Ship Painting: Current Practice and Systems in Europe
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Ship Painting: Current The underwater hull and boottop are among the most critical areas of a ship for painting. Practice (Photos courtesy of Camrex Chugoku Ltd.) COMMON METHOD OF SHIPBUILDING Current shipbuilding practice is and Systems to clean the steel in a centrifugal s long as ships blasting machine and then imme- sail the seas, Athey will need protec- diately apply a weldable shop tion from the corrosive environment primer. 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 coatings. It looks bending, stretching, and drilling, at recent developments in coating 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 cargo 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 corrosion 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 tool 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 zinc 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 pollution 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 solid base systems (resistant to alkaline condi- added to the binder 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 epoxy Since epoxy shop primers have film of uniform thickness; resins and partially hydrolysed ethyl an organic binder, they generate • have a very short dry-to-handle silicates. Reinforced wash 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 pigment- be thoroughly pretreated again (sec- affect the speed of these processes); ed mainly with iron 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 solvent-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- resin emulsion 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-solids 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 solvents classified as final painting must be done in a volatile organic compounds (VOCs), which contribute to air pollution 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