UNIT 1 CORROSION Corrosion is generally an Electro-chemical process, which results from an anodic reaction and at least one cathodic reaction. The corrosion of steel takes place at the anode. The anodic reaction is expressed as follows: MM+n+ne Where: 1. M = element involved 2. N = a number 3. E = electrons For iron and steel this would be expressed as: FeFe+++2e At least one of five basic reactions m ay take place at the cathode. The most common reaction applicable to the corrosion of steel is as follows: O2+2H2O+4e4OH Iron ore is an oxide in chemical balance with the environment; when this iron ore is converted to iron, the chemical balance is changed and the iron becomes active, i.e. it corrodes on contact with the natural environment and tries to revert back to it’s natural inert state. The natural environment usually contains moisture (which provides the electrolyte) giving the following simultaneous reactions: The products of these reactions take place in further reactions with the immediate environment leading to the formation of corrosion products, the most familiar being rust. Corrosion reactions can be accelerated by the existence of certain criteria including: 1. Variations in oxygen content on the materials surface 2. The concentration of chemical salts in the electrolyte, e.g. chlorides and sulphates: 3. Other metals or metal compounds of higher nobility (more Electro positive) in contact with the steel, e.g. millscale 4. Acids or alkalis 5. Certain types of bacteria near the materials surface 6. High temperature. The higher the temperature the greater the rate of corrosion. The following list shows some metals/metal compounds in their order of nobility in seawater at ambient temperature. The relative positions of the metals/metal compounds in the list can change in the electrolyte or temperature; this list is known as the galvanic series. Gold Silver Nickel Copper Mill scale Mild steel Aluminium Zinc Magnesium
Noble
Ignoble
If steel was in intimate contact with zinc or attached via a wire in an electrolyte, e.g. soil or water, the zinc would corrode first because the steel is more noble than the zinc. In this example the zinc becomes the anode and the steel becomes the cathode, i.e. the steel is being cathodically protected and the zinc is acting as a sacrificial anode.
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We can use the galvanic series to bring about cathodic protection to steel. There are two ways in which this can be achieved Bi-metallic corrosion When two differing metals are attached closely together the less noble one will act as an anode to the more noble one cathode and will sacrifice itself Sacrifical coating When a coating whose metallic pigment particles are less noble than the steel (zinc or aluminium) are coated onto the steel they will sacrifice themselves in order to preserve the more noble steel. When coatings are used for this purpose they must contain around 90% of the metallic pigment
Notes An electrolyte is a medium, which will allow the passage of electrical current consisting of a chemical salt dissolved in water. The greater the amount of chemical salt present, the better the conductivity of the electrolyte. Millscale is an oxide of iron produced when the steel is manufactured; it is a result of the hot steel coming into contact with the air and forming an oxide layer composed of three layers; 1. FeO nearest the steel 2. Fe3O4 3. Fe2O3 on the outside Millscale has a total thickness between approximately 25 m and 100 m It is essential for millscale to be remove from steel to be removed from the steel surface during blast cleaning for the following two reasons 1) Millscale is more noble than steel and when parts of the millscale break away the exposed areas of the steel (anode) will sacrifice themselves to preserve the more noble surrounding millscale (cathode) 2) If painted over, millscale which is loosely adhering and flaky will leave the steel, bring the paint away from the substrate thus causing early breakdown of the system due to lack of adhesion
Methods of arresting corrosion Barrier principle This isolates the substrate from the environment by means of a low permeability coating system. This type of system is usually made up from around four coats and contains an M.I.O (micacious iron oxide) coat, which helps give the paint system its low permeability Sacrifical principle This involves making a paint whose pigment particles (zinc or aluminium) are less noble than the steel onto which it is coated. The result is that the less noble coating (anode) sacrifices itself to the more noble steel (cathode) Galvanising Involves the dipping of steel components into a bath of molten zinc at around 450 0c this leaves a zinc coating on the steel component of between 85 m - 130 m and is directed under BS 429 Passivation Chemical reactions which are achieved between rust and inhibitive pigments in the primer and the substrate Sacrificial anodes Blocks of metal ignoble to steel which are attached to steel structures as a sacrificial anode
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UNIT 2 SURFACE PREPARATION 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 Waterblasting Weathering Flame cleaning Vapour degreasing The quality of a surface preparation is governed by the amount of surface contaminate 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 blast cleaning. Dry abrasive cleaning Dry abrasive blasting is carried out by projecting a highly concentrated stream of small abrasive particles onto the substrate’s surface at speeds of approximately 720 km/h (450m.p.h.). The operation removes rust, scale and any other extraneous material from the substrate and also leaves an irregular profile, which provides an ideal key for the 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 Abrasive Size Hardness Density Shape Human factors Speed Angle Distance Time Both metallic and mineral abrasives are commonly used for blasting for example Steel or chilled iron grit Steel shot Metallic grit and shot mixed Copper slag Garnet Sand The control of substances hazardous to health regulations 1994 (coshh) do not allow the use of sand containing free silica in dry blasting operations because of the risk of silicosis There are other abrasives, which are used, usually for specialised applications e.g. walnut shells, ceramic grits, crushed glass aluminium silicate.
Effect of abrasives Grit is angular in profile with sharp cutting edges: it shatters millscale and undercuts any surface contaminates resulting in a clean surface with a rough profile. The amplitude tends to be quite erratic with a large occurrence of rouge peaks, especially when blasting in one area to long.
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Notes Sand is not dangerous unless it is in dust form when it can be inhaled, e.g. after fragmentation during the dry blasting operation. Shot is spherical, it shatters millscale 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 rouge peaks but may press impurities into the surface. It is a 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) Surface profile The shape of a cross-sectioned blast finish is known as the surface profile or anchor pattern. Rogue peaks Amplitude Trough 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 m. the amplitude of a blasted surface may be measured by a number of methods including the use of surface profile needle gauge, surface replica tape, e.g. testex tape or a surface comparator. Surface profile needle gauge 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
Notes Rouge peaks are peaks that stand out above the required profile and should be avoided if applying thin coatings as they may lead to spot or flash rusting. Blast finishes produced in production should not be touched with bare hands due to contamination. All accurate measuring equipment should be issued with calibration certificates or certificates of conformance to give assurance that the readings obtained are going to be correct within a stated margin of error. Surface replica tape Testex tape is a trade name of a commonly used or 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: 1. Zero the micrometer ensuring the flat contact points are clean 2. Remove paper backing and stick Testex tape to the surface to be measured
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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 50m from the reading to obtain the amplitude. The plastic Mylar film to which the soft compound is attached is 50 m 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 fingernail, small wooden stick or similar- never with the fleshy part of the finger as this will contaminate the blast A profile grading can be given when the area under assignment is rougher than the smoother of two adjacent areas on the comparator but not as rough as the rougher of the two areas. The profile is then graded to thethan following: Fine profile - equalaccording to or rougher area 1 but not as rough as area 2 Medium profile - equal to or rougher than area 2 but not as rough as area 3 Course profile - equal to or rougher than area 3 but not as rough as area 4 If the profile is finer than area 1 it is termed finer than fine If the profile is courser than area 4 it is termed courser than course Blasting grades The grade of a blast finish relates to the amount of surface contaminate remaining after blasting. The grade of blast finish is primarily governed by blasting time and the velocity of the abrasive particles BS 7079: PART A1 BS 7079 Preparation of steel substrates before the 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. The surface under examinations visually compared with high quality photographs in the standard both before and after blasting. The preparation is then given a coding e.g. SA 2 1/2 which can be interpreted using the following extract from the standard: Rust grades A B C D
Steel covered with millscale but little if any rust Steel surface surface,largely which has begun to adherent rust and from which the millscale has begun to flake Steel surface on which the millscale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision Steel surface on which the millscale has rusted away and on which general pitting is visible under normal vision
Notes It is important to note that needle gauges, surface replica tape and surface comparators only give the degree of roughness and not the degree of cleanliness. BS 7079: Part A1 is the same as ISO 8501-1 and SIS 05-59-00 Preparation grades – blast cleaning Prior to blast cleaning any heavy layers of rust shall be removed by chipping. Visible oil and grease shall also be removed After blast cleaning the surface shall be cleaned from loose dust and debris. Sa1 light blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from poorly adhering millscale, rust, paint coatings and foreign matter. Sa2 through blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter. Any residual matter shall be firmly adhering. Sa21/2 very through blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter. Any traces of contamination shall show only as slight stains in the form of spots or stripes.
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Sa3 blast cleaning to visually clean steel when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter. It shall have a uniform metallic colour.
Comparison of blasting grades SSPC Bs 7079/ SIS 05-59-00 White metal sp5 Sa3 Near White metal sp10 Sa21/2 Grade 2 Commercial finish sp6 Sa2 Light blast and brush off sp7 Sa1 SSPC= Steel structures painting council
nace Grade1 Grade 3 Grade 4
NACE =National association of corrosion engineers Equipment Centrifugal blast units Blasting in factories is often carried out using rotating wheels, which throw the abrasive at the component. These units are known as Centrifugal blast wheels, are usually fixed installations and are commonly used for large production runs, e.g. on pipes in plate mills and large steel plates in shipyards. The main advantages of this system compared to air blasting systems are as follows 1. Lower cleaning time 2. Low abrasive consumption 3. Low energy consumption 4. Less labour used 5. More consistent and uniform blast finish 6. More environment friendly 7. Safer to implement (closed system) 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 forcewould at speeds typically betweenThe 250-350km/h. For cost reasons the abrasive normally be reusable. abrasive is recycled up to approximately twenty times providing it is free from oil and grease. An air-wash separator removes any dust contaminates from the recycled abrasive before it is fed back into the wheels. Air blasting Pressure blasting, which is a type of air blasting system, would normally be used on site work. Vacuum blast and suction blast also come under this category of air blasting but are not as widely used due to lower efficiency Pressure blasting equipment consists of 1. A compressor providing an air supply of approx. 100p.s.i. 2. A pressurised pot containing the abrasive 3. Liquid separators, i.e. moisture filters 4. A carbon impregnated hose 5. A venturi shaped blasting nozzle 6. A dead mans 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 100psi, depending upon the abrasive used. Limiting pressures to 100psi is also advantageous for safety reasons It is important to keep the pressure at the nozzle as close to 100psi as possible because for every 1% loss in pressure there is approximately 1 1/2% loss in efficiency. The pressure at the nozzle may be measured using a hypodermic needle gauge; this is placed through the wall of the hose near the nozzle with the hole in the needle facing the nozzle
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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 nozzles and the venturi shaped nozzle. Straight bore nozzles are rarely used for blasting large surface areas because they are not as efficient as venturi nozzles. The velocity of abrasive leaving a straight bore nozzle at 100 psi is approx. 350km/h whereas the velocity shaped nozzle under similar conditions would be approx. 720 km/h
Strai ht bore
Venturi shaped
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 Safety Centrifugal blast units are a closed system; i.e. human access to the blasting areas 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 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 100 psi 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 films because they do not create a dust. However, all wet blasting methods have similar disadvantages over dry abrasive blasting, including The availability and drainage of water The production and disposal of sludge (particularly with abrasive injection) The extra cost of supplying and mixing 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 7
High pressure water jetting Operates at pressures up to 60,000 psi, 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 contaminates Will remove millscale at high pressures Low pressure water plus abrasive injection Operates at 100 psi. 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. Steam blasting, with or without abrasive injection Operates at 100 psi. This method is ideal for surfaces contaminated with oil, grease, etc. disadvantages high cost and low efficiency. Air blasting withinclude water injection Water with or without an inhibitor is injected into an air/abrasive stream. Hand and power tool cleaning Hand and power tool cleaning, relates to scraping, chipping, wire brushing, sanding, grinding and needle gun cleaning. 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 A1 defines standards of wire brushed finishes along with other hand and power tool cleaning method as follows. Prior to hand and power tool cleaning, any heavy layers of rust shall be removed by chipping. Visible oil and grease and dirt shall also be remover St2 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 millscale, rust, paint coatings and foreign matter St3 Very thorough hand and power tool cleaning As for St 2 but the surface shall be treated much more thoroughly to give a metallic sheen arising from 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 adhesion. 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 Notes Bronze brushes may not be permitted because of the possibility of galvanic corrosion. Plastic bristles embedded with abrasives as an alternative. Needle gunning A needle gun or jasons hammer 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 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. After the same may check needle gunning the amplitude of the surface profile methods used for abrasive blast cleaning if the contour of the substrate allows.
Reciprocating needles Compressed air
Needle gun
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The guns shall have needles of a small cross section, the profile created must not exceed 100m, no sharp egged craters must be left and all rouge peaks must be removed. Grinding Shall only be carried out under direct supervision of the engineer. Abrasive discs May be permitted in certain circumstances. Approval for their use must be sought and particular care must be taken on pressure containing parts not to create notches Flame cleaning The application of an oxyacetylene flame to the steel surface to be cleaned is an efficient method of removing rust, millscale and other contamination. The effectiveness of the process is due to a combination of factors Differential expansion- the millscale on contact with the intense heat expands at a faster rate than the steel to which it is attached andofflakes off and moisture. As the moisture is rapidly driven off Dehydrationrust is a combination iron oxide 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 of fastener, 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: 1. Flames cleans the surface, this gives a light grey appearance on the surface when finished 2. Wire brushes the surface to remove all the dry powder 3. Primes the surface, it is often necessary to apply the paint while the metal is still warm around 400c (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 the minimum flame cleaning standards according to rust grades, i.e. A / F1 B/ F1 C/ F1 D/ F1 Disadvantages Dealing with high temperature naked flames Damage and warping to thin materials Heat causing loss of strength to steel fastenings Advantages Metal is perfectly dry for painting Chemical cleaning 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, 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 millscale to be removed. Other acids e.g. phosphric 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 (HB Footners duplex process) 1. Degrease- removes surface contaminates such as grease and oil by use of a suitable solvent, e.g. xylene, usually applied by cloth 2. Pickle- total immersion in a tank of acid, e.g.5-10% sulphuric acid at 65-700c, to remove
3. 4.
millscale, rust etc., the time taken is variable and depends on the type and degree of contamination. An inhibitor is also present in the tank Wash- a clean water wash to remove acid and surface residues, usually applied by hose or spray Phosphate- the technique involves a final treatment in a 1-2% phosphoric acid solution held at 800c for 1-2 minutes. This leaves a thin rust inhibitive phosphate coasting on the steel 9
surface to which the coating should be preferably applied while it is still warm, possible 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 hydrocarbon such as 1,1,1, trichloroethane, trichloroethylene and carbon tetrachloride were commonly used as degreasers but there use has declined, or been completely restricted, due to high toxicity. Heavy vapours of all chemical solvents are a hazard in enclosed areas. A thin film of oil invariably remains after solvent cleaning but the more solvent used 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 its solvent strength and compatibility considerations Some non-ferrous metallic surfaces such as copper, brass and galvanised steel may be coated for anti corrosion purposes but sometimes they are coated for aesthetic reasons. Relatively reactive materials, e.g. zinc and aluminium, are often coated to prolong their effective life. Non metallic materials such as glass reinforced plastic (GRP) or concrete are often coated for appearance reasons only, although if concrete is coated its effective life or time to maintenance may be prolonged. Regardless of the reason for coating the surface must be cleaned 1. Remove any oil grease by suitable solvent or proprietary degreaser 2. Remove any water soluble contaminates using water, usually mixed with 2% detergent 3. Rinse using clean water. Preferably use power washing equipment where the situation allows 4. Sweep abrasive blasting preferred. If this is not practical , abrade using emery cloth or wire brush It may not be feasible to use sweep blasting as a surface preparation method with certain surfaces such as thin gauge aluminium cladding or zinc galvanising. However using a wire brush or some other abrasion method may not always be the best alternative. Etch primers or etchants are usually on these surfaces to provide a key to the substrate. For many non-ferrous substrates including aluminium and zinc a polyvinyl-butyral PVB type etch primer or a non proprietary etchant called t-wash is used. The etchant used in both cases is phosphoric acid. It is not always a requirement to apply etch primers or etchants to zinc coatings which have a dull appearance due to an oxide layer which has formed over a period of time due to reactions with the oxygen in the atmosphere. The oxide layer usually provides an adequate key for the coating system. The specification must always be consulted to determine which preparation method to use on non-ferrous substrates. For example, a specification may require t-wash etchant to be used on unweathered zinc galvanised surfaces, PVB etch primers on aluminium surfaces and abrasion using emery cloth on copper surfaces. Tests to detect surface contamination may be qualitative or quantitative. Qualitative tests will determine whether or not contamination is present but they will not show the 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/m 2 although this value may not be the exact amount present. Soluble iron salts Colourless soluble iron salts may be present in pits within the substrate after blasting. 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 milligram’s per square meter mg/m 2
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The maximum requirement may be as low as 10 mg/m 2 although other specifications may state that 30 mg/m 2 is the critical level. Only quantitative tests could be used to determine whether these requirements are met Test results may be misleading or totally wrong if chromate or nitrate inhibitors have been used, for example in wet blasting. Potassium ferricyanide test 1. Spray a fine mist of distilled water onto a small area of the blast cleaned surface 2. Wait a moment for any water droplets to evaporate then apply a Potassium ferricyanide test paper by pressing down for 2-5 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 Note the Potassium ferricyanide test may also be referred to as the Potassium hexacyanoferrate (iii) test Merckoquant test This test is also known as the Eisen test and is a colourmetric quantitative test claimed to be 85% accurate down to 30 mg/m 2 Bresle sample patch This is a mercury (II) nitrate (mercuric nitrate) titration test clamed to be 95% accurate down to 10 mg/m 2 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 Millscale is cathodic with respect to steel. This means that if any traces of millscale 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 millscale particles left after blasting to Sa 3 the copper sulphate test may be used A fine mist of slightly acidic copper sulphate solution is sprayed onto a localised area of approximately 10mm in diameter. The steel turns a bright copper colour and any millscale 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 then examined using a magnifying glass and an assessment is made. Standards do exist which standardise the test conditions and the way in which results are assessed. Ref.: Bs7079 Part 3 Oil or grease Simple visual assessment may reveal the presence of oil or 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 fluoresce, but a dark environment is required for this method. Another method is to drip several drops of a solvent such as xylene onto the suspect area. After a few moments remove some of the solvent and drip onto a tissue or filter paper. When the solvent has evaporated any oil or grease will show up as a brown ring on the paper.
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UNIT 3 PAINT CONSTITUENTS Paints may be classified in several different ways, however one way in which paints may be subdivided is as follows Liquid paint containing solvent Solvent free liquid paint Powder paints Solvent free liquid paints and powder paints are latter developments, which eliminate the need for costly and hazardous solvents Liquid paints containing solvents are the most common, although solvent free materials are being more widely used. Water based paints have been used for sometime in industrial applications, e.g. electrodeposition paints in the motor industry Opaque liquid paints consist of a liquid medium know as the vehicle (solvent and binder) plus solid pigment particles. An unpigmented paint known as varnish Binder The binder is the film former, i.e. the component in the paint, which forms a relatively hard continuous film. It may be thought of as the adhesive that holds the pigment and other additives together. The binder contributes mainly to the durability, provides the necessary mechanical strength and physical properties and provides the adhesion, cohesion and flexibility of the coating A paint type is normally identified by its binder Acrylic Alkyd Cellulose Chlorinated rubber PVC/PVA emulsion Epoxy Ethyl and methyl silicate Vegetable oils e.g. linseed Phenolic Polyurethane Silicone
EachStyrene binder has its own characteristics, therefore a paint must be carefully selected to ensure it is able to do the work required: e.g. it would not be the right choice to coat a chemical plant with linseed oil based paint as these paints have low chemical resistance. A good choice would be an epoxy or Chlorinated rubber as these have good chemical resistance A paint binder forms polymers when drying takes place. A polymer molecule is composed of many smaller parts, contributed by similar or dissimilar molecules which are joined together until there are hundred or thousands of atoms in the polymer molecule. This process is known as polymerisation. Polymers consist of chemical compounds made up from elements of a low molecular weight, such as carbon, hydrogen, oxygen and nitrogen Most polymers are organic and may be naturally occurring, or as is more usual in modern paints, synthetic -A-A=A-A-A-A
Linear polymer A-A-A-A
-A-A-A-A-B
Branched Polymer A-A-A-A
Cross-linked polymer
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All binders are polymers and in the case of reversible or non-convertible coatings, the polymer is fully formed and therefore does not undergo further polymerisation during the drying or curing process Non- reversible or convertible binders are composed of polymers which are not fully formed and which undergo further polymerisation during the drying or curing process. Resins Natural resins are obtained from plant secretions or plant fossils and include lac, copals and dammers. Natural resins may be hard brittle or soft semi-solids, are usually quite transparent and may have film-forming properties although they are usually used to modify the properties of oils. Natural resins quite often have to be modified chemically by heating etc. before they are any use to the paint industry. Some natural resins are soluble in organic solvents but not water, some natural resins classified as gums are soluble in water e.g. gum Arabic. Almost all resins used in paint form ulations nowadays are synthetic e.g. epoxy, alkyd, vinyl etc., these have similar physical properties to natural resins but have different chemical composition. Oils Before synthetic resins made their appearance, unsaturated drying oils were used as film formers. Nowadays their use is restricted, although they are commonly used in oleoresinous varnishes (oil and resin) for special use Common drying oils are linseed and tung oil, which dry by oxidation Paints, which contain more oil than resin may be referred to as, long oil paints, these produce elastic, slow drying, paint films used for decorative purposes. Paints which contain more resin than oil may be referred to as short oil paints, these produce brittle fast drying paint films used for structural coatings Pigments Pigments are solids in powdered form, which are derived from chemical reactions, minerals, vegetables or animals. Pigments may be organic or inorganic. Most pigments used in paints are inorganic although there are some common organic pigments. Most pigment types must be chemically inert and insoluble in vehicle in which they are dispersed Pigments basically give paint film its colour and opacity (hiding power), but may also improve the paint film hardness and durability. Colour permanence when exposed to the environment i.e. light, air and moisture, is also a consideration. Opacifying pigments are typically less than 1 m per particle. There are pigments, which can apply other characteristics to paint e.g. anti corrosive properties. Titanium dioxide is a white pigment and worthy of special mention because it is present in many paints over a variety of colours. Its main characteristics are high tinting strength and hiding power, low weight, good chemical inertness and resistance to heat Pigments are usually classified by colour or by the primary characteristic they afford to the paint; listed below are some pigments classified by the latter Opacity Describes how well the paint will cover over the underneath surface and is possible and is opposite to transparency. The opacity of a paint is determined mostly by the amount of pigment in the paint. The greater the pigment quantity the better covering power the paint will possess Any piece of equipment that measures opacity is called a cryptometer Two types in common use are hiding power charts and the pfund cryptometer Opaque pigments These inert pigments are used for the purpose of providing colour and opacity Carbon black red, yellow Compounds of calcium Compounds of cobalt blue Compounds of chromium green, yellow, orange brown, red, yellow Compounds of iron Titanium dioxide white Rust inhibitive (anti corrosion) pigments Used in primers for the purpose of corrosion prevention Inhibitive pigments work by anodic and/or cathodic polarisation of the metal substrate. The soluble particles of some types of rust inhibitive pigment react with the moisture as it passes
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through to the metal making it non-corrosive. Red lead and calcium plumbate are basic and react with acidic components of the vehicle to form an inhibitor Red lead* Calcium plumbate* Zinc chromate* Zinc phosphate Boro-silicates Zinc phospho-oxide Barium metaniobate *The use of the above marked in most countries is banned or restricted due to high toxicity Metallic pigments May be used to give metallic finishes such as those used on cars. Zinc may be used to give anticorrosive properties by acting as a sacrificial anode by means of cathodically protecting the underlying steel, providing the metallic particles are in close contact with on another. In most situations, it is unlikely that an aluminium pigment even if closely packed will achieve cathodic protection Zinc Aluminium Extender pigments More often referred to as extenders; these are not opaque and are used for increasing viscosity, reducing gloss, aiding intercoat adhesion and to improve the cohesive strength of the paint film. Some opaque pigments can also provide these characteristics, but the main advantage of using extenders is lower cost Kaolin (china clay) Chalk Talc Slate dust Barytes Laminar pigments Laminar pigments are small flakes, which have a leafing effect when the paint dries; this means that the flakes of pigment overlap one another like leaves on the ground. This results in an excellent coating to resist the passage of water; tensile strength is also improved MIO (micaceous iron oxide) sometimes referred to as flaky or specular hematite, is a laminar pigment widely used in midcoats on structural steelwork and is available I many types of binder formulations. Mica, glass flakes and aluminium flakes are other laminar pigments, which have similar characteristics to MIO Solvents Usually the ability of a paint to spread over a surface is far from ideal unless the paint contains a solvent. The paint solvent must be volatile so that it evaporates from a coating of paint to leave a viscous film. The choice of solvents is important because the use of inappropriate solvents can affect the drying and gloss characteristics of a paint and in some cases can chemically react with the liquid paint which will have an adverse affect on the paint film properties The important properties of a solvent are Solvent power: strong solvents e.g. acetone, are required for complicated polymers, this enables the molecules of paint to move more easily Rate of evaporation: some solvents evaporate quicker than others, also the method of application will effect evaporation; spraying results in faster evaporation than brush applied coatings, if the rate of evaporation is to quick problems may arise In application i.e. dry spray Flash point: this is the minimum temperature of the solvent at which the vapours given off are flammable if a source of ignition is introduced Toxicity: the toxicity of a solvent can be determined from its occupational exposure limit expressed in PPM A variety of apparatus exists for measuring toxicity, a common type known as the draeger tube works on the similar principle to a breathalyser
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Weak solvent Water
emulsion, vinyls, acrylics, PVA/PVC
Aliphatics White spirit Turps+substitues Hexanes
natural oils, natural resins, isomers e.g. alkyds
Aromatics Xylene Toluene Benzene Styrene
chlorinated rubber
Ketones Acetone MEK/MIBK
epoxies
Polyurethanes use ketones and esters with aromatic solvents Strong solvent Other constituents In addition to the main ingredients of a paint, namely the binder, solvent and pigment, there are other constituents added for a variety of reasons, e.g. to aid the paint manufacturing process, to increase shelf life, to aid application, to aid film formation, to aid drying or curing, to repel bacteria or suppress plant growth, to reduce flammability and to reduce UV degradation. Other constituents include Plasticisers Added to paint to prevent a brittle film Thrixotropics Gives a jelly like structure which gives the paint build and anti-sag properties Dryers or catalyst Some binders such as oils or resins need added dryers to speed up the drying process Two pack paint rely on a chemical reaction to bring about the drying the catalyst provides this Anti-skinning agents Helps prevent oil-based paints from skinning over Extenders Cheap mineral powders which can be added to paint in order to make it floe more easily, increase opacity but most of all to lower the cost of the paint Others Anti-foaming agents, preservatives, fungicides and bactericides. Solutions and dispersions An opaque paint is dispersion; the pigment particles are suspended in the vehicle or binder. The vehicle of a paint solution of binder dissolved in solvent; a clear varnish is also a solution Solutions In a solution, a substance known a the solute, which can either be a solid or a liquid, is dissolved in a liquid known as the solvent to form a homogenous substance Sugar ( solute ) with water ( the solvent ) Alkyd binder ( solute ) with white spirit ( the solvent ) Dispersions In a dispersion there is no solubility, one component, which could be a solid or a liquid, is surrounded by a liquid. There are two types of dispersions: suspensions and emulsions Suspensions In a suspension, solid particles are dispersed within a liquid, each particle or group of particles being surrounded and wetted by the liquid, e.g. pigment and vehicle If a paint was in complete dispersion each pigment particle would be completely surrounded and wetted by the binder
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In practice, complete dispersion is rarely achieved because the pigment particle group together in small groups known as aggregates when supplied by the pigment manufacturer; these aggregates are not completely broken down by the paint mills during paint manufacture However the paint manufacturer must break down the aggregates to achieve the necessary degree of dispersion of fineness of grind for the particular paint; for example the final degree of dispersion must be high to obtain gloss paints Emulsions In an emulsion, minute droplets of one liquid known as the dispersed phase, are dispersed in a second liquid known as the continuous phase. The dispersed phase is completely surrounded by the continuous phase but is not dissolved by it E.g. Full cream milk: cream(dispersed phase) with water (continuous phase)
Salad vinegar(dispersed phase)with oil(continuous phase) Housedressing: hold emulsion PVA/PVC co-polymer(dispersed phase)with water(continuous phase)
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UNIT 4 PAINT DRYING AND CURING Drying is defined in Bs2015: 1992 glossary of paint and related terms as the change of a coating material from the liquid state to the solid state, due to evaporation of solvent, physo-chemical reactions of the binding medium, or a combination of these processes When the drying process takes place during exposure to air at normal temperature, it is called airdrying; if drying is accelerated by the application of heat above ambient temperature, but below that used for stoving, it is termed forced drying. Application of heat by using an oven or infrared is termed stoving Various terms exist in relation to the drying of a paint film Dust dry- when dust no longer adheres to the paint surface Surface dry or sand dry- (not a Bs 2015 term) when the paint is dry on the surface but is soft and tacky underneath. This term primarily applies to oil based paints Touch dry- when a very slight pressure with the fingers does not leave a mark or reveal stickiness. Freedom from residual tack is another term encountered Tack free- free from tack even under pressure Hard dry- (not defined in Bs 2015) this term is used to describe certain degrees of film hardness when tested by specified methods. The term may also be used when the drying has reached such a stage that if desired, a further coat may be applied Dry to handle- the state of drying when a coated item can be handled without damage Drying mechanisms The types of drying mechanisms are Solvent evaporation Oxidation Chemical curing Coalescence Solvent evaporation Some paint dry solely by solvent evaporation to leave a film of non-volatile solids; but a permanent chemical change does not take place. This process is known as lacquer drying Paints, which cure solvent evaporation, areorknown as non-convertible paints. These termssolely meanbythat the binder is a linear branched polymer, whichorisreversible fully formed in the can and does not undergo further polymerisation after application. It also means that if the paint solvent or other solvents in the case of house hold emulsion paints, is reapplied to a dried coating, the coating will resoften Non-convertible paints such as chlorinated rubber and cellulose lacquers are examples of paint, which dry solely by solvent evaporation (lacquer dry) Oxidation Paints based on drying oils, which includes most alkyd and phenolic paints dry firstly by solvent evaporation and then by oxidation. This is sometimes known as oxidative drying. On contact with the oxygen in the air a chemical reaction takes place- the paint polymerises, with the aid of dryers which are present in the paint, to form a relatively hard film Paints which cure by oxidation are known as convertible or non-reversible paints which means that if the paint solvent is reapplied after curing, the coating will not redissolve because of the permanent change that has taken place Paints, which dry by this process, have complex polymers compared to reversible or nonconvertible paints. Chemical curing Chemical curing paints cure by a chemical reaction between ingredients in the paint, unlike oxidation drying which is chemical reaction between the binder and the oxygen in the atmosphere. Paints employing this drying mechanism dry initally by solvent evaporation, if a solvent is present, and then by polymerisation due to a chemical reaction with the curing agent Paint which chemically cure are convertible or non-reversible paints, and therefore have complex polymers compared to reversible or non-convertible paints.
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Curing agents can be added to a paint prior to application (two-pack system), or they can already be present in a paint and then activated by an external source of energy, e.g.heat, ultra-violet light, infrared light electron beam etc. Paints which melt when heat is applied as known as thermoplastic coatings. Coatings which do not melt after the application of heat are know as thermoset coatings Two pack paints are used taking into consideration the pot life and in some cases the induction period The pot life is the maximum time period after mixing in which the paint must be used; this can vary from a few minutes to a few hours The induction period is the minimum time period during which the mixed components are left to stand before use. This is to allow for certain chemical reactions to take place. Induction periods are typically up to 30 minutes Other terms for induction period are lead time and stand time Coalescence Paints, which dry by this mechanism, dry initially by water evaporation; this allows the polymers of the paint, which are fully formed during manufacture of the polymer, to come into contact with one another and physically join together Acrylic emulsions and vinyl emulsions dry by this m echanism and are classed as reversible or non-convertible paints, although this is not very obvious in practice.
Drying and curing tests Ballotini test Ballotini, which are tiny glass balls, are dropped onto a wet painted panel. The time in hours is given at the side of the panel and where the Ballotini fail to stick to the painted panel drying has occurred Stylus test Similar to the Ballotini test, the stylus employs a series of trailing needles which pass over the wet painted panel, because the needles are set at different tensions it can be established when the paint is tack dry, hard dry and fully cured
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UNIT 5 COATING SYSTEMS Corrosion protection methods Paint systems protect the substrate from corrosion by a combination of one or more of the following methods The barrier principle: the substrate is isolated from the environment, which causes corrosion by using a coating, which has a low permeability to moisture and air. This may be achieved by applying a thick coat of paint or applying a paint having low permeability, e.g. epoxy, polyurethane; or even better applying a thick, low permeability coating. Passivation: corrosion is retarded or arrested by chemical reactions between rust inhibitive pigments in the primer, and the substrate and/or moisture passing through the paint film Cathodic protection: this is achieved by coating the substrate with a paint containing metallic pigments, usually zinc (aluminium in some cases) which are ignoble with respect to the substrate. Cathodic protection may also be achieved by means of metal coatings such as zinc galvanising Layers of a paint film Paint systems may be single layered coatings or multi layered coatings. A multi layered paint system consists of a primer, at least one midcoat and a finish coat. Each coat has its own specification Primer The function of a primer is to provide maximum and lasting adhesion to the substrate, to provide a key for the next paint layer and, in most cases, to retard corrosion by means of an inhibitive pigment when primer is applied to steel substrates Although not always practised, it is often considered good practice to apply primers with a brush as this enables the paint to be worked into the substrates surface, thereby providing optimum wetting of the substrate and mixing in any dust particles, thus achieving optimum adhesion Etch primers, also known as wash primers, are supplied either as single pack or two pack materials which contain phosphoric acid that reacts with the substrate and which also sometimes contain an inhibitive pigment such as zinc phosphate. This type of coating is often considered as an etchant or form of surface preparation rather than a primer in the conventional sense Another form of etch primer is the mordant solution. An example of this is t-wash which was developed by the former British rail. T-wash essentially consists of an aqueous solution of phosphoric acid and copper sulphate Mordant means “ of a corrosive nature ” or “ to bite into ” Many specifications will not allow etch primers to be sprayed due to their high toxicity Midcoat Midcoats may be standard undercoats or high build coats, primarily there to serve as a barrier to prevent the passage of water Primers and finish coats are often thin layered coatings and are quite permeable compared to midcoats; without a midcoat the inhibitive pigment in the primer would soon be leached out, which will lead to corrosion Midcoats also build up the film thickness to provide a more even surface by filling slight surface irregularities Finish The final coat in a system gives a surface its final appearance; i.e. colour and gloss. A final coat must also have solar protective properties and, on most structural work, must have a gloss finish to allow water to flow more readily from a surface and allow the surface to be cleaned easily
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Types of coating system General There are various ways to classify paint systems as shown in the following table Classification Options Function Anti corrosion Anti fouling Decorative Flame retardant Shop primer Moisture tolerant Road marking Binder type Alkyd
Pigment type
Colour
Cellulose Chlorinated rubber PVA/PVC emulsion Epoxy Polyurethane Vinyl Alumiumn Micaceous iron oxide Zinc rich Red oxide Zinc phosphate Black Blue Green Red White Yellow
The following table extracted from Bs5493 shows the principle coating systems identified by their binder
Type Zinc coating(except sprayed metal): bare, sealed or painted Sprayed metal: bare, sealed or painted Organic zinc-rich Inorganic zinc-rich Drying oil type Silicone alkyd One pack chemical resistant
One pack chemical resistant and drying oil type primer Two pack chemical resistant Two pack chemical resistant overcoated with One pack chemical resistant travel coat and finish Bitumens
Characteristic constituents Zinc and /or zinc-iron alloy Zinc or aluminium metal Zinc and organic binder Zinc and silicate binder Drying oil, urethane oil, alkyd, modified alkyd, phenolic varnish or epoxy ester plus pigment Silicone-modified alkyd plus pigment Chlorinated rubber or vinyl copolymer resin plus pigment Epoxy ester or alkyd primer with Chlorinated rubber finish Epoxy or polyurethane resin(including modification with coal tar)plus pigment Epoxy resin overcoated with Chlorinated rubber plus pigment Coal tar or mineral bitumen with or without pigment, coal tar enamel
Moisture curing polyurethane’s and high molecular weight linear epoxy resins which are both one pack chemical resistant materials, are not included in the product sections because of limited experience in their use at time of Bs5493 publication 20
Sacrificial coatings Sacrificial coatings contain pigments which cathodically protect the ion or steel substrate to which the paint is adhered, these pigment particles eventually corrode thereby sacrificing themselves by corroding in preference to the substrate. In order to have this property the sacrificial pigment must be ignoble to the material to be coated; zinc and aluminium are the most common types of pigment employed Zinc rich primers are two pack paints and contain metallic zinc in high concentration. A minimum zinc content of 90% (by weight) of the D.F.T. is often specified In order to work effectively the zinc particles must be held in close contact with themselves and the substrate, therefore an efficient binder is necessary Organic zinc rich primers usually have an epoxy binder Inorganic zinc rich primers often contain a methyl or ethyl silicate binder. They have excellent heat resistance properties and may be used as a single coat system or they may be overcovered with a specialised finish, e.g. silicone sealer, if required Metal coatings e.g. zinc galvanising and metal-sprayed coatings, are principally used for very long-term protection and do not usually come under the category of paint systems When a zinc rich or a zinc metal coating is subject to minor damage, e.g. a scratch, a corrosion reaction will take place which produces zinc salts that self-seal the damaged area Powder coatings Powder coatings are basically solvent free paints. They may be thermosetting or thermoplastic. Epoxy powder, which gives a thermosetting coating, is commonly used nowadays for a variety of applications including underground pipelines and domestic appliances. Each thermosetting powder particle contains base and curing agent, but they do not react together until they are activated with a heat source. The component to be coated is usually preheated, the powder may be then applied by using a fluid bed or spray technique; in both cases the powder is usually applied using electrostatic methods to achieve more uniform thicknesses and to reduce powder wastage via overspray After application, the coating m ay sometimes be post cured in a subsequent stoving operation Moisture tolerant systems The surface that requires coating may be below dew point temperature, for example, due to low temperature gas or liquid in a piping system. In situations like these, moisture tolerant systems may be specified for use on damp surfaces Various definitions may be used when moisture exists on a substrate, for example 1. Damp surface- surface temperature is below dew point but there is no detectable water 2. Moist surface- standing water and droplets have been removed but there is a thin film of moisture on the surface 3. Wet surface- droplets and free water are present on the surface Definitions differ between specifications, always consult the applicable specification for exact definitions which apply Paints for use on damp surfaces include Moisture curing- e.g. one pack polyurethane Solvent free- e.g. two pack epoxy Water displacing- e.g. some moisture tolerant chlorinated rubbers Water absorbing- e.g. some moisture tolerant chlorinated rubbers Moisture curing Moisture curing materials are mainly polyurethanes, which cure by reaction with moisture in the air; they therefore require a minimum RH % for application rather than a maximum. This value is often quite generous being as low as 35% RH, however, a maximum may still be specified- check data sheets The curing rate is temperature dependent (as with other materials), but being moisture curing the curing rate is also RH% dependent. Most figures quoted for overcoating/cure times are based on 65%RH A maximum of 80%RH is typically specified for moisture sensitive urethanes Immediately a can is opened and moisture is in contact with the paint, the curing reaction starts, so this single pack material has a pot life varying from 6 hours upwards. Some manufactures stipulate that the material must be used within 24 hours or within one working day
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Hot enamels The term enamels usually applies to a hot-applied bituminous materials, i.e. coal tar and asphalt (bitumen). Enamels were, and still are for some organisations, commonly applied to the external surfaces of pipelines, both in the factory for full pipe lengths and on site for welded joints The term enamel was srcinally a marketing term applied to hard thermosetting ready mixed paints because of their superficial resemblance to vitreous enamels The pipe is first blast cleaned or chemically cleaned then primed with the appropriate quick drying primer. Hot enamel (~200 0c) is poured (flooded) over the primed surface then a fibreglass inner wrap is applied, possibly whilst simultaneously applying a second flood coat. Another flood coat is immediately applied and then an outer wrap, consisting of fibreglass strands impregnated with the same enamel material, is wrapped around the whole surface. This system gives a homogenous, continually bonded layer with spiral wound reinforcing. The thickness range is typically 3-4mm Note: bitumen and coal tar enamels are not compatible. They are also reversible materials and so will soften and bleed with the application of hydrocarbon solvents. Heating a blade and moulding the surrounding material into the void can easily repair pinholes Tape wrapping systems Hot applied tapes In general, these are only used in conjunction with hot applied enamels. They have very poor adhesion properties when applied onto a smooth surface, e.g. plastics. The tape material is melted by blowtorch or similar and fused to the primed material. The tape is then spirally wrapped around pipes and similar usually with an overlap of up to 55% Cold applied laminate tape Usually consists of an outer polyvinyl chloride (PVC) or polyethylene (PE) film to which is bonded a mastic layer of synthetic rubber or rubber modified bitumen. Interleaving wax paper prevents adhesion between adjacent layers The tape is applied to a primed substrate and spirally wrapped around the pipes and similar usually with an overlap of up to 55%. Being thermoplastic, temperature affects the ease of application and behaviour during service Self adhesive overwrap tapes Not normally used as a wrapping system on their own but used as an overwrap system to hold other materials in place, e.g. fillers and grease based tapes These tapes are PVC or PE with a thin layer of pressure sensitive adhesive. Used only on smooth substrates because they will not adhere properly to rough substrates Grease based tapes Are composed of a synthetic fibre bandage impregnated with petrolatum grease. Highly mouldable, flexible and can be used on any substrate providing it is free of loosely bonded contamination Fillers Normally used to modify contours of valves and flanges etc. to facilitate the use of tape systems. Typically based on petrolatum grease, bitumen and rubber, consideration need to be given to substrate and overwrapping material compatibility Plastic coatings There are many types of plastic used in the coating industry, e.g. polypropylene, polyethylene and polystyrene. Common systems include heat shrinkable materials and materials, which are applied, hot then shrunk onto the component by the application of cold water Elastomeric coatings Elastomeric coatings have elastic properties and may be considered as synthetic rubber coatings. Elastomeric materials include neoprene, syntactic polyurethanes and EPDM (ethylene propylene diene monomer). After application some systems are vulcanised whilst others come in two-pack form and chemically cure
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Unit 6 PAINT MANUFACTURE Manufacturing paint is a relatively simple process. The skill in making paint is how the paint maker utilises the vast array of various ingredients at his disposal Paint is manufactured in paint mills whose main job is to grind down the pigment to the finest possible degree whilst at the same time mixing the ingredients to smooth high quality paint Types of paint mill Direct charge mills, which take and mix all the raw ingredients together. Types are Ball mill Attritor mill High speed dispenser Pre mix mills, take the ingredients which have been pre-mixed to a rough consistency and refine this mixture to a high quality paint this system is used fir large production runs. Types are Sand pearl Bead mill Colloid mill Single roll Triple roll Typical manufacturing process Select and assemble raw materials Mill the paint Carry out any quality testing required Can the paint Warehouse and distribute the paint Fineness of pigment grind During the paint milling process the pigment content of the paint is ground down to a very fine degree. This is required in order to give the finished paint film smoothness, but more importantly enables the paint to carry a high degree of gloss where this is required. If a pigment has only been coarsely ground the surface finish will be relatively rough and when subjected to a light box test will appear somewhat dull due to the reflected light being scattered rather than reflected The fineness of grind gauge, which is also called a hegman gauge, is a block of stainless steel around 175mm long by 65mm wide. It has a channel running along its length going from 0 m to 100m; the depth is identified along its length. Paint is applied to the channel and then a flat edge scraper is pulled from the 100m deep end towards the shallow end. At some point along the way as the paint in the channel becomes shallower pigment particles will break the surface. Where between 5-10 particles break the surface this is said to be the fineness of grind
100 50
0
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UNIT 7 PAINT COLOURS Bs 5252 Framework for colour co-ordination for building purposes establishes a framework within which 237 colours have been selected as a source for all building colour standards and the means of co-ordinating them. It is not itself a range of colours for any particular product and is not used to specify British Standard Colours. A colour is only standardised when it is included in another British Standard, such as those that have been derived for paints BS 4800, vitreous enamel BS4900, Plastics BS4901 and sheet and tile flooring BS4092. Such standards contain only a portion of the total colours contained in BS5252 and are selected to meet design requirements within relevant technical and economic constraints BS 4800 BS4800 –paint colours for building purposes specifies 100 colours, which have been selected from the framework of the 237 colours contained in BS5252 Each colour in BS4800 is identified by three parts as follows Hue: The first part signifies hue or colour and consists of an even number of two numerals (e.g. 04) twelve main hues are used and numbered 02 red purples 04 reds 06 yellow reds 08 yellow reds 10 yellows 12 yellow greens 14 greens 16 green blues 18 blues 20 purple blues 22 violets 24 purples Greyness: The second part signifies greyness, i.e. the apparent amount of difference in greyness between one colour with another. Five grades are used; each defines by a single letter. There are four steps of diminishing greyness from A (maximum) to D (minimum). Beyond this colours are pure of free from greyness and graded as E, e.g. pure yellows are prefixed 10-e Weight: When coded only two on the above grades it was found that the yellow hues (yellow-red and yellow green) at minimum greyness looked heavy in comparison with other colours. This was overcome by raising the value of the yellowish colours. The result was more uniform in weight. The weight is given in pairs of numbers from 01-56. Groups of colours within each of the five greyness ranges are graduated from high to low value Each of these graduations is numbered A greyness 01 to 13 B greyness 15 to 29 C greyness 31 to 40 D greyness 43 to 45 E greyness 49 to 56 E.G. Hue (colour) 02-24 in Even numbers
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E greyness A-E A is neutral; E is pure colour
53 weight Colour saturation 01-56
24
The above example signifies a deep bright blue Black, white or any grey maybe up from only black and white are not colours but are called neutrals and are always pre-fixed by 00Black is 00-E-53 White is 00-E-55 BS4800 can be used to identify existing colours when maintenance painting and is a method by which all manufactures can make exactly the same colour
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UNIT 8 PAINT TESTING GENERAL There are a large number of paint/paint film tests, which are carried out, some of which may be unique to one paint manufacturer or client’s specification. Bs 3900- Methods of Test for Paints – Is a specification widely referred to, which covers procedures, apparatus and related information on widely used test methods for paints, varnishes and similar products. The general introduction setting out the scope of the series is intended to be read in conjunction with each of the parts which are issued in loose leaf form and can be obtained separately A. Tests on liquid paints (excluding chemical tests) B. Tests involving chemical examination of liquid paint and dried paint films C. Tests associated with paint film formation D. Optical tests on paints E. Mechanical tests on paint films F. Durability tests on paint films G. Environmental tests on paint films (including test for resistance to corrosion and chemicals) H. Designation of intensity, quality and size of common types of defect: general principles and rating schemes
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UNIT 9 ARTIFICIAL WEATHERING Formerly natural weathering of painted samples provided information on how well paints performed when subjected to various environmental conditions. This however was a slow process, which sometimes took years to bring back results. In order to speed up this process a number of accelerated weathering devices were devised which brought back test results in a much shorter time Common tests in use 1) Salt spray box: simulates how paints behave under marine conditions 2) Water soak test: tests for paint film permeability 3) Tropic box: simulates how paints behave in a high humidity environment 4) Temperature cycling: tests for paint film flexibility by subjecting samples to alternate high and low temperatures 5) Cold check testing: tests for low temperature contraction cracking
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UINT 10 FLASHPOINT 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 air/vapour 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 1. Fix the Abel cup containing the substance for assessment into a bath of water 2. Apply heat source to the water bath and monitor the temperature of the substance in the Abel cup 3. Activate the source of ignition every 1/20c of rise in temperature 4. The flashpoint temperature is identified when a blue flame flashes over the substance being assessed Note: if an orange flame is observed the temperature is too high and overheating has occurred. The material under test should replaced and the test restarted
Thermometers
Ignition
Support
Water bath
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UNIT 11 VICOSITY Procedure for measuring viscosity using a ford flow cup no4: 1. Bring temperature of paint to200c+/- 0.50c 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 finger from the orifice and start stopwatch 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 taken in seconds is taken as the viscosity 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 percentage of thinners added to pain taken from a freshly opened can A thrixotropic paint needs to be worked to reach the free flowing stage, therefore the viscosity cannot be assessed with a flow cup: a rotational viscometer or another type of viscometer, which works the paint, must be used
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UNIT 12 DENSITY Density is the weight per unit volume and is therefore found by the following formula Weight Density --------Volume The unit used for measuring the density of paint is usually grams per cubic centimetre (g/cm 3) 3 1cmof water =1 millimetre =1 gram 1000 cm 3 of water =1 litre =1 kilogram The density of a paint will be higher than that of water, the density of a solvent will be lower than that of water, and the density of a curing agent may be higher or lower than that of water Vent
100 cc
Density cup Procedure for measuring density using a 100cm 3 density cup 1. Weigh the cup to the nearest decigram using laboratory balance with a 1000g capacity and a sensitivity of +/- 0.1g 2. Remove cover and fill with paint to 2.5mm 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 and re-weigh 5. Determine the weight of paint by subtraction 6. Divide weight by 100 if the density in g/cm 3 is 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-mix 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 Relative density Relative density or specific gravity is the density of any substance compared to the density of water Density of x Specific gravity (sg)=---------------------Density of water Because the density of water is 1g/cm 3 the 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 ratio 30
Example 1. What is the density of a paint if 5 litres weighs 7,35kg?
A.
weight Density --------Volume
B.
7.35kg Density --------5 litres
C.
7.35 x 1000 grams Density --------5 x 1000 cm 3
D.
Density = 1.47g/cm 3
2.
A two-pack paint is mixed at a ratio of seven parts base to two parts curing agent; the densities are 1.59g/cm 3 and 0.78g/cm 3 respectively. What is the density of paint after mixing?
a) b) c) d)
7 parts base 2 parts curing agent 9 parts combined density
1.59 x 7 = 11.13 0.78 x 2 = 1.56 11.13 + 1.56 = 12.69 12.69 / 2 =1.41 g/cm 3
Note the sg would be 1.41
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UNIT 13 WET FILM THICKNESS The wet film thickness is taken immediately after 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 The wet film thickness may be found by using a comb gauge or an eccentric wheel
W.F.T.
Comb gauge PAINT
Substrate
0
Eccentric wheel 250
Scale
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 paintfilm. The film thickness should be recorded as lying between the last tooth touching and the first non tooth as shown on he tooth calibrations marked on the gauge 3. at least two further readings should be taken in different places in order to obtain representive results over the full coated area NOTE: The w.f.t. is sometimes recorded as the average between the last touching tooth and the first non-touching tooth The wet film thickness may be found by calculation Calculation VOLUME W.F.T. = -------------AREA 100 W.F.T. = -------V.S.(%) X D.F.T.
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UNIT 14 DRY FILM THICKNESS There are four methods of determining the dry film thickness of paint non destructive test gauges destructive test gauges test panels calculation Non destructive test gauges Measuring the d.f.t. directly with a non-destructive test gauge is the most widely used method; there are a variety of gauges available with various scale ranges magnetic film gauge (banana gauge) pull-off gauge or tinsely pencil magnetic horseshoe gauge gauge current/electromagnetic eddy Magnetic film gauge (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 Scale wheel
Paint
Magnet
Ferromagnetic substrate Magnetic film thickness gauge Calibration procedure 1. choose a magnetically insulated shim of known thickness, close to the thickness of the paint you expect to find, e.g. don’t choose a 25 m shim to calibrate if you expect the coating to be in excess of 300m; this will reduce the accuracy 2. place the shim on the same substrate surface as the finish on which the paint to be measured is attached, e.g. if the paint is on a blasted surface, calibrate the gauge on an uncoated blasted surface 3. place the magnet onto the shim and press firmly on the instrument, wind the scale wheel forwards ( away from yourself ) until the magnet is definitely attached to the shim/substrate 4. Gradually wind the wheel backwards slowly until the magnet detaches itself. At this point, move the cursor on the instrument to the thickness of the shim as shown on the scale wheel. With some instruments the scale itself must be moved to line up with the fixed cursor. When using the latter type of instrument, rotate the wheel to zero to locate the position of the scale adjuster The instrument is now calibrated and may be used to measure the d.f.t. of a non-magnetic 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 other nondestructive test gauges
Scale
Magnet
The pull off gauge, or tinsley pencil as it most widely referred to, consists of a magnet at the tip of the instrument, which attached itself to the coated substrate. The gauge is then slowly pulled away from the coated substrate at normal incidence until the magnet 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 before use
Tinsley pencil
Magnetic horseshoe gauge The magnetic horseshoe type gauge works by measuring the change in magnetic flux between two poles of a magnet, the change of magnetic flux depends on the coating thickness. The accuracy of these instruments is claimed to be +/- 10% and as with the other magnetic gauges, may only be used for measuring the thickness of non-ferromagnetic coating applied over ferromagnetic substrates Eddy current and electromagnetic gauges The most accurate of the non-destructive gauges for measuring the d.f.t. are 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; electromagnetic gauges are used on ferromagnetic substrates such as ferritic steel Many Eddy current and electromagnetic gauges also have statistical capabilities and some will download and upload information from computers 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 (p.i.g.) is one such type of Destructive gauge. A small vee shaped channel is cut into the coating at a fixed angle 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 test gauges are the Saberg thickness drill or Erichsen thickness drill which work on a similar principle to the paint inspection gauge Test panels 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 carried out and measuring the d.f.t. with a micrometer
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Calculation The d.f.t. 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. is as follows V.s.% x W.f.t. d.f.t. = ----------------- -100 Example What would the d.f.t. if 15litres of paint with a volume solids of 44% is used to cover an area of 12m x 7m? To find d.f.t. a) D.f.t. v.s.% ------ = -----------W.f.t. 100
b) D.f.t. =
v.s.% x W.f.t. ---------------------100
c)
44% x W.f.t. ---------------------100
D.f.t. =
To find W.f.t.
W.f.t. is not directly given in the question, therefore must be found by calculation
Volume d) W.f.t. = --------------Area 15 litres e) W.f.t. = --------------12m x 7m
f)
15 x1000cm 3 W.f.t. = -------------- 1200cm x 700cm
convert all given units to common units, i.e. cm
15 cm 3 g) W.f.t. = --------840 15 x 10,000 m h) W.f.t. = --------840 i)
convert to m (10,000m to 1 cm)
W.f.t. = 179m
Return to d.f.t. Formula 44% x 179 m ---------------------100
j)
D.f.t. =
k)
D.f.t. = 79m
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UNIT 15 ADHESION Adhesion failures more often occur between the uncoated substrate and the prim er 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 a system; when compatibility is lacking it is often the adhesion which suffers Cohesive failure
Adhesive failure between paint films
Substrate
Adhesive failure between primer and substrate
Vee cut test With a sharp knife, cut a vee using approximately 12mm cuts forming a 30 0 angle, through the paint film and down to the substrate. Insert the tip of the 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 substrate Cross cut test (cross hatch test) Using a sharp knife or a multi-bladed cutter, cut 6 lines vertically and horizontally, 2mm apart, to produce 25 squares. Cover with adhesive tape and snatch off; the amount of segments remaining on the tape may be multiplied by four and then given a percentage value or a value may be given 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 X-cut tape test With a sharp knife or similar make an x shaped cut with the smaller angle between 30 0 and 45 0. The cuts must be made down to the substrate in a single action and are approximately 40mm in length. A piece of specified pressure sensitive tape approximately 75mm long and 25mm 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 1-2 minutes the tape is pulled off rapidly at an angle as close to 180 0 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 0a=removal beyond the area of the x Dolly test A more technical adhesion test, the pull off adhesion test or dolly test, m ay show: Adhesive failure between primer and substrate ( most likely ) Adhesive failure between paint films Adhesive failure within a individual paint film
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Load adjustment
Dolly puller
Load indicator
Dolly Paint film
Substrate
Pull off dolly test
Procedure for carrying out pull-off test 1. Clean and degrease the surface to be tested and the dolly contact surface 2. Roughen both surfaces with fine/medium grade emery cloth 3. Mix regular araldite and stick dolly to surface, leave for 24 hours at 250c 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 a reading from position of cursor when dolly detaches. Values will be typically obtained in either Mpa, N/mm 2 or p.s.i. Note alternative adhesives are possible, see test procedure sheets A minimum pull-off value for the paint type used should ideally be specified in the specifications 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 as 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 re-usable and contain a hole down the 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
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UNIT 16 HOLIDAY DETECTION Holiday detection or pinhole detection is an operation, which detects any holes/holidays 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 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 25mm thick, e.g. high frequency spark testers. For coatings ranging from approximately 0.5mm to 4mm thick AC/DC or pulsed DC Holiday detectors, usually powered by a 6volt battery, would normally be used Note holiday detection must not be carried out on wet surfaces or in the rain High voltage Holiday detectors Voltage selection Prior to carrying out holiday detection the correct voltage must be selected because too much voltage may indicate the presence of holidays where 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 voltmeters 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/multi-meter or proprietary calibration voltmeter supplied by the detector manufacturer Note Holiday detectors should be checked throughout the working day to ensure correct set-up 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. 0-5kV for accurate voltage selection. For thicker coatings 0-20kV 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 The correct voltage is ideally determined by detecting the presence of a known pinhole, which has been induced diagonally through the coating to the bare metal. However, the voltage is normally selected by measuring the coating/wrapping thickness and applying a formula, e.g. 125V per 25m of thickness (same as 5kV per mm), or following other specification requirements Note it is preferable to ensure the coated surface 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 Operation When operating a Holiday detector on a coated surface, an earth wire from the main unit is 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 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. 300mm/sec 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. The kV dial will drop B. An alarm will sound e.g. a buzzer C. A light will come on 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
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Water is used to wet a sponge, which is connected to the positive terminal on test instrument. When the sponge passes over a pinhole, the water is drawn into it, which allows the DC. 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 9V and 90V, whereas others have only a single setting e.g.9V There is no hard and fast rule for voltage to use these instruments but it is generally accepted that up to ~300 m the 9V setting is adequate; up to ~500 m would require the 90V setting. The specification or written instruction should state the voltage to be used
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UNIT 17 SAMPLING Sampling refers to taking paint samples for analysis/testing and is concerned with taking the correct samples, ensuring correct identification of the samples and maintaining the samples in correct condition until assessed Bs3900: Part A1 – methods of tests for paints. Sampling (same as en21512) Generally samples may be taken and analysed at four stages 1. During manufacture- taken from the final manufacturing vessel 2. During canning(drums, barrels and tanks) 3. On delivery to purchaser- in which case the material should be unaltered 4. At the point of application- to ensure adherence to manufactures recommendations(data sheet) For items 3 & 4 an independent laboratory may be used for analysis, e.g. to determine density, viscosity, thinner type used etc. When taking samples at the application point, e.g. on site, the sample as supplied is termed sample A; the sample taken at the point of application is termed sample B The following points should be noted during sampling: a) The paint should be thoroughly mixed to provide a homogenous sample b) The sample should be truly representative, i.e. not confined to surface areas of the batch c) Two types of apparatus will be needed for mixing and taking the sample. Preferably broad bladed stirrers of mechanical mixers and special sampling tubes or dip cans d) Containers should preferably be metal or glass. Metal containers should be uncoated internally with tight closures. Containers should have closures which are not affected by the material e.g. solvent attack e) All sampling equipment should be scrupulously clean and dry so as not to contaminate the sample f) Take into consideration safety requirements, e.g. the material under test may be toxic or flammable g) Method of sampling is related to the type of material, e.g. liquid, highly viscous or powder products. When sampling from bulk storage, several small samples from differing depths and locations should make up a representative sample h) The number of samples taken upon delivery would largely depend on the amount received. Bs 3900: part 1A recommends n/2 Where n = the number of containers, undamaged, unopened containers would normally be chosen unless specified Labelling and sealing of the samples should be done as soon as possible and information on the label should include the following i. Manufactures name / product description ii. Quantity and other delivery of delivery iii. Batch numbers or other reference, e.g. tank numbers iv. Date of Manufacture and date sample taken v. Total number of samples taken vi. A reference number for the sample (for bulk delivery) vii. The name of the consignor viii. The place where the sample is to taken ix. Name of sampler Sample should be analysed as soon as possible and stored according to Manufactures recommendations Reporting The sampling report should ideally contain the information given on the label and refer to Bs3900: part 1A or other specification used. Any abnormalities should be noted, for example a) b) c) d) e) f)
Container defects Visible foreign matter Abnormal colours Abnormal odours Errors of labelling etc. Presence of skin, settling etc 40
g) Any difficulty in re-incorporation Information on the tests conducted and the results also requires recording, but the details will depend on the tests conducted and the requirements of the specification
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UNIT 18 WEATHER CONDITIONS The coating specification should always state the weather conditions in which a coating can or cannot be applied. A typical painting specification extract is as follows It is not permissible to apply paints when the following conditions apply During rain, snow, or high winds When the air temperature is not at least 3 0c above the dew point temperature When the air or metal temperature is below 5 0c When the relative humidity is above 90% Relative humidity RH% and dew point 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 dew point is the temperature at which water vapour in the atmosphere would form condensation. Therefore, if the temperature dropped to the dew point temperature the Relative humidity would rise to 100% and condensation would be formed on any objects at, or below that temperature Note the capacity of air to hod water doubles every 11 0c rise in temperature Measuring R.H.% AND Dew point Both Relative humidity RH% and dew point are measured using a hygrometer of which there are many types 1. Aspirated hygrometers a) The screen hygrometers and masons hygrometers 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 airflow over a wet wick c) Whirling hygrometers is a portable and dynamic type which operates by physically moving air through the air 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 absorption properties of paper 3. Digital hygrometers are split into two categories: i. Rh meters which give digital readouts of R.H. and D.P. only ii. Thermo-hygrometers which give digital readouts of R.H.,D.P. 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 Note the transport of mercury by air is not permitted, therefore coloured alcohol- in-glass thermometers may be specified for work, which involves equipment being transported by air The frame is rotated by hand as fast as possible for at least 90 seconds, or otherwise specified, so that the bulbs pass through the air at least 4ms -1. 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 Note the dry bulb temperature is the air temperature with a wind chill factor Repeat the operation until consecutive readings of each bulb temperature agree to within0.2 0c If it is 100% Relative humidity the wet bulb will be the same 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 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% R.H region and above 42
Metal temperature The metal temperature is measured with a magnetic temperature gauge, sometimes known as a limpet gauge, or electrical contact thermometer.
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UNIT 19 PAINT APPLCATION Brush application There are many types of brush, which may be used to apply paint; the flat paintbrush is the most common type encountered in the UK and USA. The oval type tends to be favoured in continental Europe Brushes may have fillings of the following types; Bristle, I.e. hogs hair Horsehair Natural fibres Synthetic fibres mixture of above Bristles have a natural taper allowing the brush to maintain its form; they also have scales along the length of each hair, which allow the brush to hold more paint. Bristle brushes are usually high quality brushes and are quite expensive Brush applied coatings often do not have the uniformity of thickness usually encountered with spray coatings, but they do not produce spray for or overspray, i.e. there is less of an environmental hazard, less wastage and less spotting etc. on nearby structures Brushing also works the paint into a substrates surface, which gives optimum coverage and mixes in any dust particles, thus helping to achieve optimum adhesion Roller application Roller application is a quicker method than brush application and is useful for large flat areas, but unlike brush application, the paint does not get worked into the surface to the same extent and there is also a lack of uniformity in film thickness. Roller application is not permitted in certain specifications for certain work Curved roller exist i.e. for pipes; some types of roller have a paint fed via a fluid line connected to the handle Spray application Spray application produces a production rate well in excess of that achieved by brushing and is therefore a very common paint application method For a paint to be sprayed successfully it must first be atomised, i.e. it must be broken down into very fine droplets. Atomisation is achieved by both the conventional spray and airless spray application methods but by different methods in each case Conventional spray The paint is held in a container attached to the top of the gun in the case of gravity feed; underneath the gun in the case of suction feed; or remote from the gun in the case of pressure feed Pressure fed conventional spray guns are able to cover much greater areas without the need to continually refill the container and therefore the most common type of conventional spray systems encountered. Two lines feed the spray gun; one carrying the paint at low fluid pressure, usually under 20 p.s.i. and the other carrying an air supply at a pressure of approximately 40-75 p.s.i.. The air supply leads to an air cap on the gun and blows onto the paint stream as it exits the nozzle resulting in very fine atomisation Airless spray One line feeds the spray gun carrying the paint at a pressure typically between 2000 and 4000 p.s.i.. There is no air cap on the gun, hence the term airless. The paint is atomised by forcing it through a small orifice at high pressure; when the paint meets the air it splits up into fine droplets due to the air resistance The unpressurised container containing the paint is remote from the gun; the paint is sucked up using a fluid pump and fed to the gun by way of a special reinforced high pressure fluid line. A compressor supplies the air up to approximately 100p.s.i. to the fluid pump; this air inlet is adjustable. The fluid pump multiplies the pressure by a ratio governed by the pump, e.g. 35:1 ratio pump supplied with an air pressure of 100 p.s.i. would lead to a fluid pressure of 3500 p.s.i In addition to the conventional compressors supplying compressed air, other methods exist to achieve the fluid pressures necessary for airless spray, e.g. the diaphragm pump and the electric pump, both these supply hydraulic pressure
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The spray tips on airless spray guns are usually lined with tungsten carbide; some have the advantage of being reversible by turning a lever 180 0 to clear out any blockages in the small orifice. There is also a tip known as the titian tip which has an adjustable hole size The orifice size governs the throughput of paint whilst the orifice angle governs the size of the pattern Typical requirements for tip orifice sizes used with airless spraying are shown in the table Paint type Chlorinated rubber High build epoxy Zinc rich paints
Tip size 13-21 thou” 17-23 thou” 17-23 thou”
Airless pressure (p.s.i.) 2400 3000 2800
Using airless adhere spray equipment can be very dangerous due to the high fluid pressure; operatives must always to the following
use tips designed for airless spray use fluid lines in good condition designed for airless spray ensure there are no kinks in the line ensure the safety catch is on when the gun is put down never point the gun at anybody or yourself never attempt to clean or change nozzles when the fluid is pressurised ensure the equipment is earthed to prevent static shock wear an airfed helmet Comparison of airless and conventional spraying Airless spraying conventional spraying Usually coarser atomisation Fine atomisation for high quality finishes Method of atomisation Pressure drop when fluid exits By air jets on the nozzle small orifice Factor atomisation
Air pressure Fluid pressure Fluid delivery Air contamination Materials sprayed
Maintenance
Up to 100p.s.i. to the fluid pump 600 to 6000 p.s.i. Medium to high higher production More overspray but less fog and rebound Uniform fine grinds necessary, although can handle relatively high viscosity’s More required especially tips, equipment is more sophisticated
40 to 75 p.s.i. to the gun Less than 20 p.s.i. Low to medium Lower production less overspray but More fog and rebound Basically any materials that flow well can be sprayed Less require; equipment is more basic
Electrostatic spray This method of paint application requires the use of a special spray gun, which applies a charge to the paint when applied. The article to be coated is earthen so that the charged particles are attracted towards it; when an area on the component has been coated to a particular thickness there is less of an attraction, due to an insulating effect the coating has, although the paint may still be attracted by unquoted areas on the article which may not be in the direct line of the application nozzle This type of application results in uniform coating thickness with a substantial reduction in overspray. Electrostatic spray and other Electrostatic application methods are widely used in
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factories for coating all types of components including pipe, fridge’s, washing machines etc using both liquid and powder coatings Other methods of paint application Other methods of paint application include dip coating padding hot spraying spin rotating flow coating/curtain coating aerosol Galvanising Used for structures, fitting and cladding. The components are degreased, e.g. with warm caustic soda, acid cleaned, washed, fluxed with ammonium chloride then immersed in a bath of molten zinc between 420-450 0c to achieve a coating thickness of approximately 85-130m Sheardising Used for fittings, fasteners and small items, which are likely to be distorted by, hot dip Galvanising. Sheardising is particularly suited to threaded components where only a small change in dimension is acceptable. The items are first degreased and pickled then tumbled for a few hours in hot zinc dust at a temperature just below the melting point of zinc. Coating thicknesses of approximately 15-30m are typically achieved Calorising Same principle as Sheardising but using aluminium powder Anodising An electrolytic method of coating aluminium with a dense oxide. The component to be Anodised is dipped into a bath of weak acid usually sulphuric and oxidation is induced electrically. For adhesion of subsequent paint films, etching may be required Electroplating The plating of small parts by the electrolytic deposition of metal, e.g. zinc, from metal salt solutions, coating thickness of up to 25 m are usually applied Hot metal spraying Metallisation, or the method of hot spraying one metal with another, is a widely used system for preventing corrosion of metal structures. The most common sprayed metals used are aluminium and zinc Application is normally carried out using one of the following methods 1) Powder fed system the coating metal is supplied in fine powder form, and blown through a heat source onto the substrate. This is a very wasteful method which often requires a recycling facility 2) Electric arc system the wire is used as one electrode (similar to the welding process), the energy from the arc melts the wire and resulting molten metal is blown onto the substrate by means of an air jet. The arc method of metal spraying is now widely used for mechanical applications, e.g. production lines, where thousand of components of identical shape are to be coated 3) Wire andofpistol system thiswith is the commonmotor, method of metal in use. It consists a hand held gun anmost air-powered which drawsspray wire application through knurled feed rollers into an oxygen/fuel gas flame where the metal is melted. This molten metal is then projected onto the prepared surface by the products of combustion augmented by an air blast from the compressed air driven motor Metal-sprayed coatings are often sealed to prolong their life due to their porous nature
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UNIT 20 COATING FAULTS The following have been extracted from BS2015 glossary of paint terms Bittiness: The presence of particles of gel flocculated material or foreign matter in a coating material or projecting from the surface of a film Note 1: the term seedy specifically denotes the presence of bits that have developed in a coating material during storage Note 2: the term peppery is sometimes used when the bits are small and uniformly distributed Bleeding: The process of diffusion of a soluble coloured substance from, into or through a coating material from beneath, thus producing an undesirable staining or discolouration Blistering: The formation of dome shaped projections or blisters in paints in the dry film of a coating material by local loss of adhesion and lifting of the film from the underlying surface Note: such blisters mat contain liquid, vapour, gas or crystals Bloom: A deposit resembling the bloom on a grape that sometimes forms on the gloss film of a coating, causing loss of gloss and dulling of colour Chalking: The formation of a friable, powdery layer on the surface of the film of a coating material caused by disintegration of the binding medium due to disruptive factors during weathering Note: chalking can be considerably affected by the choice and concentration of pigment Cissing: The formation of small areas of the wet film of a coating material where the coating material has receded leaving holidays in the film Cracking: Generally, the splitting of a dry paint or varnish film, usually the result of ageing. The following terms are used to denote the nature and extent of this defect 1) Hair cracking: cracking that comprises of fine cracks which may not penetrate the top coat; they occur at random 2) Checking: cracking that comprises of fine cracks which may not penetrate the top coat and are distributed over the surface giving the semblance of a small pattern 3) Cracking: specifically, a breakdown in which the cracks penetrate at least one coat and which mat be expected to result ultimately in complete failure 4) Crazing: cracking that resembles checking but the cracks are deeper and broader 5) Crocodiling or alligatoring : a drastic type of crazing producing a pattern resembling the hide of a Crocodile or alligator 6) Mud cracking: a network of deep cracking that form as the film of a coating material dries especially when it has been applied to an absorbent substrate. Mud cracking is associated primarily with high pigmented water borne paints Cratering: The formation of small bowl shaped depressions in the film of a coating material Curtaining, sagging: A downward movement of a paint film between the times of application and setting, resulting in an uneven coating having a thick lower edge. The resulting sag is usually restricted to a local area of a vertical surface and may have the characteristic appearance of a draped curtain; hence the synonymous term curtaining Dry spray: The production of a rough or slightly bitty film from sprayed coating materials where the particles are insufficiently fluid to flow together to form a uniform coat Effloresence NOT A PAINT DEFECT. It is the development of a crystalline deposit on the surface of brick, cement, etc., due to water containing soluble salts, coming to the surface, and evaporating so 47
that the salts are deposited. In some cases the deposit may be formed on the top of any paint film present, but usually the paint film is pushed up and broken by the Effloresence under the coat Flaking Lifting of the coating materials from the substrate in the form of flakes of scales Flocculation The development of loosely coherent, pigment agglomerates in a coating material Grinning The showing through of a substrate due to inadequate hiding power of the coating material Holidays A defect due to faulty application techniques seen as areas where the film of a coating material is of insufficient thickness or where there is a complete absence of coating materials on random areas of the substrate Lifting Softening, swelling or separation from the substrate of a dry coat as the result of the application of a subsequent coat Orange peel effect The uniform pock-marked appearance, in particular of a sprayed film, resembling the peel of an orange due to the failure of the film to flow out to a level surface Note see also spray mottle and pock-marking Pinholing The formation of minute holes in the wet film of a coating material that form during application and drying due to the air or gas bubbles in the wet film which burst, giving rise to small craters that fail to coalesce before the film has set Residual tack The degree of stickiness remaining in a film of a coating material which, although set, does not reach the true tack-free stage Ropiness Pronounced brush marks that have not flowed out because of the poor levelling properties of the coating material Saponification Not specifically a paint defect term. The formation of a soap by the reaction between a fatty acid ester and an alkali Note: in painting practice Saponification refers to the decomposition of the medium of a film by alkali and moisture in the substrate, e.g. new concrete or rendering based on cement, sand and lime. Saponification films may become tacky and discoloured. In very severe cases film may be completely liquefied by Saponification Wrinkling, rivelling The development of wrinkles in a film of a coating material during drying, usually due to the initial formation of a surface skin Note: see also crinkling and finish
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UNIT 21 DEFINITIONS Compliant coating A coating which complies with the environmental protection act of 1990 E.P.A. Contractor A person, firm or company which enters into a contract Long term protection Typically 10 years Medium term protection Typically 5 years Short term protection Typically 2-3 years Damp surfaces Temperature below dewpoint but on which water is not readily detectable Moist surfaces Where standing water and droplets have been removed but which still displays a noticeable film of water Wet surfaces Surfaces on which standing water or droplets are present (when these may be in evidence final surface preparation is not normally carried out) New galvanising A galvanised steel surface upon which a cohesive oxide layer has not yet formed (bright and shiney, less than three months old) Fully weathered galvanising A galvanised steel surface upon which a cohesive oxide layer has formed by natural weathering (dull and lacking in sheen) Dewpoint The temperature at which condensation would form on a substrate T Wash An etch primer for zinc metal surfaces. Blue in colour it turns black upon drying if it has been successfully applied Hot duty surfaces Metal surfaces that will reach a temperature in excess of 99 0c when in use Test areas May be requested in order to demonstrate that the selected system is capable of meeting its requirements. This refers to the method and standard or preparation, equipment used, paint specified and on the same substrate Access equipment A green tag identifies safe scaffolding. Unsafe scaffolding by a red tag. Scaffolding is inspected by a competent at least every seven days and always after bad weather Sheeting for protection Sheeting of a non-flammable nature must be used to protect areas not being prepared or painted from contamination, (in particular tarpaulins must not be used) Metallic zinc paints Special care must be taken not to allow zinc rich paints to contaminate stainless steel nor to be applied within 75mm of weld end preparations Tie coat Where adhesion problems, solvent stripping or bleeding might occur, a tie coat would be employed to prevent these particular problems arising
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UNIT 22 DUTIES OF A PAINT INSPECTOR The job of a paint inspector is to inspect and report. He must act with integrity at all times and be true to the specification requirements The following is a list of points, which form the basis of typical activities performed by painting inspectors 1 General Obtain or gain access to the specification Learn the specification Ensure your instruments are In proper working order Get to know the plant Get to know the personnel Check work for conformance to the specification Keep the engineer/supervisor informed at all times especially if there is any departure from the specification Make written reports at an agreed frequency Attend site meetings when required 2 At the beginning of the day Check the environmental conditions Check the equipment (preparation, application and inspection for conformance to specification) Check materials Check the previous days work where applicable ( film contamination and dft if wet films have been left overnight) Establish with the contractors senior personnel the days work programme 3 During the day Check environmental conditions Check equipment Check usage of materials (when specifically required to do so) Check each operators work 4 On completion of the work at each stage Ensure that the work meets the specification by carrying out or witnessing tests Check for any application faults 5 At the end of the day Check the days work Check housekeeping Complete reports 6 On completion of the job Ensure as objectively as possible, that the specified requirements have been satisfied Write a summary report if required to do so Ideally, painting inspection personnel should be issued with relevant procedures and work instructions to enable them to carry out inspection and associated activities in accordance with the clients or organisations requirements. The procedures should leave the inspector in no doubt as to what is to done. Unfortunately, this documentation rarely exists!
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Typical inspectors duties Before work commences 1. Determine your duties and responsibilities. Duties may include those, which relate to health and safety aspects talking into consideration mandatory requirements. You may also be required to check that rejected paint or used abrasive is disposed of correctly 2. Ensure the contractors supervisor is aware of your duties and authority 3. Ensure you have correct applicable specifications and data sheets. Also ensure you have at least have access to relevant referenced normative documents 4. Determine the order of precedence for narrative documents if the specification does not make it clear 5. Learn the specification, procedures, work instructions etc. 6. Approach the senior inspector or supervisor if you are not sure of what is intended of any requirement 7. Ensure you have copies of any applicable documentation, e.g. correspondence, minutes from meetings, concessions etc. 8. Liase with the contractors supervisor to determine whether the contractors personnel are familiar with the work requirements 9. When required, confirm that the contractors operators are properly trained and conversant with the equipment, materials and application techniques being used 10. Agree with the client /supervisor the level of liaison that is required and determine reporting/recording requirements 11. Ensure you have test instruments etc. that are required and that they are properly calibrated and in correct working order
Surface preparation 1. a) b) c) d) e) f) g) h)
Check the specification, procedures and/or work instructions to establish Standard against which work will be measured Methods by which work is to be assessed, e.g. surface comparator Degree of surface cleanliness required Surface profile requirements (where required) Any special test to be carried out Requirements regarding equipment and consumables Ambient conditions required Recording/porting requirements
2.
Check the condition of the substrate before cleaning and make a note of rust grade, general contours (sharp edges, burrs etc.), spatter or flux residue on welds, algae or mould growth, zinc salts etc. any areas suspected to be defective e.g. cracked, laminated or mechanically damaged, should be reported immediately to the supervisor or client Note do not allow surface laminations, cracks and similar to be dressed without the permission of the supervisor/client 3.
Ensure ambient conditions allow surface preparation to take place. The following may have to be assessed/measured a) Air temperature b) Steel temperature c) Relative humidity d) Dew point e) Moisture on substrate f) Potential sources of contamination, i.e. chemicals, salt spray, fumes, etc. g) Potential changes in the weather to adverse conditions 4.
Identify areas being prepared
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5. a) b) c) d) e)
Check that the correct materials and equipment are being used, e.g. correct type, correct size, consumables are free from contamination, etc. examples Abrasive type, size and cleanliness. No reuse of expendable material Correct wire brushes Correct needles in needle guns Presence of carbide tips on scrapers Correct chemicals for cleaning
6.
Carry out inspection of prepared surfaces as required by the specification
7.
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
8.
Ensure that all concerned are clear about the reasons for any rejections
9.
Where remedial work has been necessary, re-inspect for conformance to the specification
Paint Material 1.
Check the specified requirements
2.
Check that the paints 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 paint container
3.
Check that all paints to be used on a surface are supplied by one paint manufacture (unless otherwise specified)
4.
Check that the paint is the correct type for application method being used, i.e. brush grade or spray grade
5. Check that the paint storage conditions are correct Note: any warranty on the material is likely to depend upon proper handling and storage 6.
Determine whether the paint is being withdrawn from the store in proper rotation
7.
Ensure paint is not being used beyond its shelf life
8.
Monitor material usage to determine whether there is sufficient paint in storage for the completion of the job (or part job). This is not always the responsibility of the inspector
9.
Check that the paint is being mixed and stirred correctly. Any permitted addition of thinners must be monitored to ensure correct type and amount. For two pack paints a) Check that the materials are mixed strictly in accordance with the paint manufactures data sheets b) Confirm that any induction time is strictly adhered to or time is allowed for gas bubbles to escape (if applicable) c) Confirm that mixed material is not used after pot life 10. Conduct all necessary paint sampling procedures and tests; or confirm that such tests have been carried out prior to the commencement of work. Record batch numbers of paints tested Paint Application 1.
Check the specified requirements
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2.
Check that the surface to which the paint is being applied is free from contamination, i.e. oil/grease, dust, spent abrasive. Corrosion products etc. any areas suspected as being defective, e.g. cracked, laminated or mechanically damaged, should be reported immediately to the supervisor or client
3.
Ensure that the ambient conditions allow painting to take place. The following may have to be assessed/ measured a) Air temperature b) Steel temperature c) Relative humidity d) Dew point temperature e) Moisture on the substrate f) Potential sources of contamination, i.e. chemicals, salt spray, fumes, dust etc. g) Potential changes in the weather to adverse conditions Note: check that the particular paint being applied does not have any special restriction on its application 4.
Confirm that paint is not being applied to coated substrates either before or beyond the specified overcoating times for the existing coating
5.
Check that the correct application method is being used
6.
Identify areas being painted
7.
Confirm that stripe coats have been applied correctly if specified
8. Carry out inspection of painted surfaces as required by specification. For example a) Check each coat of paint is uniformly applied and is free from curtains, sags, runs, holidays or other visible defects b) Measure the wft immediately after application c) Measure the dft Note: it there are significant differences between the dft calculated from the wft and the measured dft, check the material to ensure that only permitted additions of thinners were made to the paint if possible 9.
Ensure that any areas of defective coating are identified for remedial work
10. Ensure that all concerned are clear about the reasons for any rejections 11. Re-inspect any remedial work carried out to ensure that it conforms to the specified requirements 12. Check that the completed work is uniform in colour and finish 13. Unless the specification states otherwise, check that the handling of recently coated items is carried out in such a way that the coatings are not damaged 14. 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 bee rejected. The reasons for any rejections should be clearly identified Typical contractor malpractice’s Typical contractor malpractice’s which the inspector should be aware of are as follows 1.
Use of unskilled operators. This may relate to surface preparation, application of paint or safety considerations e.g. unsafe scaffolding 53
Note: the painting inspector cannot normally report on unskilled operator as something, which does not conform, to specification 2.
Use of unsuitable equipment, which may be worn brushes, poorly maintained and leaking compressors, damaged ladders contaminated equipment from previous contract etc.
3.
Painting or preparing surfaces during inclement weather conditions such as rain, snow, fog, mist etc.
4.
Hand mixing paint which should be mechanically mixed
5.
Painting before inspection of substrate preparation or previous coat
6.
Applying two paint coats or more to the same area in one day assuming the specification does not allow this
7.
Missing out a coat
8.
Use of wrong solvent or an excessive amount of solvent
9.
Use of incorrect paint type or mixing different manufactures products
10. Storing paint incorrectly, e.g. where the specification requires paint to be stored In a temperature controller environment 11. Paint used outside expiry date, suspected by condition of can, excess binder on top of freshly opened can, pigment settlement. Check the date by way of the manufactures coding: inform engineer 12. Re-using expendable abrasives 13. Insufficient blasting or painting in difficult areas such as under pipes 14. Cleaning surfaces with contaminated cotton waste or rags or using materials for cleaning which are not permitted 15. Applying a thickness of paint which is less than the specified minimum
Note: using diesel in the paint (as a plasticiser or thinner) has been done on many occasions! This will prevent proper drying
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QUESTION PAPER 1 1. 2. 3.
4. 5. 6. 7. 8. 9 10
11 12 13 14 15
Describe why and how corrosion occurs? Describe what is meant by Bi-metallic corrosion? Describe the following a) The galvanic series? b) Millscale? c) Rust grades to SIS 05-59-00? d) Blast cleaning grades to SIS 05-59-00? Describe the difference between water blasting and pressure washing? Name two tests used to determine the presence of hygroscopic salts left on a blast cleaned surface? Name four expendable abrasives and state the difference between an expendable and a reusable abrasive? Name the three basic constituents of a solvent carrying paint and the functions that each performs in the paint? Name six pigments and there respective colours? Describe what qualities a laminar pigment gives to a paint and name four laminar pigments? Describe a) A barrier paint system? b) A sacrificial paint coating? Describe what a tie coat is and give three reasons why we may require one? Describe a typical paint manufacturing process and give two types of paint mill, which could be used? Describe what you know of viscosity and outline one method of testing? Describe what you know of density? Can you use a banana gauge over a) An M.I.O coating? b) An aluminium substrate? c) A galvanised surface?
16 17 18 19 20
Describe a destructive test for determining the dry film thickness of a paint? Give four artificial weathering devices and describe what they simulate? What is the name of an instrument, which measures opacity and gives two examples? Describe a dolly test for adhesion? How do the following work a) Conventional spray set-up? b) Airless spray system? c) Electrostatic spray system? 21 How is B.S. 4800 used in the construction industry? 22 What are the essential differences between a blister and an osmotic blister? 23 How do the following display their characteristics a) Bleeding? b) Blooming? c) Chalking? d) Cissing? e) Grinning? f) Flocculation? g) Lifting? h) Ropiness? i) j) k) l)
Solvent stripping? Oxidation? Chemical curing? Overspray?
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24 Define the following a) Dewpoint? b) R.H.? c) Induction period? d) Pot life? e) Flash point? f) Shelf life? g) Batch number? h) Masking out? i) Rust blooming? j) Water borne coating? k) B-SA2 ½? l) Feathering? m) A-ST3
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QUESTION PAPER 2 1. 2. 3. 4. 5. 6. 7. 8. 9.
What kind of process is corrosion? What particular problems relate to maintaining of an effective coating system in the splash zone? What factors influence the rate of corrosion? What is millscale and why is it a particular problem when attempting to protect steel? What is the number of the Swedish standard contained within BS 7079 and list the rust and blasting grades together with their brief titles? Name three tests for locating hygroscopic salts and mill scale on a blast cleaned substrate? What factors influence how clean and how rough the surface becomes after blast cleaning? Regarding hand and power tool cleaning state what always concludes this process and to what standards? What three basic ingredients make up a traditional solvent carrying paint?
10. What are the functions of each of these three in a solvent carrying paint? 11. Name five items that can be included in paint to improve its performance or suitability for a certain use? 12. Identify six paints by their binder names? 13. Name four binder /solvent combinations? 14. Name six opaque pigments together with their respective colours? 15. Describe what a laminar pigment is? 16. What is the difference between a convertible and a non-convertible coating? 17. Define the term induction period? 18. Define the term pot life? 19. Define a barrier system of coatings? 20. Define the term tie coat and give an example of its use? 21. What is the difference between thermosetting and thermoplastic? 22. Name two types of paint mill and give two examples of each? 23. What is the title of BS3900 a) How many tests are set in BS3900? b) How are the tests identified (name, number, letter) which? 24. What does a fineness of grind gauge measure? 25. What is flashpoint and in what apparatus is it determined? 26. What is viscosity and what equipment is used to measure the viscosity of a) Free flowing paint b) Thrixotropic paint? 27. What is a density cup and give an example of its use? 28. Calculate the volume solids of a paint it the w.f.t. was 186 m and the d.f.t. was 93 m? 29. Calculate the w.f.t. of a paint if the vs% was 66% and the d.f.t. was 88 m? 57
30. Calculate the d.f.t. of a paint if the vs% was 44% and the w.f.t. was 234 m? 31. Calculate the density of a paint if a 5ltr tin weighed 15 kg? 32. Calculate the density of a two pack material if part A paint was 1.5g/cm 3 and part B catalyst was 0.5 g/cm 3 assume a mix ratio of 1:1? 33. Name an example of a non-destructive d.f.t. gauge? 34. Name an example of a destructive d.f.t. gauge? 35. Name four artificial weathering devices and say what they are designed to simulate? 36. Name two drying / curing test and stare hoe they are operated? 37. What does a cryptometer measure and give two examples of a cryptometer? 38. What determine the degree of gloss a paint may possess? 39. Name three adhesion tests and describe how one of them is carried out? 40. Name eight duties of a paint inspector? 41. eight-contractor 42. List Name three methodsmalpractices? of applying paint and compare the advantages and disadvantages of each in terms of quality and cost effectiveness? 43. What does BS2015 glossary of paint terms refer to and give three examples? 44. What is BS 4800 and how is it used in the construction industry?
45. a) b) c) d) e) 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64.
Define the following Long term protection Medium term protection short term protection new galvanising weather galvanising What is the procedure for removing oil and grease from a surface before preparation commences? What is the procedure for removing oil and grease from a surface if found after preparation has taken place? What is the procedure for removing algae and mould growth from pipework? Under what conditions must paint be stored? What is ‘t’ wash, what is it used for, what colour is it, and how is it checked for correct reaction when using it? What distance must be left at areas to be welded when painting? How much must overlap old repairs? What is the maximum allowed time for priming to take place following preparation and by what method is primer applied? What must all pneumatically and electrically equipment operated power tool be? What type of sheeting may and may not be used for protection against spillage and spotting? What must not be used to clean out paint kettles or to dispose of rubbish? Give the rules for the mixing of paints regarding sizes? What are osmotic blisters and what can cause them? Some metals are difficult to prepare name three and describe the way you might prepare them? Where a pipeline leaves the ground what problems would you find and how might you treat this? Name five paints and state their drying or curing method? Name a black pigment in common use apart from coal tar? What is a holiday detector what does it detect and how does it work? If you worked on a contract where frequent stoppages occurred for poor weather, what additional information would you record?
65. What is important about selecting a paint system suitable for certain environment? 66. State the principles of being able to use a banana gauge for taking d.f.t. readings? 67. Give one advantage of using testex tape and disposable w.f.t. combs?
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ANSWERS PAPER 2 Example answers 1.
Corrosion is an electro chemical process
2.
Problems include: growth of algae, never dries properly, excess electrolytic solution
3.
Oxygen content on the surface, R.H. above 60%, temperature, presence of impurities and higher nobility metals
4.
Millscale is loosely adhering and will flake off and also higher nobility
5. Sis-05-59-00 Rust grades a) Millscale and no rust b) Rust and little millscale c) Rust and light pitting d) Heavy pitting and rust
Blast grades sa1 light blast clean sa2 thorough blast clean sa21/2 very thorough blast clean sa3 blast to visually clean steel
6.
Potassium ferrocynaide, silver nitrate and copper sulphate for millscale
7.
Abrasive – size, shape, hardness and density of blast Human – speed, angle, distance and time of the nozzle
8.
Sis 05-59-00 St2 manual wire brushing St3 mechanical wire brushing
9.
Solvent, binder and pigment
10. Binder- hold particles together, gives finish, gives adhesion, gives flexibility and drying Pigment- gives colour and opacity, resistance to acids/alkalis Solvent- evaporating part of process, thins paint, cleans equipment and degreaser 11. Extenders- improve opacity help flow and lower cost 59
Anti-skinning- to stop paint skinning over Stabiliser- to stop paint separating in can Thrixotropic agent- to enable high build Dryer- to improve drying 12. Acrylic, cellulose, epoxy, emulsion, polyurethane, chlor rubber and alkyd 13. Epoxy – acetone Chlor rubber – xylene Alkyd – white spirit Emulsion – water 14. Red lead- red Zinc chromate – yellow Carbon – black Titanium dioxide – white Calcium plumbate – white Coal tar – black 15. A pigment that has leaf shaped particle that interlock together to form an impervious coating when dry 16. A non-convertible coating undergoes a chemical change upon drying and cannot be damaged by its own solvent, a convertible coating can be damage by its own solvent as this change does not happen 17. The period when a two pack paint must be left after mixing to allow for reactions and escape of air 18. The time 2 pack paint remains usable after mixing 19. A method that isolates the substrate using a low permeable coating 20. Used to tie two incompatible systems together used for when there are adhesion problems, solvent stripping or bleeding 21. Thermosetting cannot be changed once set thermoplastic can be heated and changed 22. Direct charge – ball mill, high speed disperser Pre mix – sand mill, pearl mill 23. Method of test for paints, eight tests, lettered with title 24. Fineness of the pigment grind in paint 25. Is the point at which vapours will ignite measured in an able type cup 26. Viscosity is the measurement of a liquids resistance to flow free flowing paint is measured in ford flow cup and thrixotropic is measured using a rotational viscometer 27. Small alloy cup holding 100ml which can be used for checking if 2 pack paint has been mixed correctly Dft x 100 28. Vs% = ---------------Wft
93 x 100 -----------186
50%
60
100 x dft 29. Wft = --------------Vs
100 x 88 -----------66
133.35m
Vs x wft 30. Dft = ----------------100
44 x 234 ----------100
103m
31. weight --------volume
32. a 1.5 ---b 0.5
15 --5
3g/cm3
2/2 = 1 g/cm 3
2.0 33. non-destructive test gauge is a banana gauge 34. destructive gauge is a paint inspectors gauge (P.I.G.) 35. water soak test – to test permeability Cold box – to test for cold cracking Salt spray – to test under marine environment Temperature box – to check under hot and cold conditions for flexibility
36. Ballotini, which are tiny glass balls, are dropped onto a wet painted panel. The time in hours is given at the side of the panel and where the Ballotini fail to stick to the painted panel drying has occurred. Stylus test, the stylus employs a series of trailing needles which pass over the wet painted panel, because the needles are set at different tensions it can be established when the paint is tack dry, hard dry and fully cured 37. Measures opacity, hiding power charts and pfund cryptometer 38. Fineness of pigment grind, amount and type of binder and application skill 39. Cross cut, cross hatch and dolly test Dolly test a) Clean and degrease the surface to be tested and the dolly contact surface b) Roughen both surfaces with fine/medium grade emery cloth c) Mix regular araldite and stick dolly to surface, leave for 24 hours at 250c d) Cut paint around the dolly down to the substrate using special cutter e) Attach pull-off instrument and apply pull-off force f) Take a reading from position of cursor when dolly detaches. Values will be typically obtained in either Mpa, N/mm 2 or p.s.i. 40. 61
a) b) c) d) e) f) g) h)
Learn specification Check all consumables Check temperatures and humidity Assess condition of substrate Check wft and dft Witness work going on Make reports Ensure safe working is carried out
41. a) b) c) d) e) f) g) h)
using unqualified personnel using unsafe plant incorrect paint incorrect thickness applied re-using expendable shot incorrect storage of paint using paint after specified pot life incorect use of thinners
42.
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