material, cold applied lanlinate tape 01 a glease based kdpe uvel wrapped will1 a self ad~esiveuvel wlap lape as specified. Multi-component liquids could also be used. The method employed should be governed by specification. Patch type repairs with tapes are not advisable; if tapes are specified then full circumferential wraps will be required.
Cold applied laminate tapes Cold applied laminate tapes (CALT) are commonly used to wrap welded joints primarily because they are easy and relatively clean to use. They may be also be used to wrap fittings, full pipe lengths and may be used for full circumferential repairs. Patch repairing with these tapes should be avoided. These tapes are compatible with most coating materials and they are often used in conjunction with a primer.
Constituents Cold applied laminate tapes are made up from a polyethylene (PE) or polyvinyl chloride (PVC) carrier tape with a coating of self adhesive, anti-corrosive bituminous compound or self adhesive synthetic rubber compound. Over width silicone coated or wax coated interleaving paper exists in each tape roll to make it easier to unroll the tape during application and it also reduces the amount of dirt contaminating the edges of the rolls.
Advantages and limitations Advantages 1.
Available in many widths.
2.
No extensive training required for application.
3.
Very few compatibility problems and will adhere to virtually any coating.
4.
Can be applied manually or by a special application machine.
Limitations 1.
More difficult to apply at low temperatures (needs heating).
2.
User unfriendly at high temperatures (mastic viscosity).
3.
Can be over tensioned.
4.
Susceptible to wrinkling during soil stressing - poor lap shear strength.
5.
Does not adhere well to sharp contours, e.g. edges of welds.
Surface preparation Factory Not commonly applied in factories - if done, blast cleaning or chemical cleaning may be encountered..
Site Welded joints are blast cleaned using pressure blasting equipment and dry expendable abrasive. O Ruane & T P O'Neill Issue 1 : 28/04/97
Ruane & T P Oweill Typical requirements (welded joints): Remove solar protective coating and all extraneous material from the existing coating (150 mm is typical requirement). Degrease only if necessary. Chamfer coating by 50 mm or 100 mm as specified, using a blow torch and scraper. Blast clean using dry expendable abrasive -profile requirements - Sa2%, medium profile or 50-75 pm. Inspect blasted areas for surface laminations (slivers); if any exist then remove with a grinder, check for correct contour and wall thickness then reblast the area. Check for disbonded coating, if any exists then remove and reblast the area. Substrates other than steel are usually wire brushed or abraded to give a key and then appropriate primers applied as per specification or manufacturer's instructions.
Application Factory When CALT is applied at a factory as the primary anti-corrosion system, machine application with a static roller rig is likely to be the method used. The pipe lengths are rotated and geared to the rig which carries the spool of tape, correctly tensioned, to wrap neatly and evenly along the pipe. A primer as specified by the tape manufacturer would be used before tape application. The primer usually has to be in a dry state before the tape can be applied.
Site Although line travel machines do exist for CALT, application is more likely to be manual. A primer as specified by the tape manufacturer would be used before tape application. The primer usually has to be in a dry state before the tape can be applied. The tape is then applied in a spiral fashion, usually with a 55% overlap, maintaining a uniform tension. Under tension can result in unsightly creases and wrinkles, which if left, will lead to early failure (or their existence will increase the CP current used). Over tension shows up as discoloration of the (normally black) tape. The tape stretches and narrows and shows up light bluelgrey in the area of the stress.
Safety requirements There are no significant hazards associated with CALT
Testing Tests on the primer 1.
Viscosity measurement using a flow cup at a specified temperature.
2.
Film thickness measurement.
Tests on the final wrapping 1.
Holiday detection.
2.
Bond strengthladhesion test.
3.
Visual check for uniformity of coating contour.
Repairs Repairs to CALT wrappings, e.g. after adhesion tests, can be carried out by overwrapping with CALT. Patch type repairs are not advisable; full circumferential wraps are normally specified.
Q Ruane & T P O'Neill
Issue 1 : 28/04/97
Grease based tapes Constituents Grease based tapes usually consist of a bandage of woven or non-woven synthetic material or glass fibre. This carrier is impregnated with petrolatum grease - refined petroleum condensates to which are added moisture displacing agents and sometimes inhibitors.
Advantages and limitations Advantages 1.
Compatible with all pipeline coatings.
2.
Easily mouldable for awkward shapes.
3.
Very low standard of surface preparation requirements.
4.
Very simple to apply.
Limitations 1.
Can be very messy to use.
2.
Does not give a coating of high quality.
3.
Usually requires overwrapping, especially on buried pipelines.
Surface preparation Factory Not normally applicable. Site Blast cleaning would not normally be specified when grease based tapes are used. In most instances, as long as loose and flaky materials are removed, that will suffice. Wire brushing to St2 or St3 is commonly specified. All traces of moisture should be removed.
Application Factory Not applicable. Site Grease based primers are normally recommended but not always specified. The tape is applied in a spiral fashion usually with a 55% overlap. Grease based can also be applied doubled or twisted to fill in sharp contours because of its mouldability and may sometimes be used in place of more conventional filler materials, i.e. mastics and putties.
Safety requirements There are no significant hazards associated with grease based tapes.
Testing The are no testing requirements other than visual examination for contour. Holiday detection and DFT measurements are not practical propositions unless the grease based tape is overwrapped. Even if overwrapped these tests are not always carried out because it will be very unlikely that any specification deviation will be found. Holiday
0 Ruane & T P O'Neill Issue 1 : 28/04/97
detection on overwrapped grease based tapes which have been in service is a normal requirement.
Repairs 10
Repairs to grease based tapes are easily applied by using the same product. Patch type repairs are not advisable; full circumferential wraps are normally specified.
Self adhesive overwrap tapes
,,
As the title suggests, these tapes are usually only used to overwrap other coatings such as grease based tapes and cold laminate tapes.
Constituents A strong pressure sensitive adhesive is bonded to a tough carrier tape of PE or PVC. This medium is not designed as a corrosion protection system but to add mechanical strength to other mediums and to stop migration of constituents in grease based tapes and mastics and fillers.
Advantages and limitations Advantages 1.
Clean to use.
2.
Simple to use.
3.
Available in various widths.
4.
Excellent adhesion to itself and other smooth surfaces.
5.
Compatible with all other coatings.
Limitations
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1.
Not a suitable anti-corrosion coating system in its own right.
2.
Wide tapes are physically difficult to apply - correct tensioning is important.
3.
Not tolerant of uneven surfaces.
Surface preparation Factory Not normally applicable.
,
Site Blast cleaning would not normally be specified when grease based tapes are used. In most instances, as long as loose and flaky materials are removed, that will suffice. Wire brushing to St2 or St3 is commonly specified. All traces of moisture should be removed.
80
Application Factory Not normally applicable but could be applied mechanically or manually as overwraps for other mediums.
90
Site Primers are sometimes recommended but not always specified. The overwrap tape is applied in a spiral fashion usually with a 55% or 25 mm overlap. It may be specified that the direction of spiralling must be made in the opposite direction of the spiralling on the existing tape wrapping.
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Ruane & T P o' ~ e i l l If welded joints or repairs are being made with either grease based tape or cold laminate tape, the overwrap tape must be firstly adhered to the existing coating before overwrapping the joint wrap and beyond onto the existing pipe coating on the other side of the joint. Overlap requirements on either side of the joint wrap is typically a minimum of 50 mm, 75 mm or 100 rnm depending on the specification.
Safety requirements There are no significant hazards associated with self adhesive overwrapping tapes.
Testing Tests on the primer 1.
Viscosity measurement using a flow cup at a specified temperature.
2.
Visual check for coverage - DFT not normally measured.
Tests on the final wrapping 1.
Holiday detection.
2.
Bond strengWadhesion test (in rare circumstances).
3.
Visual check for uniformity of coating contour.
Repairs Repairs to overwrap tapes are easily applied by using the same product assuming the underlying coating material is not significantly affected. Patch type repairs are not advisable; full circumferential wraps are normally specified.
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Brushing mastics are high viscosity coatings which may be built up to the required thickness, as specified by the manufacturer, by a number of applications (usually two.) after primer application. Each layer must have an appropriate thickness as specified by the manufacturer. For most products a total dry film thickness of 500 pm will give satisfactory results. The following points should be taken into consideration when using brushing mastics: a. b. c. d. e.
30
Not suitable for overlapping onto polyolefin pipe coatings. Ideally suitable for components of complex shape. Very susceptible to rocWstone damage and will normally require the use of sand or similar padding around the finished coating if buried. Not suitable for overlapping onto certain tapes, although the tapes may overlap onto the mastic. See manufacturer's data sheet for compatibility. Damage due to scuffing may easily be patch repaired.
Application The following text shows a typical application procedure: 1.
After surface preparation, apply primer by brush, overlapping onto the factory applied coating, to attain a total dry film thickness of approximately 25 pm.
2.
As soon as the primer has dried, e.g. after 1 hour at 20°C, apply one coat of the specified mastic, laid on evenly by brush, to give an appropriate dry thickness specified by the manufacturer overlapping on to the existing factory applied coating by a minimum of 75 mrn.
3.
Allow to dry for at least 4 hours at 20°C before applying a second coat of the specified mastic, by brush, to a minimum dry film thickness specified by the manufacturer.
4.
Before the coated components can be handled or buried, a specified time must elapse which can be anything from 24 hours to 7 days depending on the product type and ambient conditions.
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I lo
I
Fillers are based upon a range of materials, e.g. petrolatum grease, bitumen and rubber. They are also available in a number of forms such as tape, extrusion or bulk packaged in containers or sacks. They are normally intended to be used for modifying the contours of valves, flanges and similar components, in such a manner that allows for the components to be wrapped with conventional cold applied tapes or heat shrinkable material. Primers are normally used. Application of the filler is normally carried out by hand, although trowels and knives may also be used.
I
The following points should be taken into consideration when using fillers: a. b.
c.
d.
Materials must be carefully chosen having due regard to their suitability for use with any existing coating and the overwrapping tape to be used. Suitability for use on components operating at elevated temperature or in situations where soil stressing might occur will be related to the base product of the material. Some material will not exhibit particularly good adhesion to the primed surface, therefore reliance will be placed upon the overwrapping to provide an impervious barrier. Material containing bitumen or petrolatum grease should also contain biocides to prevent microbial degradation.
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Internal pipe coatings may be applied for the following reasons: a. b. c. d. e.
An example of a starzdard which deals with interturl pipe coatings is: BS ISO/CD 15741 - 'Interr~nl paint coating of pipelines for the conveyance of mn-corrosive gas:
Prevents corrosion during pipe storage. Aids gas flow - reduces friction. Reduces vibration caused by turbulence. Reduces fatigue stress - which could cause failure. Reduces noise pollution.
The coating chosen is normally 2-pack polyamide cured epoxy therefore the inspection approach is similar to that for a Painting Inspector. Many of the following requirements have been taken from the draft standard identified in the side note.
1
Surface preparation Surface preparation is normally carried out by dry abrasive blast cleaning to Sa2% medium grade. Slivers, laps etc. to be removed by grinding. Dust removal to class 0 in accordance with I S 0 8502-3. Use high pressure water to remove contaminants such as salt and dirt. Any organic contaminant such as grease or oil to be removed by using detergent or an approved organic solvent. The remaining salt level and methods for measurement should be agreed between contracting parties. Other standard specifications may specify chemical cleaning (pickling) as the preparation method, especially on small diameter pipes where blasting will be impractical.
I
Coating application Temperature shall be at least 3°C above the dew point temperature - heat pipe if necessary. Other ambient requirements would be as specified by the coating manufacturer. Paint will be spray applied. An accelerated curing procedure may be used by heating the pipe up to a maximum of 100°C.
10'
Testing Tests on the wet paint a. b. c. d. e. f. g. h. i. j. k.
Viscosity. Density. Solid content (by weight). Solid content (by volume). Drying time (to handle). Pot-life. Pinholes (porosity) - using glass plates. Ash content. Fineness of grind. Sieve retention. Flash point.
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Tests on cured paint film a. b. c. d. e. f. g. h.
1
j.i. k. 1.
Buchholz hardness. Adhesion test (cross-cut test). Salt spray resistance. Bend test (conical mandrel). Water resistance. Methanovwater 1:1 resistance. Methanol resistance. Gas blistering. Hydraulic blistering. High temperature resistance. Dry film thickness. Visual - sagging etc..
Repairs Significant defects are repaired using the same procedure as used with the original coating, except that pinholes and small damaged areas may be wire brushed instead of blast cleaned. In both cases lightly abrade onto existing coating by at least 10 mm. Damaged internal coatings are not often repaired when found on site.
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Overall, there are large number of tests which are carried out on raw coating materials and the resultant coatings. Tests may be carried out forfitness for purpose reasons or for quality control. Coating inspectors, especially if working on behalf of a client/customer, would not normally conduct any test associated with raw materials other than the gel time test associated with FBE or possibly viscosity tests for quality control reasons. A test associated with the final coatings are more likely to be performed by a coating inspector but not if it is a laboratory type test. However, a coating inspector requires a certain degree of knowledge about laboratory type tests because the inspector working on behalf of the client/customer may be required to verify that test results meet specification requirements.
Factorynaboratory based tests Viscosity Viscosity is defined ar a @id's resistance tojloiv.
See also Unit P7E. All liquids in use in the coatings industry would be subject to viscosity tests at some stage of a production and sometimes prior to application. For ambient temperature application primers, either rotational viscometers or flow cup types could be used, e.g. I S 0 or Ford flow cups. For hot application enamels (coat tar and bitumen) and hot melt type mastics and adhesives, the Zahn cup or Frikmar cup are most likely.
If a flow cup is being used, the correct selection of flow cup type, hole size and test temperature are important factors. Viscosity tests are occasionally encountered on site.
Gel time (FBE powder) The gel time test is a test may be conducted on epoxy powders prior to application to ensure that the powder is in a suitable condition to be used in order to achieve the correct degree of cure from the final coating, i.e. in order to obtain the expected properties from the coating. The gel time will be affected by the age of the powder, i.e. there will be a limited shelf life. During transport and storage, the raw powder material will be subjected to different temperatures and compaction forces. This allows sintering and a small degree of molecular freedom resulting in chemical action (cross-linking). This means that when the powder is applied, the cure on the substrate will not allow full wetting, may entrap gasseslair and will cure too quickly or improperly.
If test results show that the gel-time is below the stated minimum time then the powder must not be used because the curing of the coating would probably be adversely affected. To determine gel time: 1.
Measure the temperature of a hot plate to ensure it meets the requirement of the data sheet or specification.
2.
Apply a small amount of epoxy powder (approximately half a teaspoonful) onto the hot plate using a spatula.
3.
Start stopwatch when powder hits hot-plate.
4.
Stir epoxy (now liquid) using spatula and lift. Epoxy liquid will be runny, but after a number of repeated attempts the liquid will gel (string like toffee); at this stage stop the stopwatch.
The indicated time is the gel time at the recorded temperature. O Ruane & T P O'Neill Issue 1 : 28/04/97
(
The manufacture of the material used will state a gel time on the product data sheet and the time achieved at test must be longer than that stated on the product data sheet or it must lie within a specified time range for acceptance. The gel time test may be conducted on site, e.g. in a cabin or site office, but it will rarely be a requirement.
Differential scanning calorimetry (DSC)
I
DSC is a type of differential thermal analysis (DTA). It is a method of assessing the degree of cure of an FBE coating by using the thermal characteristics of the supposedly cured coating material. When FBE is heated, by application to a hot substrate, it melts and cross-linking starts immediately (due to molecular freedom). The reaction rate drops with a corresponding drop in temperature until eventually no reaction at all is occurring. Hence the material can be at various degrees of cure depending on temperature loss rates. Material at differing degrees of cure will be composed of different chemical compounds and have a different molecular structures and therefore will display different thermal characteristics. By comparing two glass transition temperatures (Tg's) of a supposedly cured material, obtained by testing the same sample twice, it is possible to determine the degree of cure. The two Tg's should be the same, however, providing that Tg2 comes within a specified temperature range of Tgl the degree of cure will be accepted. The range specified may include a tolerance to allow for the sensitive nature of the test. The test procedure basically entails micronizing and conditioning a small coating sample, approximately 10-15 mg as required, placing it in a small aluminium pan and applying heat to it at a rate of 20°C per minute up to 240°C. The sample is cooled to room temperature then the test is repeated, although it is not necessary to attain a temperature too far above the glass transition temperature for the second run.
The glass transition temperature (Tg) of a material is defined as being the temperature at which the substance changes its physical state from a brittle (glassy) solid to a rubbery solid.
Penetration Most coating materials, especially those which are thermoplastic (enamels, polyethylene, laminate tapes etc.), are subjected to penetration tests. The penetration test simulates contact with sharp objects, e.g. stones or welding electrodes, which the coating may make contact with when in service. The test apparatus consists of a metal rod, weighted and machined to a contact diameter of typically 2.5 mm. This is placed through a collar through which it can freely move. The rod is placed touching a panel which is coated with the material under test. A fixed static gauge is operated by a shoulder on the rod to monitor accurately any vertical movement of the rod. The weight used can be varied accordingly to specification requirements and the profile of the contact point can also be changed by using a different metal rod. A sharp point is specified for coal tar enamel because the material oxidises quickly and forms a tough skin; a flatter profile would be used for bitumen.
1
After a specified time at a set load, the amount of penetration is measured from the gauge and compared to specification requirements.
Softening point This is a test conducted on raw materials for thermoplastic systems, e.g. enamels and polyolefins. Ring and ball apparatus is used to determine the temperature at which two steel balls pass through prepared discs of the test material by virtue of the material softening sufficiently to allow displacement.
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Water soak (absorption) Coated panels are totally immersed in water at a stated temperature for a given length of time. The panels are then removed and re-weighed. The amount of water absorbed is then calculated. Maximum requirements are usually specified as a percentage by weight, e.g. max. 5%.
'
Elongation This test is mainly on plastics, e.g. tapes, polyethylene cladding and polypropylene, but also used on detached films of FBE and fibreglass reinforcements. The test is done to determine how far a material will stretch before it breaks, sometimes called elongation at break, and is expressed as a percentage. A bar bell shaped sample is used which is clamped at both ends. A pulling force is applied at a specified rate (expressed in mm per minute). Two lines are scribed on the narrow section of the bar bell and the distance between them measured before and after the test. A minimum elongation would normally have to be achieved, e.g. at least 350% for a material such as polyethylene.
Tensile strength 40
Fibreglass reinforcements detached epoxy coatings and most plastic coatings are subjected to a tensile strength test. The test is similar to the elongation test except that the maximum load the sample withstands before failure is recorded. Applicable units: ~lmrn'or MPa.
Impact resistance
4/,(+ ;I klW
TO P
Most pipeline coatings are subjected to this test.
An impact tool with a specified end profile of a specified weight is dropped over a specified distance onto the coating sample. The area is then assessed for damage using a holiday detector. Each material has its own required energy absorption impact rating measured in joules (J). A typical requirement for FBE is 5 J for 0.5 kg dropping from a height of 1 m.
Cathodic disbondment test See Unit P7Q.
Strain polarisation test During cold bending operations, large stresses are imposed on the coating. The result of this imposing stress is strain. When cathodic protection is applied, these areas of strain may fail prematurely, therefore this test is performed to determine whether a strained coating is susceptible to breakdown because of the application of cathodic protection. To conduct a strain polarisation test, test samples are prepared by forcing a curved mandrel onto a coated test plate; the coating is on the larger radius produced (on the outside of the bend). The coated samples are then subjected to a current to make cathodic, usually using the same procedure as described for the cathodic disbondment test but without the hole in the coating. After the specified period has elapsed, the sample is then tested by holiday detection to determine any areas of breakdown.
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Tests performed by inspection personnel (factory and site) Thickness Most coatings are subjected to thickness checks usually performed manually using specified gauges (magnetic, electromagnetic or ultrasonic). Automatic systems do exist in some factories, e.g. for use on polyethylene cladding.
20
The methods used to determine thicknesses vary because of limitations associated with each method when trying to deal with the huge range of thicknesses a coating inspector may encounter, e.g. 20 pfor a primer and 25 mm for a thick elastomer type coating. A destructive check performed by taking a coupon or using a heated needle on a needle type gauge may be specified for some thicker coatings.
Adhesion 30
See also Unit P7N. Various forms of adhesion tests exist for different types of material and situations. Qualitative and quantitative type tests may be specified according to procedural requirements. Adhesion tests which may be encountered on pipeline coating systems include: a. Vee cut tests. b. X cut tests. c. Bond tests. d. Primary bond tests - enamels (factory). e. Dolly test. f. Hydraulic adhesion test. g. Peel creep (usually identified as a separate type of test). Only selected tests are briefly described below. Vee cut test
This is a quick and easy qualitative test for thinner coatings, e.g. primers, MCL's and FBE.
X cut test A test more likely to be used on B E . This test is not the same as the X-cut tape test. This is a test with the same principle as the vee cut test but uses an X instead of a V. A metal bar or similar is used to provide a fulcrum and an attempt is made to peel the coating off by using the point of a utility knife at the intersection. The intersection is then assessed for damage and rated against a standard set of diagrams in the applicable specification. Bond test 80
A qualitative test usually done on enamels on coated pipes, similar principle to the vee cut but uses a rectangular test area, e.g. 30 rnrn wide 100 mm long.
Peel creep Used largely on polyolefins and other plastic coatings.
A method of measuring the average force required to peel off a coating from a substrate at a constant rate of pull. A strip typically 20 mm wide is cut circumferentially into the pipe coating, e.g. 100 mm long or as otherwise specified. The end of the strip is lifted and placed in the jaws of the pull off mechanism. A rate of pull is specified, e.g. 10 mm per minute, and the
O Ruane & T P O'NeiU h u e 1 : 28104197
force required to maintain this rate is recorded; alternatively, some specifications require the time taken to peel off a stated length at a given force. Other variables are the test temperature and the angle of pull which can be perpendicular or tangential. lo
Holiday detection See Unit P7R.
20
I
Awareness of other tests A pipeline coating inspector is less likely to have any involvement in tests not mentioned above. Other tests or test results which may be encountered are listed in the Units which apply to the coating/wrapping materials.
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I
Duties of a pipeline coating inspector The duties of inspection personnel are essentially those inspection duties which the client or employer wants them to perform. A significant problem in industry is that different organisations use inspection personnel in different ways, or use inspectors for functions additional to inspection. For some, this has led to a misunderstanding as to the defined role of inspection. The definition of inspection to BS EN 28402 : 1991 : Quality Vocabulary - "Activities such as measuring, examining, testing, gauging one or more characteristics of a product or service and comparing these with specified requirements to determine conformity. " The definition of inspection to BS EN 45020 : 1993 : Standardisation and Related Activities - "Evaluation for conformity by measuring, observing, testing or gauging the relevant characteristics. " ... "Evaluationfor conformity" is defined in the standard as: "Systematic examination of the extent to which a product, process or service fulfils specified requirements. "
1
Inspection is not supervision and inspection is not a substitute for supervision. It is not the duty of an inspector to deviate from specified requirements; generally speaking, if the specification is inadequate the work will be inadequate. Inspector qualification schemes do not require, or test for, a sufficient depth of corrosion engineering, coating technology or design knowledge which would enable an inspector to pass judgement on the correctness of an application specification. It could be argued that experienced senior inspectors may be in a position to take certain engineering decisions, but it is dangerous to generalise on this point.
The agreed spec@cation(s) for the contract may consist of a combination of one or more of thefollowing:
.nationallinternational
-
Inspection may be performed for fitness for purpose or quality control purposes, and may be carried out by the contractor, the client or a third party.
spec@cation(s) client spec@cation(s) job specification(s) procedure spec@cation(s)
Accurate reporting is an important duty for any inspector, but what constitutes an accurate report can differ between organisations and projects. Who the inspector actually reports to is also an important consideration.
60
It should be made clear to all workers, including inspectors, as to what is expected from them for the activities they are to perform - this is a basic quality assurance requirement. This is not to say a pipeline coating inspector should not perform duties outside the scope of inspection, this may be acceptable providing the person is competent to perform the work and providing it has been made clear what is required from the outset. Ideally, pipeline coating inspectors should be issued with relevant procedures and work instructions to enable them to carry out inspection and associated activities in accordance with the client's or organisation's requirements. The procedures should leave the inspector in no doubt as to what is to be done. Unfortunately, this documentation rarely exists!
I
Typical inspector's duties Before work commences 1.
1
2. 3.
Determine your duties and responsibilities. Duties may include those which relate to health and safety aspects taking into consideration mandatory requirements. You may also be required to check that rejected coatinglwrapping material or used abrasive is disposed of correctly or quarantined. Ensure the contractor's supervisor is aware of your duties and authority. Ensure you have correct applicable specification(s) and data sheets. Also ensure you at least have access to relevant referenced normative documents.
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4.
1
Determine the order of precedence for normative documents if the specification does not make it clear.
5. 6.
Learn the specification, procedures, work instructions etc.
7.
Ensure you have copies of any applicable documentation, e.g. correspondence, minutes from meetings, concessions etc..
8.
Liaise with the contractor's supervisor to determine whether the contractor's personnel are familiar with the work requirements.
9.
When required, confirm that the contractor's operators are properly trained and conversant with the equipment, materials and application techniques being used.
10
Approach the senior inspector or supervisor if you are not sure what is intended of any requirement.
10. Agree with the client/supervisor the level of liaison that is required and determine reportinglrecording requirements.
/
11. Ensure you have test instruments etc. that are required and that they are properly calibrated and in correct working order. Surface preparation Check the specification, procedures andfor work instructions to establish: standard against which work is to be measured; methods by which work is to be assessed, e.g. surface comparator; degree of surface cleanliness required; surface profile requirements (where applicable); any special tests to be carried out, e.g. for detectingimeasuring degree of surface contamination, sieve analysis of abrasives; requirements regarding equipment and consumables; f. g. ambient conditions required; h. recordinglreporting requirements. 2. Check the condition of the substrate before cleaning and make a note of rust grade, general condition of pipelfitting, spatter or flux residue on welds etc.. Any areas suspected to be defective, e.g. cracked, laminated or mechanically damaged, should immediately be reported to the supervisor or client. 1.
a. b. c. d. e.
Note: Do not allow surface laminations, cracks and similar to be dressed without the permission of the supervisoriclient.
3. a. b. c. d. e. f. g.
1
Ensure ambient conditions allow surface preparation to take place. The following may have to be assessed/measured: Air temperature. Steel temperature. Relative humidity Dew point temperature. Moisture on substrate. Potential sources of contamination, i.e. chemnicals, salt spray, fumes, dust. Potential changes in the weather to adverse conditions.
4.
Ensure any areas requiring protection from abrasive damage are shielded or masked off.
5.
Identify pipeslfittingslwelds being prepared.
Check that the correct materials and equipment are being used, e.g. correct type, correct size, consumables are free from contamination. etc.. Examples: a. Abrasive type, size and cleanliness. No re-use of expendable material. b. Correct wire brushes. C. Correct chemicals for chemical cleaning. 7. Cany out inspection of prepared surfaces as required by the specification including any overlap requirement onto existing coating.
6.
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8.
I
1
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Record the results of the inspection. The areas inspected must be identified in the report ensuring that it is clear what has been accepted and what has been rejected. The reasons f i r any rejections should be clearly identified.
Ensure that all concerned are clear about the reasons for any rejections. 9. 10. Where remedial work has been necessary, re-inspect for conformance to the specification.
I
Coatinglwrapping material
I
1.
Check the specified requirements.
2.
Check that the materials delivered to the work place correspond to the requirements of the specification and data sheets. The specification may require certain information to be displayed on each material container or package.
3.
Check that any primer or liquid material is the correct type for the application method being used, i.e. brush grade or spray grade. Check the material storage conditions against manufacturer's recommendations.
30
Note: Any warranty on the material is likely to depend upon proper handling and storage. 5.
Especially when dealing with materials having a short shelf-life, determine whether the material is being withdrawn from the store in proper rotation, i.e. usually on ajirst in, jirst out basis.
6.
Ensure material is not being used beyond its shelf life.
7.
Monitor material usage to determine whether there is sufficient material in storage for the completion of the job (or part job). (This is not always a responsibility of the inspector).
8.
Check that liquid coating materials are being mixed and stirred correctly. Any permitted addition of thinners must be monitored to ensure correct type and amount. For two pack coating materials: a. check that the materials are mixed strictly in accordance with the paint manufacturer's data sheets, e.g. add Part B to Part A in the correct ratio; b. confirm that any induction time is strictly adhered to; c. confirm that mixed material is not used after its pot life.
9.
Conduct that all necessary sampling procedures and tests; or c o n f m that such tests have been carried out prior to the commencement of work. Record batch numbers of materials tested.
40
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I
Coatinglwrapping application
1.
Check the specified requirements.
2.
Check that the surface to which the coatinglwrapping is being applied is free from contamination, i.e. oillgrease, dust, spent abrasive, corrosion products etc.. Any areas suspected as being defective, e.g. cracked, laminated or mechanically damaged, should immediately be reported to the supervisor or client.
Note: Do not allow surface laminations, cracks and similar to be dressed without the permission of the supervisorlclient (not usually the remit of a coatinglwrapping inspector).
3. a. b. c. d. e.
Ensure that the ambient conditions allow coatinglwrapping to take place. The following may have to be assessed/measured: Air temperature. Steel temperature. Relative humidity Dew point temperature. Moisture on the substrate.
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f.
I 4.
Confirm that coatinglwrapping material is not being applied to coated substrates either before or beyond the specified overcoating times for the existing coating.
10
1 1 1
Check that the correct application method is being used.
5. 6.
Check that correct application temperatures are being used (where applicable). Identify areas being coatedwrapped.
7. 8. a. b. c. d. e. f. g.
9. 401
I
Potential sources of contamination, i.e. chemicals, salt spray, fumes, dust. Note: Check that the material being applied does not have any special restrictions on its application.
Carry out inspection of coatedwrapped surfaces as required by specification. For example: Check that primers and coatingslwrappings used on field joints or repairs overlap onto the existing coating by the correct distance. Check that tapes are applied on risers from the bottom upwards. Check that tapes are being applied in spiral fashion with 55% overlap or as otherwise specified. Check that coatingslwrappings possess uniform contours after application. Measure the dry film thickness (d.f.t.) where required. Carry out holiday detection. Carry out adhesionlbond tests. Take any test samples required.
10. Ensure that any areas of defective coating are identified for remedial work. 11. Ensure that all concerned are clear about the reasons for any rejections. 12. Re-inspect any remedial work carried out to ensure that it conforms to the specified requirements.
50
13. Record the results of the inspection. The areas inspected must be identified in the report ensuring that it is clear what has been accepted and what has been rejected. The reasons for any rejections should be clearly identified.
Miscellaneous 1.
Check that the handling, transport and storage of coatedwrapped pipes and fittings is carried out in a manner approved by specification which does not cause damage to the coating.
2.
When required, attend appropriate meetings, such as periodic on-site meetings or those meetings called to provide solutions to a particular problem that has arisen. You may also be in a position where you need to arrange a meeting to resolve problems that have arisen.
3.
Ensure that you effectively organise your time so that you are available for inspections when required. Do not give the contractor an excuse to say, "we were waiting for the inspector to carry out inspection".
4.
Check the work area housekeeping. For example, equipment and consumables should be cared for (correctly handled, stored and maintained) and the site or shop should be tidy (free from empty containers, worn brushes, spent abrasive etc..).
5.
On completion of the worklcontract, ensure that all records (specifications, procedures, work instruction, permits, concessions, plans, report sheets etc..) are collated and filed in the appropriate location. This is only required when it is your designated responsibility.
6.
Do not seek confrontation. Try and avoid arguments. Never be condescending, patronising or arrogant. Remember the main duty of an inspector is to inspect against specified requirements and report findings. If the specification is not clear on a particular requirement, seek advice from the supervisor or client. Do not accept or reject work based on your opinion alone. Be objective at all times.
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Reports and records The reporting requirements of quality control associated with pipeline coatings and the actual information recorded can differ considerably from job to job. The daily inspection report is conlrnon Lo mosl jobs and is ollen wrillen oul on a Daily Inspection Report Form which has a format designed by the organisation that you are representing, i.e. the inspection agency, contractor or client. Progress reports are often required and these may have to be produced on plain paper or on specially designed forms. Ideally, the exact reporting and recording requirements should be specified in a procedure or in the job specification itself. Always liaise with the supervisor or client verify what is required to be recorded or reported. Regardless of specification requirements, the inspector should always make a detailed log of work performed, observations, relevant conversations and similar; include applicable times, dates, people involved etc.. You may find this information very useful in future disputes. The following list shows the documentation that may exist on a project involving pipeline coating inspection and that which the senior inspector is more likely to be responsible for collating and controlling effectively until final completion of the work.
4011
1.
The applicable specification(s).
2.
Procedures and related work instructions.
3.
Quality plans.
(
4.
Method statements.
5. 6.
Concessions (waiver or variation orders).
1
I
Daily inspection report forms.
7.
A daily log (this may be stand-alone document or one in addition to a daily inspection report).
8.
Lists of remedial action.
9.
Progress reports.
10. Minutes of meetings. 11. Correspondence. 12. Calibration certificates. 70
13. Copies of work permits. 14. Site instructions. 15. Mechanical completion certificates (hold-point release forms or inspection request forms). 16. Audit reports a. Internal. b. External. 17. Non-conformance reports. 18. Certificates of conformity.
Q Ruane & T P O'Neill h u e 1 : 28/04/97
Ruane & T P 0'Nei//
Examples of possible contractor malpractice 1.
Use of unskilled operators. This may relate to surface preparation, application of coatinglwrapping or safety considerations. Note: The in~pecto~. cannot narmii~llyrnpcrrl on unskilled crperntor ns something which does not conform to specification.
2.
Use of unsuitable equipment, e.g. worn brushes, poorly maintained and leaking compressors, contaminated equipment.
3.
coatingtwrapping on site or preparing surfaces during inclement weather conditions such as rain, snow, fog, mist etc..
4.
Re-using expendable abrasives.
5.
Insufficient cleaning or poor coating in difficult access areas such as under pipes (on site).
6.
Coatinglwrapping before inspection of substrate preparation or previous coat.
7.
Missing out the primer (when a primer is specified).
8.
Use of wrong solvent to clean application toolslequipment.
9.
Not filling irregular contours when using wrapping tapes.
10. Not enough tensioning when using wrapping tapes. 11. Incorrect material application temperatures (or hold temperatures) when using hot applied materials. 12. Storing coatingtwrapping materials incorrectly, e.g. where the specification requires material to be stored in a temperature controlled environment. 13. FBE or liquid paint materials used outside the expiry date.
14. Applying a thickness of coating which is outside the specified range.
0 Ruane 8: T P O'Neill
Issue 1 : 28/04/97
Pipes and fittings must be handled, transported and stored in accordance with specified requirements. The following text offers some typical requirements.
Handling The two main considerations for handling pipe correctly are (1) personnel safety and (2) prevention of damage to the pipe or coating. Where practicable, pipes should be lifted using a spreader beam with suitable slings (nylon type), as this is the safest method. Where a spreader beam is not practicable, two leg chain slings (brothers) could be used, these should be fitted with properly designed profiled hooks fitted with guide ropes, and the inner edge of the hooks should be coated in nylon or soft alloy. The use of vacuum lifts and magnetic devices may be permitted. Chains must not be slung around pipes even if padded. When handling large bends or tees, a nylon sling should be passed through the bore.
II Transport When chains or straps, pipe cradles, batten carriers, or stanchions are being used during transport, they must be padded, e.g. with at least 12 mm of rubber at contact points. Pipes may be stacked in pyramid or parallel fashion when transporting. Stacking in parallel fashion will require the use of padded pipe cradles or battens between the pipe layers.
Stacking at permanent or temporary storage sites Pipes are stacked pyramid fashion on either hard standings or soft standings. Hard standings, which may only be used on flat firm ground, consist of bearers (skids or runners), padded with wood wool pads or similar. The number of bearers required for each pipe on the base layer will be governed by the size of the pipe (weight) and the type of coating applied. Soft standings consist of two parallel sand rows separated by approximately 3/4 of the pipe length. Polyethylene sheeting is often required between the sand and the pipes. Stacking should be done in such a way as to avoid accumulation of water inside the pipes, a fall of -150 mm is desirable. The maximum number of tiers and any requirements for preventing pipe to pipe contact between tiers should be specified.
O Ruane & T P O'Neill Issue 1 : 28/04/97
Ruane & T P O 'Neil/
10
I
Ditching
I
Ditching is the lowering in of a pipe, or string of pipes, into a trench.
,,
Side booms are normally used for ditching, the number of which depends on size, weight and number of pipes to bc ditchcd. It is good practice to have the booms (jibs) lined with rubber, e.g. car tyres, to prevent damage to the pipe coating if the pipe comes into contact with the boom.
If the trench is excavated in rock, a well rammed bed of approved sandlgravel mixture would be applied to the base of the ditch to a depth required by specification, e.g. at least 150 rnm deep. The base of the ditch must be evenly bedded and the inspector must make sure there are no stones, welding electrodes etc. present in the ditch which could cause damage to coating or interfere with cathodic protection.
,,
When thermoplastic coatings have been used, e . g coal tar enamel, lowering-in should not take place if the ambient temperature is high, e.g. above 27°C or as otherwise specified. When the pipes are lifted from the skids prior to lowering-in the coating must be checked by holiday detection to ensure freedom from skid damage. It is normal to ensure earth has been achieved by testing the bare bevel ends of the pipe string.
I
Backfilling
I
The important considerations when backfilling are as follows: a. b. c.
d.
f
70
The pipe and pipe coating must not be damaged. Foreign material which may cause damage to the coating or interfere with cathodic protection must not be placed in the trench. The backfill should be well compacted, otherwise the pipeline may not be adequately protected by the CP system or a much higher current may be required to achieve adequate protection. Environmental considerations may apply - ideally the excavated material should be returned in layers which correspond to how it was originally. When the excavated material contains rocklstones, it will not be permissible to return this material in the layers around the immediate viscinity of the pipe.
Backfill material, consisting of either the excavated material or imported fill, is placed into the trench in layers of a specified depth, e.g. 300 mm each layer. Each layer must be well compacted with hand rammers or mechanical vibrators prior to the deposition of the next layer. Stone free layers are deposited until the backfill material is a specified distance above the pipe, e.g. at least 300 mm. The remaining excavated material is then returned to the trench in layers and compacted as above. The minimum depth of cover between the top of the pipe and the surface of the normal ground will be specified. Water courses, road crossings, railway crossings and similar require special considerations.
Note: To protect the coating against stones and soil stressing, a plastic grid mesh is sometimes wrapped around the pipe prior to backfill.
O Ruane & T P OINeill Issue 1 : 28/04/97
I lo
The Pearson survey is an above ground survey technique used primarily to locate coating defects in buried pipelines. Pearson survey equipment may be used to determine: a. b. c. d.
the presence of holidaysldamage; the existence of metallic objects; the depth of the pipe; thc position/direction of the pipe.
The survey compares the potential gradients along the pipeline measured between two movable electrical ground contacts. The potential gradients result from an injected a.c. signal leaking to ground at coating defects or metallic objects located near the pipeline. A transmitter is electrically connected with one lead to the pipeline, e.g. at a cathodic protection test point and the other lead to a good remote earth and then energised. j0I
The receiver can be used in a pipe locating mode only. The section of pipe to be surveyed should f i s t be located and identified to enable the survey operators to follow the route of the pipeline. Stakes can then be inserted at measured intervals if required. 30
40
50
The receiver is connected via a cable harness to earth contacts worn or held by the two operators such that at all times earth contact is maintained. A connecting cable provides a separation of 6-8 m between the operators. The two operators follow the route of the pipeline; when the leading man (front contact) passes over a defect, a higher signal level is indicated in the earphones by an increase in volume and also visually on the receiver if a signal level meter if fitted. As the front contact passes the defect, the signal fades and then increases again as the rear contact passes over the defect. Where the signal is not easily interpreted or where there may be more than one defect within the span of operators, this may be clarified by surveying at right angles to the pipeline, i.e. one operator walks over the pipeline and the second walks parallel to the pipeline 6-8 m from the pipeline. In this mode each defect is indicated, as the operator over the pipeline traverses the fault. The survey may be carried out with an impressed current cathodic protection system energised. However, sacrificial anodes, bonds to other structures or similar should be disconnected prior to the survey as these can mask defect areas or drastically reduce the length that may be surveyed from one injection point.
O Ruane & T P O'Neill Jsue 1 :28/01/97
.
Pipeline Coatings Inspector ICorr Level 2
Part B
TABLE OF CONTENTS
PART B
.......................................................................... GKAPHlTISATION ........................................,........ ................... BASIC CHEMISTRY ................................................................... SURFACE PREPARATION ............................................................ CORROSION
.. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . ..... . . .. . . ..... . . . . . . . . . . . . . . . . . . . . Wet blasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. ... . .. . . . . . . . . . , .. . . . . . . . . . . . . . . . . . .. . Hand and power tool cleaning . . . . . . .. . . . . . . . ... . . . . . .. . . . . . . . . .. . . . . . .. . . . .. . . . . . . . . . . . . . . Flame cleaning . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . .. , . . . . .. . .. . . . . . . . . . . . . . . . . . . . Chemical cleaning . . . . . .. . . . . . . . . .. .. . . . . . .. . . . . . . .. . . .. . . .. .. . . .. . .. .. . . . . . ... . . . .. . . .. .. Dry abrasive blasting
................................... FLASHPOINT ...............................................................,.......... VISCOSITY ............................................................................ DENSITY ............... ............................................ ................... WET FILM THICKNESS (WFT) ........................................................ DRY FILM THICKNESS (DFT) ......................................................... ADHESION ............................................................................. CATHODIC DISBONDMENT TEST .................................................... HOLIDAY DETECTION ................................................................ WEATHER CONDITIONS ......................................... ................ HEALTH & SAFETY ................................................................... TESTS T O DETECT SURFACE CONTAMINATION
COSHH Regulations 1994 . . . . . . . . . . . . . . . . . . . ... . . .. . .. . . . . .. . . . . . . . . . .. . .. . . . . . . .. . . . . . . . . Occupational Exposure Limits (EH40) . . . . .. . . .. . . . .. . .. .. . . . . . .. . . . . . ... . . . . . .. .. . . . . .. . .. Volatile organic compounds . . . . . . . . . . .. . . . . . . . . . . .. . . . . . .. . . . . . .. . . ... .. . . . . .. . . . . . . . .. . . . Health & Safety data sheets . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . .. . . . . .. . . . . . . . . ... . .. . ... . . . . FLAWS ON T H E SUBSTRATE
........................................ QUALITY ASSURANCE ................................................................ NORMATIVE DOCUMENTS ........................................................... CATHODIC PROTECTION (INTRODUCTION)
Ruane & T P 0'Neil/ Corrosion is generally an electro-chemical process which results from z reaction and at least one cathodic reaction. The corrosion of steel takes plrrt a e,,t anode. y
1
G n o d i c reaction is expressed as follows:
M = element involved FC n = a number e = electron(s) At least one of the following cathodic reaction(s) takes place at the cathode: Where:
l0I
1
a
1.
Oxygen reduction in acid solutions:
02+4H++4e+2H20
2.
Oxygen reduction in neutral and alkaline solutions:
02 + 2H20 + 4e + 40H-
1
3.
Hydrogen evolution:
2H++2e + Hz
4.
Metal ion reduction:
~ e +*e + ~ Fe+'
301
5.
Metal deposition:
C U ++~2e
1
%Y~(OYM~N
+ Cu
Iron ore is an oxide of iron in chemical balance with the environment; when this iron ore is converted to iron, the chemical balance is changed and the iron becomes i.e. it corrodes on contact with the natural environment and tries to revert back to its natural inert state. The natural environment usuallv contains moisture (which ~ r o v i d e s the electrolyte) giving the following simultaneous reactions:
-
An electrolyte is a substance which, when in solution (usually water or in its fused (moltett)state), will conduct u current and be broken down by it.
active,
Anodic reaction: Fe
+ Fe* + 2e-
i.e. iron gives fcrrous (iron) ions and electrons
I A positive ion may be called a cation whereus a negative ion m y be called an anion. These rennr can cause confusion because cathodes are thought of us being negative and anodes positive!
a
Cathodic reaction: 2H20 + 02 + 4e-
+ 40H-
i.e. water + oxygen + electrons give hydroxide ions. The products of these reactions take part in further reactions with thc immediate environment leading to the formation of corrosion products, the most familiar being rust: 61
Fe*
+ 20H- + Fe(OH)2
I
i.e. ferrous ions plus hydroxide ions gives iron hydroxide.
(
i.e. iron hydroxide plus oxygen gives rust.
SkO
Corrosion reactions can be accelerated by the existence of certain criteria including:
Mill scale is an oxide of iron produced when the steel is rrmufactured; it is a result of the hot steel 8, coming into conract with air and forming an oxide composed of three layers: FeO nearest the steel, Fe,O, then Fe,O, on the outside. Mill scale has a total thickness bemeen approximately 25 pm and
1.
variations in oxygen content on the material's surface;
2.
chlorides and sulphates;
3.
other metals or metal compounds of higher nobilip (more electro-positive) in contact with the steel, e.g. mill scale;
4.
acids or alkalis;
5.
certain types of bacteria near the material's surface.
6 . I nCw6* L*
The following list shows some metals/metal compounds in their order of nobility in sea water at ambient temperature. The relative positions of the metals/metal compounds in the list can change with a change in electrolyte type or temperature; this list is known as the galvanic series. The galvanic series may show the potential of each metaumetal compound measured in volts against a specified type of reference electrode. If absolute potential values of metal elements only are shown, which are independent of the electrolyte used, the list becomes known as the electrochemical series.
-
8 Ruane & T P O'Neill h u e 3 : 1911UY6
Ruane & T P 0'Neil/ Gold
NOBLE (ELECTRO-POSITIVE)
Silver '
The electrochemical series is nroduced under standard conditions and is useful for theoretical assessnlents or in laboraror-)i siriratiurrs.
Nickel Copper 10
Mill scale
-
Mild steel Alum~nium Zinc Magnesium
5g& &v
I
IGNOBLE (ELECTRO-NEGATIVE)
Example: If steel was in intimate contact with zinc or attached to zinc via a wire in an electrolyte, e.g. soil or water, the zinc would corrode first because steel is more noble than zinc. 30
In this example the zinc becomes the anode and the steel the cathode, i.e. the steel is being cathodically protected and the zinc is acting as a sacrificial anode.
8 Ruane & T P O'Neill h u e 3 : 19IIUY6
Ruane & T P 0'Neil/ Carbon and graphite are inert in many corrosive environments, their tensile strengths are very low, e.g. 500 to 3000 psi, impact resistance is nil and abrasion resistance is poor. Resistance to alkali's and most acids is good but oxidising acids such as nitric acid (HNO,), sulphuric acid (H2S04)and chromic acid (H,CrO,) attack it. They also have a low resistance to reaction with halides and halogens (fluorine, chlorine, bromine and iodine). Graphitisation is the process where the iron component is removed from the metal leaving a network of carbon particles - a de-alloying process. The residual carbon retains the shape of the original object and, unless the weak structure is fractured.
,,
30
40
50
The mechanism of this corrosion process is believed to be based on cathodic depolarization caused by removal of hydrogen from the surface, thereby reducing iron sulphate (FeSO,"'') to iron sulphide (FeS), which is itself corrosive. This leads to bacterial attack from sulphur reducing bacteria (SRB) called Desulphovibrio Vulgaris, sometimes referred to as metal eating microbes (MEM) of which there are two types - aerobic and anearobic. Graphitisation most commonly occurs in salt waters, acidic mine waters, dilute acids and those soils containing sulphates and SRB. It is possible to mimic these conditions and cause graphitisation in a laboratory by immersing in diluted H2S04. Graphitisation usually progresses directly into the surface in a uniform manner, leaving a porous matrix of the remaining alloying element carbon. There is no outward appearance of damage but affected material loses mass and becomes porous and brittle. The porous residues may retain appreciable tensile strength and also have moderate resistance to erosion, e.g. a completely graphitised pipe may continue to hold water until fractured by impact. It is essential that all graphitisation is removed because the exterior of a structure or pipe containing graphitisation becomes very noble in any galvanic coupling, therefore any other metal in contact or in close proximity will have its corrosion rate accelerated. Anti-graphitisation measures include efficient barrier coatings and for new systems the addition of several percent of nickel during the manufacturing process.
8 Ruane & T P O'Neill h
e l IY/lU96
Types of chemistry Chemistry is essentially the study of the composition of substances that are made up of elements and of the changes that substances undergo. The subject of chemistry is usually divided into three main branches; organic chemist~y,inorganic chenzistry and physical chenlistry.
Organic chemistry is the study of carbon which has the unique ability to bond with itself to form long chains of atoms, e.g. polyethylene. Most of the chemistry involved in paints and coatings is organic chemistry. Inorganic chemistry is the study of non-organic aspects of chemistry, i.e. the elements and their compounds. Physical chemistry is the study of the physical properties of elements and compounds.
Atoms The smallest part of an element that can have the elements properties. All matter is composed of atoms which are tiny particles, so small that 100 million placed end-to-end would measure 1 cm. An atom as a whole is electrically neutral.
Molecule When atoms are bonded together in a fixed whole number ratio, they are known as molecules.
A molecule is defined as the smallest particle of an element or compound. Examples of molecules are: 0 2
co2
Oxygen
Carbon dioxide
HCI Hydrogen chloride
H20
Water
Element An element is said to be a pure substance which cannot be broken down into anything simpler by chemical means. There are 110 elements known to man. Below is a table of the elements most commonly encountered in paint technology and corrosion technology.
]
-ier
r
r
I
1
symbol
I
Element
H
I
Hydrogen
16
S
Sulphur
17
C1
Chlorine
26
Fe
Iron
30
I
~n
I
Zinc
~ 1
AC I
Ruane & T P OINei//
Compound A pure substance which is made up of two or more elements chemically bonded together is called a compound. 10
The chemical bonds by which the elements are joined together may be either ionic or covalent. Examples of compounds are: CH.4 Methane
20
H20
Water
NaCl Sodium Chloride
Ions When a compound such as sodium chloride is dissolved in water it dissociates into ions, i.e.: NaCl+ Na+ + C1-
30
An ion is a positively or negatively charged particle. Positively charged ions are called cations, e.g. Na+, and negatively charged ions are called anions, e.g. C1-. Groups of atoms can also possess a charge, e.g. hydroxide OH-, sulphate SO4 and these are known as radicals.
Chemical bonding Atoms are held together in molecules and large chemical structures, such as a cross linked epoxy, by chemical bonds. There are various kinds of bonds such as co-ordinate, covalent, double, hydrogen, ionic, trip Le. These bonds are the forces that exist between the atoms and are produced by electrons. Electrons are either shared between atoms, or atoms gain or lose electrons forming ions. The sharing of electrons is called covalent bonding and this occurs when similar atoms bond together. Losing or gaining electrons occurs when different types of atoms bond together, e.g. a metal such as sodium Na+ and a non-metal such as chlorine CI-. This form of bonding is known as ionic.
Covalent bonding
,,
Covalent bonds are formed by two similar atoms coming together and sharing their electrons, e.g. hydrogen, which only has one electron. The two hydrogen atoms form a molecule of hydrogen (Hz) by the two electrons coming together to form a bond.
Hydrogen molecule
8 Ruane & T P O'Neill ksue 1 2 Z W 4
Covalent bonds are usually found in compounds which only contain non-metallic elements, for example: HCl Hydrogen chloride
Co2
Carbon dioxide lo
0 2
NH3
Oxygen
Ammonia
Covalent bonds which contain two electrons are called single bonds. Many molecules such as unsaturated fatty acids contain double bonds. Some molecules such as acetylene H-C = C-H, contain triple bonds. Covalent bonds are not as strong as ionic bonds.
20
Ionic bonds These chemical bonds occur because of the electrostatic attractive forces between negatively and positively charged ions, i.e. between anions and cations. Ionic bonds are found in compounds of certain non-metals, e.g. Na+Cl- and also in compounds involving radicals, for example:
Copper sulphate
Potassium nitrate
Chemical formulae A chemical formula is a way of representing a substance or compound by using the symbols of the elements present in the formula. 50
There are two commonly used forms of chemical formulae: (1) molecular and (2) structural. The molecular formula of a substance shows the number and types of atoms in the molecule but it does not show how the atoms are arranged, for example: C3H,0 Acetone
60
A structural formula is one that shows the bonds between atoms and the position of the atoms with respect to each other, for example:
'= I O
Acetone
Structural formulae are sometimes written in an abbreviated form, for example: CH,. CO
80
. CH3
Acetone
To summarise, it will be seen that the formula for acetone can be written in any of the following ways, all of which are correct: y
C3H,O
3
C=0 I CH3
0 Ruane & T P O'Neill h e 1 22/04/94
CH,
CO
CH,
I
Correct surface preparation is a vitally important stage for most coating systems, it is often the process which governs the service life of the coating system. There are various ways to prepare a surface prior to coating: Abrasive blast cleaning Wire brushing Scraping Grinding Needle gunning Chemical cleaning Water blasting Weathering Flame cleaning Vapour degreasing
30
I I
50
The *of a surface preparation is governed by the amount of surface contaminant remaining on the substrate after cleaning although it may also relate to the resultant surface texture, e.g. the surface profile on a substrate after abrasive blast cleaning.
-
Dry abrasive blasting Dry abrasive blasting is carried out by projecting a highly concentrated stream of small abrasive particles onto the substrates surface at speeds of up to approximately 720 km/h (450 m.p.h.). The operation removes rust, scale, dirt and any other extraneous material from the substrate and also leaves an irregular profile which provides an ideal key for coating adhesion. Dry abrasive blasting is often the best method of surface preparation for long term protection coating systems.
Abrasives The degree of surface roughness and rate of cleaning is partially governed by the characteristics of the abrasive used; these being:
60
1 Both metallic shot and metallic grit particles tend to ricochet and so the blasting area needs to be totally enclosed for efficient recovery and cleansing.
Size Hardness Density Shape Effect of abrasives Grit is angular in profile with sharp cutting edges; it shatters mill scale and undercuts any surface contaminant resulting in a clean surface with an irregular profile. The amplitude tends to be quite erratic with a large occurrence of rogue peaks, especially when blasting in one area for too long. Many materials both metallic and non-metallic are used to manufacture grits. Shot is spherical, it shatters mill scale, but does not have sharp cutting edges to cut into a surface, however, the visual appearance of a shot blasted finish is similar to a grit blasted finish although there is less roughness to the touch. Shot blasting work hardens a steel surface to a greater degree than grit, which has the effect of reducing the chance of any stress corrosion cracking which could otherwise occur in the future. Shot also reduces the occurrence of rogue peaks but may press impurities into the surface.
Shot is usually made from cast steel although a natural mineral called staurolite is also available in shot (spherical) form. It is common practice to mix metallic shot and grit to obtain a blast finish close to the ideal (a typical mix being 70-80% shot and 20-30% grit).
O Ruane & T P O'Neill Issue 3 1YllllY6
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Ruane & T P O1Nei/
I
Chilled iron grit Used in enclosed blast units and open blasting where recovery systems can be employed. The abrasive is relatively cheap and efficient in use. On impact, small chips are removed from the abrasive which exposes new cutting surfaces for the next cycle. Because of this factor, excessive wear can occur due to larger quantities of fines which need to be extracted. Cast steel grit
Used mainly in blast pens and grit recycling systems. Tends to round off during use as the sharp edges wear down and is therefore not as efficient as the chilled iron varieties but is easier on moving parts. High-carbon steel is used because low-carbon steeI cannot be crushed into a grit. Cast steel shot
For use mainly in enclosed recovery systems because of the ricochet characteristics (some mechanised floor blasting systems employ this). It has a tendency to impress impurities into the surface as it peens or work hardens the top few nanometers. Available in high-carbon steel (HCS) and low-carbon steel (LCS). Cut steel wire
Normally used in enclosed systems only to facilitate recovery and cleansing. Sharp cutting edges round off as quickly and evenly as the steel abrasives mentioned above.
1
Synthetic slags and fused aluminium oxide
Synthetic slags include copper refinery sIag, nickel refinery slag, iron furnace slag (calcium silicate slags), coal furnace slag (aluminium silicate slag). These are expendable abrasives all having a grit profile and are used in open blasting systems. Cheap with no silicosis hazards (typically less than 1% free silica). Breaks down quickly to fine particles. Extremely fine particulate matter embeds into the profile resulting in slight discoloration. Natural mineral abrasives
This type of abrasive includes garnet, silica sand, olivine sand and staurolite. All are available in grit form although staurolite is also available in shot form. Natural mineral abrasives are used mainly in open blast systems and gives a high degree of surface cleanliness.
Sand is not dangerous unless it is in dust form when it can be inhaled, e.g. after fragmentation during dry blasting operations.
Note: The Control of Substances Hazardous to Health Regulatiotls 1994 (COSHH Regulations) do not allow the use of sand containing free silica in dry blasting operations because of the associated health hazard of silicosis. Agricultural by-product abrasives This classification includes corn cob, walnut shell, egg shell, peach husk, coconut shell and many more. Used for stone cleaning or sensitive substrates and has a tendency to absorb water.
1
Abrasive analysis The use of large particle sizes does not increase blasting efficiency or economy. The large particles tend to bounce and ricochet along the blast hose loosing speed and hence impact energy. Also large particles cannot always clean out the corrosion pits within the substrate or clean out the pits of the profile cut by abrasive which has already impacted. The most efficient medium contains a controlled mix of large and small particles known as a working mix. Because non-metallic abrasives are normally expendable, the working mix should be as required on supply. Metallic abrasives, being recyclable, gradually reduce in size cycle by cycle until the particles are small enough to be drawn out by the air cleansing system. It therefore
8 Ruane & T P O'Neill Issue 3 1Y01/Y7
Ruane & TP o
weill becomes necessary to do an analysis of the abrasive particle sizes to ensure that the optimum mix is being used. Procedures for laboratory sieve tests for the determination of particle size distribution of metallic abrasives are deta~ledin BS 7079 : Part E7 (IS0 11125-2), for non-metallic abrasives - BS 7079 : Part F12 (IS0 11 127-2). Because non-metallic abrasives are usually expendable and are supplied as an ideal working mix, tests for the determination of particle size distribution are very rarely required in the field. Conversely, abrasive analysis is often required when using recyclable metallic abrasives.
,,
In order to obtain a representative sample of recycled abrasive, it is advisable to use a sample point near to the point of re-application on a gravity feed line, e.g. immediately before the top hat on a Wheelabrator or on a free-fall area into the blast pot. A simple test to determine working mix would be as follows: 1.
Select the sieves as specified in the relevant standard for the abrasive size denomination and assemble, in descending order of mesh size - largest at the top and ensure there is a receiver under the smallest.
2.
Weigh out a 200 g sample taken from the points as suggested above and place into the top sieve; position the lid and shake well for five minutes.
3.
Separate the sieves and remove any abrasives remaining on top of each individual mesh. It is essential to remove every single particle entrapped in the mesh by brushing with a stiff, short bristled brush. Weigh the abrasives remaining on each sieve and, if applicable, any falling through the smallest.
4.
Divide the weight in grams by 2 (because of the 200 g sample) to express as a percentage.
5.
Consult the relevant specification/standard to check compliance with required size ratios. Note: Generally, this analysis conducted on a recycled working mix will indicate a shortage in large particle ratios and is likely to require addition of new abrasives.
30
The laboratory method requires mechanical agitation. 15 minutes for angular abrasives and 10 minutes for spherical abrasives
40
1I
Checking for contamination of abrasives Abrasive blasting is done to remove contaminants from the substrate and to increase the surface area which in turn improves adhesion and the life of the coating. If contaminants are present in an abrasive mix, the substrate can actually be coated rather than being cleaned. Several tests can be done to ensure that this does not occur although some are intended for laboratory use, e.g. tests for the determination of moisture content and the amount-of water soluble chlorides present. Test for compressed air cleanliness
Pass the compressed air supply through a white cloth (at reduced pressure) for several seconds. Discoloration of the cloth indicates contamination. Wet stain shows oil or water (water will dry off, oil will not).
I
Excessive dust or finings in a working abrasive mix
A simple test is merely to toss a hand full of abrasives into the air and observe the dust carried away on the breeze. A more definite test is to drop a hand full of recycled abrasives into a beaker of water. The finingsldust etc. will float (held in the surface tension of the water) and so can be roughly quantified.
A laboratory test also exists for determining the foreign matter present in metallic abrasives to BS 7079 : Part E l 1 (IS0 11 125 : Part 6).
Q Ruane & T P O'Neill Issue 3 1911UY6
(
Oil or grease in a working mix A quick and simple test for thls is to place an amount of abrasive into a beaker, pour on sufficient water to just cover and stir vigorously for a few seconds. Observe the surface of the water for the purple/blue/green sheen characteristic of oil on water. Another test is to pour hydrocarbon solvent (typically xylene) into the abrasive, agitate, then pour some of the solution onto a clean glass plate. Any residual smear indicates oil or grease present in the mix after the solvent has evaporated.
Rogue peaks are peaks which stand out above the required profile and should be avoided if applying thin coatings us they may lead to spot or flush rusting.
Surface profile The shape of a cross-sectioned blast finish is known as the surface profiIe or anchor pattern.
SURFACE PROFILE
Blast finishes should not be touched with bare hand.^ otherwise contamination will result.
The size of the profile as measured from the peaks to the troughs is known as the amplitude or peak to trough height, and is primarily governed by the size of abrasive used, although other factors are important, e.g. angle of impingement, hardness of surface and other characteristics of the abrasive itself. Maximum amplitudes or amplitude ranges would normally be quoted in specifications, a typical amplitude range for liquid paints would be in the region of 30-75 pm. The amplitude of a blasted surface may be measured by a number of methods, including the use of a surface profile needle gauge, surface replica tape, e.g. Testex tape, or a surfnce comparator. Surface profile needle gauge
A11 accurate measuring equipment, e.g. dial micrometers, should be issued with calibration certificates or certificates of cotgfonnance to give msurarzce that the readings obtained are going to be correct within a stated murgin of error.
This relies on a needle reaching the bottom of the troughs on the surface profile. Because there are so many troughs of different depth, it is normal, and necessary, to take ten or twenty readings and calculate the average amplitude. Before taking any readings it is necessary to zero the gauge on a flat piece of glass.
SURFACE PROFILE GAUGE
O Ruane & T P O'Neill Issue 3 1911UY6
Ruane & T P O'Neill
Surface replica tape Testex tape is a trade name of a commonly used surface replica tape. It is used in conjunction with a dial micrometer and although quite costly, has the advantage of providing a permanent record. The procedure for carrying out this test is as follows:
\
101
(
Zero the micrometer ensuring the flat contact points are clean.
1
Remove paper backing and stick Testex tape to the surface to be measured.
2. 3.
Rub the Testex paste into the troughs using a blunt instrument, until the peaks can be seen butting up to the transparent plastic.
4.
Remove the Testex tape from the surface and measure the overall thickness with the dial micrometer.
5.
Deduct 50 pn (2 thou") from the reading to obtain the amplitude. The plastic (Mylar) to which the soft compound is attached is 50 pn thick.
Surface comparator The roughness of the surface to be assessed is compared to the different areas on the comparator by visual examination and if necessary by scraping with a finger nail, small wooden stick or similar - never with the fleshy part of the finger as this will contaminate the blast.
3
It is important to note that needle gauges, surfoce replica tape and surface comparators only gives the degree of roughness and not the degree of cleanliness.
41
A profile grading can be given when the area under assessment is rougher than the smoothest of two adjacent areas on the comparator but not as rough as the rougher of the two areas. The profile is then graded according to the following: Fine profile: Equal to or rougher than area I but not as rough as area 2. Medium ~ r o f i l e :Equal to or rougher than area 2 but not as rough as area 3. Equal to or rougher than area 3 but not as rough as area 4. Coarse If the profile is fine;than area 1 it is termed finer than fine.
BS 7079 :Part A1 is the same as I S 0 8501-1 and SS 0 5 59 00.
-
If the profile is coarser than area 4 it is termed coarser than coarse.
I
Blasting grades
The grade of a blast finish relates to the amount of surface contaminant remaining after blasting. The grade of blast finish is primarily governed by blasting time and the velocity of the abrasive particles. BS 7079 : P a r t A 1 BS 7079 - Preparation of steel substrates before application of paints and related products. Part A1 of this standard is pictorial and shows rust grades prior to blasting and the degree of surface cleanliness after blasting. 70
I 80
The surface under examination is visually compared with high quality photographs in the standard both before and after blasting. The preparation is then given a coding, e.g. C Sa2% which can be interpreted using the following extract from the standard:
B
-
C
-
D-
01
-
Rust grades: A - Steel surface Iar~elyc o v w with adherent mill scale but little, if any, rust.
Steel surface which has begun to rust and from which the m ~ l scale l has begun to flake. Steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision. Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.
Preparation grades - blast cleaning. Prior to blast cleaning, any heavy layers of rust shall be removed by chipping. Visible oil, grease and dirt shall also be removed. After blast cleaning, the surface shall be cleaned from loose dust and debris.
O Ruane & T P O'Neill h u e 3 1911Y96
C
I
Sal
-
Sa2
-
Light blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter. Thorough blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from most of the mill scale, rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering.
Sa2% - Very thorough blast cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes.
LSa3
-
Blast cleaning to visually clean steel. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and shall be free from mill scale, rust, paint coatings and foreign matter. It shall have a uniform metallic colour.
BS 7071 (SS 05 59 00)
SSPC
1
NACE
White metal (SP5)
Sa3
Grade 1
Near white metal (SP 10)
Sa2%
Grade 2
Commercial finish (SP6)
Sa2
Grade 3
Light blast and brush, off (SP7)
I
Sa 1
I
1
Grade 4
SSPC = Steel Structures Painting Council NACE = National Association of Corrosion Enmneers
I
Equipment
(
Centrifugal blast units
I
80
90
Blasting in factories is often carried out using rotating wheels which throw the abrasive at the component. These units, known as centrifugal blast wheels, are usually fixed installations and are commonly used for large production runs, e.g. on pipes in pipe mills and large steel plates in shipyards. The main advantages of this system compared to air blasting systems are as follows: a. lower cleaning time, b. lower abrasive consumption, c. lower energy consumption, d. less labour used, e. more consistent and uniform blast finishes, f. more environment friendly, safer to implement - closed system. g. The abrasive is fed into the centre of the wheels and to the inner edges of the attached blades by means of an impeller. The abrasive is then accelerated to the end of the blades and onto the component by centrifugal force at speeds typically between 250-350 kmfh (160-220 m.p.h.). For cost reasons the abrasive used would normally be reusable. The abrasive is recycled up to approximately twenty times providing it is free from grease or oil contamination. An air-wash separator removes any dust contaminants from recycled abrasive before it is fed back into the wheels.
Ruane & T P O 'Neil/
I
Air blasting Pressure blasting, which is a type of air blasting system, would normally be used on site work. Vacuum blast and suction blast equipment also come under the category of air blasting but are not as widely used due to lower efficiency. Pressure blasting equipment basically consists of: a cnrnprewnr, providing an air supply of approximately 0.7 MPa (100 p.s.i.), a pressurised pot containing the abrasive, l~quidseparators, i.e. moisture filters (knock-out pots), a carbon impregnated hose, a Venturi shaped blasting nozzle, a dead rnurzs handle for direct operator control.
The velocity of abrasive particles leaving a blasting nozzle is primarily governed by the pressure at the nozzle; the higher the pressure the higher the velocity and therefore the higher the rate of cleaning. There is a point at which an increase in pressure does not increase the velocity substantially, this is at approximately 0.7 MPa (100 p.s.i.) depending on the abrasive used. Limiting pressures to 0.7 MPa (100 p.s.i.) is also advantageous for safety reasons. It is important to keep the pressure at the nozzle as close to 0.7 MPa (100 p.s.i.) as possible because for every 1% loss in pressure there is approximately a I%% loss in efficiency. The pressure at the nozzle may be measured using a hypodermic needle gauge, this is placed through the hose near the nozzle, with the hole in the needle facing the nozzle.
I ,,
60
Blasting nozzles Blasting nozzles are available in a variety of materials and orifice sizes. Sometimes the nozzles are lined with relatively abrasive resistant materials, e.g. tungsten carbide, for a longer working life. Two types of nozzle which exist are the straight bore nozzle and the Venturi shaped noule. Straight bore nozzles are rarely used for blasting large surface areas because they are not as efficient as Venturi shaped nozzles. The velocity of abrasive leaving a straight bore nozzle at 0.7 MPa (100 p.s.i.) is approximately 350 k m h (220 m.p.h.), whereas the velocity for a Venturi shaped nozzle under similar conditions would be approximately 720 km/h (450 m.p.h.).
I
Straight Bore Nozzle
I
Venturi Shaped Nozzle
Venturi shaped nozzles also produce a larger blast pattern with the whole area receiving a relatively equal amount of abrasive, whereas, a straight bore nozzle concentrates most of the abrasive in the central area of the blast pattern, resulting in a fringe area of lower blasting efficiency.
0 Ruane & T P O'Neill Issue 3 1YIlUY6
Safety Centrifugal blast units are a closed system, i.e. human access to the blasting area is limited. When using an open system, e.g. for site blasting applications using pressure blasting equipment, access is not usually restricted, therefore warning signs are necessary and regular inspection of the equipment is required. Other safety considerations relating to pressure blasting are as follows: Use of carbon impregnated hose to reduce the chance of static shock. Use of a dead-mans handle to stop the flow of abrasive when the operator lets go of the nozzle. Keeping hoses as straight as possible to prevent kinks which may lead to a blow-out. Use of hoses of the correct type, i.e. reinforced. Use of external couplings if joining hoses together. Internal couplings reduce the bore and the eroding action of the abrasive could lead to a blow-out. Restricting the pressure to 0.7 MPa (100 p.s.i.). The wearing of protective clothing, including an air fed helmet, boots, leather apron and gloves.
Wet blasting Wet blasting methods are good for removing soluble salts such as chlorides from surfaces and are good for the removal of toxic coatings, e.g. red lead paint films, because they do not create dust. However, all wet blasting methods have similar disadvantages over dry abrasive blasting, including: a. b. c. d.
the availability and drainage of water; the production and disposal of sludge (particularly with abrasive injection); the extra cost of supplying and mixing a corrosion inhibitor (assuming the specification allows the use of an inhibitor); the problems associated with drying large surface areas or the higher cost of water miscible primers compared to conventional primers.
High pressure water jetting Operates at pressures sometimes in excess of 200 MPa (-30,000 p.s.i.) which can be extremely dangerous. The advantages of this method are as follows: Simple to operate. Highly flexible and mobile in use. Suitable for removing soluble contaminants. Will remove mill scale at high pressures.
High pressure water plus abrasive injection Operates at pressures up to 140 MPa (-20,000 p.s.i.) which can be extremely dangerous. The advantages of this method are the same as for high pressure pure water blasting, but will also remove firmly held contamination and will create a surface profile.
Low pressure water plus abrasive injection 90
Operates at -0.7 MPa (100 p.s.i.). It is claimed that this technique is very controllable and will remove one coat of paint if required. Disadvantages include high cost and low efficiency.
8 Ruane & T P O'Neill h e 3 lYflUYh
1
a
Steam blasting, with or without abrasive injection Operates at -0.7 MPa (100 p.s.i.). This method is ideal for surfaces contaminated with oil, grease, etc.. Disadvantages include high cost and low efficiency.
lo
Air blasting with water injection Water with or without an inhibitor is injected into an aidabrasive stream.
Hand and power tool cleaning Hand and power tool cleaning, relates to scraping, chipping, wire brushing, sanding, grinding and needle gunning. This method of cleaning, although not as effective as blast cleaning, is often used for short term protection coating systems, maintenance work, or where access for blasting is restricted or damage from abrasive to the surrounding environment would occur. Wire brushing is a widely used surface preparation method but it only cleans up an existing surface, it does not re-cut a new profile. BS 7079 : Part A l defines standards of wire brushed finishes along with other hand and power tool cleaning methods as follows: l any heavy layers of rust shall be removed by Prior to hand and power ~ o v cleaning, chipping. Visible oil, grease and dirt shall also be removed. S t 2 - Thorough hand and power tool cleaning. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter.
1 1
-
Very tl~oroughhand and power tool cleaning. As for St2, but the surface shall be treated much more thoroughly to give a metallic sheen arisi,ng f r o 9 the metallic substrate.
St3 is usually obtained by mechanical wire brushing and St2 is usually achieved by hand wire brushing. Care must be taken to avoid over brushing a particular area causing burnishing, a condition with a highly polished surface which has an adverse effect on coating adtesion. Z
Bronze brushes m y nor be permitted because of
thepossibilifyofgulvanic
*
St3
60
corrosion. with embedded abrasives are available as an alternative.
For safety reasons, it may be specified that wire brushes used must be of the non-sparking type, i.e. phosphor-bronze or beryllium-bronze.
Needle gunning A needle gun, or Jason's hanlnler as it is sometimes referred to, consists of many air operated reciprocating tungsten needles. It is usually preferable for the needles to have a small cross-section. Needle guns are useful for cleaning difficult surfaces such as rivet heads and welds, they also peen (hammer) and stress relieve the surface. Their disadvantages are that they can leave sharp edged craters and rogue peaks and they also have a tendency to push impurities into the surface.
Reciprocating needles
I O Ruane & T P O'Neill Issue 3 19/1YY(( I
1/ Compressed aif NEEDLE GUN
1
Ruane & T P 0'Neil/ After needle gunning the amplitude of the surface profile may be checked by the same methods used for abrasive blast cleaning if the contour of the substrate allows.
I
Flame cleaning The application of an oxyacetylene flame to the steel surface to be cleaned is an efficient method of removing rust, mill scale and other contamination. The effectiveness of the process is due to a combination of factors: Differential expansion - The mill scale on contact with the intense heat expands at a faster rate than the steel to which it is attached and flakes off.
-
Dehydration Rust is a combination of iron oxide and moisture. As the moisture is rapidly driven off the rust is dehydrated and converted to a dry powder which can be removed by wire brushing. Heat penetration - The heat from the flame penetrates all the surface irregularities and removes all traces of moisture, oil, grease, etc.. The flame cleaning of any form of fastener, e.g. rivets or bolts, should be avoided as a loss of mechanical strength may be caused. Flame cleaning often requires three operatives who work in a team as follows: No.1No.2
-
No.3
-
flame cleans the surface, this gives a light grey appearance on the surface when finished. wire brushes the surface to remove all the dry powder. primes the surface; it is often necessary to apply the paint while the metal is still warm, around 40°C (which is about the maximum to which the hand can be comfortably applied).
The warmth of the plate lowers the paint viscosity enabling it to flow more easily into irregularities and also ensures that condensation will not form on the surface. BS 7079 : Part A1 shows minimum flame cleaning standards according to rust grades, i.e. A F1, B F1, C F1 and D F1.
60
Chemical cieaning Pickling and phosphating Pickling is a chemical cleaning process which is widely used in a factory environment for preparing items such as pipes and steel plates. The process usually involves immersing the steel in a bath of hot acid such as sulphuric acid (H,SO,) which has been inhibited to reduce attack by the acid on the steel. The acid dissolves a thin oxide layer at the interface with the steel causing the rust or mill scale to be removed. Other acids, e.g. phosphoric acid and chromic acid, are used to passivate the substrate to retard corrosion reactions and also to promote adhesion. The acids react with the steel to form a thin layer on the surface which passivates the surface and provides corrosion resistance. Procedure (H B Footner's duplex process): 1.
Degrease: Removes surface contaminants such as grease and oil by the use of a suitable solvent, e.g. xylene, usually applied by cloth.
2.
Pickle: Total immersion in a tank of acid, e.g. 5 1 0 % sulphuric acid at 65-70°C to remove mill scale, rust etc., the time taken=iable and depends upon the type and degree of contamination. An inhibitor is also present in this tank.
O Ruane & T P O'Neill
Issue 3 IYIlYYh
3.
Wash: A clean water wash to remove acid and surface residues, usually applied by hose or spray.
4.
Phosphate: The technique involves a final treatment in a 1 to 2% phosphoric acid rust inhibitive solution held at 80°C. for 1 to 2 minutes. This elavnphosphate coati-e steel surface to which the coating should be preferably applied while it is still warm, possibly after a final wash.
Hydrocarbon solvent cleaners The removal of oil or grease from a substrate using hydrocarbon solvents involves proprietary brands of degreasers which usually use solvents such as xylene, toluene and solvent naptha. Other solvents known as halogenated hydrocarbon solvents such as perchloroethane and perchloroethylene are also used.
Note: Halogenated hydrocarbons such as 1,1,1 trichloroethane, trichloroethylene and carbon tetrachloride were commonly used as degreasers but their use has declined, or been completely restricted, due to high toxicity. Heavy vapours of all chemical solvents are a hazard in enclosed areas, e.g. inside tanks. A thin film of oil invariably remains after solvent cleaning but the more solvent used and the more frequent the operation, the less residual matter there is present. Xylene is a commonly used degreaser but its use on painted surfaces is limited due to solvent strength and compatibility considerations.
Emulsion cleaners Emulsion cleaning uses oil emulsifying agents (soaps) which form a suspension of oil droplets within a liquid. It is usual to follow emulsion cleaning with water or steam cleaning to remove the soap and emulsified oil or grease.
Alkali cleaners Economic and efficient in use and therefore popular but operator safety is a serious consideration. Vegetable oils are saponified and mineral oils are emulsified. This category is designed to preferentially wet the substrate, thus displacing any contaminants.
Q Ruane & T P O'Neill
Issue 3 19/12P)6
Tests to detect surface contamination may be qualitative or quanthtive. Qualitative tests will determ~newhether or not contamination is prese'?t b s n o t show thc exact quantity, although an idea of the extent of contamination will normally be determined. There are many tests for detecting contamination but some of these require a chemist or other suitable qualified person to perform; these tests tend to be mainly quantitative, i.e. a quantity is determined, e.g in mg/m2, although even this value may not be the exact amount actually present.
10
Soluble iron salts 20
,,
Colourless soluble iron salts may be present in pits within the substrate after blast cleaning. If salts are present, they will accelerate corrosion causing rust spots which may in turn break the bond of any applied coatings leading to the failure of the coating system. Some specifications state the maximum levels of salts permissible on a surface and express the quantity in milligrams per square meter (mg/m20r mg.m-'). The maximum requirement may be as low as 10 mg/m2 although other specifications may state that 30 mg/m2 is the critical level. Only quantitative tests could be used to determine whether these requirements are met.
Note: Test results may be misleading or totally wrong if chromate or nitrate inhibitors have been used, for example in wet blasting.
-
The porarsium ferricyanide test may also be referred to U S the potarsium hexacyanoferrate (111)test.
40
Potassium ferricyanide test 1.
Spray a fine mist of distilled water onto a small area of the blast cleaned surface using a scent-spray type of bottle.
2.
Wait a moment for any water droplets to evaporate then apply a potassium ferricyanide test paper by pressing down for 2 to 5 seconds.
3.
Remove the test paper and check to see if any salts have been drawn by capillary action. They show as prussian blue spots.
Merckoquant Test
,,
This test is also known as the Eisen test and is a colormetric quantitative test claimed to be 85% accurate down to 30 mg/m2. Small test pads on plastic strips are soaked in a solution of 2,2' bipyridine which reacts with Fe++to produce a red colour. A colour change through white to dark red is shown on the test kit container with corresponding values in parts per million (ppm).
70
Procedure It is generally accepted that by using the areas and quantities mentioned below, a direct conversion to mg/m2 can be obtained.
I
1. 2.
80
There is a laboratory test to determine the quantity of chlorides on a cleaned surface to BS 7079 :Part 8 2 which also invloves mercury (11) nitrate.
Tape off an area of 150 rnm x 150 mm using masking tape or similar. Soak a swab of cotton wool in a pre-measured 22.5 ml of distilled water and swab the masked off area, replacing the cotton wool into the distilled water.
3.
Dry off the area using a fresh piece of cotton wool, removing as much moisture as possible and place this second swab into the distilled water.
4.
Stir the water containing the cotton wool swabs well and immerse the test strip into the solution, after approximately ten seconds, shake to remove the excess water and monitor the resulting colour change with the master chart.
Bresle sample patch This is a mercury (11) nitrate (mercuric nitrate) titration test claimed to be 95% accurate down to 10 mg/m2. (Reference I S 0 8502 : Part 6). 0 Ruane & T P O'Neill Issue 2 lYIlUY(I
I/
Ruane & T P O'Nei
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Salt contamination meters These normally give a digital readout and work by directly measuring the ionised metal salts dissolved in a quantity of water.
Mill scale Mill scale is cathodic with respect to steel. This means that if any traces of mill scale are present on the surface after preparation they can accelerate the corrosion of the underlying steel and disbond, leading to the eventual failure of any coating system applied. To test for the presence of any mill scale particles left behind after blast cleaning to BS 7079 grade Sa3 the copper sulphate test may be used. Procedure
A fine mist of slightly acidic copper sulphate solution is sprayed onto a localised area of approximately 100 mm in diameter. The steel turns a bright copper colour and any mill scale particles show as black spots.
Dust The presence of dust may be determined by applying transparent pressure-sensitive adhesive tape to the test surface and then removing. The tape is examined using a magnifying glass and an assessment of the degree of dust contamination is made. Standards do exist which standardize the test conditions and the way in which the results are assessed. For example, the pressure applied to the tape and its degree of stickiness will partly govern the results. (Reference BS 7079 : Part B3).
Oil or grease Simple visual assessment may reveal the presence of oil, dr grease, however, a cotton wool swab wiped over the surface may reveal oil or grease which was not directly visible when on the surface. The use of an ultraviolet lamp may also detect oil or grease by causing it to fluoresce, but a dark environment is required for this method. Another method is to drip several drops, using an eye type dropper, of a solvent such as xylene onto the suspect area. After a few moments remove some of the solvent with the dropper and drip the solvent onto a tissue or filter paper. When the solvent has evaporated any oil or grease removed by the solvent will show up on the paper as a brown ring.
Ruane & T P O'Nei! Flashpoints give an indication of fire risk and are defined as, 'the lowest temperature at which solvent vapour from the product under test in a closed cup gives rise to an airlvapour mixture capable of being ignited by an external source of ignition'. Flashpoint determination of paints or solvents may be carried out in accordance with BS3900 part A9 using a closed cup of the Abel type. Procedure:
F a n orange f i m e is observed, the temperature is ' roo high and overhearing has occurred The material under resr should be cooled or reploced and the resr restarted.
1.
Fix the Abel cup containing the substance for assessment into a water bath.
2.
Apply a heat source to the water bath and monitor the temperature of the substance in the Abel cup.
3.
Activate the source of ignition every Yi°C rise in temperature.
4.
The flashpoint temperature is identified when a blue flame flashes over the substance being assessed.
Q Ruam & T P O'Neill I s v c 2 2OI1m
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Ruane & T P 0'Neil
1 The s ~ u d yofthe flow of liauids is known us
A fluid with a high - viscosity has a high - resistance to flow and therefore has a thick consistency; the frictional' forces between the molecules are greater. The opposite is true with a low viscosity fluid.
Temperature affects viscosity, therefore any comparative tests must be carried out at a specific temperature, e.g. 20+0.5"C. The S.I. unit for dynamic viscosity is the Pascal second ( P a s ) which is equivalent to the newton-second per square metre (N.s//n2). The old c.g.s. unit, the poise, is still commonly used. 20
(
,,
The viscosity of water is approximately 1 centi-poise. Another c.g.s. unit which may be encountered relating to kinematic viscosity is the stoke. A fluid having a viscosity of one poise and a density of 1 g/cm3 has a viscosity/density ratio of one stoke. The instruments used for measuring viscosity are known as viscometers of which there are many types. Viscometers in paint laboratories are usually of the rotational type and include the Krebs-stornzer viscometer, the cone and plate viscomerer and the rotatlzinner.
Cone and plate
1 %
Rotathinner
Krebs storrner
-
ROTATION VISCOMETER ROTOR ENDS
A simple method for measuring the viscosity of free flowing paints is by using aflow cup; again there are may types including the /SO, Ford and Zahnflow cups.
6o
8 Ruane & T P O'Keill Issue 2 ZII/IUYfi
I
I
Procedure for nleasuring viscosity using a Ford flow cup No. 4: 1.
Bring temperature of paint to within 20+0.S°C.
2.
Level the apparatus, then with the end of one finger over the orifice of the cup, rapidly fill it with paint.
3.
Allow a moment for air bubbles to rise, then draw a flat edge across the top of the cup to wipe off the paint level with the edges.
4.
Remove the finger from the orifice and start the stop watch simultaneously with the commencement of the paint stream. The watch is stopped when the first distinctive break in the paint stream occurs.
5.
The time in seconds is taken as the viscosity.
'OI
201
This procedure can be used to determine the quantity of any added thinners. There is no direct relationship between the time value obtained and the percentage of added thinners. A comparison has to be obtained by preparing a number of control samples using different percentages of thinners added to the paint taken from a freshly opened can. A thixotropic paint needs to be worked to reach the free flowing stage, therefore the viscosity cannot be assessed with a flow cup; a rotation viscometer or another type of viscometer which works the paint must be used.
0 Ruane & T P O'Neill
Issue 2 2tl11UYfi
Ruane & T P 0'Neil/ Density is weight per unit volume and is therefore found by the following formula: weight Density = volume
10
The unit used for measuring the density of paint is usually grams per cubic centimetre (g/cm3). 1 cm3 of water
= 1 millilitre
1000 cm3 of water = 1 litre 20
-
= 1 gram
= 1 kilogram
The density of a paint will be hi her than that of water; the density of a solvent will be that lower than that o m e r ; the ensity of a curing agent may be higher or =than mater.
hp
DENSITY CUP
Procedure for measuring density using a 100 cm3density cup:
1.
Weigh the cup to the nearest decigram using a laboratory balance with a 1000 g capacity and a sensitivity of +O. 1 g.
2.
Remove the cover and fill with paint to within 2.5 mm of the brim.
3.
Carefully replace the cover so that air and any excess paint is expelled through the vent.
4.
Wipe off any surplus paint from the cover then reweigh.
5.
Determine the weight of the paint by subtraction.
6.
Divide weight by 100 if the density in g/cm3is required.
This procedure can be applied to determine the quantity of any added thinners. The weight of a sample of paint taken from a paint kettle could be compared with control samples which have been prepared by adding differing percentages of thinners to the paint taken from a freshly opened can. There is a relationship between the obtained weight and the percentage of added thinners if the pre-mixed density of thinners and density of paint is known. It is also possible using this procedure to determine whether two-pack paints have been mixed in the correct proportions.
8 Ruane & T P O'Neill Issue 2 08/11J/Yh
Ruane & T P 0'Neil/
Relative density 10
Relative density or specific gravity is the density of any substance compared to the density of water:
Specific gravity (SG) =
density of x density of water
O'
Because the density of water is lg/cm3the figure obtained from the SG formula will be the same as that obtained from the density formula, the difference is that the answer for the SG formula will have no units, i.e. it is a dimensionless & atr.
"
Example formulae What is the density of a paint if 5 litres weighs 7.35 kg?
1.
a.
weight Density = volume
b.
7.35 kg Density = 5 litres
C.
Density =
d.
Density = 1.47 g/~nz'
40
50
60
70
7.35 x 1000 grams 5 x 1000 cnz'
A two pack paint is mixed at a ratio of seven parts base to two parts curing agent; the densities are 1.59 g/cm3 and 0.78 g/cm3 respectively. What is the density of paint after mixing?
2.
a.
7 parts base
1.59 x 7
= 11.13
b.
2 parts curing agent
0.78 x 2
=
c.
9 parts combined
d.
Density
Note: SG would be 1.41 80
90
O Ruane & T P O'Neill Issue 2 13t01N7
11.13
+ 1.56
12.69 + 9 parts
1.56
= 12.69 =
1.4 1 g/cm3
The wet film thickness is taken immediately after a coating has been applied so that any deviation from the specified thickness range can be immediately rectified while the paint is still wet, thereby reducing the amount of dried coatings which are outside the specified thickness tolerances. Also any calculations based on volume solids will be meaningless if a lot of solvent has evaporated. '0
The wet film thickness may be found by using a comb gauge or an eccentric wheel.
Substrate
'
Scale
ECCENTRIC WHEEL
1
The w.$t. is sometimes recorded as the average behveen the last touching tooth and the first non-touching tooth.
2jk.I
Procedure for measuring w.f.t. using a comb gauge
1.
Immediately after application of the paint, the comb-gauge should be placed firmly onto the substrate in such a way that the teeth are normal to the plane of the surface.
2.
The gauge should then be removed and the teeth examined in order to determine the shortest one to touch the wet paint film. The film thickness should be recorded as lying between the last touching tooth and first non-touching tooth as shown on the tooth calibrations marked on the gauge.
3.
At least two further readings should be taken in different places in order to obtain representative results over the full coated area.
The wet film thickness may be found by calculation: w.f.t. =
, , volume
0 Ruane & T P O'Neill Issue 2 13/01/!47
Ruane & T P 0'Neil/
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Adhesion failures more often occur between the uncoated substrate and the primer due to inadequate wetting of the substrate which may be as a result of insufficient surface preparation, insufficient dust removal after surface preparation or contamination All paints within a system should have compatibility between coats and with the substrate. It is advisable to obtain all the components for a paint system from one manufacturer otherwise it may not be possible to guarantee the system; when compatibility is lacking it is often the adhesion which suffers.
Paint system
Cohesive failure
\
/
eslve failure (between paint films)
'i-
Substrate
Adhesive (between failure primer
and substrate)
Vee cut test
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I
With a sharp knife, cut a vee using approximately 12 mm cuts forming a 30" angle, through the paint film and down to the substrate. Insert the tip of the knife blade under the tip of the vee and attempt to lever the paint away from the substrate. If the integrity of the coating is sound it should not peel cleanly from the surface
Cross-cut test (cross hatch test) Using a sharp knife or multi-bladed cutter, cut 6 lines vertically and horizontally, 2.0 mm apart, to produce 25 squares. Cover the area with adhesive tape and snatch off; the amount of segments remaining may on the tape may multiplied by four and then given a percentage value or a value may be given based a scale in accordance with the applicable specification. The tapes degree of stickiness will be relevant to this test and the number and size of the squares may vary, therefore always consult the relevant specification for precise instructions.
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X-cut tape test A sharp knife or similar is used to make an X shaped cut with the smaller angle between 30" and 45". The cuts must be made down to the substrate in a single action and are made approximately 40 mm in length. A piece of specified pressure sensitive tape approximately 75 mm long and 25 mm wide is placed over the cut and pressed down in the central area first using a finger. An eraser on the end of a pencil is then used to firmly rub the tape so full adhesion is achieved. Within 1 to 2 minutes the tape is pulled off rapidly at an angle as close to 180" as possible. The X-cut area is then examined and the adhesion is rated using a scale from 5A = no peeling or removal through to OA = removal beyond the area of the X .
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Ruane & T P 0'/Veil/
Dolly test A more technical adhesion test, the pull-ofladhesion test or dolly test, may show: adhesive failure between primer and substrate (most likely); adhesive failure between paint films; cohesive failure within an individual paint film.
Load indio Dolly puller
Paint film
Substrate
401I 1 Altert~ativeadhesives ctrz possible, see test procedure sheefs.
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PULL OFF DOLLY TEST
Procedure for carrying out pull-off adhesion test: Clean and degrease the surface to be tested and the dolly contact surface.
I. 2.
Roughen both surfaces with finelmedium grade emery cloth.
3.
Mix regular araldite and stick dolly to the surface, leave for 24 hours at 25OC.
4.
Cut paint around the dolly down to the substrate using special cutter.
5.
Attach pull-off instrument and apply pull-off force.
6.
Take reading from position of cursor when dolly detaches itself. Values will be typically obtained in either MPa, N/mm2 or p.s.i..
A minimum pull-off value for the paint type used should ideally be specified in the specification(s) for the work being carried out. In the absence of such criteria, a minimum pull-off value should be obtained from the paint manufacturer who should also state categorically whether or not all values less than the minimum pull-off value are deemed a failure.
Hydraulic adhesion test This test uses a similar principle to the dolly tester, but usually gives more accurate test results. The dollies used are reusable and contain a hole down their centre through which a hydraulically operated rod applies force directly to the coated surface in order to pull the dolly away from the surface. The opposing force is supplied by the end of the adhesion tester which grips the top of the dolly.
8 Ruane & T P O'Neill Issue 2 21IlUY6
This test may be performed on a sample cut from the coated structure or coated samples which simulate the coating applied to a structure which is cathodically protected.
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The test will indicate whether the coating system is susceptable to disbondment if hydrogen gas bubbles are given off from the surface of the substrate - which is likely to be the case in service if the substrate is excessively cathodic, i.e. an excessive amount of cathodic protection to a coated structure can lead to disbondment of the coating. Cathodic disbondment is a significant problem when coating defects are present due to a stripping action caused by the hydrogen bubbles and is especially prominent when the impressed current is way in excess of the corrosion current; this condition is purposely produced for the cathodic disbondn~enttest. The test incorporates a coated test panel (or sample taken from the coated structure) with a hole drilled into the coating; this simulates a paint film defect. Surrounding the hole, a plastic tube is glued down and filled with a specified electrolyte, e.g. a 3% wlv sodium chloride solution. Wires from a battery or transformer are attached to the test panel and to a inert electrode, e.g. platinum rod, set into the lid of the plastic tube and making contact with the electrolyte; a current is then impressed to make the test panel cathodic. The potential required will be specified, -1500 mV is typical. The coating is assessed after a given period of time, e.g. a few weeks, for the amount of stripping which has occured from the boundary of the hole.
Sodium chloride solution Plastic ring
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Q Ruane & T P O'Keill
Issue 2 2111UY6
6mm hole drilled into substrate CATHODIC DISBONDMENT TEST
Holiday detection or pinhole detection is an operation which detects any holesholidays in a coating or wrapping; the instrument used for this is known as a holiday detector or pinhole detector. Substantial lack of thickness and inclusions in the coating may also be detected in some cases. Visual inspection in addition to holiday detection is still a very important part of inspection.
Holiday detection must not be carried out on wet surfaces or in the rain.
There are various types of holiday detector, some used for thin paint coatings, e.g. the wet sponge type, whilst others may be used for coatings over 25 mm thick, e.g. high-frequency spark testers. For coatings ranging from approximately 0.5 mrn to 4 rnrn thick, AC, DC or pulsed DC holiday detectors, usually powered by a 6 volt battery, would normally be used.
High voltage holiday detectors Voltage selection
Holiday detectors should be checked throughout the working day to ensure correct set up.
Prior to carrying out holiday detection the correct voltage must be selected because too much voltage may indicate the presence of holidays when they do not exist, or really excessive voltages may even burn a hole into the coating. Not enough voltage may result in holidays not being detected. The volt meters or voltage settings on holiday detectors should be checked for accuracy by using a method recommended by the holiday detector manufacturer. This may involve using a calibrated volt meterlmulti-meter or proprietary calibration voltmeter supplied by the detector manufacturer. When relatively thin coatings are being tested, e.g. fusion bonded epoxy coatings, it is usually necessary to have a fine scale on the machine, e.g. 0 to 5 kV for accurate voltage selection. For thicker coatings 0 to 20 kV is normal. Correct holiday detection voltage is governed by the thickness and dielectric strength of the coating. The method to use for selecting voltage should be specified for each type of coating.
It is preferable ro ensure the coated structure is properly earthed by testing for the presence of a known pinhole. This may not be permitted due to the repair which will have to be made on the pinhole.
The correct voltage is ideally determined by detecting the presence of a known pinhole which has been induced diagonally through the coating to bare metal. However, the voltage is normally selected by measuring the coatinglwrapping thickness and applying a formula, e.g. 125 V per 25 pm of thickness (same as 5 kV per mm), or following other specification requirements.
Operation When operating a holiday detector on a coated structure, an earth wire from the main unit is either clipped to the structure or trailed along the ground. If the earth lead is to be trailed along the ground, the structure must be earthed, usually via a crocodile clip to a wire with a metal spike attached which is hammered into the ground. The electrodes (brushes) used, which are attached to the end of an insulated hand stick, are normally of the wire brush type although carbon impregnated neoprene brushes also exist but are not as effective. Spring wrap around coils are commonly used on pipes. The maximum travel speed for brushes or coils may be quoted in specifications, e.g. 300 m d s e c . When the brush or coil comes into contact with a holiday, a spark will jump across between the gap which completes the circuit. One or more of the following indications will warn the operator of its presence: a. b. c.
The kV dial will drop. An alarm will sound, e.g. a bleeper. A light will come on.
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Ruane & T P 0WeiI1 When a holiday is detected it should be marked/circled with a waterproof marker, but the marking should be sufficient distance from the holiday so as not to interfere with the adhesion of the repair.
Wet sponge pinhole detectors Only low voltages are required for these instruments because water, sometimes containing a wetting agent such as washing up liquid, is used as an electrolyte to conduct the current from an electrode (wet sponge) through a pinhole to the conductive substrate. Water is used to wet a sponge which is connected to the positive terminal on the test instrument. When the sponge passes over a pinhole, the water is drawn into it, which allows the d.c. current to pass through to the substrate and back along the return wire to complete the circuit. Some wet sponge pinhole detectors have a variable voltage setting between 9 V and 90 V, whereas others have only a single setting, e.g. 9 V. There is no hard and fast rule for voltage to use with these instruments but it is generally accepted that up to -300 pm the 9 V setting is adequate; up to -500 pm would require the 90 V setting. The specification or written instruction should state the voltage to be used.
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The coating specification should always state the weather conditionsin which a coating can or cannot be applied. A typical painting specification extract is as follows:
I
'It is not permissible to apply paints when the following conditions apply: During rain, snow, or high winds. When the air or metal temperature is not at least 3OC above the dew point temperature.
10
When the air or metal temperature is below 5°C. When the relative humidity is above go%.'
I
Relative humidity (RH%)and dew point Definitions
*
Relative humidity is the amount of water vapour in the air expressed as a percentage, compared to the amount of water vapour which could be in the air at the same temperature. The higher the air temperature the greater the amount of water vapour which can be held in it.
The capacity of air to hold water doubles every 11°C rise in tempemture.
The dew point is the temperature at which water v w n the atmosphere would form condensation Therefore, if the temperature dropped to the dew point temperature the y& -t would rise to 100% and condensation would be formed on any objects at, or below, that temperature.
Measuring R.H. % and dew point Both relative humidity and dew point are measured using a hygronieter of which there are many types. 50 I
1.
Aspirated hygrometers The screen hygrometer and Masons hygrometer are static types which rely on a natural airflow over a wet wick. b. Assman and psychrodyne hygrometers are also static types which work by a fan driven air supply over a wet wick. c. Whirling hygrometer is a portable and dynamic type which operates by physically moving a wet wick through the air. a.
The transport of mercury 80 by air is not permitted, therefore coloured alcohol in glars thermometers m y be specified for work which involves equipment being transported by air.
,,
The dry bulb temperature is the air temperature with a wind chill factor.
2.
Dial hygrometers come in two main forms: hair and paper. Hair hygrometers operate by expansion and contraction of hair, usually human (treated), and are extremely accurate and fast in operation. Paper hygrometers also work on absorption but this time the absorbtion properties of paper.
3.
Digital hygrometers are split into two categories: (1) RH meters which give digital readouts of RH and DP only and (2) thermo-hygrometers which give a digital readout of RH, DP and ambient dry bulb temperatures.
The whirling hygrometer, or psychrometer, is the most common type used by coating inspectors consisting of two mercury-in-glass thermometers set side by side in a frame which is provided with a handle and spindle so that the frame and thermometers can be rotated quickly about a horizontal axis. The bulb on one of the thermometers, called the wet bulb thermometer, is covered with a closely fitted cylindrical cotton wick, the end of which dips into distilled water or clean rainwater contained in a small cylinder attached to the end of the frame. The frame is rotated by hand as fast as possible for at least 90 seconds, or as otherwise specified, so that the bulbs pass through the air at least 4 ms-'. This causes the water to evaporate from the wet bulb. The wet bulb cools down to a constant wet bulb temperature due to the evaporation rate of water from the wet wick. Always read the wet bulb temperature before the dry bulb temperature immediately after rotation.
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Ruane & TP 0'~ei// Repeat the operation until consecutive readings of each bulb temperature agree to within 0.2"C. If it is 100% relative humidity the wet bulb will be the same temperature as the dry bulb, because no evaporation can occur, i.e. the air is saturated. If the wet and dry bulb temperatures are the same, the current temperature is the dew point. The relative humidity and dew point cannot be read directly from the apparatus, hygrometric tables or special slide rules must be used. Hygrometric tables are more accurate in the 90% RH region and above.
Metal temperature The metal temperature may be measured with a magnetic temperature gauge, sometimes known as a limpet gauge, or electrical contact thermometer.
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COSHH Regulations 1994 Scope Thc Control of Substances Hazardous to Health (COSHH) Regulations 1994 (SI No. 3246) define a substance hazardous to health as: a.
b.
c. d. e.
a substance listed in part 1 of the approved list as dangerous for supply within the meaning of Chemicals (Hazard Information and Packaging) Regulations 1993 and for which an indication of danger specified for the substance in Part V of that list is very toxic, toxic, harmful, corrosive or irritant. one which has a maximum exposure limit (MEL) in Schedule 1 of COSHH or if the H & S Commission has approved an occupational exposure standard (OES). a biological agent. dust in air - when substantial. a substance comparable with the above.
The COSHH regulations are not applicable to the control of lead, asbestos, radioactivity, explosive or flammable properties of materials, high or low temperatures, high pressures, medical treatment or below ground work (mining). Other Regulations deal with these areas. 40
Responsibilities The exposure of an employee to substances hazardous to health is under the control of the employer. A training organisation is responsible for exposure by trainees. Employers must prevent exposure to substances hazardous to health, or control exposure when total prevention is not reasonably practicable. Personal protective equipment, e.g. masks, are a second choice for control. Prior to work commencing, employers must always carry out a risk assessment for all substances hazardous to health to which employees may be exposed. Employees have a duty to report any problems in exposure control procedures or any defects found in protective equipment. 6O
--I The occupational exposure limit for xylene is an occuparionaI exposure standard (OES), therefire the 80 OES is 100 ppm.
I
Employers must keep records of examinations/monitoring tests carried out. These are kept for 5 years; 40 years for identifiable employees.
Occupational Exposure Limits (EH40) The Guidance Note EH40, entitled Occupational Exposure Limits, is a document published by the Health and Safety Executive and updated each year which gives occupational exposure limits for substances hazardous to health. An organic solvent, which is a substance hazardous to health, has its own occupational exposure limit as given in EH40. The toxicity value of a solvent is expressed in parts per million (ppm), e.g. the occupational exposure limit for xylene is 100 ppm, this means to say that if the air contained xylene exceeding 100 ppm the air would be considered to be a significant hazard to health. There are two types of occupational exposure limit: 1.
Maximum exposure limit (MEL): 'is the maximum exposure limit for that substance set out in Schedule 1 in relation to the reference period specified (in COSHH) when calculated by a method approved by the Health & Safety Cornrnision.'
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Ruane & T P 0Vlileill -
2.
Occupational exposure standard (OES): 'the standard approved by the Health & Safety Commision for that substance in relation to the specified reference period when calculated by a method approved by the Health & Safety Commision.' When a MEL is specified, exposures must be kept as low as is reasonably practicable, but always below the specified value. Long term exposure limits are averaged over an 8 hour reference period. Short relm exposure limits are taken over u I5 minute reference period.
An OES should not be exceeded, but, an exposure over the limit is acceptable providing that the reason for exceeding the OES has been identified and measures are taken to reduce the exposure below the OES as soon as is reasonably practicable. Examples of solvents with their corresponding long term exposure limits (OES's unless otherwise specified) can be seen in the following table: -
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Solvent Examples with Corresponding OEL's Group
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Name
OEL (ppm)
Alcohols
Methanol Ethanol
Ethers
Ethyl ether Isopropyl ether
400 250
Esters
Methyl acetate Ethyl acetate Isobutyl acetate
200 400 150
Ketones
Acetone M.E.K. M.I.B.K.
750 200 50
Hydrocarbons (Aromatic)
Xylene Toluene Benzene
Hydrocarbons (Aliphatic)
n-Octane Hexane White spirit
Chlorinated hydrocarbons
]
, Miscellaneous
1, 1, 1-~zchloroethane Trichloroethylene Carbon tetrachloride Methyl chloride Water Nitromethane Nitrobenzene
200 1000
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n/a 100 1
"EL. "Maximum short term exposure limit: 450 ppm. 'Maximum short term exposure limit: 150 ppm.
Depending on the context of : ; k sentence. VOC cun mean :_jtiie corn,,ounh . .-k.5'!torganic content
Volatile organic compounds Volatile Organic Compounds (VOC's) are toxic and harmful to the environment. It is estimated that in 1992 in the United Kingdom alone, 1.8 million tons of VOC's were released into the atmosphere - 35% to 50% of which came fiom paints. As an example of weight volume ratio, each litre of car paint contains 0.5 kg (1.25 Ib) of VOC's. Each gallon (4.55 litres) of industrial coating can contain 4-5 Ib (1.8-2.3 kg) of VOC's. The European Community directives are demanding a reduction of 30% VOC emissions by 1999, with further reductions thereafter. The United Kingdom Environmental Protection Act of 1992 goes even further, requiring a reduction of 38% 6 Ruane & T P 0 > i l l h e 3 3112%
Ruane & T P 0Neil/ of VOC by 1998. COSHH Regulations also require paint manufacturers to screen all raw material used in the manufacture of their product and eliminate, where possible, all materials which may be dangerous to manufacturing operatives and to applicators. The alternatives to standard paints containing organic solvents ate solvent free paints (100% VS), high volume solid paints, i.e. over 65% VS, water borne paints and powder coatings. Most paint manufacturers have chosen their particular product path, some have opted for acrylics and vinyls, some for new formulations of water borne epoxies and some for 100% VS. Each of the systems have their advantages and disadvantages, e.g. 100% VS urethanes have no VOC's; they are mainly fast curing; highly resistant to chemical attack; chalking and natural erosion is virtually nil but the activator chemicals are extremely toxic. High volume solid paints still contain VOC's and therefore in the future their use is likely to be restricted. Water borne coatings are environmentally friendly and biodegradable, therefore extra costs are not incurred in disposing of containers and sludges etc. but application areas are significantly reduced because of the slow evaporation rate of the solvent water. Water borne epoxies have been in use for some time now, but when used in high humidity environments their successful application and attainment of intended properties is difficult to achieve.
Waferborne epoxies are offen referred fo as new generation or third generation epoxies.
Powder coatings can be applied as thin as 25 pm electrostatically with a utilisation yield of 98%. The disadvantage is that costly heat is required for the reaction. The component needs to be heated to a minimum of 70°C (over 200°C in some cases) to melt the powder in order to form a film and for the reaction to take place. Powders can be formulated to melt at much lower temperatures but this would create manufacturing problems and storage stability problems.
Health & Safety data sheet The information typically present on a health and safety data sheet is as follows: 1.
Date of issue.
2.
Product namelreference and manufacturer.
3.
Intended use.
4.
Health hazards: a. OEL (8 hours & 15 minutes). b. Respiratory; skin; eye; long term effects.
Flammability - fire prevention, fire fighting. a. Flash point. b. LEL and UEL. RAQ (to 10% of LEL), e.g. 100 m3 of air per litre of paint applied. c. Requirements when handling - barrier creams, masks etc.. 6. 5.
7.
First aid procedure.
8.
Storage.
9.
Spillage.
10. Environmental. I 1. Additional information.
0 Ruane & T P O'Neill Issue 2 lYOlN7
Ruane & T P 0'/Veil/
Tenm such as defects, flaws, discontinuities or imper;fectionr may be used to describe faults on rhe substrate.
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There are many faults which may exist on the surface of the material to be coated. The potential consequences of coating over these faults are ( I ) premature failure of the coating system and (2) failure of the component. The painting inspector is normally expected to evaluate whether sharp contours such as edges are acceptable to paint, however, it is rarely ever a painting inspector's duty to determine what remedial action, if any, needs to be applied to inherent material defects such as laminations or cracks. It is important for the painting inspector to be aware that flaws in the material may still exist after inspection and acceptance by other QC personnel. Flaws may also have been induced by secondary processing, e.g. grinding, or may have been induced in service, e.g. fatigue cracking. Substrate faults may only reveal themselves to visual inspection after surface preparation, it is at this stage the inspector should be vigilant.
Inspectors are sometimes used to carry out dual inspection roles, e.g. welding inspection und puinring inspection, In this situution, the inspector used should be &alified fir both activities. These dual approved inspectors often are authorised to evuluate substrate faults and to request remedial action.
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When a surface flaw is revealed, the painting inspector should not try and evaluate its type and significance, and should not permit the painting contractor to remove the fault before it has been evaluated by somebody qualified and authorised to do so. It should be made clear to painting inspectors from the outset of work what action they should take if surface breaking defects are found in the material to be coated, e.g. report immediately to the senior inspector or client before carrying out further work on the affected item. The most common defect on the surface of steel that the painting inspector is likely to encounter, especially after abrasive blasting has been carried out, is the surface lamination or sliver. Other terms are used to describe this flaw but most of them used are incorrect when compared to terms used in national and international standards.
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Surface laminations are not necessarily shallow flaws, they may extend deeply into the material. Disc grinding is usually used to remove them, and in the case of critical components such as pressure vessels, this is followed by magnetic particle inspection or penetrant testing to confirm complete removal and ultrasonic testing to determine whether the maximum wall thickness reduction has not been exceeded. Any cracks found on the material's surface may result in the complete rejection of the component or permission for a localised repair using welding may be granted. Flaws in metals may be divided into three categories: a. b. c.
Primary processing flaws. Secondary processing flaws. In-service induced flaws.
To give an idea of the scope and complexity of the subject as a whole the following text is provided. It is not necessary for a painting inspector or senior painting inspector to know the detail.
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Service induced and secondary processing cracks Service induced cracks are attributable to some external influence during service such as vibration or cyclic thermal stresses. There are many ways to categorise or term cracks which have occurred in service or during secondary processing. The following list identifies the main types: a. b. c. d. e. f. g.
Heat treatment cracks. Grinding cracks. Hydrogen cracking. Brittle fracture. Ductile fracture. Fatigue fracture. Stress corrosion cracking.
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There are three common causes for cracking during the heat treatment process: a. b. c.
during reheating - cracking due to thermal shock; during quenching - cracking due to rapid contraction; after a hardening process - when the component is not tempered soon enough.
Grinding cracks Grinding cracks usually occur in groups at right angles to the direction of grinding or as a network of cracks when rotary grinding wheels are used. They are commonly caused by using the incorrect grade of grinding wheel, by applying too much force or by loss of grinding fluid (if used) and only occur in materials which can be hardened.
Hydrogen cracking Hydrogen cracking may not only occur during welding but may occur in service or due to secondary processing. A sufficient quantity of hydrogen has to be available to enter the material or the inherent hydrogen content has to be high enough. The material must also have a grain structure which is susceptible to cracking. Hydrogen cracking may be in the form of a fine network or may exist as a linear or wandering line. Hydrogen can also form blisters on the surface. The following list shows common sources of hydrogen which may lead to hydrogen cracks: a. b. c. d.
Chemicals used during etching - etching cracks. Chemicals used during the plating process - plating cracks. Acids used during pickling -pickling cracks. Hydrogen contained within chemicals being transported via pipework or contained within vessels, e.g. sour gas.
Fatigue cracks
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Fatigue cracking is a service failure which occurs under cyclic stress conditions. It normally occurs at a change in section, e.g. groove, radius, step, weld toe etc., therefore design and workmanship are important to minimise failure by fatigue. Them are two main sources of stress cycles: a. b.
Mechanical stress cycles - caused by vibration or movement. Thermal stress cycles - repeated heating and cooling creates repeated expansion and contraction.
All metals are susceptible to fatigue failure. Since design and workmanship play a major part, ferrous based materials have an endurance limit applied to one grade of steel in a specific heat treated condition operating within specific parameters, below this limit fatigue is unlikely to occur. Other metals will all have the potential to fail by fatigue given the required conditions. Fatigue failures start at a specific point and propagate with each stress cycle at a rate that depends on the applied stress. Fatigue failure is easily identified by beach markings on the fractured face. Final failure can be any other mode of fracture, e.g. brittle or ductile failure.
Stress corrosion cracking This type of cracking sometimes occurs in materials in a state of tensile stress and in contact with a corrosive medium. The level of stress which can cause the cracking may be well below the yield point of the material.
/
Stress corrosion cracks are surface breaking and are usually found at any sharp change in section, notch or crevice, especially in structures which have not been stress relieved. Both ferrous and non-ferrous materials are susceptible to stress corrosion cracking.
Ruane & T P 0'Neil/
Basic principles and methods The first line of defence against corrosion on a buried or immersed iron or steel structure is usually the coating. If the coating totally isolates the structure from the environment which causes corrosion, the structure will not corrode. However, coatings are not totally impermeable and they are also likely to contain some defects, therefore corrosion would take place unless a secondary system protected these areas. Cathodic protection prevents the structure corroding at areas where coating defects exist. The coating defects must be of the type which allows an electrical path to exist from the soillwater to the metal surface. Therefore, cathodic protection is usually a back up to the coating for anti-corrosion purposes, although it should be noted that cathodic protection can also be applied to uncoated structures.
All coatings are permeable to water, to (1 greater or lesser extent.
Both anodic and cathodic areas would be present on the surface of a structure without cathodic protection. Anodic areas on the structure corrode, current flows from anodic areas through the electolyte to cathodic areas. When any exposed metal on the structure's surface receives current, the area becomes cathodic; this prevents corrosion. Cathodic protection is defined in British Standard BS 7361 : Part 1 : 1991 - Cathodic protection (Code ofpractice for land and marine applications) as: 'A means of rendering a metal immune form corrosive attack by causing a direct current to flow from its electrolytic environment into the entire metal surface.'
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There are two methods of applying cathodic protection: 1.
Using sacrificial anodes.
2
Using impressed current.
Both methods achieve cathodic protection by passing small d.c. currents through the electrolyte from anodes which are installed close to the metallic structure. The anodes will corrode and are manufactured in materials which provide a long service life and consistency of corrosion characteristics. The natural potential of steel or castlductile iron in soil is approximately -550 millivolts (mV) when measured at ambient temperatures against a standard known as a saturated copper/copper sulphate reference electrode. Cathodic protection is considered to have been achieved to a level where corrosion activity is arrested - when the potential has been shifted in a negative direction by the application of cathodic protection current by 300 mV or to a minimum level of -850 mV when referred to a saturated copperlcopper sulphate reference electrode. Detailed and further information with respect to cathodic protection levels are shown in BS 7361 : Part I: Section 2.
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Considerations for protection of coated structures It is essential to obtain a sound, holiday free, coating on the structure because this minimises the current required for protection. Current has to be substantially increased if there are many bare steel areas on the surface of the structure - this is also the case if other metallic items come into contact with the protected structure. Coating damage also increases the chance of electrical interaction (inteverence) with other buriedlimmersed metallic structures. On buried structures it is especially important to have coatings which are uniform in thickness and homogeneity so that coating damage or coating deterioration will be more easily detected. It is possible for the cathodic protection to be too great, i.e. the structure can be over negative. This can cause excessive amounts of hydrogen gas to be given off from the
8 Ruane & T P O'NeillIACEL
metal substrate resulting in coating disbondment known as cathodic disbondment; the more negative the structure, the more hydrogen gas is evolved. It is not usually an easy task to attain uniform protection over the full surface area of a structure. Non-uniformity in protection tends to be increased in the following situations: 10
1.
1 1
2.
3. 4.
High c.urrent density necdcd for protection. Electrolyte resistivity is high. The anodes are too close to the structure. Non-uniform coating quality.
Determination of adequate protection To determine whether a structure is receiving the required protection a device known as half-cell reference electrode is used - this measures the structure-to-electrolyte half-cell potential. From the readings taken from a voltmeter attached to the reference electrode, the cathodic protection technician or engineer can determine whether or not the structure is being adequately protected. There are various types of reference electrode see Unit CP9.
Aim of quality assurance The aim of quality assurance is to improve quality whilst keeping costs to an acceptable level. The objective of a system used to implement quality assurance, i.e. a quality system, is to prevent the occurrence of problems which result in remedial action. If problems do occur the objective is to determine and rectify the root cause(s), thereby reducing faults and wastage in the future. This will in turn, improve quality and reduce costs. The emphasis is on prevention rather than detection and cure.
Benefits of adopting quality assurance A properly implemented and managed quality system should: help to ensure that the company focuses on market needs and requirements. a. make the company more competitive in the market place due to an increased b. customer confidence in the company's output, i.e. a product or service that a customer wants - this includes timing. lead to a reduction of costs due to a reduced number of faults and wastage. c. give a measure of performance which will enable any areas for improvement to d. be identified. induce a more organized way of thinking which makes management more e. organized and effective. f. provide motivation; motivated employees provide a better working environment in addition to the product or service output benefits.
What is quality assurance? The definition for quality assurance given in I S 0 8402 : 1986 (BS 4778 : Part 1) entitled Quality vocabulaty: 'All those planned or systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality.' The quality of a product or service is attained only by working in a controlled manner, following formalised procedures which are designed to eliminate the occurrence of problems. Quality assurance provides the objective evidence needed to give maximum confidence for quality. Quality assurance may be considered as a management tool when used within an organization. A supplier that implements and maintains a system for assuring quality, is providing maximum confidence to a purchaser, or potential purchaser, that the supplied product or service attains, or is going to attain, its fitness for purpose. Different people have different concepts for what is meant by a quality product or service, therefore it is very important to be aware of the customers' requirements a n d o r expectations. Contract documents or purchasing specifications should clearly define a company's requirements for a product or service. The quality of the product or service is deemed to have been achieved when the exact requirements have been met completely and consistently - assuming correct specification!
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Scope of quality assurance Quality assurance should encompass all parts of an organization and all phases of an activity, i.e. planning, design, production, maintenance, administrative etc. in order to give a purchaser, or potential purchaser, maximum confidence that the client's expectations for quality have heen mr,l Crnllnhnration with suppliers and purchase.rs should also be part of an organization's quality system.
QA, QC and inspection compared Quality assurance is not inspection. Inspection is one of the important elements within a system for quality assurance. Inspection requires continuing evaluation in the same way as the other elements, e.g. planning, designlspecifications, production etc ... Inspection is defined in BS 4778 : Part 1 as: 'activities such as measuring, examining, testing, gauging one or more characteristics of a product or service and comparing these with specified requirements to determine conformity.' Inspection is also defined in EN 45020 : 1993 : Glossary of terms for standardization and related activities: 'Evaluation for conformity by measuring, observing, testing or gauging the relevant characteristics.' Evaluation for conformity is defined in the same standard as: 'Systematic examination of the extent to which a product, process or service fulfils specified requirements.' Quality control is defined in BS 4778 : Part 1 as: 'the operational techniques and activities that are used to fulfil requirements for quality.' This definition can be vague, so modifying the term to be more specific is advantageous, e.g. manufacturing quality control. Quality control is involved with the monitoring of a process and eliminating the causes of any deficient output of a process, or any phase during a contract, which has an effect on quality. The information obtained from inspection, as defined above, is used for quality control. Quality control deals with the actual measurement of quality performance which is compared against what is required and action is taken on the difference. Quality control is asking the question, "is the worWaction being performed correctly"? Quality control does not reach all elements which affect quality, e.g. quality control will rarely do anything to correct problems relating to management, traceability, training and staff motivation. Quality assurance applies to all areas which have an affect on quality, and asks the question, "has the worWaction been performed correctly"? This question can only be asked after information has been obtained from quality control and all other departmentslareas which affect quality.
QA standards BS EN I S 0 9000 series - Quality systems. BS 4778 - Quality vocabulary. [EN 28402; I S 0 84021 BS EN 3001 1 - Guidelinesfor auditing quality systems.
Normative document: Document that provides rules, guidelines or characteristics for activities or their results. Note: The term normative document is a generic term that covers such documents as standards, technical specifications, codes of practice and regulations. [IS0 GUIDE 2 & BS EN 4.50201.
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1. Standard: Document, established by consensus and approved by a recognized body, that provides, for common and repeated use, rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context. [BS EN 450201.
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2. Code of practice: Document that recommends practices or procedures for the design, manufacture, installation, maintenance or utilization of equipment, structures or products. Note: A code ofpractice may be a standard a part of a standard or independent of a standard. [BS EN 450201.
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3. Specification: The document that prescribes the requirements with which the product or service has to conform. A specification should refer to or include drawings, patterns or other relevant documents and should also indicate the means and the criteria whereby conformity can be checked [BS 4778 : Part 11.
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4. Technical specification: Document that prescribes technical requirements to be fulfilled by a product, process or service. Note: A technical specification should indicate, wherever appropriate, the procedure(s) by means of which it may be determined whether the requirements given are fulfilled. A technical specification may be a standard or part of a standard or independent of a standard [BS EN 450201
5. Waiver, concession, dispensation, variation:
Documents which amend a
specification or a contract in part or in its entirety. 6. Regulation: Document providmg binding legislative rules, h s is adopted by an authority. Documents setting out legal laws, sometimes referred to as statutory instruments, i.e. the instrument by which we are advised of the law. Note: An authority is a body that has legal powers and rights [BS EN 450201.
7. Procedure (1): Specified way to perform an activity [IS0 100051. 70
8. Procedure (2): A written description of all essential parameters and precautions to be observed when applying inspection or a test method to a specific item or quantity of items, following an established standard, code or specification. [ICORR REQ DOC].
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9. Instruction: Provision that conveys an action to be performed [BS EN 450201. 10. Written instruction: A detailed written description of the inspection(s) or test(s) to be performed [ICORR REQ DOC]. 11. Technical drawing: When approved for construction. Note: 'As built' drawings are not normative as they are merely records.
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Issue 2 30/08/04
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There are four methods of determining the dry film thickness of a paint: non-destructive test gauges destructive test gauges
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test panels calculation
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NON-DESTRUCTIVE TEST GAUGES Memurhg the cl.f.1. clireclly with a n u n - c l t ~ u c i vla31 gaugt! iu UIU lrlusl wicluly uu~cl method; there are a variety of gauges available with various scale ranges:
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magnetic film thickness gauge electromagnetic gauge eddy current ultrasonic gauge
Magnetic film thickness (banana) gauge The banana gauge, as it is most widely referred to, may only be used for measuring the thickness of non-ferromagnetic coatings applied over ferromagnetic substrates. Prior to use, the gauge must be calibrated.
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Scale wheel\
Ferromagnetic substrate
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MAGNETIC FILM THICKNESS G A U G E Calibration procedure:
1. Choose a magnetically insulating shim of known thickness, close to the thickness of the paint you expect to find, i.e., do not choose a 25pm shim to calibrate if you expect the coating thickness to be considerably greater as this will reduce the accuracy. 2. Place the shim on the same substrate material type and profile as that on which the paint is to be measured, i.e., if the paint is on a blasted surface, calibrate the gauge on an uncoated blasted surface and visa versa. 3. Place the magnet onto the shim and press f m l y on the instrument, wind the scale wheel anti-clockwise (away £tom yourself) until the magnet is definitely attached to the shimlsubstrate. 4. Gradually wind the wheel slowly clockwise until the magnet detaches itself. Repeat to confirm the reading (the reading is taken opposite the fixed cursor). If the reading is not accurate to the shim the scale itself must be moved by adjusting the calibration bar to move the scale the appropriate amount.
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The instrument is now calibrated and may be used to measure the d.f.t. of any nonmagnetic paint films to within a claimed accuracy of *5% in some cases.
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Pull-off gauge This type of gauge may only be used for measuring the thickness of non-ferromagnetic coatings applied over ferromagnetic substrates. They are not very accurate compared to the other non-destructive test gauges.
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The pull-off gauge, or Tinsley pencil as its most widely referred to, consists of a magnet at the tip of the instrument which attaches itself to the coated substrate. 'l'he gauge is then slowly pulled away from the coated substrate at normal incidence until the magnet Scale detaches itself, at this point the indicator on the body of the gauge is read (you have to be quick because the magnet and indicator are spring loaded!). Calibration is required Magne' before use.
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TINSLEY PENCIL
( Magnetic horseshoe gauge The magnetic horseshoe type gauge works by measuring the change in magnetic flux between the two poles of a magnet, the change in magnetic flux depends on the coating thickness. The accuracy of these instruments is claimed to be *lo% and as with the other magnetic gauges, may only be used for measuring the thickness of nonferromagnetic coatings applied over ferromagnetic substrates.
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Eddy current/electromagnetie gauges Amongst the most accurate of the non-destructive gauges for measuring d.f.t. are the eddy current and electromagnetic gauges of which there are many types. If calibrated correctly, accuracy is likely to be within *5%. Eddy current gauges are used on non-ferromagnetic conductive substrates, electromagneticgauges are used on ferromagnetic substrates such as ferritic steel. Many eddy current/electromagnetic gauges also have statistical capabilities and some will download and upload information fiom computers.
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DESTRUCTIVE TEST GAUGES Destructive test gauges cut into the paint film and should therefore only be used where necessary due to the cost of repairing the damaged coating. They are sometimes used on paint films containing M.I.O. pigment; M.I.O. is ferromagnetic and therefore non-destructive test gauges, which rely on a non-magnetic coating, cannot be used. The paint inspection gauge (PIG) is one such type of destructive test gauge. A small vee shaped channel is cut lnto the coating at a f ~ e angle d governed by a cutter built into the gauge. The width of the channel is then measured on a graticule scale by means of a microscope which is again built into the instrument. Other destructive thickness gauges are the Saberg thickness drill or Erichsen thickness # drill which work on a similar principle to the paint inspection gauge. 7
O Ruane & T P O'Neill Issue 2
Test panels, e.g. metal plates of a known thickness, may be used to measure the d.f.t. indirectly, by coating them in the same way as the work being camed out and measuring the d.f.t. with a micrometer.
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CALCULATION The daft. may be assessed indirectly by measuring the w.f.t. of the paint, and providing the volume solids (v.s.%) content of the paint is known, calculating the d.f.t. as follows:
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Example: 30
wJt. is not given directly in the question, therefore must be found by calculation.
What would be the d.f.t. if 15 litres of paint with a volume solids content of 44% is used to cover an area 12 m x 7 m? To find d.f.t:
c. d.f.t.
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To find w.f.t.:
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volume area
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15 litres 12m x 7m
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15000 1200 x 700
Convert all existing units to common units i.e. cm.
w.f.t.
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g. w.f.t. = 0.017858cm x 10,000
Convert to pm (10,000pm = 1 em).
h. w.f.t.
1
=
178.58pm
Return to d.f.t. formula:
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Paint
--'Water' thinned I
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r:vater
soluble types
-I'emulsion' types
Solven tless
'Solvent' thinned
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Non-drying
Drying
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I waxes fish oils, etc. petroleum jelly
Convertible
Air drying
Non-Convertible
Heat convertible
Chemically cured
phenolics epoxy poyuret hanes acrylics (stoving)
two-pack paints e.g. epoxy po1yure:hanes tarlepoxy tarlurethaze zinc silicates
I Oleoresinous (linseed and/or tung oil with synth~ticresin) Alkyds Epoxy esters
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I air drying
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heated (plastisols)
I organosols rubber, vinyls
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