Ship Paint Systems

Coal tar pitches are less readily available than they used to be their content of polynuclear aromatic hydrocarbons makes their safe use suspect. They have been replaced to some extent by the less effective petroleum bitumens whose mechanical properties are improved by combination with poly(vinyl chloride) and other chlorinated polymers as one-pack, nonconvertible coatings, or by combination with epoxy or polyurethanes in high-performance two-pack convertible coatings. Similar materials are preferred for coating ballast spaces and double bottoms in vessels, where economical systems are required that do not have to be attended throughout the lifetime of the vessel (15-25 years). Multiple coats with a total thickness of 250-400 pm are usually employed. 

The environment in chemical tanks is amongst the severest to which marine coatings are subjected. Each tank may have to transport some 1500 bulk, liquid cargoes that include crude oil, refined gasoline, aviation spirit, diesel oil, solvents, vegetable oils, or wine. Inorganic cargoes are also carried in solution (e.g., alkalis and acids). New demands also appear such as methanol as a fuel and feedstock, and methyl rert-butyl ether as an additive for lead-free petrol. 

No single type of coating is universally applicable, and nonpaint alternatives may also be used (e.g., rubber linings and stainless steel). Vessels that are not dedicated to a single type of cargo (parcel tankers) usually have a number of coating systems. 

3) To facilitate cleaning and avoid cross-contamination of one cargo by another 

Variants of six main paint systems are currently used (Table 11.1). 

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Stripping of top coats soaked by rain or sea-water has occurred with alkyd-resin-based paint systems, mainly on ships. The risk of such intercoat failure is reduced if the time interval between application of coats is reduced, but is best controlled by modification of the alkyd resin with a proportion of a different material. 

The total paint system of a typical underwater ship hull is shown in Figure 6. 

Figure 6 A sketch showing the different layers in a paint system used on the underwater hull of a ship. Usually the whole system is composed of 5-6 layers.

When sacrificial anodes (usually Zn or Al) are used on ship hulls fliey are combined with a paint system. The anodes are mounted close to eaeh other on the area around the propeller, partly because the propeller steals some of the eurrent and partly because of turbulence leading to higher demand of eathodie eurrent density. Zn and Al anodes are also used in ballast tanks on tankers and bulk earriers. 

Regarding selection of paint on steel in different atmospheric environments, some proposals are given in Table 10.18. More extensive guides for selection of paint systems are given in References [10.37, 10.48]. Particularly for ships and marine structures. Chandler [10.27] has presented a clear survey comprising structure parts above water as well as submerged and in the splash zone. 

In bilges and naval ships, zinc spray plus a chlorinated rubber paint system is used, for example, in Royal Navy Sheffield Class destroyers based on 8-10 years service before maintenance (Hunter, 1981). Galvanizing alone was used in earlier RN ships. 

Painting System Guide No. 19.00 Guide for Selecting Painting Systems for Ship Bottoms... 

This guide covers painting systems for the protection of the exterior boot-top areas (the area from the light load line to the deep load line) of steel ships. It should be noted that boottops are rarely used with today s commercial ships, and bottom systems may extend up to the deep load line. In general, the anticorrosive and antifouling paints covered in SSPC-PS Guide 19.00 are applicable to boot-top areas. 

This guide covers painting systems for the protection of the topside or exterior area of steel ships. This includes the area from the deep load line to the rail, more commonly... 

Surface estimating aids presented by many paint suppliers are not valid for economic evaluation of paint systems - the true surface areas form the base for estimates (texture, undulation, fasteners, flanges, etc.). For appropriate evaluation of a particular paint system, the most important factor is its maintenance cost per square meter per year, and this consideration should be coupled with the serviceability of the paint system in variable conditions (e.g. change of cargo in a ship or similar). 

Fouling organisms attach themselves to the underwater portions of ships and have a severe impact on operating costs. They can increase fuel consumption and decrease ship speed by more than 20%. Warships are particularly concerned about the loss of speed and maneuverabiHty caused by fouling. Because fouling is controUed best by use of antifouHng paints, it is important that these paints be compatible with the system used for corrosion control and become a part of the total corrosion control strategy. 

These coatings provide the most effective fire-resistant system available but originally were deficient in paint color properties. Since, historically, the intumescence producing chemicals were quite water-soluble, coatings based thereon did not meet the shipping can stability, ease of application, environmental resistance, or aesthetic appeal required of a good protective coating. 

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