Cargo Tank Coatings

School of Marine Science and Technology Cargo Tank Coating for Chemical Tankers

By: Neofytos Giannakopoulos Student No.: 027022476 Responsible Supervisor: A. P. Roskilly Dissertation submitted to the School of Marine Science and Technology in partial fulfillment of the requirements for the degree of Master of Science (MSc) In Marine Engineering Newcastle upon Tyne Summer 2003

Cargo Tank Coatings for Chemical Tankers

Table of contents INTRODUCTION .....................................................................................................................3

Chapter 1 WORLD FLEET OF CHEMICAL CARRIERS .......................................................................7

Chapter 2 STRUCTURE OF TRANSPORT SYSTEM.............................................................................8

Chapter 3 SHIP DESIGN AND ARRANGEMENTS..............................................................................10 3.1 Number of tanks.................................................................................................................11 3.2 Vertical Bulkheads.............................................................................................................11 3.3 Loading/Discharging System.............................................................................................13 3.4 Tank ventilation .................................................................................................................13 3.5 Cargo heating .....................................................................................................................14 3.6 Tank cleaning System ........................................................................................................14

Chapter 4 MECHANISMS OF CORROSION.........................................................................................15

Chapter 5 THE ROLE OF COATINGS ...................................................................................................18

Chapter 6 COATING COMPONENTS ...................................................................................................21 6.1 Pigment ..............................................................................................................................21 6.2 Resin ..................................................................................................................................21 6.3 Solvent ...............................................................................................................................21

Chapter 7 COATING SYSTEMS AND TYPES......................................................................................23 7.1 Zinc Silicates......................................................................................................................23 7.2 Epoxy coatings...................................................................................................................26 7.2.1 Pure Epoxies .................................................................................................................27 7.2.2 Epoxy Phenolics............................................................................................................29 7.2.3 Epoxy Isocyanates ........................................................................................................30 7.2.4 Cyclosilicon Epoxies ....................................................................................................31

Chapter 8 Mechanical properties of coatings ...........................................................................................33 8.1 Flexibility...........................................................................................................................33 8.2 Toughness ..........................................................................................................................34 8.3 Adhesion ............................................................................................................................34 8.4 Hardness.............................................................................................................................35 1

Cargo Tank Coatings for Chemical Tankers 8.5 Abrasion.............................................................................................................................35 8.6 Slip .....................................................................................................................................36 8.7 Internal Stresses .................................................................................................................36

Chapter 9 MAINTENANCE AND REPAIR ACTIONS.........................................................................37 9.1 Surface pre-treatment.........................................................................................................38 9.1.1 Metal works ....................................................................................................................38 9.1.2. Sand blasting................................................................................................................39 9.2 Abrasives............................................................................................................................41

Chapter 10 COATING APPLICATION ....................................................................................................45

Chapter 11 T he role of ventilation and dehumidification..........................................................................53 1.1 Ventilation during Blast Cleaning .....................................................................................53 1.1.2 Ventilation during Application and Curing ..................................................................55 1.1.3 Dehumidification ..........................................................................................................57

Chapter 12 ABSORPTION – DESORPTION COATINGS CHARACTERISTICS.................................59

Chapter 13 CLEANING CARGO COMPATIBILITY..............................................................................67

Chapter 14 ECONOMIC COMPARISON BETWEEN COATED MILD STEEL AND STAINLESS STEEL .....................................................................................................................70

Chapter 15 Statistical Analysis of measurements.......................................................................................74 15.1 Histograms and mean values ...........................................................................................74 15.1.2 Results analysis...........................................................................................................80 15.2 3D Representation of d.f.t................................................................................................81 15.2.1 Results of 3D d.f.t. representation ..............................................................................85 15.3 Conclusions and proposals (recommendations)...............................................................85 Abbreviations Appendix Glossary References

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INTRODUCTION During the 19 th and early 20 th century, the manufacture of synthetic organic chemicals depended principally on coal, limestone, cellulose and molasses. However, early in this century more attention was given to petroleum because it was found to be cheap, plenty and a rich source of elements required in chemical industry. [Corkhill M. Fairplay Publ.,1981] Following the tremendous growth of chemical industry in our days, the chemical shipping industry carries more than 50,000 different chemical substances and taking into account that the world petrochemical demand will have a growth of 80% by 2010, it is obvious that the demand for more sophisticated and efficient chemical tankers will be increased too.[Advanced Polymer Coating,2002] Furthermore, the seaborne trade in liquid cargoes can be broadly segregated into three main sections as the following fig.1 show’s:

Fig.1: Seaborne trade in liquid cargoes, Source: International Coatings Ltd,2003 We can see that approximately 30% of liquid trading cargoes (chemicals and chemical products) is carried by chemical carriers.

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Cargo Tank Coatings for Chemical Tankers A chemical tanker is defined as a tanker, which carries liquid cargoes except crude oil and cargoes requiring no significant cooling or pressure tanks. The range of cargoes carried by these ships is vast and includes not only chemical products but, commodities such as vegetable oils, animal fats, molasses, wine, solvents and some clean petroleum products and lubricants. Additionally a chemical tanker can carry inorganic substances like sulphuric acid, phosphoric acid and caustic soda. The first chemical tanker was built in the early 1960s and since then they have become a fast growing sector of shipping industry. Those ships can be categorised into three types: •

Parcel Tankers, which have been described as “liquid cargoes” are built up to 40,000 dwt and are able to carry up to 50 different consignments.

•

Bulk Chemicals Carriers, they have similar size to the Parcel Tankers but, they have fewer tanks and they usually carry smaller number of cargoes simultaneously, mainly easy chemicals like vegetable oils or acids.

•

Small Chemical Tankers, generally they are under 10,000 dwt and used in short sea trades. Despite their size these ships can also be highly complex and expensive to be built.[Corkhill M., Fairplay Publ.,1981]

The chemical tanker is a very special type of ship due to the complexity and the particularity of the cargo. And that’s because the cargo is extremely corrosive like methanol, sulphuric acid, caustic soda, acetic acid and virgin naphtha. So most of the times, much attention is given to the cargo tanks and to their ability to ensure the integrity and the purity of the cargo. Many times chemical tanker owners have invested large amounts of money building ships with stainless steel cargo tanks. In general stainless steel is considered to be the ideal material of construction, being non corrosive and easy to clean. However, not all cargoes can be carried in stainless steel tanks. So many vessels carrying chemical substances have cargo tanks made of mild steel coated with special coatings systems able to prevent corrosion of the steel and protect the cargo from contamination by contact with the steel. Of course the coatings cannot stand forever, so a periodically survey and maintenance should be carried out. 4


Cargo tank coatings can be categorized into two main groups: a) Inorganic coatings – zinc silicates and ethyl silicate types b) Organic coatings- epoxy and modified epoxy systems Each of them has good and bad characteristics: Inorganic coatings are one-layer coatings, comprising of inorganic silicates pigmented with high percentage of zinc powder. The paint film is porous so the cargo, after the discharge of the ship, can be completely removed from the coating by evaporation but it cannot be happened the same with high-density cargoes like lube oils. Generally the life of those coatings is proportional to the thickness of the coat. Organic coatings consist of an organic resin system which when is mixed with a hardener it forms a cross-linked array of chemical bonds between the resin molecules. Those types of coatings have the ability to resist in more strong acids or alkalies than inorganic coatings and they tend to absorb significant quantities of cargo and contamination problems can still occur.[Jones D., 2002] Today’s state of coatings can be categorized as: •

Pure epoxy

•

Epoxy phenolics

•

Epoxy isocyanates

•

Alkaline zinc silicate

•

Ethyl zinc silicate

•

Cyclosilicon epoxy [Ackermann N.,1998]

One important factor, which determines the performance of coating, is the curing process and the adhesion of coating on the metal surface. Curing is done in ambient temperature or with high velocity hot air applied into the tanks. Very important factor regarding curing is the time and relative humidity. If we consider a coating system as an epoxy/amine coating where the epoxy is the first component of the coating and the amine is the curing agent, then the epoxy and the amine will react together. As the curing continues the molecules will become greater and they will continue to grow until a “gel” is formed. When the “gel” is 5

Cargo Tank Coatings for Chemical Tankers formed, the epoxy molecules cannot be more soluble to the solvent. This means that the molecules do not move free as previously. When the greatest amount of reaction has taken place and the biggest amount of the solvent has escaped the system then the system can be consider to be cured. The coating system can be further be cured in order to achieve better properties of the coating by increasing the “environment” temperature. In this way we obtain to assist further the molecule mobility and thus further reaction between epoxy and amine molecules. At the end most of the solvent has escaped the system. This heat treatment is known as the “post-curing” and the final film has better properties than the simple cured film. Many case studies have been made about the cost of building a chemical tanker with stainless steel or building it with coated mild steel. The results show that a vessel with mild steel tanks costs almost the half and further more applying the latest coating technology, the income could greater than the one with stainless steel cargo tanks. But the final decision is up to the ship owner to choose the most suitable-reliable coating system for each cargo type. [Advanced Polymer Coatings,2002] At the second part of this project, a statistical analysis of the dry film thickness has been made. The purpose of this analysis was to explain any irregularities of the final d.f.t. by using the given data. This analysis is composed by calculation of mean values of d.f.t of each surface and understanding of the distributions of measurements. Also a 3D representation of d.f.t. has been done for each surface of the tank in order to specify the variation of coating thickness with respect to the distance of measurements The coating applied on a chemical’s tanker cargo tanks (not a modified chemical tanker), which were constructed of flat welded plates and corrugated bulkheads. A three layer epoxy coating system was applied on a blasted metal surface with a roughness profile of 60-80 microns. The air temperature into the tank was 37 C for the first layer, 34 C for the second and 36 C for the third. The values of relative humidity were 33%, 40% and 24% respectively. The data, measurements were provided by a subcontractor. The specified d.f.t according manufacturer’s specifications was 300 microns. The maximum allowed diminution was 10% or 290 microns. 6


Chapter 1 WORLD FLEET OF CHEMICAL CARRIERS As the demand for chemical commodities keeps growing every year, the number of new building chemical carries starts growing too. Thus the number of trading carries has risen from 1859 ships to 2534 into one decade (1992-2002), which corresponds to an increase of 36%. And taking into account that the world petrochemical demand will have a growth of 80% by 2010, it is obvious that the demand for more sophisticated and efficient chemical tankers will increase too. At the following graph 1 this increase can be illustrated:

No. of ships

No. of ships

3000 2500 2000 1500 1000 500 0 1990

1992

1994

1996

1998

2000

2002

2004

Years

Graph 1.1:World chemical carrier fleet, Source: World Fleet Statistics

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Chapter 2 STRUCTURE OF TRANSPORT SYSTEM The structure of the transport system can be represented by the following fig .2.1

Fig. 2.1:Chemical carriers transport system structure, Source: Stopford,2003 On the right side are listed the commodities which appear to be carried by parcels of under 10,000 dwt. On the left there are three fleets which transport these parcels. The fleet of small tankers in which each vessel has a dwt. under 10,000 tones, the fleet with vessels of dwt. between 10,000-20,000 tones and the parcel tanker fleet in which each vessel has a dwt. range between 10,000 tones to 50,000 tones. The parcel tankers are often operated by big companies and offer liner services for large parcels. They arrange transport on a contract of affreightment in order to meet the needs of the trade. However, they also take cargoes from the spot market when the destination fits in the operating pattern and the freight rate is satisfactory. 8

Cargo Tank Coatings for Chemical Tankers The medium size vessels operate on the spot market, grouping together several spot parcels on a voyage basis. Finally the smaller tankers operate on the spot market, picking up whatever parcels are available. These small tankers tend to operate particularly in the region of Europe and Asia. [Stopford, 2003]

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Chapter 3 SHIP DESIGN AND ARRANGEMENTS The design of a chemical tanker is regulated by the IMO Convention on the Carriage of Dangerous Chemicals by Sea. This divides the chemical cargoes and the cargo tanks into three categories: •

IMO Type I

•

IMO Type II

•

IMO Type III

The most hazardous liquids must be carried in Type I tanks. This type of cargoes, when they are released in the sea they build up in marine environment and marine organisms. Tankers approved for the carriage of these chemical must have double bottoms and be located not less than one-fifth of the ship’s breadth from the ship’s sides measured at the water line. So most of the times the Type I tanks are located in the centre of the ship. Type II tanks must also have a double skin to be protected by collision. Type III can be carried in standard tanks. [Stopford,2003] The above arrangements are illustrated in the fig.3.1

Type I

Type II

Type III

Fig.3.1: Cargo tank arrangements with respect to the type of cargo, Source: Polish Register of Shipping, 2001

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Cargo Tank Coatings for Chemical Tankers 3.1 Number of tanks The number of cargo tanks has to be carefully evaluated so as many as possible of them to be filled up to 98% of the volume, during the voyage. There are limitations referring the volume of the tank with respect to the type of cargo they are design to carry. So for a Type I the tank size should have a max. volume of 1,250 m3 and for a Type II should not exceed 3,000 m3 . However, the final dwt. of the ship will play the most effective role in order the number of tanks to be determined. [Corkhill M.,1981] 3.2 Vertical Bulkheads There are four types of vertical bulkheads used in chemical tanker construction: 1. Vertically corrugated bulkheads This bulkhead is very efficient. They can be used to border tanks in both longitudinal and transverse direction. However, when the length exceeds 6 m. additional strengthening should be applied. 2. Horizontally corrugated bulkheads This type has become common for tankers grater than 5,000 dwt. They intend to be installed in only one direction 3. Plane with horizontal stiffeners This type is most popular in larger tankers especially as longitudinal bulkheads with horizontal stiffeners and vertical webs arranged on the outboard side of the bulkhead in the wing tanks. 4. Sandwich or Double skin Bulkheads This type has many advantages in that provides a cofferdam to segregate incompatible cargoes and to give an effective barrier between two cargoes being carried at different temperatures. This type is commonly used for neighbour tanks which are made of different steel (stainless steel, mild steel). Some of the above types are illustrated in the following fig. 3.2

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Fig.3.2: Types of bulkheads, Source: Stopford, 2003

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3.3 Loading/Discharging System The loading / discharging system is consisted from cargo pipelines, pumps and valves. Some requirements should be taken into account during the deign of a such system. 1. The system should have minimum length of pipes 2. The charging of the tanks should be direct 3. The cargo pumps and their position should be considered 4. Closing and safety valves should be taken into account 5. The system should be easily cleaned and accessed Also the types of materials used for the pipelines should be compatible with the cargo type. 3.4 Tank ventilation All cargo tanks should be provided with a venting system appropriate to the cargo being carried and this system should be independent of the air pipes and venting systems of all other compartments of the ship. The tank venting system should be designed so as to minimize the possibility of cargo vapour accumulation above the decks, entering accommodation and machinery spaces. Also entrance of water should be avoided into the cargo tanks and at the same time neither pressure nor vacuum created in the cargo tanks during loading or discharging. Two types of venting systems are used: 1. The open tank venting system. It is consisted of individual vents for each tank 2. The controlled tank venting system. It is a system in which pressure – vacuum relieve valves are fitted to each tank to minimize the pressure or vacuum.[IBC Code,1998]

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3.5 Cargo heating For some cargoes is required to be carried at certain temperatures. For that reason heating coils are installed in the cargo tanks to keep the cargo at a relative temperature. The heating substance is oil or water coming from a heat exchanger, so that the cargo will be carried at a desired range of temperatures. [ExxonMobil, 2002] 3.6 Tank cleaning System The tank cleaning system is very important because the possibility of cargo contamination is depended on great percentage from the cleanness of the tank. A vessel caring two products requires only a simple system. But, a parcel chemical tanker needs a more sophisticated cleaning system, which incorporates: 1. Tank cleaning pump 2. Tank cleaning heat exchanger 3. Tank washing machines 4. Water distribution pipeline The washing solution might be fresh water or chemical substances suitable to clean the tanks at relative temperatures. [Corkhill M. Fairplay Publ.,1981]

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Chapter 4 Mechanisms of Corrosion Corrosion can be defined as “the destruction of a metal by an electrochemical reaction with its surroundings”. Experiments have shown that iron will not rust when it is in dry air, nor in water which is free from dissolved oxygen so both oxygen and water are necessary in the corrosion process. The process of corrosion will be accelerated with the presence of an electrolyte into the solution, especially when it is acid or base. The chemistry of corrosion is described above. In simple terms corrosion can be expressed by the chemical reaction Aa+bB=cC+dD where A is the metal and B the non –metal reactant (reactants) and C, D are the products of the reaction. In other words it is an electrochemical reaction of a metal with its environment. What actually happens can be seen at the following fig 4.1

Fig.4.1 Corrosion kinetics. Source: http://www.marineengineering.co.uk, 2003 The iron reduces to iron ions at anode, the oxygen is reduced by combining with water and electrons passed from the anode (by iron changing to ions) to hydroxyl ions. The oxygen reacts with the Fe2+ to form ferrous oxides (Fe2O3, FeO), which are a reddish brown loose deposit.

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Cargo Tank Coatings for Chemical Tankers Generally corrosion is categorized into two groups: •

‘Dry’ corrosion, where a reaction takes place between the metal and the gas or vapour. The gas could be air, halogen, etc. A characteristic of this type is that the initial oxidation of the metal, reduction of metal and the formation of compound must occur at one and at the same place at metal/non-metal interface.

•

‘Wet’ corrosion, which is the oxidation and reduction of the metal in a solution. The oxidation here occurs at different areas of the metal. In this case what happens is a transfer of electrons through the metal which is the anode to electron acceptor reducer which is the cathode. [Shreir L.L.,1994]

There are many types of corrosion. Some of them are: atmospheric corrosion, chemical, galvanic, stress, electrical, pitting. So corrosion may take a variety of form that range from a uniform loss of thickness to a highly localized attack resulting in pitting and cracking of structure. The environment is an essential feature in determining the corrosion behaviour of metals. Sometimes slight changes can have significant effect on corrosion performance. The following fig. 4.2 shows the effect of PH on corrosion rate of steel.

Fig.4.2Corrosion Rate vs PH , Source: Tomashov N.D.1966 In chemical tankers most of the cargoes are organic and inorganic substances so it is essential to coat them in order to prevent corrosion of tanks. However, due 16

Cargo Tank Coatings for Chemical Tankers to break down of coating system corroded areas and spots may be developed. The following pictures are representative examples of corroded steel of cargo tanks of modified chemical tanker.

Pic.4.1:Corroded metal plaques, Source: Maroudis G., 2003

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Chapter 5 The role of coatings Basically the main purpose of coatings used in marine environment is the protection of hull from corrosion. In operational terms, it is a way of increasing the “life” of a metal structure or plate. In the case of cargo tanks the role of a coating is double. Firstly the coating system has to generate an isolation barrier between the mild steel plaque and the corrosive cargo substance. Secondly it must have very smooth surface to provide easy tank cleaning. Additionally the absorption/desorption characteristics must be good enough so that minimum cargo substance is absorbed, and this quantity should be desorbed in its greatest percentage and the faster as possible. Furthermore the coating system should ensure the following: •

Prevent or minimize the penetration of film coating from any element of cargo.

•

Resist corrosion products that may develop from or on the substrate

•

Form full and continuous film with a high level of integrity and low levels of moisture vapour transmission

•

Performing excellent adhesion and cohesion

Although most coatings are manufactured having the above features, most of the times fail because of several reasons like bad application or weather conditions. So the scope is to minimize the failures and maximize the life of coatings. [Dromgool, 1996] The coating protection mechanism is composed by three sub-mechanisms: First mechanism: By the barrier protection, the dried film blocks oxygen, moisture and any corrosive environment from the metal. However, all coatings cannot prevent by 100% their penetration by the corrosive elements and this property is called permeability. Typical barrier coatings are two-part epoxies (e.g. epoxy amines).

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Second mechanism: Coatings that protect by inhibition contain special pigments to inhibit the corrosion reactions on the steel surface. Third mechanism: Coatings with sacrificing action contain zinc in powder form. Zinc is more active than steel. So if zinc is in contact with steel and this system is present into a corrosive environment, then the zinc will corrode to protect steel. Most of the times zinc is contained in the primer. [Barnhart R.,1997] Depending the kind of coating we are going to use, we define the number of layers that steel is going to be coated. For example, zinc silicate coatings are applied as one layer while epoxies can be applied as two or three. So if we examine a three layer coating system, then each layer will have its purpose. The following fig.5.1 shows the coating system

Fig.5.1: Three layer coating system, Source: Caridis P.,1993 The primer is the first coat applied to the surface. It is very important because it ensures the adhesion of coating on the surface. Surface preparation helps the coating stick by removing contaminants that interfere with bonding and by creating a profile

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Cargo Tank Coatings for Chemical Tankers or roughened surface. Many primers for steel also contain anti-corrosive pigments that actively assist the control of corrosion. The intermediate coat or undercoat, is required in many coating systems and may provide one or all of the following functions: improve chemical resistance, serve as an adhesion coat between the primer and topcoat when the primer and topcoat are not compatible, and increase the thickness of the coating system. The finishing coat is the final coat applied. Topcoats are formulated to improve the chemical and weather resistance of the coating system, and provide characteristics such as: colour, gloss and wear resistance.

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Chapter 6 Coating components Generally the coating is consisted by three components: • The pigment • The resin • The solvent Each one has its purpose and their mixture provide the final coating properties. 6.1 Pigment The pigment is used to distinguish the types and number of coatings. Is a relatively insoluble element of the coating system. It is well known for the colour characteristic, which gives to the coating. Additionally, it assists corrosion resistance, adhesion characteristics and decreases moisture permeability. They can be categorized as inorganic and organic. They are used because they enhance the anti-corrosive coating characteristics and for aesthetic purposes. [Weldon D., 2001] 6.2 Resin The resin plays the most important role at the film formation. It holds the pigment particles together and binds the coating to the metal surface. Also it has significant effect on the durability, strength and chemical resistance of the final film. Additionally forms the final membrane upon which depend many of its basic physical and chemical properties. Generally the coating systems are categorized according the type of resin. [Chandler K., 1985] 6.3 Solvent The primary role of solvent is for application. The solvent provides the coating to be taken out of the can and be applied on the surface, dissolves the film-forming ingredients and provides flow out of the coating once it is on the surface and contributes to the drying, adhesion, of the final film. Furthermore the solvent is the main component, which helps the resin and cure agent molecules to react. 21

Cargo Tank Coatings for Chemical Tankers Additionally the solvents evaporate, in a their greatest percentage, and they are not taken into account as parts of the dried film coating. Some times the coating system might contain more than one solvent each of which has a certain role like to dissolve the resin and control evaporation. [Barnhart R.,1997]

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Chapter 7 COATING SYSTEMS AND TYPES During the last 30 years, several types of coating have been used for tank lining service in the sea trades. Some of these coating materials have stopped to being used for lining materials and new more reliable and flexible in performance have been developed. Today’s typical coating systems can be categorized as follows: 7.1 Zinc Silicates Zinc silicates are generally a two system formulations, consisting of zinc powder which has particles size of 5~9 microns and inorganic or organic binder. The zinc powder may be blended with lead and iron oxides to provide improved spray application properties. The silicate binder may be water based with potassium (inorganic) silicate blend or alcoholic (organic solvent) solution, in order curing take place. Post-cured silicates normally have an aqueous base and require application of a chemical curing solution to harden properly. Self- cured silicates may be aqueous or solvent -based and do not require application of curing solutions.[Goldie H.,1973] Most of the times are applied as one coat, which acts as a barrier between steel and corrosives. However, they are not resistant to strong acids and bases. This means that in practice these coatings are suitable only for cargoes, which have PH range of 5.510. [Haga Per Roar ,1983] Zinc silicates are unusual coatings, are one of the few coatings which are designed so that all of the solid pigment particles are not coated with polymer and all of the gaps between particles are not filled with polymer, i.e. they are designed to be porous films.

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Fig.7.1 Zinc Silicate coating system, Source: Mitchell M. & Summers M.,2002 At the following fig 7.2 we can see that as the percentage of solid zinc is increased the percentage of gaps is decreased.

Fig.7.2 Zinc Silicate coating system with respect the zinc percentage, Source: Mitchell M. & Summers M.,2002 It is obvious that the best performance in chemical resistance will be achieved with the maximum zinc percentage. Coatings, which are water based are the Alkaline Zinc Silicates, they may be composed of water-dissolved sodium silicate, potassium silicate or lithium silicate. The curing of coating occurs by the reaction between the zinc powder (pigment) and the binder silica gel (binder). The binder is supposed to react also with the steel substrate, forming a chemical form that results in outstanding adhesion. This chemical bonding to steel surfaces avoids undercutting of coating. The curing mechanism and coatings formation is supposed to occur in three stages: 24

Cargo Tank Coatings for Chemical Tankers 1. Initial reaction involves concentration of the components by water evaporation. This brings the zinc and silica into close contact, providing a moist coating on the substrate. During this stage, wetting agents in the paint enhance contact of the coating film with the steel surface. 2. At the second stage insolubilisation of the coating film, caused by the reaction of zinc ions with the silicate, and formation of the initial zinc silicate will occur. After this reaction a solid coating on the metal surface has formed. The mechanical and chemical properties are acceptable but the film has a porous structure. 3. The third stage of reaction is composed by the action of carbonic acid formed by the carbon dioxide and moisture on the coating surface. The carbonic acid, when penetrates the coating film reacts with the free zinc particles completing the formation of a dense zinc silicate matrix. Curing or hardening of the coating takes place by hydrolysis of the soluble silicate followed by interaction with the zinc to form an insoluble zinc/zinc silicate complex. For self -cure formulations only atmospheric moisture is needed to complete the chemical reaction since atmospheric CO2 creates carbonic acid with moisture. On the contrary, at the cure of post- cure types, hydrolysis requires application of an acidic solution by spraying it on the coating. The acidic solution (acid phosphate) is sprayed after evaporation of water and forms insoluble silicates and phosphates. The excess acid is removed from the surface.[Jones D.,1992] Generally, the curing depends on the silica to alkali ratio (the higher the ratio the faster the curing), the type of alkaline metal (lithium silicates offer the earliest insolubilisation) and the size of zinc particles (finer particle sizes may cause cracks). Ethyl Zinc Silicates are solvent borne coatings consisting of ethyl silicate and zinc powder. The curing procedure is similar to the Alkaline Zinc Silicates but now instead of water, solvent is evaporated. Despite there are similarities in coating formation there are differences at application. 25

Cargo Tank Coatings for Chemical Tankers So Alkaline Zinc Silicates, cannot be applied by airless spray equipment due to high content of metallic zinc powder, dehumidification and ventilation during the drying and curing stages 1&2 are critical, they are applied in a single coat with a range of 75~125 microns due to crack formation. On the other hand, Ethyl Zinc Silicates can be applied by airless spray since organic zinc silicates have lower content of zinc powder, the drying conditions are less critical with relative humidity be greater than 60%. The main problem is the difficulty of respraying low dry film thickness areas since adhesion problems may occur at the first coat.[Ackermann N.,1998] Although the physical properties (i.e. hardness and abrasion resistance) vary according to the type of silicate used, chemical resistance and cargo compatibility are very similar. These coatings are normally applied as a single coat of 75~125 microns to a blast clean metal surface. They are sensitive to quality of surface preparation and blast cleaning to a white metal finish is necessary (Sa3).[Rogers J.,1971] Generally, the above coatings have an extremely high resistance and tolerance to aromatic hydrocarbon solvent such as benzene and toluene, alcohols and ketones. They are not resistant to acids or alkalis, including sea water which has a slow deteriorating effect. Vegetable oils and animal fats are unsuitable but halogenated compounds are suitable provided that tank surfaces are free of moisture. Any moisture will react with the cargo and release acids, which will damage the coating. Also the cargo should not contain any moisture for the same reason. So it is important that both tanks and cargo will be free of moisture. [Corkhill M. Fairplay Publ.,1981] 7.2 Epoxy coatings Epoxy coatings are generally suitable for the carriage of alkalis, glycols, seawater, animal fats and vegetable oils but, they have limited resistance to aromatics such as benzene and toluene, alcohols such as ethanol and methanol. In other words are blends of polymers of varying molecular weights. They contain curing agents in order to cure fast and they are 75~90 % solids by volume. They have very good chemical resistance and they applied as two or three layers.[Salem Linda S,1996] 26

Cargo Tank Coatings for Chemical Tankers These coatings have a tendency to pick up slight traces of the product carried, especially those chemicals which have only a limited suitability. Alcohols, esters, ketones have a tendency to soften the coating and in this condition the coating is more likely to absorb small amounts of cargo. A “fingernail test” can be used to establish the hardness of the coating. If the fingernail is able to penetrate the coating, it is still considered to be soft, in that case the tank is vented thoroughly before water washing is carried out. Generally these coatings are suitable for the carriage of animal and vegetable oils provided the acid value does not exceed 10 (i.e. free fatty acid content of 5%). However, oils or fats with acid value between 10 and 20 may be acceptable for limited time of carriage. Molasses is acceptable in epoxy provided the PH is above 4, although dilute solutions may become acidic and attack the coating. Such situation is remedied by adding an alkali to keep PH in acceptable level. [Corkhill M. Fairplay Publ.,1981]. Epoxy coating can be categorized according to the resin that they will be mixed as follows: 7.2.1 Pure Epoxies Pure epoxy coatings are based on bisphenol and epichlorhydrin resins reacting, through their terminal epoxide groups, with hardeners having polyfunctional –NH2 groups which are called polyamines. H2N-CH2 CH2-NH- CH2- CH2NH2 [Source: Rogers J., 1971] These reactions allow chain extension and/or crosslinking to occur without the elimination of small molecules such as water. Therefore epoxy resin products have lower curing shrinkage than many other types of thermosetting plastics. There is a wide range of epoxy resins and a great diversity of crosslinked products can be obtained. The chemical structure of epoxy resins consists of epoxy and non-epoxy parts. The non-epoxy part may be aliphatic, cycloaliphatic or highly aromatic hydrocarbon. In practice the reaction products of bis-phenol A and epichlorohydrin dominate the commercial market. The epoxy is a highly reactive functional group and 27

Cargo Tank Coatings for Chemical Tankers can be crosslinked to form a network structure in the presence of curing agents or hardeners[ Virt-u,2003]. The properties of cured pure epoxy products depend on: •

The type of epoxy

•

The type and quantity of hardener

•

The degree of cross-linking

•

The nature and quantity of additives Z Y

Chemical resistance and mechanical properties of epoxy coatings may vary. The factors which, influencing these properties are the molecular weight of resins, the type of hardener (curing agent) and the pigmentation and solvent mixture. Low molecular weight epoxy resins results in coating films with a higher density of three-dimensional crosslinkings as well as a lower number of hydroxyl croups. Therefore, low molecular weight epoxy resins offer better chemical and water resistance than medium molecular weight epoxy resins, which, on the other hand offer better mechanical resistance and flexibility. The most valuable property of epoxy resins is their ability to transform from the liquid state to tough, thermoset solids. The conversion is accomplished by the addition of a chemical compound, the curing agent. Depending the type, curation may occur at ambient temperature or may require post heating. Amine cured agents provide good chemical resistance to epoxy coatings, while polyamide-cured epoxies show more surface tolerance and better mechanical properties. The later are more preferable because they offer superior solvent resistance. Generally this type of coating is used because of its versatility, resistance range and application properties. A pure epoxy coating can be applied by airless spray at medium to high dry film thickness without sagging, cracking or pinholing. However, the maximum overcoating intervals are relatively short (three to five days), requiring a tight application schedule.[Ackermann N.,1998]

28

Cargo Tank Coatings for Chemical Tankers 7.2.2 Epoxy Phenolics They are multifunctional epoxy resins made by the epoxidation of phenolics resins with epichlorhydrin. This type of amine-cured resins result’s in polymers with very high crosslink density, offering outstanding chemical resistance. However, most epoxy phenolic coatings require heating to 50 ~70 C for four to five days to reach their full resistance range. Generally, the chemical resistance of heat-cured epoxy phenolics against strong solvents and fatty acids is better than pure epoxies. From practical point of view, however, heat post-curing poses several problems. To keep the cargo tanks at the required temperature, they must be loaded with an inert cargo (i.e. lube oil) and heated with the heating coils. This procedure is usually insufficient to reach 50~70 C in areas such as deck –heads and bulkheads, requiring the use of auxiliary heaters in the double skin compartments as well as the construction and heating of provisional air casings (void spaces made of staging which trap into blown hot air) on the deck areas above tank ceilings. Today the heating can be easily achieved by blowing hot air into the tanks but it is quite difficult to ensure that all tank areas are kept constantly and uniformly at the required temperature for long periods. It has been observed that without heat treatment, the chemical resistance of epoxy phenolics improves after a service tome of at least three months if only moderately aggressive cargoes are carried, but it does not acquire the full resistance range of heat –cured coatings. Properly formulated epoxy phenolics coatings have application properties similar to pure epoxies but usually longer overcoating times, making recoating less critical. On the other hand, they may create more overspray due to their stronger solvents, which are evaporated faster. A coating system with a dry film thickness of more than 700~ 800 microns, which may occur at critical areas such as angular welding seams on bottoms and ceilings may cause cracking through the whole coating film. Usually this phenomenon appears only after a salt-water test and cannot be detected during application. [Ackermann N.1998]

29

Cargo Tank Coatings for Chemical Tankers 7.2.3 Epoxy Isocyanates Higher molecular weight epoxy resins can be crosslinked with polyisocyanate with polyisocyanate compounds. This reaction occurs at room temperature and the isocyanate reacts with the hydroxyl groups of epoxy resin. So a densely crosslinked structure with excellent chemical resistance is obtained. Cured epoxy isocyanates offer a resistance range similar to heat-cured epoxy phenolics, the only exception is that cannot carry alkaline cargoes with high concentrations. Most cargoes can be carried after a curing of ten days. Very aggressive cargoes such as methanol can be carried after a three-month service period. It has been mentioned the cure occurs at ambient temperature however, epoxy isocyanates are more difficult to apply than pure epoxies or epoxy phenolics and they have more critical application properties. For example, because they need rapidly evaporated solvents, overspray may be a problem and they are sensitive to overthickness. So the dry film thickness of the whole system is small and crack may occur at 150 microns. Most of the times the crack can be observed with naked eye after drying, however some times it is visible only with magnifying lens because the crack does not split the whole coating film. Therefore, it will not result in rusty spots during the salt-water test. [Ackermann N.,1998] Areas usually affected by cracking are angular welding seems and corroded spots (pitting). Stripe coated areas, if overcoated before they are completely dry, can cause cracking or blistering. To eliminate this problem, each coat must be inspected for cracking and defected areas should be repaired. Paint defects such as sagging and orange peel must be also eliminated because they are associated with cracking. These application problems as well as health problems are the main reason for reduced usage of epoxy isocyanates. However, if they are used they can offer excellent resistance to aggressive cargoes, especially in the case of newbuildings. In the case of repair they may not be recommended because at heavily corroded steel it will be difficult to avoid overthickness on pitted areas.

30


7.2.4 Cyclosilicon Epoxies These coatings are based on a new resin, which is essentially a cyclic silicon structure with five epoxidised phenol groups, that are cured by means of catalyst to give a highly crosslinked polymer. In other words these coatings are a two-component paint based on Siloxirane, a patented polymer with an organic/inorganic matrix. More precisely Siloxirane consists of SiO- rings as a backbone forming a homopolymerized thermoset (heat cured) coating resin with high chemical resistance and good mechanical properties. [Advanced Polymer Coatings,2002] The homopolymerized thermoset resin has an oxygen to carbon linkage with high dense and cross-linked molecular structure. Also the absence of –OH eliminates the failure of building other types of polymers. [Keehan Don,2001] Polymerization occurs with transformation of epoxy groups into ethers resulting in this way and after curing in strong ether (carbon-oxygen -carbon) linkages without hydroxyl or ester groups. So in this way we avoid the attack of acid or hydrolysis to the coating system. The combination of a very densely crosslinked structure and strong chemical bonds makes the resin to perform excellent resistance to penetration by the most aggressive solvents, acid and alkaline cargoes. In contrast to the other coatings, cyclosilicon coatings are much more resistant because of the two following main reasons: 1. The higher bond strengths of the Si-O bond, which form the backbone of the inorganic polymer chain make them more resistant compared to the C-C bond strength of the organic polymer chain. 2. The Si-O bonds are already oxidised making them resistant to atmospheric oxygen and most oxidising chemicals. [Andrews Adrian F.,2002] 3. The measured internal stresses give very low values when compared to high solid epoxies. This could be due to the lack of diluents which are often found in epoxies. [Mitchell M. & Andrews A.,2002]

31

Cargo Tank Coatings for Chemical Tankers Manufactures claim that cyclosilicon coatings can resist up to 98% of the seatrade cargoes, including cargoes which are unsuitable for stainless steel. Additionally these coatings have very low absorption characteristics. As a result they can offer significant advantages over conventional coatings regarding the cargo range, cargo handling and tank cleaning. The coating is applied as a two-component paint. It can be applied like a conventional organic coating with partial curing taking place at room temperature, then the curing time will range from four to five days. However, for the full chemical resistance range, heat curing at 80 C for at least eight hours with hot air is necessary. Moreover, the coating system is sensitive to overthickness. Maximum dry film thickness should not exceed 500 microns because of the risk of solvent entrapment or cracking. Also overthickness could be a problem when recoating older tankers with corroded structures since the barrier of 500 microns could be exceeded. The coating systems can be summarised at the following table7.1: Coating Systems for Cargo Tanks Surface Preparation ISO 8501-1 Sa 2.5

Coating System

Minimum Number of DFT stripe Coats Epoxy primer 100 microns 2 Epoxy undercoat 100 microns Epoxy finish 100 microns ISO 8501-1 Sa 2.5 Epoxy phenolic primer 100 microns 2 Epoxy phenolic undercoating 100 microns Epoxy phenolic finish 100 microns ISO 8501-1 Sa 2.5 Epoxy isocyanate primer 90 microns 2 Epoxy isocyanate undercoating 90 microns Epoxy isocyanate finish 90 microns ISO 8501-1 Sa 2.5 Zinc Silicate 80 microns 1 ISO 8501-1 Sa 2.5 Cyclosilicon epoxy 150 microns 1~2 Cyclosilicon epoxy 150 microns ISO 8501-1 Sa 2.5 : Preparation of steel Substrates before application of paints and related products-Visual assessment of surface cleanliness. Table 7.1. Coating Systems for Cargo Tanks: Source: Ackermann N.,1998

32


Chapter 8 Mechanical properties of coatings Generally, the performance of a coating describes how well it is carrying out its function in service. No matter how, good the chemical properties of a coating are, in order to fulfill its function, has also to provide adequate mechanical properties. The mechanical performance of a coating describes how it responds to stresses and strains imposed during service. The main mechanical properties of coating can be categorized as : 8.1 Flexibility Flexibility is the ability of a coating to be bent or flexed in forming operations without cracking, losing adhesion, or failing in some other manner In the case of a ship the whole structure, including cargo tanks, are subjected to deformations during service. Additionally, stresses on the structure are appeared during the loading of ship or when close tanks are charged with different kind of cargoes. The following fig.8.1, show’s the elastic deformation of a chemical tanker’s structure due to uniform loading.

Fig.8.1:Deformed plot, uniform loading, Source: Lloyd’s Register Technical Association, 1995 33

Cargo Tank Coatings for Chemical Tankers So the coating should have a degree of elasticity to remain on the steel surface without any crack to occur. The same elastic behaviour of coating system should be provided during plastic deformation of tank components. The following picture 8.1, show’s that no crack of coating has occurred during the plastic deformation of a stiffener.

Pic 8.1:Un-cracked coating after plastic deformation, Source APC, 2002 8.2 Toughness Toughness can be defined as the ability of a coating to withstand an impact without cracking or breaking. It is dependent on the nature of coating used and on adhesion. 8.3 Adhesion The importance of adhesion is the ability of a coating to resist removal from the surface to which it is applied. Such adhesion can be between substrate and coating, between a primer coating and a top coating or between coatings applied to an existing coating. In addition, the coating must adhere under various weathering and cleaning, usually aqueous, conditions. The mechanism of adhesion problems can be represented on the following fig.8.2

34


Fig.8.2: Adhesive and cohesive failure of coatings, Source: Caridis, 1993

8.4 Hardness Coating hardness is the ability to resist permanent indentation, scratching, cutting, and penetration by a hard object. Different methods of evaluating hardness yield different results because they measure different qualities of the material. There is no absolute scale and each method has its own scale of defined hardness.

8.5 Abrasion Abrasion resistance is the ability of a coating to resist having its original appearance and structure altered when it is subjected to the influence of erosion, rubbing, scraping, or other ablative action. Both temperature and environment can have an effect on abrasion resistance, but the relationship between these factors and interrelated mechanical properties is not simple. For example, hardness and modulus increase with decreasing temperature, and this may be detrimental to abrasion resistance if the coating loses flexibility or toughness. Increases in humidity around an object or subjecting an object to a moist environment as in washing a wall can soften a coating and alter its resistance to abrasion. [ Koleske J.,1997] 35


8.6 Slip Coatings are said to have good slip when they have a low coefficient of friction and poor slip when they have a high coefficient of friction. Slip indicates the ease with which two contacting surfaces can move by each other. Coatings are said to have slip when they have a tack-free surface and behave as if they were lubricated. Slip is an important characteristic of coated tanks for it is the property that allows easy removal of cargo during tank cleaning. 8.7 Internal stresses When a solid coating film forms, a liquid is changed into a solid. While the film is liquid, the coating is mobile and volume contraction takes place. As a solid coating film forms, contraction continues but is restricted by adhesion. As a result of this restriction, tensile stresses develop within the coating. However, as soon as stress develops, the molecules seek to relieve the stress and a relaxation process begins. Therefore, as film development continues, stresses within the film can increase, decrease, or remain constant depending on the rate of stress development and of stress relaxation. It should be noted that stress development begins when the glass transition temperature (Tg) of the changing system is reached.[Marrion A.,1994]. Tg is the temperature where a polymer changes phase from soft form to glassy form

36


Chapter 9 MAINTENANCE AND REPAIR ACTIONS Before each maintenance and repair action takes place, it is essential to ensure safety of people on board the ship. So the owner has to deliver each tank in a clean and gas free condition, the shipyard or the subcontractor should fit explosion-proof, low voltage lighting so that good working light is achieved in all parts of tank during work operations and inspections. Additionally, shipyard should supply and maintain at all times adequate ventilation and proper dehumidification during all phases of blasting, coating and curing.[Advanced Polymer Coatings,2002] In order to eliminate every possibility of damage on steel or on coating the following considerations must be taken into account. Only combination “pipe and cable ladder-type” with expanded metal grate staging, suspended from specially fitted stainless steel overhead lugs or rigid knock down pipe staging, is acceptable. A representation of the used staging can be seen at the below picture 9.1

Pic. 9.1 : Scaffolding erection, International- Marine coatings, 2002 All pipe ends shall be fitted with plastic plugs to prevent grit accumulation within the pipe. Staging is to be of such construction that it can be disassembled and removed from the tank without damage the lining. [Devoe Coatings,1993] 37

Cargo Tank Coatings for Chemical Tankers Items not to be blasted and coated such as heating coils or pumps should be removed from the tank or protected against over-blast and over-spray by coveringwrapping. During the blasting process the best protection is offered by a wrapping in heavy duty rubber or plastic with overlaps and joints on the underside of the coils to prevent dust and blast media collecting in the wrapping. During application the covering should be made of paper material. [Hempel,2002] Finally, suitable protection from water should be provided above the hatch openings of all tanks, which will be lined. So water guards are to be installed to prevent rainwater from draining into the tank. 9.1 Surface pre-treatment 9.1.1 Metal works The highest level of possible surface preparation is strongly recommended in all tank lining work and that’s because the effective life of any tank lining system largely depends on how the surface of the steel has been prepared prior to application of the paint. Before abrasive blast takes place, all sharp edges and welds must be ground to a smooth finish of radius of 2 mm and all traces of rust and dirt to be removed as the following fig 9.1 shows

Fig. 9.1:Sharp edges, Source:International-marine coatings,2002

38

Cargo Tank Coatings for Chemical Tankers Weld metal should be of good quality free from pores, undercuts and notches in order to avoid pinholes and surface irregularities.

Fig.9.2: Undercuts, International-marine coatings,2002 Of equal importance is the removal of surface laminations, which may be present in the steel plate.

Fig.9.3: Plate defects: Source: International-marine coatings,2002 9.1.2 Sand blasting During blast cleaning, a large number of small metal or mineral particles impact the steel surfaces that are being cleaned at high speed. There are several ways of achieving a clean surface with blasting whose differences lie in the size of particles used, the manner of “transportation”, (e.g.air), and in the type and kind of mechanical equipment required. The choice of method depends on the kinds of dirt present and the various types of old, worn, and damaged protective coatings. For cargo tanks, the abrasives are forced on the metal surface by air. The resulting surface should not be excessively smooth but rough enough to achieve satisfactory adhesion of the paint to the metal surface.[Caridis P., 1993] The compressed air used for abrasive blasting should be oil free, be cooled after compression and must not have higher temperature and humidity than the air fed into the tanks by the dehumidifiers. By balancing the ventilation of the tanks, oxidation of the blasted surface is effectively prevented. The abrasive should to be used should be dry, sharp, of good quality with a content of soluble salts which should not exceed a specific limit.[Carboline Int.,1995] After the end of blasting an inspection of steel roughness is done by using specific tools and standards. There are many test methods to measure roughness of the 39

Cargo Tank Coatings for Chemical Tankers surface.

Among

the

most

suited

for

the

field

are

the

comparators.

In ISO 8503, two comparators are specified, one with profiles corresponding to blastcleaned surfaces using grit abrasive (reference comparator G) and one corresponding to blast-cleaned surfaces using metallic shot abrasives (reference comparator S). The nominal values for these profiles identify the limits of the three grades fine, medium and coarse. The following stage is the inspection for contamination of the metal surface by oil, grease or salt substances, which may lead to early lining failure due to osmotic blistering. To detect water soluble salts, the most user-friendly method available today is to dissolve the salts being present at the surface and to measure the conductivity of the water sample. Conductivity is an indication of how well a liquid solution will conduct electricity and is measured in micro-Siemens (µS). The conductivity can through calculation be converted to a corresponding salt content on the surface. [Jotun, 2003] The procedure can be seen at the following fig. 9.4

Fig.9.4: Contamination test, Source: Forsgren A.,2000 First we attach the patch on the metal surface, after that we use a needle to inject deionised water into the patch and finally we measure the solution conductivity. An easy test for detecting oil/grease on a surface is the "water break method", where a drop of water is added on to the prepared surface. The drop will spread out rapidly on the surface if no oil/grease is present, but will remain on the surface in a 40

Cargo Tank Coatings for Chemical Tankers drop-shaped form in the presence of oil/grease. This method is not a standard. [Jotun, 2003] The type and the size of abrasive are important to the work. The abrasive should produce the profile shape and height required to ensure acceptable lining adhesion to the substrate (metal). After the finish of blasting we use suitable industrial-type of vacuum cleaners in order to remove residual grit and dust from surfaces. Also the staging should be vacuum cleaned. Since we have ensure that the metal surface is completely clean and the temperature and humidity of air into the tank the most appropriate we can start apply the coating keeping these conditions constant at the same time. 9.2 Abrasives Abrasives should be chosen with characteristics that minimize shattering and embedment into the metal, which is the most important. Many of the heavy angular grits, particularly the steel grit, can damage an old lining but their disadvantage is that they may embedded in the substrate. Usually for marine applications we use non-metallic abrasives because of their low dusting, low fragmenting and ability not to penetrate the substrate. The most effective abrasive is the one, which produces uniform, jagged tooth with the greatest increase in effective surface area but with a height, which is approximately to 30 % of the final dry film thickness.[Dromgool M.,1996] A projection of a blasted and coated surface can be seen at the below fig.9.5

41


Fig. 9.5 : Blasted and coated surface profile, Source:Rogers J., 1971 However, the quality of blast-cleaning can vary widely depending on types of abrasives, air pressure, distance and angle of nozzle from metal surface and speed of blasting. The following fig.9.6 show’s the usage of several types of abrasives on metal surfaces and the impact direction

Fig. 9.6: Abrasive types, Source:Caridis P., 1993 42

Cargo Tank Coatings for Chemical Tankers Standards of blast-cleaning are described in various specifications laid down by different authorities. For example, the steel preparation according the Swedish Standards Association is: Sa 1: Light blast cleaning When the surface is examined using the naked eye, it has to seen to be free of traces of oil, grease, dirt and lightly attached mill scale, rust, old layers of protective coatings and other bodies. Sa 2: Thorough blast cleaning When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. Any remaining dirt has to be well attached to the surface. Sa 2.5: Very thorough blast-cleaning When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. Any attached dirt has to have the form of light stains in the form of narrow strips or points. Sa 3: Blast-cleaning to visually clean steel When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, old layers of protective coatings and other bodies. The surface should have a metallic shine. In the shipbuilding industry, we often encounter an initial condition A (good condition) or B (rust condition), whereas the required preparation grades are, according to regulations Sa 2, Sa 2.5 and Sa 3. Preparation grade Sa 3 is desirable but requires expensive cleaning installations whereas at the same time it produces an increase in surface roughness beyond the desired level. Thus, in superstructures a 43

Cargo Tank Coatings for Chemical Tankers preparation grade of Sa 2 is often sufficient whereas for underwater regions of the hull and the inner structure of compartments a preparation to grade Sa 2.5 is required. Table 9.1 that follows includes the preparation grades of steel surfaces using blast- cleaning methods, and the symbols used in four different regulations. International Standards Organisation

British Standard BS 4232-1967

Steel Structures Painting Council, USA

ISO 8501 -1 (1998)

National Association of Corrosion Eng. NACE TM-0 1 -70

Swedish scale SS 05 59 00-1988 2

German scale DIN 5592 1977

Sa 1

SSPC-SP7

Brush off

NACE No. 4

blast -cleaning

Sa 2

Grade 3

SSPC-SP6

NACE No. 3

Commercial-blast cleaning

Sa 2.5

Grade 2

SSPC-SP5

Near-white

NACE No. 2

blast-cleaning

Sa 3

Grade 1

SSPC-SP4White metal NACE No. 1

Table 9.1: Blasted surface Standards, Source: Caridis P., 1993

44


Chapter 10 Coating application Regardless of the coating material that is finally chosen the correct implementation of coating is extremely important. It has been mentioned previously that steel temperature and relative air humidity in the tank are the two basic factors, which ensure the correct application of coating. Steel temperature, can be measured by a contact thermometer. It is an electronic instrument to measure the steel temperature. When steel temperature is measured and we have found the dew point, we can determine if it is possible to start the paint application. The steel temperature should always be 3 C above the dew point. A contact thermometer can be seen at the below picture 10.1

Pic. 10.1: Contact thermometer, Source: Jotun, 2003 The dew point is the highest temperature at which moisture will condense from the atmosphere. The dew point is essential to determine if the climatic conditions are acceptable for paint -work. The calculation, can be done from tables or by a so-called dew point calculator. There are two slightly different types available today. They are based on similar principles and consist of two seals, which are set against each other, so that the required information can be read. When you have measured the dry and wet bulb temperature, the dew point and relative humidity can be read from the dew point calculator. 45


Pic. 10.2: Dew point calculator, Source: Jotun, 2003 The relative humidity is measured with a sling hygrometre. The sling hygrometre, picture 10.2, consists of two thermometers, one dry and one wet (wet cotton wool wrapped around the sensor). On rotation, the water in the cotton wool will evaporate, thus cooling the thermometer in ratio to the dry one, which measures the temperature of the air. From the temperature readings, the relative humidity can be calculated

Pic. 10.2: Sling hygrometre, Source: Jotun,2003

In our days, the use of dehumidification (DH) and temperature control systems has been shown to provide a number of benefits during surface preparation and coating/lining operations in a variety of ambient conditions. For example, by creating air dew points well below surface temperature, thus reducing relative humidity at the surface, properly employed DH systems can prevent the development of rust during and after surface preparation and prior to coating application. Properly sized DH 46

Cargo Tank Coatings for Chemical Tankers equipment can also improve worker health and safety conditions by providing increased air flow and oxygen levels, and lower explosive limits and toxicity levels within storage vessels and other closed areas.[SSPC,2003] Since all conditions are satisfied the application of coating may start. The lining material, which is stored under controlled temperature is mixed and thinned in the correct proportion before use, and after mixing must be used within the specified “pot life” set by the manufacturers. To avoid errors in mixing ratios the components are supplied appropriately sized containers. The application of coating starts from the bottom of the tank to the ceiling, because during application the evaporated solvents go to the bottom of the tank. So the air in the tank is both renewed and dehumidified to keep clean atmosphere and steady temperature and humidity conditions.

The used equipment is an air-less spray system, as shown in the following fig.10.1

Fig 10.1: Air-less spray system, Source : Perez A., 2003 The advantages of air-less spray include the provision of a smooth paint film with less change of air entrapment, greater versatility for the operator, less turbulence in the spray pattern and greatly reduced risk of contamination with moisture and oils from improperly cleaned compression equipment.[Goldie H.,1973] The following picture 10.3 shows the coating application. 47


Pic.10.3: Application of coating, Source: PCE,2000 Also important role play’s the sequence of coating application. A brief representation of coating sequence can be seen at the following fig. 10.2

Fig. 10.2: Coating application sequence, Source: Perez A.,2003 If we consider a coating system of two parts (2 coatings), then we apply the first coating to all tank surfaces for a specific dry film thickness. At this stage, as we approaching the ceiling we must cover the tank bottom to avoid any overspray. After the first coat is sufficiently dry the tank should be inspected and any uncoated spots should be coated. However, there are some critical areas where due to their structure it is impossible to achieve the appropriate dry film thickness. For that reason stripe coats are applied with roller or brush. Typical stripe coat areas are:

48


• backs of stiffeners • cut outs i.e. scallops, etc • welds • areas of difficult access (corners etc) • ladders and hand rails • areas of properly prepared pitting Some of these areas are show at the following fig.10.3

Fig 10.3:Key areas for stripe coat applications, Source: Internationalmarine,2003 Some stripe coatings can be seen at the following picture 10.4

49


Pic. 10.4: Stripe coatings Source: Maroudis G.,2002 As the stripe coatings have been applied and inspected we apply the second coating, which has different color from the first, using the air-less equipment. When the coating system has adequately dried then dry film thickness measurements are done using appropriate instruments like the one at the following pic.10.5

Pic. 10.5: Instrument for d.f.t. measurement, Source: Jotun,2003 At the same time hardness of the coating are taken using a hardness-pen and inspection for any uncoated areas is done for the whole tank. The following pictures show how a coated tank looks like.

50


Pic.10.6Coated cargo tank of modified chem. Tanker, Source: Sigma Coatings Marine,2003

Pic.10.7: Coated cargo tank of a chemical Tanker, Source: APC2003 The tanks may be charged with sea -water and then discharged in order any uncoated areas to be discovered. These areas will be corroded by the sea -water. At the last stage all heating coils and pumps are fitted on their initial places. Hot air is applied into the tank continuously to enhance curing of coating. This procedure is called post-curing and it can be done even while the ship is in service

51

Cargo Tank Coatings for Chemical Tankers with these tanks empty. The curing time depends on temperature as the following graph show’s.

Temperature

(C)

Cure Schedule @ Different Ambient Temperatures 50 40 30 20 10 0 3

4

5

6

7

8

Cure Time (Days)

Graph 10.1: Cure time with respect toTemperature, Source: International-marine,2003

52


Chapter 11 T he role of ventilation and dehumidification It has been stressed previously that in order to be a coating system efficiently applied air temperature and humidity should be under control, the same controlled conditions should be achieved during blasting. Temperature determines the maximum amount of moisture which air can hold. When warm air is trapped in the tank, it is cooled by contact with the cold structure of the vessel so the relative humidity level rises until the dew point is reached and water droplets begin to form on the cold surfaces. So in order to avoid the above mentioned phenomenon we try to keep steel temperature few degrees of C above the air tank temperature.[ Doughty P.,1998] There are three primary purposes for ventilating tanks and enclosed areas during cleaning and coating operations: -

operator’s health and safety

-

operator’s visibility

-

curing of coating. [Appleman B.,2000] Ventilation is required during each stage of the process: blast cleaning,

application of coating, and curing of coating. It can be described in terms of airflow and the exchange of clean incoming air and dirty outgoing air. The balancing of incoming and outgoing air is an important feature of a ventilation system. If a high volume of clean air is blown into the tank while a lower volume of dirty air will be extracted then air turbulence will be created. 11.1 Ventilation during Blast Cleaning During abrasive blasting, the air is filled with dust from the abrasive material, which breaks down as it hits the rust-coated surface and from the surface that is being cleaned. The dust creates visibility problems for workers as well as risks to their respiratory systems and to their eyes. In addition, dust that settles on the surface after

53

Cargo Tank Coatings for Chemical Tankers blasting, effect’s the proper adhesion of the coating system. The amount of ventilation or the number of complete exchanges of incoming and outgoing air required during blast cleaning depends primarily on the volume of the tank. Other important factors are the number of blast operators, the amount of corrosion on the tank’s surface, the characteristics of abrasive and surface material, which will be removed. The following fig.11.1 show’s the arrangement for the tank blasting

Fig.11.1:Set-up of blasting equipment, Source: Appleman,2000 There are no specific rules for required air changes. However, some ventilation requirements have been adopted from experience. Some characteristic figures are pointed below. • Spaces of 2,000 ft3 (60 m3) and less shall have an air change every minute. • Spaces from 2,000 ft3 to 30,000 ft3 (60 m3 to 850 m3) shall have an air change every three minutes. • Spaces from 30,000 ft3 to 100,000 ft3 (850 m3 to 2,800 m3) shall have an air change every five minutes. • Spaces over 100,000 ft3 (2,800 m3) shall have an air change every ten minutes. [Appleman B.,2000] 54

Cargo Tank Coatings for Chemical Tankers 11.2 Ventilation during Application and Curing During painting operations in tanks, the air becomes laden with paint overspray and solvent vapour. Like dust from abrasive blasting operations, airborne overspray particles can create respiratory risks for workers. In addition, overspray particles that settle on the surfaces, which are being coated can interfere with the adhesion of the coating. Solvent vapors can pose health risks. They can also create fire and explosion atmosphere. The health and safety hazards presented by these conditions dictate that ventilation requirements must be carefully calculated and that the air inside the space should be subsequently monitored throughout the painting operation. The ventilation rate should sufficient to dilute solvent vapour to 10% or less of the lower explosive limit (LEL) of the specific solvents being sprayed. LEL is the lower limit of flammability or explosiveness of a gas or vapour at ordinary ambient temperature. It is expressed in percent of the vapor in air by volume. Additionally we force the solvents to leave the coating system so that molecules can move easily and further hardening reaction take place. Proper ventilation is obtained with equipment for moving air, directing the air, and the efficient set-up of the equipment. The major air movement components of a ventilation system are fans, ducting, and system layout. A typical ventilation system during coating application and curing is represented below.

55


Fig.11.2:Set-up of ventilation during painting and curing, Source: Appleman B.,2000

The reason of setting the suction pipes near the bottom of the tank is because dust particles and vapour solvents are heavier than air they concentrate at the lower tank height. Additionally as both blasting and painting are applied from the tank bottom to the top, we keep the atmosphere for workers clean.

56

Cargo Tank Coatings for Chemical Tankers 11.3 Dehumidification Dehumidification may be required or desired for three reasons. One reason is that a coating specification may require a maximum relative humidity (RH) that is below the ambient RH of the tank. For example, U.S. Navy specifications allow a maximum RH of 50% during application and cure of solvent-free epoxies. For most shipyards in the USA, Europe, and Asia, the ambient RH will normally be higher than 50%, so dehumidification is needed to meet the specification. A second reason for requiring dehumidification is to prevent condensation on a steel substrate. Condensation will occur when the dew point is at or above the surface temperature. Most coating specifications require the surface temperature to be at least 3 degrees C above the dew point temperature. If these conditions are not met, dehumidification can be used to lower the dew point. These conditions will then allow blasting and painting to proceed. Some shipyards prefer a spread of 6 or 8 degrees C, especially for tanks. In many locations, the surface temperature is less than 3 degrees C above the dew point temperature, so without dehumidification, there would be a substantial risk of condensation. A third reason for requiring dehumidification is that it can create working conditions that can improve productivity. Dehumidification can raise or lower the ambient temperature while reducing the RH in a tank. Coating work on tanks under dehumidification can continue despite cooler ambient temperatures and high RH. Dehumidification can also lower the ambient air temperature, so it can reduce hot and humid conditions inside a tank that make the workers tired. A dehumidification system can be characterized by the following parameters: • volume of airflow • air velocity through the dehumidifier • power requirements • external static pressure • moisture removal capacity • initial and final temperatures. A typical dehumidification system is shown at the following fig.11.3 57


Fig 11.3 Dehumidification system, Source: Appleman B.,2000

58


Chapter 12 ABSORPTION – DESORPTION COATINGS CHARACTERISTICS

Generally the coatings are porous, this means that cargo can physically penetrate the film and be captured into it. The sequent of this behaviour is the possible reaction between the previous and the following cargo, which might lead to cargo contamination. The potential of cargo contamination exists with all types of coating and the rules of Food & Drug ministries of several counties are quiet severe regarding tests for extractable constituents in paint film. [Jackson P.R, 1996] Regarding inorganic coatings (i.e. zinc silicate coatings), very volatile cargoes can be easily removed using evaporation-ventilation techniques from the coating because this type of coating does not absorb large quantities of cargo. However, “heavier” cargoes like lube oil cannot be easily removed from the film. That might cause contamination of the next cargo, especially when the next cargo is a “good” solvent. [Jones D., 2002] The following fig.shows the condition of a zinc silicate coating before and after the cargo storage:

Fig 12.1: Advanced Polymer Science. Absorption characteristics of Zinc Silicate coatings,2002

59

Cargo Tank Coatings for Chemical Tankers The organic tank coatings, despite they are more resistant to corrosive environments they tend to absorb greater quantities of cargo than zinc silicate. The following fig. 12.2 show’s the condition of an epoxy coating before and after the cargo storage:

fig.12.2 : Absorption characteristics of Epoxy coatings, Advanced Polymer Coatings  Despite the fact that organic coatings absorb and desorb cargoes is not new only the last years few projects have been done related to this attitude of the coatings. The main factors influencing absorption-desorption characteristics are: •

Coating thickness

•

Temperature

•

Tank cleaning

Also some coating absorption/desorption characteristics are influenced by water. Some coatings have considerably lower rate of absorption when they are saturated with water.[Woods W.,1994] At the following pictures we can see the difference between a new coating and a coating breakdown.

60


Pics 12.3 &12.4 : New and old coating, Source: Trevor P.,1998 The coating breakdown has a form of blistering, which increases the tendency of the coating to absorb cargo. It has been observed that the absorption rate of a substance into a coating film is rapid and increases in a linear way and then falls to zero when the film becomes saturated. In other words no more cargo is absorbed by the coating. On the other hand the desorption rate is rapid too, at the first stages, and at the end it falls to a steady value. That means than the absorbed substance has not fully escape the film. .[CWA Consultants Ltd, London, 2002] The following fig.12.4 shows the absorption/desorption characteristics of an organic tank lining.

Fig.12.4 :Absorption/Desorption characteristics, By David R. Jones, Oil & Chemical Dept., CWA Consultants Ltd, London

61


It can be seen that absorbed cargo quantity becomes maximum during the first three days and until the 13th day the absorption rate relatively does not change. By the 13th day desorption occurs, and lasts four days. As we can see the desorption rate does not change during the last two days (17-19), which means that an amount of absorbed substance will retain into the coating. If we make a comparison between different types of coatings and one type of carried cargo we can see that different types of coatings show a variation in absorption/desorption characteristics. The following fig. 12.5 shows those variations between pure epoxy, phenolics epoxy and epoxy isocyanate coating

Fig.12.5: .[Absorption/Desorption characteristics, By David R. Jones, Oil & Chemical Dept., CWA Consultants Ltd, London] One of the methods which has been used in order to measure the adsorption and desorption was similar to the Standard Test Method for Water Absorption of Plastics, ASTM D570. According this method coated test panels were immersed into cargo substances. The panels were initially blasted cleaned to Sa 3 roughness level and then

62

Cargo Tank Coatings for Chemical Tankers were coated with appropriate thickness, the coatings were cured according to manufacturer’s instructions. Before the immersion, measurements of panels weight were taken. Thus observing the panel weight change as the panel was immersed, conclusions were made about when the absorption has stopped. Similar conclusions about the desorption were made about the desorption which occurred at 25 C and relative humidity (RH) 50%. The results shown that: •

For several types of coating in various types of cargoes that absorption/desorption curves follow the above mentioned format. Fig12.6

•

Cargoes having small molecules are able to penetrate organic coatings easier than those cargoes with greater molecules. For this reason methanol is one of the most aggressive cargoes.

•

Each coating has different performance at different type of cargo. For that reason careful examination of coating performance should be carried out before the choice of coating.

Fig 12.6: Parry Trevor, Absorption/Desorption Characteristics of Organic Tank Lining Systems 63


It has to be stated that during the above experiment the temperature was constant. For variations of temperature the coating performance is expected to change too. [Parry T. Dec.1998] After some years in service (i.e. 5~10), depending the type and the sequence of cargo, the coating stops having its protective characteristics. The following picture shows the condition of an epoxy phenolic coating used to protect tanks from methanol.

Picture 12.3:Christopoulos N., General Shipping, Destroyed protective coating of a chemical tanker

64

Cargo Tank Coatings for Chemical Tankers The latest technology coatings claim’s outstanding performance. They prevent the film penetration and desorption of absorbed cargo is very small. The molecular size is smaller and the density of links is grater. .[Advanced Polymer Coatings,2002] This can be shown to the following fig.12.7

Fig.12.7: Chemical characteristics and absorption of coatings,Source:Advanced Polymer Coatings,2002 Due to high cross-linked molecule structure of coating the percentage of adsorbed cargo is reduced. Fig 12.8   

NOTE: Percent absorption in parenthesis (0.18%)

  Fig.12.8Chemical characteristics and absorption of coating, Source:Advanced Polymer Coatings,2002 65


So the general considerations, which should be taken into account can be summarized as follow: • The absorption/desorption characteristics of the paint systems differ significantly. Some paints absorb less amount of cargo than others and desorb the cargo more efficiently. The selection of such coating system reduces the risk of contamination. • Allow coatings to desorb as long as possible. The rate of desorption increases as the tank temperature increases. An important point is that continuous ventilation of tank is not as effective as the increased air temperature in the tank. • Avoid the stowage of “sensitive” cargoes such as ethanol, methanol, isopropanol etc., in tanks where incompatible cargoes have been previously stowed.

66


Chapter 13 CLEANING CARGO COMPATIBILITY It is often necessary to clean or ventilate cargo tanks when changing cargoes in order to prevent undesired interactions between cargo residues and the next cargo. Such interactions can form substances that may attack the coating system, enhance danger of steel corrosion and contaminate or discolour the next cargo. For example, when residues of a cargo, which contains ester groups in its chemical composition, may create acetic acid by hydrolysis. Hydrolysis will take place as soon as the residues of a cargo will come in touch with water molecules. This reaction will cause corrosion and may attack the coating. The same will happen when, cargo residues, contain chlorinated hydrocarbons. They can form hydrochloric acid upon contact with water or water containing cargo. To avoid such interactions, all esters and chlorinated hydrocarbons must be transported in dry cargo tanks. Methanol cargoes can be especially problematic. Besides having a softening effect on organic coatings, methanol residues in a coating can cause water vapour permeability, causing osmosis between coating and steel substrate. In addition, methanol can extract residual solvent and low molecular weight materials from the coating. This induces stresses in the coating that can lead to cracking. Only highly crosslinked coatings are resistant to methanol. Most coatings suppliers do not allow transportation of water-containing cargoes after transportation of methanol. [ Berendsen A.M.1998] Additionally cargo-compatibility tables result of collaboration of chemical companies and organizations are available to ensure the cargo purity and corrosion steel prevention. The following table 13.1, has been created by US coast guard and chemical companies:

67


Table 13.1: Cargo Compatibility, Source: Corkhill M. Fairplay Publ.,1981 68


Commodity lists from coating manufacturers commonly indicate which cargoes may be transported in tanks coated with their systems and under what conditions (e.g.,cargo temperatures, transport times, and types of subsequent cargoes). The following Table 13.2, shows the type of cargo according to IMO standards and the temperature range that cargo should be carried

IMO

Product Cargo /

Code Code

3

2

1 3

3

8 9 488 1344 891 645 3510 5 229 9 10 2728 1246 3511 2729 11 2730 1370 12 1370

Temperature

Product

Rating

ACETONITRILE ACETOPHENONE (PHENYL METHYL KETONE) ACETOXYETHANE ACETOXYETHYLENE,1ACETOXYPROPANE,1ACETOXYPROPANE,2ACETYL CHLORIDE ACETYL OXIDE ACETYL TRIBUTYL CITRATE ACETYLBENZENE ACETYLENE DICHLORIDE ACETYLENE GAS ACETYLENE TETRACHLORIDE ACID MIXTURES (NITRATING ACID) ACIDULATED OILS (SOAP STOCK) ACIDULATED VEGETABLE OILS ACINTOL ACRALDEHYDE ACROLEIC ACID ACROLEIN

A A A A A A A A A A A A A A A (80) A (80) A A A A

Table 13.2: Cargo/Product- Carriage Temperature range. Source APC,2003 Guidelines for tank cleaning procedures when changing cargo should be followed carefully to ensure that cargo residues are sufficiently removed before loading a new cargo. Organic tank lining systems can absorb materials from cargoes, and the amounts after different time periods are not well defined. Variable and unpredictable absorption/desorption characteristics are found not only among different coating types but also within the same generic type of coating from different manufacturers. In

69

Cargo Tank Coatings for Chemical Tankers addition, different rates of absorption/desorption are found among different cargoes. This can make it difficult to select the correct cargo tank coating system.

Chapter 14 ECONOMIC COMPARISON BETWEEN COATED MILD STEEL AND STAINLESS STEEL Stainless steels are good materials for chemical tanks, because of their ability to create a passive layer on their surface. A thin, both non- penetrative and protective film is created on the steel surface. This passive layer is mainly consisted by chromium oxide, which is very resistant to corrosive environments. A typical stainless steel composition illustrated in the following Table 14.1

Composition

of Composition %

316LN St. steel Chromium

17.5%

Nickel

11%

Molybdenum

2.9%

Carbon (max)

0.03%

Nitrogen

0.14%

Iron

66% Table 14.1:compasition of St. steel

It has a relative high percentage of chromium (17.5% Cr). So when the metal is exposure to air the chromium (Cr+3) will react with oxygen (O-2) forming the chromium oxide (Cr2O3). The film thickness is generally 15-50 Armstrong [A] and free from pores.[Goldie,1973] However, in some environments like strong hot acids, chloride solutions and generally solutions which contain halogens, the passive film can break down locally and prevention of new film formation can occur. Generally 70

Cargo Tank Coatings for Chemical Tankers the halogens block the atoms of oxygen to react with chromium. Alloys containing molybdenum show improved performance towards this type of attack. [Sheffield A. ,2000] However, stainless steel is an expensive metal, its quality can vary from supplier to supplier, and special skills are necessary for constructing large volumes tanks. Particularly special attention should be given to the welds. [Keehan D., 1996] A case study was made for a new 37,000 dwt double hull chemical tanker in order to determine the cost and profitability when it would have been built throughout with stainless steel tanks or with coated mild steel. The survey initially shown that the tanker with stainless steel tanks would cost 75 million US$, almost the double than the one with coated mild steel tanks. Additionally some assumptions were made: 1.

Both ships would operate 275 days per year.

2.

The cargo would be methanol

3.

The assumed interest over 20 years was 6%

4.

Same operating costs of 6,162 USD

The result shown that with amortization of the financing costs, including the above interest, the 75 million USD ship would represent a daily amortization cost of 23,433 USD, on the other hand the 30.5 million USD ship would have a daily amortization cost of 11,810 USD. Thus the total daily cost would be 29,595 USD for the ship with stainless steel tanks and 17,972 USD for the ship with the coated mild steel tanks. For the above cargo and for a freight rate of 0.69USD/dwt/day there would be generated a revenue of 25,530 USD per day for both ships. So finally we have a daily loss of 4,065 USD for the ship with stainless steel tanks and a profit of 7,558 USD for the ship with the coated mild steel tanks. A summary of the above results can be seen at the Table 14.2

71

Cargo Tank Coatings for Chemical Tankers 37,000dwt double chemical tanker Daily

hull Stainless

Steel

Tanks Coated Mild Steel Tanks

75M$

30.5M$

amortization 23,433 $

11,810 $

operation 275days/year Operating Cost/day

6,162$

6,162$

Total Cost/day

29,595$

17,972$

for 25,530$

25,530$

Revenue/day

methanol and 0.69$/dwt Profit or (Loss)/day

(4,065$)

7,558$

Table 14.2: Economic comparison of steels, Source: Advanced Polymer Coatings,2002 So in order to make profit the stainless tank vessel should be ‘fixed’ at higher freight fates. This would probably lead the vessel out of competition or to be chartered at low freight rates caring few types of cargoes. Finally the case study shown that in order the to generate the same operating profit the freight rare should be increased to 1.09USD/dwt/day or 58%. [Advanced Polymer Coatings, 2002]

72


Chapter 15 Statistical Analysis of measurements As it has been stated at the introduction, measurements of d.f.t. have been taken from all tank surfaces and each reading had a distance of 50 cm from the other. According to the coating manufacture specifications, the coating should have a specified d.f.t. of 300 microns and the minimum d.f.t. should not be below the 290 microns. In other words, the final d.f.t. should not have a diminution more than 10% of the specified. The tank was constructed by three corrugated bulkheads and three flat surfaces, all welded together. (Rough drawings are given in the appendix) The total area to be coated was 440 m2 and tank’s dimensions are given below: Height H= 7.5 m Length L=12 m Breath B= 4.9 m 15.1 Histograms and mean values Since a large number of readings were available we are able to analyze the data by constructing a distribution histogram. This is accomplished by dividing the horizontal axis into intervals of appropriate size and constructing a rectangle over the ith interval with area proportional to the number of observations.[ Hunter W.,1987 ] Also we can define the relative frequency of observation, which is given as the fraction of observations over the population of measurements.

Fi= ni/N Generally the higher the frequency of observations, the higher the number of observations. 73

Cargo Tank Coatings for Chemical Tankers Based on these measurements, we obtain information about the value of mean ( µ or

). The mean value of measurements (x) is given as the fraction of summation

of all measurements over the population of measurements [Jonhson R.,1996 ]

So the results for each surface are given below: Centreline BHD:

Frequency

of

observations

This surface is a corrugated bulkhead. Distribution of measurements on CL BHD 180 160 140 120 100 80 60 40 20 0 290-310

310-320

320-340

340-355

Intervals

Measurement Population N Mean value

N=322

Max value

354

Min value

290

µ=332

Relative Frequency 1st interval

0.034

2nd interval

0.108

3rd interval

0.52

4th interval

0.335

We can see that the greatest percentage of readings, which corresponds to a relative frequency fi of 0.52, is around the mean value. This means that the biggest area of the CL BHD surface is coated with a d.f.t. range of 320 to 340 microns. However, the 74

Cargo Tank Coatings for Chemical Tankers second biggest percentage of observations has a value of relative frequency of 0.33 for the range of 340-355 microns. The reason of this is the weld seams and corners. As the coating dries has a tendency to become thinner due to internal stresses. This phenomenon becomes more intense on sharp edges. For this reason we try to have greater d.f.t. than the flat surfaces. At the corners we try to have again grater d.f.t. than the specified, because on these areas it is difficult the coating to be applied due to the construction. Finally the two lower percentages express the BHD’s rounded areas, as the following fig. 15.1 show’s, where the coating has less thickness due to the above mentioned phenomenon of stresses. Round areas

Fig. 15.1: Rounded areas on a corrugated bulkhead Longitudinal BHD:

Frequency of observations

This surface is flat. The results are shown at the below graph.

Distribution of measurements on Long BHD 180 160 140 120 100 80 60 40 20 0 290-310

310-320

320-330

330-340

340-350

Intervals

75



N=322

Max value

349

Min value

294

µ=332

Relative Frequency 1st interval 0.2248 2nd interval 0.776 3rd interval 0.229 4th interval 0.478 5th interval 0.189

Again we can see that the biggest percentage of measurements is around the mean value and the relative frequency of this interval is 0.478. There is a range of measurements from 340 to 350 microns, which corresponds to weld seams and corners. The relative frequency of this interval is 0.189. The intervals which contain values from 290 to 330 are accepted because the specifications are satisfied. Main Deck plating:

Frequency

of

observations

This is a flat surface. Distribution of measurements on Main Deck Plating 200 150 100 50 0 315-325

325-335

335-345

345-355

Intervals

Measurement

N=230

76

Cargo Tank Coatings for Chemical Tankers Measurement Population N Mean value Mean value Max value Max value Min value Min value

N=230 µ=341 µ=341 356 356 316 316

Relative Frequency 0.030 0.034 0.739 0.195

1st interval 2nd interval 3rd interval 4th interval

Here, the biggest percentage of population has values from 335 to 345 microns. So the greatest percentage of the area is coated with a d.f.t. of a range from 335 to 345 microns. Again values greater than the mean are due to corners and weld seams. Also on the bottom tank there are some areas which are coated using brush, so the d.f.t is increased at these points. The reason is because at these areas there were the scaffolding foundations.

Tank Top: This is a flat surface.

observations

150

Frequency

250

of

Distribution of measurements on Tank Top

200

100 50 0 315-325

325-335

335-345

345-355

Intervals

77

Cargo Tank Coatings for Chemical Tankers Measurement Population N Mean value

N=230

Max value

348

Min value

316

µ=339

Relative Frequency 1st interval 0.030 2nd interval 0.073 3rd interval 0.865 4th interval 0.030

Like the main deck, the tank top show’s a similar type of distribution, around the mean value. As previously, there are points which have high d.f.t. due to welds and corners. Transverse AFT BHD: This surface is a corrugated bulkhead. observations

60

Frequency

100

of

Distribution of measurements on Transvesre AFT BHD

80

40 20 0 295-315

315-335

335-345

Intervals


N=126

Max value

345

Min value

294

µ=335

Relative Frequency 1st interval 0.103 2nd interval 0.18 3rd interval 0.714 78


The greater percentage of the area is coated with a range of d.f.t. between 335 to 345 microns. The d.f.t does not exceed 345 as in other surfaces without to exclude the possibility of existence of these values. Values less than 335 are due to areas of bulkhead which have been explained previously. Transverse FWD BHD:

Dstribution of measurements on Transverse FWD BHD 100

of

80 60

Frequency

observations

This surface is a corrugated bulkhead.

40 20 0 295-315

315-335

335-345

Intervals


N=126

Max value

345

Min value

294

µ=336

Relative Frequency 1st interval 0.039 2nd interval 0.13 3rd interval 0.825

Again the greater percentage of the area is coated with a range of d.f.t. between 335 to 345 microns.

79

Cargo Tank Coatings for Chemical Tankers 15.1.2 Results analysis If we examine the formats of the first four graphs we can reach to the conclusion that the distribution of measurements is a type of normal distribution. However, the distribution is not symmetrical as in regular normal distribution, having a symmetrical “Bell” shape format around the mean value, fig15.1

Fig 15.1: Normal distribution curve, Source Johnson R., 1996 The graphs have a “Bell” shape format, which is displaced to the right. This type of distribution is called “long tail to left” or “skewed” normal distribution. [Jonhson R.,1996] Despite the format of the two last graphs, if we examine the mean value for FWD BHD and AFT BHD we can see that in both cases it is 335 and 336 respectively. However, this is the lower limit of the third interval and because this interval has the biggest relative frequency 0.71 and 0.82 respectively, the distribution of the last two graphs is also “long tail to left” normal distribution. Generally we can see that all graphs have a displacement to the right. The main reason is that the average d.f.t of the whole tank, it is greater than the specified value of 300 microns. Generally the higher the film thickness, the longer the “life” time of a coating. So in this way we can explain the displacement of the mean from 300 to 340.

80

Cargo Tank Coatings for Chemical Tankers Also at the critical points and areas, where more attention is needed, In the d.f.t. has to have higher values than the average to avoid or minimize the possibility of coating failure. One important factor, which should be taken into account regarding the increased d.f.t. and the possibility of solvent entrapment into the film, is the climatic conditions. The high weather temperatures and relative low values of relative humidity decrease the solvent entrapment. The above ship’s tanks were maintained in Greece during summer time, where the above conditions are satisfied. Additionally these vessels are equipped with ventilation systems, so they can dry the coating while they are in service, leaving empty the fresh coated tanks. 15.2 3D Representation of d.f.t. The following 3D graphs show the thickness variation of the dried coating system. The vertical axis indicates the value of d.f.t and the other two the coordinate system of measurements. The areas coloured gray are the metal surfaces. Centreline BHD: D.F.T variation on CL BHD

350-370

350

330-350

330

310-330

310

290-310

290

270-290 250-270

270 250

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150

Value of D.F.T.

370

X

Coordinate

650 500 350 200

Y

Coordinate

50

81

Cargo Tank Coatings for Chemical Tankers From the graph we can see the variation of thickness between 322 measured points on this surface. The “valleys” which are shown on the graphs represent the rounded points, which were explained above. The pick points indicate the weld seams and corners. Also the thickness around the bulkhead’s surface, which is welded with the other BHD’s and plates, is almost equal to the thickness on welds.(330~350 mic.) Longitudinal BHD: D.F.T. variation on Long BHD

Value of D.F.T.

350 340

340-350

330

330-340

320

320-330 310-320

310

300-310

300

290-300

290

280-290

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150

280

X

Coordinate

650 450 250

Y

Coordinate

50

Here the pick points represent the welds on the surface. Since this surface is flat is easier to distinguish these points. Again areas close to the welds with the other BHD’S and welds have similar thickness to the thickness on the welds.

82

Cargo Tank Coatings for Chemical Tankers Main Deck Plating:

D.F.T variation on Main Deck Plating

Value of D.F.T

360 340

340-360 320-340

320

300-320 280-300

300 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150

280

X

Coordinate

450 350 250 150 Y Coordinate 50

Here the 3D surface is smoother than the other two. The main reason is that this area is easier to be coated. However, there are some pick points. These points are areas where have been coated with brush. At these areas there were the scaffolding foundations, which have been removed at the last stage of coating inspection.

83

Cargo Tank Coatings for Chemical Tankers Tank top plating:

D.F.T. variation on Tank Top

Value of D.F.T

350 340

340-350

330

330-340

320

320-330

310

310-320 300-310

300

290-300

1050

1150

850

Coordination

450 350 250

950

650

750

450

550

250

350

50

X

150

290

150 50

Y

Coordination

This is also a smooth surface like the main deck plating but there are no such great picks on this surface. Transverse AFT BHD: D.F.T variation on Transverse AFT BHD

340-360

Value of D.F.T.

360

320-340

340

300-320

320

280-300 260-280

300 280

250 150

650

450

350

150

Coordinate

350

550

Y

450

250

50

260

50

X Coordinat e

Like the CL BHD we observe valleys and picks for the reasons discussed previously.

84


Transverse FWD BHD : D.F.T. variation on Transverse FWD BHD

Value of D.F.T.

350 340

340-350

330

330-340

320

320-330

310

310-320

300

300-310 290-300

290

280-290

250 150

650

Coordinate

550

350

250

450 350

450

Y

150

50

280

50

X Coordinate

Again we have the phenomenon of valleys and picks as it has been observed previously on the other two corrugated bulkheads. 15.2.1 Results of 3D d.f.t. representation Regarding the flat surfaces like main deck plating, tank top and longitudinal bulkhead, we can see that the variation of thickness does not change a lot due to the format of the plate. The only exception is the main deck plating where we have the scaffolding support areas. At the weld seams, thickness is also increased to avoid possible rapid damage of coating. The same increase is observed at the weld points with the other bulkheads or plates. On the corrugated bulkhead surfaces we observe the phenomenon of “valleys” and picks. The “valleys” are explained as a result of round areas of the bulkheads, where we have thinner coating due to internal stresses as it dries. 85

Cargo Tank Coatings for Chemical Tankers The pick points appearance is explained as a result of weld seams and weld points with other plates or bulkheads. 15.3 Conclusions and proposals (recommendations) By doing the above literature review we reach to the conclusion of how important a coating system is for a chemical tanker. However, in order a coating system to be selected many factors should be taken into account. The most important are listed below:  Chemical resistance of coating system  Cargo compatibility  Cargo sequence  Absorption and desorption characteristics of coating system  Mechanical properties Also the conditions into the tank, during blasting and coating, are very critical. The stages and elements, which need special attention are:  Selection of appropriate abrasives with respect to tank condition  Monitoring of air temperature and humidity into the tank during blasting  Application of coating system and selection of sequence, with respect to time  Monitoring of temperature and humidity into the tank during application and curing time  Inspection of areas and points where poor thickness is expected due to surface irregularities and construction difficulties.

By doing the statistical analysis we come to the conclusion that in order to maximise the life of a specific coating system we can increase the thickness. This could be done by spraying an area more time or from a closest distance. The displacement of the mean value to the right (see bar graphs) show’s the above mentioned increase. 86

Cargo Tank Coatings for Chemical Tankers Additionally the increase of thickness reduces the possibility of coating failure, regarding chemical resistance. However, by increasing thickness we increase the amount of solvent, which will be contained into a coated area. So if we assume that we have two areas coated with the same coating system, but different thickness, then for the same time of drying the coating which is thinner would have better properties than the thicker due to the greater percentage of solvent evaporation. In other words the thinner coating system will have been better cured. In this way we conclude that in order to achieve a better coating system with respect to the thickness we would like to achieve, temperature and humidity are important aspects. An integrated temperature and humidity system could help to optimise tank conditions. This system could also be installed on board the ship to control the above mentioned conditions and it can be combined with the already existing ventilating system, which is used for cargo tanks. So the ship can be in service while some of her fresh coated tanks a will still be post-cured. A simple drawing of the system is represented below:

87


“In” and “Out” arrows show air circulation. Some temperature and humidity sensors can be attached on the tank plates using magnets in order to monitor air temperature and humidity. These sensors can be removed after the completion of curing. The readings, which we are going to take, can be compared to the desired values and so, the incoming air quantity, its temperature and its humidity can be steady and optimum. The optimum conditions can be defined by the coating company taking into account parameters like type of coating, weather conditions, size of tank. Generally we can achieve better performance of coating since we will improve its resistant characteristics even at the areas where the coating is poor.

88


Abbreviations d.f.t.: Dry film thickness RH: Relative humidity DP: Dew point CL BHD: Centre line bulkhead Long BHD: Longitudinal bulkhead AFT BHD: After bulkhead FWD BHD: Forward bulkhead

89


GLOSSARY ACID: Any chemical compound containing hydrogen (-OH) capable to be replaced by positive element. ALCOHOLS: Organic substance, which is characterized by the presence of hydroxyl (-OH) in the chemical structure. ALKALI: A strong base. A substance, whose water solution yields a great volume of hydroxyl ions. AMBIENT CURE: A curing reaction which takes place at the temperature of site. AMINES: Substances derived from ammonia ANIMAL OIL: Oily material deriving from animal substances. BINDER: Component which provides the cohesion of a coating. CROSSLINK: That part of a coating system, which links chains, often formed previously, to one another. CURE: The process by which a freshly applied coating becomes intractable. DEW POINT: The temperature at which the liquefaction of a vapour begins. ETHANOL: Organic substance, which contains on its chemical structure two molecules of C and one –OH group. EPOXY: Epoxide. A polymer or reactive species containing epoxy or oxirane groups

FATTY ACID: Acid which is derived from fats or oil. HUMIDITY: The percentage of water evaporated in the atmosphere. IBC: International code for the construction and equipment of ships carrying dangerous chemicals in bulk. IMO: International maritime organisation. KETONES: A class of organic compounds produced by the oxidation of secondary alcohols. METHANOL: Organic substance which contains on its chemical structure one molecule of C and one –OH group. 90

Cargo Tank Coatings for Chemical Tankers PH: An indicator which specifies the acidity of a solution. Acidous solutions have PH<7, alkaline have PH>7 and those who have PH=7 are called neutral. PHENOL: Hydroxybenzene PIGMENT: A powdered material used in coating compositions to impart various properties. POLYMER: A high molecular weight species formed from many monomer residues. POST CURE: A procedure of drying a coating using hot air in order to maximize the evaporation of solvent from the coating system. The characteristics of coating are improved. PRIMER: A coating applied to a bare substrate to prepare it for subsequent coats. RESIN: A film forming material, usually polymeric or oligomeric. SOLVENT: Products used in chemical industry to dissolve other substances. THERMOSET: A polymeric system which can undergo a chemical crosslinking process, usually on heating. TOP COAT: The last layer on a coating system. VEGETABLE OIL: Oil derived from plants. VISCOSITY: A measure of the internal friction or the power in resisting a change in the molecular structure of a substance. VOLATILITY: Liquids which evaporate readily are known as volatile liquids.

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References Ackermann N.,(1998), “Choosing the Correct Coating for Cargo Tanks”, April 1998, In: PCE vol. 6 no13,pg 44, ISSN 02453-469. Appleman B.,(2000). “ Ventilation and Dehumidification of ship ballast tanks for blasting and coating work”. In: PCE, v 5, No 4, April 2000, pg 43-50, ISSN 02453469. Andrews Adrian F(2000).” Polysiloxane topcoats – product choice for optimum performance”, International Protective Coatings, Akzo Nobel, England. Barnhart R.(1997), “How Coating Protect Steel”, In: JPCL, May 1997, p 26-29, vol 14 ,No5, ISSN 8755-1985. Berendsen A.M.(1998) “ Ship painting: Current practice and systems in Europe” In: PCE, Sep. 1998, Vol.3 No 9,pg 24, ISSN 02453-469. Carboline International, Carboline Phenoline 373 system, coating specification ,1995. Caridis P.,(1993) “Inspection, repair and maintenance of ship structures”: Protection of the hull structure using coatings and hull preparation, Witherby Publishers,1993, pg 300-331,ISBN 1856091910. Chandler K., (1985) “Marine and offshore corrosion”, Butterworths,1985, pg 208, ISBN 0-408-01175-0. Chistopoulos N.,General Shipping, Service director, Greece. Corkhill Michael,(1981).”CHEMICAL TANKERS the ships and their market” Fairplay Publications, introduction, pg 4. ISBN 0-905045-02-5. Devoe Coatings, Devchem 253, coating specification, 1993. Doughty P.,(1998). “Moisture control”. In : The Motorship, v 79, No 933, April 1998, pg 72. ISSN 00721-456. Dromgool M.(1996), “Maximizing the Life of Tank Linings”, In: JPCL, March 1996,vol 13, no 3, pg 62-74, ISSN 8755-1985. ExxonMobil (2002) “Marine Environmental & Safety Criteria for industry vessels in Exxonmobil service”, Exxonmobil, p 45. Forsgren A.(2000), ”Detection and removal of hard-to-see surface contaminants” In: PCE, vol.5 ,No6, June 2000, pg64-66, ISSN 02453-469. 92

Cargo Tank Coatings for Chemical Tankers Goldie H. (1973). “High performance materials and coatings for cargo and storage tanks”, Joint Corrosion Conference, 1973, London, ppr. No 4, pg 63~73. Haga Per Roar (1983), “The state of the art of tank coatings: getting-maintaining a good coating”.In :Shipping News Int.11/1983,vol 39,no 15, pg 34 ISSN 0029-3709. Hempel, Technical standard for tank coating work, Newbuilding and repair,2002 Hunter B.,(1978), “Statistics for Experimenters”, Wiley J., pg26, ISBN 0-471-039157. IBC Code(1998), IMO, “International Code for the construction and equipment of ships carting dangerous chemicals in bulk”, 1998, p36, ISBN 92-801-1456-5. Jackson P.R.,(1985), “Epoxies advance to combat widest range of aggressive cargoes”, In: MER, v 7, No13,p 46 ISSN 0067551. Jones D.,(2002), CWA, Consultants Ltd, Jones David, Oil & Chemical Dept., London http://www.gard.no [the role of tank coating in cargo contamination claims, full text, confirmed 10-10-2002]. Jones D.(1992). “ Principles and prevention of corrosion”, Ch. 14: Coatings and Inhibitors.1992, pg 484.ISBN 0-13-359993-0. Johnson R. (1996), “Statistics Principles and methods” Wiley J., pg 224-225, ISBN 0-471-04194-7. Keehan D. (2001).”Aerospace technology, meeting the anti-corrosion needs for chemical & products tanker ship owners” In :HSB ,1/2001,vol 49,no11, pg 44,ISSN 0923-6661. Keehan D, (1996), “A viable coating competitor to stainless steel?” In: MER, June 1996, vol. 15, no.10, p 44-46, ISSN 0067551. Koleske J.,(1997), Encyclopedia of Analytical Chemistry :Mechanical properties of solid coatings, Wiley & Sons, 1997, pg 400-415, ISBN 0471 97670 9. Lloyd’s Register Technical Association, (1995), Modern Chemical Tankers, Paper No 5, Session 1994-95. Maroudis G., Naval Architect , Greece. Marrion A., (1994), The chemistry and physics of coatings: Film formation, Chapter 5, PSC, 1994, pg 49-51, ISBN 0-85186-994-7. 93

Cargo Tank Coatings for Chemical Tankers Marine Engineering, (2003), “http://www.marineengineering.co.uk” [Corrosion, confirmed 25-4-2003]. Mitchell M. & Andrews A.(2002),”Inorganic Hybrid Coatings – Their Position, Importance and Future”, International Protective Coatings, Akzo Nobel, England. Mitchell M. & Summers M.(2002), “How to select zinc silicate primers”, International Protective Coatings, Akzo Nobel, England ,2002 Perez A.(2003) “Future Naval Capability: Total ownership cost reduction, Corrosion control technologies”, Office of Naval Research, Material S&T Division, US Marine Corps. Polish Register of Shipping,(2001), “Rules, Hull surveys of chemical tankers”, Publication no. 46, 2001, Polish Register of Shipping. Rogers J. (1971).”Tank coatings for chemical cargoes”. Trans. I. Mar.,1971 London, Vol.83, pg 139~147. Salem Linda S. (1996) ‘ Epoxy for steel’. In: JPCL. Sept .1996, vol 13 ,no 9,pg 77, ISSN 0032-4593. Sheffield A. (2000) “Chemical Tank Protection”, In: MER, June 2000, vol.11, no.5, p31-33, ISSN 0067551. Shreir L.L,(1994), Ra Jarman, GT Burstein Corrosion 1, “Metal/Environment Reactions 3rd ed”.. Corrosion as a chemical reaction at a metal/environment interface, Wiley & Sons, pg1: 7~1:8,1:11~1:12, ISBN 0-9055360-38. Stopford (2003), “Maritime Economics: Chemical Parcel tankers”, 2003, p 409, ISBN 0-415-15309-7. Tomashov N.D.,(1966), “Theory of corrosion and protection of metals- The science of corrosion”, Butterworths, pg 300,314, ISBN 65-16566. Trevor P., (1998), “Absorption/Desorption Characteristics of Organic Tank Lining Systems”, In: PCE December 1990, vo.3, no.12,p 29-29,ISSN 02453-469. Weldon D., (2001) “Failure analysis of paints and coatings”, Wiley, 2001,pg32 ISBN 0-471-49072-5. Woods W., (1994), “Tank Coatings-An Absorbing Subject”,In: Ship Repair and Marine Maintenance, March 1994 vol6, No4, New Orleans, LA, USA. 94

Cargo Tank Coatings for Chemical Tankers World Fleet Statistics (1992-2002), “World fleet of chemical carriers”, table 8, 1992-2002, ISBN 1900839504. Web sites: Advanced Polymer Science (2002), http://www.adv-polymer.com [full text, confirmed 10-10-2002]. International Coatings, (2003), http://www.international-marine.com [full text, confirmed 10-2-2003] Jotun marine coatings, http://www.Jotun.com [full text, confirmed 3-4-2003] Sigma Coatings, http:// www.sigmacoatings.com [full text, confirmed 17-5-2003]

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