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The underwater and boottop are among the most critical areas of a ship for painting. Practice (Photos courtesy of Camrex Chugoku Ltd.) COMMON METHOD OF Current shipbuilding practice is and Systems to clean the steel in a centrifugal s long as blasting and then imme- sail the seas, Athey will need protec- diately apply a weldable shop tion from the corrosive environment . This process is normally in which they operate. automated. in Europe This article presents an overview After mechanical treatments, of current practice in the selection such as rolling flat, cutting to size, and use of marine . It looks bending, stretching, and , at recent developments in the shop-coated plates and profiles By A.M. Berendsen systems for ships and describes the are welded into block sections, Coatings Consultant most critical parts of a ship for which are transported to the slip- Moordrecht, coating protection, including the way for assembly. underwater/boottop areas, ballast Unlike earlier days, many yards The Netherlands tanks, and tanks. Particular now build ships completely under attention is given to the role of anti- cover. Consequently, the yards are corrosive and antifouling systems less dependent on the climate, for the underwater hull and boottop which is a considerable advantage areas and recent developments in for preventing premature antifouling protection. Also includ- during construction. It also means ed are other areas of a ship such as secondary surface preparation (i.e., the topside and superstructure and cleaning of corroded areas, welds, the decks. or damaged and burned areas) is At the outset is a summary of limited, and since power clean- current shipbuilding practices, ing can be used for this purpose including the role of shop primers instead of total blasting, the result and methods of painting a ship dur- is less dust emission and consider- ing assembly. able savings of costs.

24 PCE September 1998 Copyright ©1998, Technology Publishing Company Shop Primers • have little or no effect on the active corrosion-inhibiting pig- The main function of a prefabri- homogeneity and strength of welds; ments, such as and calcium cation, preconstruction, or shop • not emit noxious or toxic fumes phosphates. Zinc (potassium) chro- primer is to protect steel against during welding and flame cutting (a mate, which has better anticorro- corrosion and during safety certificate is required); sive properties, is generally no the building stage. It usually func- • be able to withstand rough longer used in Europe and other tions as a base for the final coat- mechanical handling of the steel, parts of the world because of the ing system. However, it some- including bending; release of dangerous fumes during times is removed by blasting first, • be suitable as the base for the welding and because it could form such as when the steel has cor- final coating systems; blisters in underwater systems. roded during construction or • be highly resistant to water and Ethyl silicate shop primers are when the shop primer is too compatible with cathodic protection pigmented with zinc dust, which is weathered to form a base systems (resistant to alkaline condi- added to the solution shortly for the coating systems. In cargo tions); before application. Most of these tanks for aggressive chemicals, a • have no adverse effects on the products are not zinc-rich but have shop primer must be removed to environment during application and a reduced zinc content. This is ensure full chemical resistance of use; and necessary for satisfactory perfor- the final coating system. • be approved by classification soci- mance in automatic, semiautomat- eties. ic, and robotic welding processes. The dry film thick- Moreover, primers with a reduced ness (DFT) of shop zinc content produce less haz- primers is a compro- ardous fumes during welding and mise between various also less zinc salts during weather- requirements (i.e., ing, which means less risk of weldability, limit of osmotic blistering. Consequently, welding fumes, weld they provide better recoatability. quality, and drying Reducing the zinc content also time), which all reduces the corrosion-inhibiting require less thickness, properties, but they generally are and corrosion protec- sufficient to protect steel during tion, which requires building, especially because many more thickness. The ships are built under cover. How- An aesthetic topcoat usually is applied to the topside and superstructure for good appearance. prescribed DFT for ever, the best option would be to most shop primers is develop a zinc-rich shop primer Shop primers should meet the fol- 15-25 µm. with good automatic welding lowing requirements:1 For shipbuilding, the most com- properties or to develop a suitable • provide adequate corrosion pro- mon types of shop primer now are automatic welding method for such tection during shipbuilding; based on unsaponifiable binders, primers. • be sprayable in a continuous thin such as two-component Since epoxy shop primers have film of uniform thickness; and partially hydrolysed ethyl an organic binder, they generate • have a very short dry-to-handle silicates. Reinforced primers large amounts of fumes during time for transportation of steel parts based on polyvinyl butyral/phenolic welding and flame-cutting, which by conveyor rollers, magnetic resins are no longer used because can burn a large area of primer in cranes, or vacuum hoists without they contain chromate, which pro- the process. In addition, large heat- damage to the primer; duces hazardous fumes during affected zones adjacent to the burnt • be fully compatible with advanc- welding or cutting, and because areas and on the back of the steel ed welding/cutting processes (there they are not fully compatible with are an unreliable base for a paint should be no need to remove the cathodic protection systems. system. Therefore, these areas must shop primer first, and it should not Epoxy shop primers are - be thoroughly pretreated again (sec- affect the speed of these processes); ed mainly with oxide and ondary surface preparation) by

Copyright ©1998, Technology Publishing Company PCE September 1998 25 blasting or power tool cleaning. a ship, they are painted in one of to take control of their environment Zinc silicate shop primers, which two ways: and to handle advanced -free are totally inorganic after curing • as soon as possible after assem- and water-borne systems, even in and very heat-resistant, show less bly, partially or completely (the winter. In practice, however, if damage around welds and cuttings. “block painting” or “paint-then- building halls are not climate-con- Therefore, they need less secondary weld” method), in which each trolled, delays may be expected after surface preparation, which limits block is painted prior to welding of a change in the weather when no costs. These facts, coupled with the blocks to form the ship struc- coating work can be done for sever- their better resistance to cathodic ture, or al days because of condensation. protection, explain the popularity of • after a long period, e.g., after The main disadvantage of block zinc silicate shop primers. erection on the slipway (the “build- painting is a risk of mechanical In an effort to decrease the usual- ing in the shop coat” or “weld- damage and burns during transport ly high levels of volatile organic then-paint” method), in which the of the sections to the slipway and compounds (VOCs) found in shop shop-coated blocks are welded into during fitting-out. However, this primers, water-borne products the ship structure and painted later. damage can be minimised by ade- based on alkali silicate or epoxy The main advantages of block quate planning during the con- have been devel- painting are that premature corro- struction phase. Subsequent pre- oped, but they have not yet found sion and weathering of the shop- treatment and painting of the sec- wide acceptance. primed surfaces are avoided and tion welds is unavoidable, of that the final coating systems can course, and these areas must be left Assembly and Painting be applied under cover. In theory, unpainted during block painting. After assembly of the sections of block painting allows shipbuilders Premature damage of the final paint systems plays no role when building in the shop coat. The main Keywords disadvantage of building in the • Epoxy mastic: a surface-tolerant modified high- epoxy coating suitable shop coat is that shop-coated steel for application over aged coatings is subject to weathering and corro- • High-build coating: a coating that can be applied in thick layers without sag- sion during construction of the ging on vertical surfaces ship. Another disadvantage is that • High-solid(s) coating: a coating with a reduced level of classified as final painting must be done in a volatile organic compounds (VOCs), which contribute to late stage of the building procedure. • Interval-free epoxy coating: a modified epoxy coating that does not have a This may be more difficult from an accessibility viewpoint, or it may be limited overcoating time undesirable with respect to other • Modified epoxy coating: a coating based on an epoxy resin that is modified with activities or the spreading of paint another binder (e.g., a coal tar or hydrocarbon resin) to make a less expensive dust. However, building ships in the coating or to improve properties like water impermeability or recoatability shop coat completely under cover • Reinforced coating: a coating that has special materials added, such as hard minimises corrosion considerably. fillers or glass flake, to improve abrasion resistance, mechanical strength, flexi- bility, impermeability, or other properties COATING SYSTEMS • Solvent-free coating: one that in principle contains no VOCs but in practice The most important recent devel- often has a very low level of VOCs, perhaps 1–2% by volume opments in coating systems for • Solventless coating: one that has a low level of VOCs (e.g., not more than 5% ships have resulted from increasing by volume) regulations on the use of hazardous • Surface-tolerant epoxy coating: an epoxy coating that can be applied success- raw materials and the prevention of fully to substrates that are not optimally pretreated and are perhaps damp or environmental pollution. Seeking better performance and economic slightly corroded advantages have also played impor-

26 PCE September 1998 Copyright ©1998, Technology Publishing Company tant roles. last tanks, and cargo tanks. also be used. However, polyure- Many countries now regulate Considerable financial losses can thane products are becoming less coating materials that adversely result from premature damage to popular because of the toxicity of affect health or pollute the the coating systems of these parts. the isocyanates they contain. environment, and more legislation Following is a summary of the Most anticorrosive coatings are can be expected in the future. For most common coating systems for high-solids and sometimes high- shipbuilding, this can mean these and other parts of a ship. build materials. Use of epoxy mas- (depending on the country) avoid- tics is increasing. Apart from good ing open blasting and open paint Underwater Hull and Boottop surface tolerance, they generally spraying, adhering to drydock dis- Coating systems for the under- have long maximum overcoat times charge guidelines, and restricting water parts of a ship should be cor- and good recoatability. Total DFT of or prohibiting the use of coatings rosion-inhibiting, antifouling, abra- underwater hull and boottop sys- containing hazardous materials, sion-resistant, smooth, and compat- tems ranges from 250-400 µm. such as VOCs, anticorrosive pig- ible with cathodic protection. Anticorrosive properties are ments containing or chro- To minimise bunker (fuel) costs, obtained by barrier protection, mate, asbestos, organotin- the underwater hull should remain which means that the water vapour containing biocides, coal tar, and smooth during service. Conse- transmission of the system should hardeners based on aromatic quently, a coating system should be be very low. amines and isocyanates. applied as evenly as possible, and it Vinyl and chlorinated rubber Developments in coatings for should provide long-term protection paints, which can be applied and shipbuilding have focused on the against corrosion and fouling. dried at lower temperatures than reduction of VOC content. Best Increased hull friction due to foul- two-component paints, are not used results have been obtained with ing can result in up to 40% more very often anymore because of their two-component and fuel consumption compared to a high VOC levels. . In addition, water- clean hull and greater air pollution Minimum curing temperatures of borne coatings for steel have been because of the extra fuel burned to two-component epoxy products are developed, and much work has maintain a ship’s speed. 5-10 C. (Special “winter qualities” been done in developing antifoul- Systems for the underwater can cure at temperatures down to ings that have minimal adverse hull/boottop areas consist of an –10 C, but they have long curing effects on the environment. Very anticorrosive paint and an antifoul- times and usually somewhat higher high-solids epoxy siloxane hybrid ing paint on top of it. Sometimes a VOC content.) coatings introduced recently2 are sealer or tie coat is applied between In The Netherlands, a ban on coal said to offer excellent anticorrosive these two paints, especially when tar began in June 1997 because of properties and extended gloss and tar-containing anticorrosives are its hazardous polycyclic aromatic colour retention. According to the used. The sealer prevents the tar hydrocarbons. A provisional excep- supplier, use of such a product in from bleeding into the antifouling, tion was made for ship painting, a marine coating system could thereby improving its effectiveness and so two-component coal tar result in a reduced number of and adhesion. epoxy paints are still allowed for anticorrosive layers of paint, and it seagoing vessels. Other countries, could be suitable to replace ANTICORROSIVE SYSTEMS such as Germany, also regulate the isocyanate-curing In modern high-performance use of tar. It seems realistic to topcoats where they are restricted coating systems, the anticorrosive expect other countries to follow. or banned. system usually consists of at least Increasing use of glass flake in two layers of chemically curing coatings for underwater hulls and CRITICAL AREAS FOR PAINTING two-component epoxy or coal tar especially for boottop areas is evi- The most critical parts of a ship epoxy. Polyurethane/coal tar com- dent. Glass flake improves a coat- from the viewpoint of painting are binations, which cure at lower tem- ing’s mechanical strength and water the underwater/boottop areas, bal- peratures than epoxy/coal tar, can vapour impermeability. Mechanical 27 Copyright ©1998, Technology Publishing Company PCE September 1998 strength is especially important for Self-Polishing Antifoulings each vessel type4,5 based on its the boottop, which requires coat- So-called contact-matrix antifoul- typical trading pattern, speed, and ings to be very resistant to impact ings have nearly disappeared from length of stationary periods. High and scratching. The use of glass the market in recent years. They activity or high-speed ships need a flake in epoxy and coat- offer limited service life (1-2 years) slower polishing product than low- ings is very popular. and form an empty matrix on their speed ships or those that are sta- surface that consists of the tionary for long periods. ANTIFOULING SYSTEMS leached-out, insoluble part of the Increasing environmental concern The antifouling part of an under- binder. This matrix provides a cer- over use of tin has resulted in leg- water coating system consists of tain roughness to the hull, and islative restrictions in many coun- two or three layers of paint contain- forms a poor base for subsequent tries.6,8 In light of the positive bio- ing toxic materials (biocides) to pre- paint layers. logical effects of existing restrictions vent fouling of the hull by grass These products have been gradu- on marine life in coastal waters, (seaweed, algae) and shells (bar- ally replaced by more expensive further legislation may be expected. nacles, tubeworms, polyzoans, self-polishing antifoulings, which In fact, there may be a worldwide mussels, etc.). are high-build coatings based on ban on use of TBT antifoulings organometallic copolymers, often within 5-10 years. Copper and Tributyltin Toxins tributyltin acrylate. The organotin is Consequently, TBT-free antifoul- The main toxins used are copper chemically bound to the acrylic ing paints and biocide-free products compounds (mainly cuprous backbone of the copolymer and known as non-stick, low surface oxide, cuprous thiocyanate, or released in contact with seawater energy, or foul release coatings3,4,5,6 metallic copper), organometallic by a combination of hydrolysis and have been developed. compounds such as tributyltin ion exchange. The remaining back- oxide, and other biocides that are bone is then dissolved and washed TBT-free Antifoulings used mainly as herbicides in com- away by the movement of the ship. TBT-free erodible/ablative anti- bination with the other toxic The erosion occurs at the rate of foulings, also known as controlled materials. about 0.2-0.3 µm/day.1 This grad- depletion polymer antifoulings,5 Copper and tributyltin (TBT) com- ual erosion or self-polishing of the are based on a seawater-soluble pounds offer a broad range of pro- antifouling protects against fouling binder (e.g., rosin) combined with tection against fouling organisms.3 growth and keeps the underwater insoluble polymers that control However, microalgae and amphora hull smooth. Because no leached- the dissolution of the soluble are tolerant to copper, and brown out matrix remains, fresh toxic binder. In seawater, the biocide weed, seagrass, and certain diatoms material is continuously available. dissolves with the soluble binder, are tolerant to TBT. Most herbicides An advantage of tin-containing and the ingredients controlling are highly bioaccumulating and self-polishing antifoulings is their dissolution are washed away.7 The strongly absorbed by sediments. five-year service life, which is the main biocide is cuprous oxide, TBT is highly toxic to marine maximum time between dry-dock- which often is combined with one organisms such as oysters, mussels, ings permitted by classification or more “boosting biocides” for and crustaceans. For human societies. For this service life, at better efficiency. beings, it is a skin irritant, and it least two coats of paint with a DFT TBT-free products do not erode as may also be a sensitizer. As a of 150 µm each should be applied. ideally as TBT self-polishing result, TBT antifoulings are haz- Cuprous oxide often is added to the antifoulings. They produce an ardous to workers because of the TBT copolymer for maximum effec- empty matrix, which affects their possible inhalation of spray mist or tiveness against barnacles. long-term performance. The matrix blasting dust. Copper, on the other To limit costs and the amount of should be removed before applying hand, is less toxic to and TBT released into the environment, subsequent paint layers. Also, high much safer than TBT for non-target an optimum level of biocide and copper contents are essential.8 species. polishing rate are determined for Recently developed TBT-free self-

28 PCE September 1998 Copyright ©1998, Technology Publishing Company polishing antifoulings contain silicone contamination (leading to ships and the deletion by classifica- hydrolysing polymers. In addition, cratering/pinholing in nearby dry- tion societies of the allowance for they are rosin-free, which means ing paint layers that may be acci- reduced scantlings for new vessels, they have better stability to ultravio- dentally contaminated by overspray optimum long-life protection is let light. This property is especially of the non-stick coating), poor vital. important for boottops. These prod- resistance to mechanical damage, Regulations from the U.S. gov- ucts generally are based on copper poor adhesion to conventional ernment, the classification soci- acrylates combined with boosting coatings, and high cost. Practical eties, and the International biocides. Zinc acrylates also are use of self-cleaning, non-stick Maritime Organisation since 1990 used. The polymers react with antifoulings up to now has been have had a clear and positive seawater, similar to TBT self-polish- limited to very fast vessels such as effect on the structural protection ing copolymers, and the paint some naval ships and ferries. of ships.9,10,11,12 They deal with polishes away with a controlled With respect to the further devel- the duty to coat ballast tanks release of biocide.4,5,6 opment of environmentally com- (with light-coloured, hard coatings Because of their relatively fast patible antifoulings, research is combined with cathodic protec- polishing rates, the maximum in- underway to produce coatings tion), the minimum width of dou- service period of TBT-free systems is based on natural compounds with ble hulls, the height of double- about three years, although one antifouling properties developed bottom tanks, the deletion of the supplier says it produces a self- from marine organisms. allowance for reduced scantlings polishing tin-free product as good as when tanks are coated with an TBT self-polishing types.5 Ballast Tanks approved system, and a har- Overall, however, it generally is Coating systems for ballast tanks monised system of survey and cer- agreed that for large, fast vessels, should be resistant to (polluted) tification from the classification which are particularly sensitive to seawater, corrosion inhibiting, free societies. increases in fuel consumption, TBT from pores, and resistant to the An additional international regu- self-polishing systems are still indis- side effects of cathodic protection. lation likely to come into force in pensable because of the savings in According to the classification 1999 would require steelwork in fuel, the reductions in maintenance, societies, the life-determining factor ballast tanks to be designed to min- and the extended dry-docking inter- for a ship is the condition of its imise awkward-to-coat surfaces, to vals that they offer. ballast tanks. Serious corrosion give easy access for personnel and damage in them is a main reason equipment, and to facilitate clean- Biocide-free Antifoulings for taking a ship out of service, and ing and drying the tanks.12 Non-stick, low , or a ship’s second-hand price is large- In ballast tanks, pitting corrosion fouling release coatings are biocide- ly determined by the condition of can occur easily because of coating free. The most promising types are its ballast tanks. irregularities, mechanical damage, silicone elastomers based on polydi- The inner water ballast tank area and poorly coated areas. Pitting cor- methylsiloxane. Coatings based on of a ship is extremely large. For a rosion is often promoted by the fluorinated epoxy and polyurethane single-hull very large crude carrier presence of a more noble material materials are found to be less effec- (VLCC), the water ballast tank area than steel and by one or more tive. might be 140,000-160,000 m2; for a stainless steel bulkheads from adja- Non-stick coatings provide a modern double-hull design, it could cent cargo tanks. To prevent pitting smooth, non-polar, low-energy sur- be 240,000-280,000 m2 or even corrosion, sacrificial anodes are face to which the fouling species larger.9,10 sometimes installed in addition to cannot easily stick or from which it Double-hull design prevents the coating system. is easily removed by water move- leakage into the sea in case of Ballast tank coatings may be ment when the ship is sailing or by damage to a ship’s hull. Because of applied over the shop primer, mechanical cleaning.3,4,5,6 the difficulty of recoating complex which should be cleaned thorough- Disadvantages include the risk of steelwork in modern double-hull ly first, although it is far better to

Copyright ©1998, Technology Publishing Company PCE September 1998 29 remove the shop primer by blasting, Modern systems for ballast tanks impact. Marine coating regulations especially when the primed surfaces usually consist of at least two coats favour hard coatings, which offer are in a poor condition. of high-solids coal tar epoxy, properties opposite those of soft To avoid osmotic blistering, the straight epoxy, or modified epoxy coatings, particularly in light soluble salt (Cl-) content on the sur- with a total DFT of at least 250 µm. colours as an aid for inspection. face should be very low. A maxi- Because of the decreasing populari- Det Norske Veritas guidelines11 mum concentration of 20 mg/m2 ty of coal tar, modified epoxy often for ballast tank coatings include before coating application is gener- is used, preferably in light colours three categories: epoxy-based ally acceptable. to enhance the ease of inspection. (light-coloured); epoxy coal tar (no Ballast tank coatings should pro- Solvent-free epoxies are also used longer recommended due to their vide very good edge coverage. This in ballast tanks. They are applied in dark colour); and other recognised reduces the need to round the edges one or two layers with a minimum coating systems. The guidelines inside the tanks. However, the stripe DFT of 300-350 µm. (A one-layer also include three specification lev- coating of sharp edges and irregular coating should be used in conjunc- els requiring increasing degrees of or rough welds is good standard tion with cathodic protection.) surface preparation: painting practice before spray appli- Besides being environmentally • Specification I: 5 ± 3 years use- cation of each coat. friendly, they present no risk of ful life and 1 coat of 200 µm DFT; Most ballast tank coating systems explosion or fire. A disadvantage of • Specification II: 10 ± 3 years provide barrier protection against solvent-free epoxies is their very useful life and 2 x 200 µm DFT; corrosion. Consequently, they short pot life. and should have very low water vapour Other systems sometimes speci- • Specification III: 15 ± 3 years permeability. Some products are fied for ballast tanks include high- useful life and 2 x 200 µm or 3 x reinforced with micaceous iron solids vinyl tar, water-borne 130 µm DFT. oxide or similar to reduce asphaltic emulsion, and acrylic- moisture and oxygen penetration. reinforced cementitious coatings. Cargo Tanks The water vapour transmission For high-temperature bulkheads or Coating systems for cargo tank rate of coatings is given by their µ- places sensitive to mechanical dam- interiors should resist the follow- value (the ratio of water vapour age, water-borne or solvent-borne ing: cargo to be transported and transport through a layer of air to zinc silicate coatings also are substances the cargo might release; that through a coating at the same prescribed sometimes. For mainte- tank cleaning procedures; and thickness). For a good water ballast nance purposes, epoxy mastics suit- cross-contamination between dif- tank coating system, the µ-value x able for hydrojetted or mechanically ferent cargoes and also ballast the DFT (µm) should be at least 25 pretreated surfaces are used more water. These coatings also should m.13 Therefore, an epoxy coal tar and more. be corrosion-inhibiting, free from coating with a µ-value of 100,000 Soft coatings based on petroleum pores, and easy to clean. Finally, requires a DFT of 250 µm. Since tar- derivatives containing sulphonates they must not contaminate or free coatings generally have lower or wool grease containing penetrat- affect the colour or taste of the µ-values, they should be applied at ing additives also are used some- cargo, particularly cargoes intended higher DFTs. For optimal corrosion times. Both types contain water for human consumption and pure protection, a ballast tank coating repellents.14 They can be applied to chemical cargoes (which require also should offer long-lasting adhe- marginally prepared surfaces, but coatings systems approved by the sion under wet conditions. they should be reapplied frequently. appropriate regulatory agency). Conventional coating systems for Special attention should be paid to ballast tanks, such as bituminous the risk of water pollution in har- CLEANING/COMPATIBILITY coatings and solvent-free bitumi- bours when soft coatings are used It is often necessary to clean or nous compositions that should be in ballast tanks. Soft coatings ventilate cargo tanks when chang- heated before application, are sel- remain soft after application and ing cargoes in order to prevent dom used anymore. wear away under low mechanical undesired interactions between

30 PCE September 1998 Copyright ©1998, Technology Publishing Company cargo residues and the next cargo. and types of subsequent cargoes). However, crude oil tanks of Such interactions can form sub- Guidelines for tank cleaning proce- smaller tankers are usually com- stances that may attack the coating dures when changing cargo should pletely coated with a system con- system, promote steel corrosion, be followed carefully to ensure that sisting of two coats of polyamine- and contaminate or discolour the cargo residues are sufficiently or polyamide-cured coal tar/epoxy next cargo. removed before loading a new with a total DFT of at least 250 µm. For example, when residues of a cargo or ballasting.16 Modern systems are based on high- vinyl acetate monomer cargo are Organic tank lining systems can solids products. Also, one-coat sys- still present in the lining of a tank absorb materials from cargoes, and tems of solvent-free epoxy (after filled with a water-containing cargo, the amounts after different time stripe coating) are possible. The these residues will hydrolyse and periods are not well defined. DFT should be at least 300 µm. form . This reaction will Variable and unpredictable absorp- For clean petroleum products cause corrosion and may (e.g., white , aliphatic attack the coating. Likewise, hydrocarbons, etc.), systems cargo containing ethylene for crude oil tanks are used dichloride can form except for products contain- hydrochloric acid upon con- ing coal tar. tact with water or water- For more aggressive prod- containing cargo. To avoid ucts (e.g., chemicals, veg- such interactions, all etable and animal oils, and and chlorinated hydrocar- aromatic hydrocarbons), the bons must be transported in system often consists of two dry cargo tanks.15 coats of (high-build) Methanol cargoes can be polyamine (adduct)-cured especially problematic. epoxy with a DFT of at least Besides having a softening 250 µm. Polyurethane and Ballast tank protection is critical to a ship's service life. This effect on organic coatings, coating is being applied to the fore peak tank. isocyanate-curing epoxy sys- methanol residues in a coat- tems are available but not ing can promote water vapour tion/desorption characteristics are commonly used now because of the permeability, causing osmosis and found not only among different trend toward isocyanate-free coat- corrosion of the steel substrate. In coating types but also within the ings. Shop primer usually is addition, methanol can extract same generic type of coating from removed from these tanks by blast- 1 residual solvent and low molecular different manufacturers. In addi- ing to at least Sa 2 ⁄2 for good adhe- weight materials from the coating. tion, different rates of absorp- sion and to avoid undesired inter- This induces stresses in the coating tion/desorption are found among action of the cargo with the shop that can lead to cracking. Only different cargoes.17 This can make primer. highly crosslinked coatings are it difficult to select the correct cargo When the highest chemical resistant to methanol. Most coat- tank coating system. resistance is required (e.g., for car- ings suppliers do not allow trans- goes with high acid values), three- portation of water-containing car- VARIOUS APPLICATIONS layer systems based on phenolic goes after transportation of Crude oil tanks of VLCCs often epoxy are used with a total DFT of methanol. are left uncoated (apart from shop 300 µm. Commodity lists from coating priming), or only the bottom area One-layer zinc silicate systems manufacturers commonly indicate (on which acidic water settles) and (either solvent-borne, moisture- which cargoes may be transported the deckhead area (on which water curing zinc-rich ethyl silicate or in tanks coated with their systems condenses) are coated. A cathodic water-borne, self-curing zinc-rich and under what conditions (e.g., protection system also is often alkali silicate) with a DFT of 75-100 cargo temperatures, transport times, installed. µm are used for tanks carrying very

Copyright ©1998, Technology Publishing Company PCE September 1998 31 aggressive solvents such as esters be applied to the topside and super- are wet) and resistant to impact, and . There are pH restric- structures. This finish contains an scratching, and abrasion, as well as tions for zinc silicate coatings, active pigment that chemically com- resistant to (sea) water, fuel oils, because acidic or alkaline cargoes bines with rust to produce a colour- lubricating greases, cleaning agents, will affect the tank linings. less, water-soluble material. As a and cargo spills. For the highest chemical resis- result, rust stains are not visible. To minimise damage that may tance, environmentally friendly Although not often mentioned in occur to a deck coating before cargo tank coatings with reduced painting specifications, water-borne delivery, the best procedure may be VOC levels have been unavailable coatings based on alkali zinc sili- to apply a recoatable epoxy holding until now. One of the latest develop- cate, styrene acrylate dispersion, or primer during construction and the ments, based on cyclosilicone epoxy epoxy or emulsion are suit- final coating system as soon as pos- resins, is said to offer significant sible before delivery. advantages in terms of cargo The most common deck range, cargo handling, and coating systems are two-com- tank-cleaning operations, espe- ponent epoxies, polyure- cially for light chemical thanes, and zinc silicates with tankers.15 Since 1993, it has a DFT of 250-300 µm for been applied to about 30 epoxy/polyurethane systems tankers, mainly for the and 75-100 µm for zinc sili- methanol trade. For full chemi- cates. cal resistance, heat curing at 80 Epoxy/polyurethane sys- C with hot air or steam is nec- tems often consist of a primer, essary. a thick midcoat, and an easily A light-coloured coating, such as on this fresh water tank, recoatable topcoat, preferably is an aid to effective inspection. COATING OTHER SHIP PARTS all high-solid products. The For other parts of a ship, the able for the topside and superstruc- topcoat can be made anti-skid same anticorrosive coatings ture. However, their use generally is by adding an aggregate such as described for the outer hull are limited to the mid- and topcoat lay- non-sparking silica, pumice pow- applied, perhaps with some reduc- ers. Suitable products include der, or aluminium oxide. tion in film thickness. For conven- acrylic dispersions and epoxy For zinc silicate, the deck sur- ience and efficiency, it is advanta- acrylic or (silicone) alkyd emul- faces should be blast cleaned to Sa 1 geous to keep the number of speci- sions. However, their gloss levels 2 ⁄2 (or sweep blasted when an fied products as low as possible. are usually lower than those of sol- intact silicate shop primer is pre- vent-borne paints. sent) directly before coating appli- Topside and Superstructure Water-borne systems can be suc- cation. Due to their limited For the topside and superstruc- cessfully applied, especially for resistance to acids and alkalis, zinc ture, an aesthetic topcoat of aliphat- ships built completely under cover. silicates should not be used on the ic polyurethane or aliphatic However, this requires at least mod- decks of chemical tankers. polyurethane/acrylic may be used. erate temperature and humidity Special, heavy-duty systems Isocyanate-free alternatives include levels and strict planning. Also, based on thick, solvent-free elas- epoxy/acrylic or other modified painters experienced in applying tomeric coatings applied by trowel epoxy coatings, although they gen- water-borne paints are needed. or roller over a thin primer to a erally have reduced gloss and DFT of 1-3 mm also are available. colour retention compared to Decks In addition, water-borne systems polyurethanes. Polysiloxane epoxy Paint systems for decks should be consisting of an alkali zinc silicate hybrid coatings also can be used as very resistant to corrosion and the primer and an epoxy emulsion aesthetic topcoats. influences of weather. They should mid- and topcoat can be considered A special antirust-stain finish may be non-slip (even when the decks for decks.

32 PCE September 1998 Copyright ©1998, Technology Publishing Company SUMMARY and chrome-containing pigments, 8. “Biocidal Products Directive May As a result of regulatory pres- and asbestos. This work will go on. Impact Antifouling Coatings” sures, developments in coating sys- At the same time, new materials Protective Coatings Europe (January tems for ships in recent years have such as epoxy siloxane hybrid coat- 1996) 16-17. emphasised reduced VOC levels. ings have begun to find their way 9. Rodney H. Towers, “Impact of Traditional high-VOC coatings such into the marine painting field. Also, New Rules on Structural Protection of as those based on vinyl resins and high-performance, surface-tolerant Ships,” The Royal Institution of Naval chlorinated rubber have gradually mastics have been developed that Architects, International Conference been replaced by chemical-curing, perform well on less than ideally on Marine Corrosion Prevention, two-component coating systems. prepared substrates. Further devel- October 1994. However, use of low-VOC coat- opments are expected in this area as 10. J. Eliasson and R. Malfanti, ings gives rise to specific problems, well as in the improved performance “Design Life Specifications—Cost such as a reduction in substrate of tin-free antifoulings and other Analysis—Cost-Effective Specification: wetting, which can reduce adhe- ways to combat fouling, such as the Their Relation” in Achieving Cost sion, corrosion resistance, chemical use of low surface energy or anti- Effectiveness in Coatings Work, pro- resistance, and mechanical proper- stick coatings. ceedings of PCE ’98, April 1998, The ties. Other difficulties include shor- Hague, The Netherlands (Pittsburgh, ter pot life times, longer drying REFERENCES PA, USA: Technology Publishing times, more difficult film thickness 1. A.M. Berendsen, Marine Painting Company) 212-225. control, and sometimes the need Manual, ISBN 1 85333 286 0, 1989. 11. Erik Arskheim, “Ballast Tanks and for special application equipment. 2. Ko Keijman, “High-Solids Cargo Holds in DNV’s Guidelines for These disadvantages play a smaller Coatings: Experience in Europe and Corrosion Protection of Ships” role with paints containing some USA,” Paper No. 40 in Protecting Protective Coatings Europe (June solvent rather than no solvent at Industrial and Marine Structures 1997) 26-35. all, which explains the popularity with Coatings, proceedings of PCE 12. Jan Aubert, “Ballast Tank Integrity of high-solids products. ’97 conference, March 1997, The and the New IMO Coating Rule” Further development is needed, Hague, The Netherlands (London: Drydock (September 1995) 18-20. however, for solvent-free epoxy Paint Research Association). 13. J.C. Moree, “Tank Coating underwater hull coatings and high- 3. “Critical Review of Current & Failures,” TNO Industrial Research solids/solvent-free cargo tank coat- Future Marine Antifouling Coat- Report C92.1246, December 1992. ings with high chemical resistance. ings,” Lloyd’s Report 14. “Guidelines for the Corrosion Water-borne coatings have 93/TIPEE/4787, November 1993. Protection of Ships” Det Norske played a limited role in marine 4. Richard Chapman, “What’s Veritas, July 1992. applications, probably because of Involved in Selecting the Correct 15. Norbert Ackermann, “Choosing their sensitivity to humidity and Antifouling?” Protective Coatings the Correct Coatings for Cargo Tanks” low temperatures during applica- Europe (March 1998), 32-36. Protective Coatings Europe (April tion and curing. Also, apart from 5. Colin D. Anderson, “Develop- 1998) 44-51. alkali zinc silicate, most water- ments in TBT-Free Antifouling 16. Verweij’s Tank Cleaning Guide. borne coatings remain more or less Technology” Protective Coatings 17. Trevor Parry, “The Absorp- water sensitive even after full cur- Europe (April 1998) 22-31, 68. tion/Desorption Characteristics of ing, which makes them less suit- 6. J.E. Hunter, “Antifouling Coatings Organic Tank Lining Systems” in able for immersion service. and the Global Environmental Achieving Cost Effectiveness in Much work has been done in Debate” Protective Coatings Europe Coatings Work, proceedings of PCE Europe and countries throughout (November 1997) 18-24. ’98, April 1998, The Hague, The the world to substitute hazardous 7. Eva Bie Kjaer, “Bioactive Netherlands (Pittsburgh, PA, USA: coating materials such as coal tar, Materials in Antifouling Coatings,” Technology Publishing Company) aromatic amine and isocyanate Progress in Organic Coatings 20 251-261. hardeners, certain plasticizers, lead (1992), 339-352.

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