Corrosion protection of steel ships - Localized high strain failures of protective coating
Paper in proceedings, 2011
The advances of corrosion protection over the last decade has been considerable due to mutual efforts of ship owners, TSCF, IMO, IACS, research institutes and paint manufacturers. Now there are well-described corrosion protection systems that give 15 years of surface protection without much maintenance work. However, there are deterioration mechanisms that are not understood. Further research efforts are needed and new understanding should be turned into procedures for implementation. Tankers and bulk carriers are designed for about 25 years. In trans-ocean trade these ships may make as few as 300-1000 voyages in the lifetime. The loading and discharge cycle ballast-cargo-ballast causes high local stresses in some bracket corners and cutouts. During the ocean voyages wave loads causes high stresses in the same spots. When stresses are superimposed the yield point of the material is frequently reached and there will be local yielding in many such places. In the paper it is shown that the steel sustains high strain better than the aged coating. Spots at risk are not systematically identified today despite the very high probability of localized failure of the coating. Once the coating has failed locally corrosion starts and accelerated crack propagation takes place in the steel for the wave-induced stresses. The paper gives a state of the art description concerning corrosion protection systems and focuses on the localized strain induced coating failures, some methods under development. In order to avoid localized cracks in the coating due to high strain followed by corrosion one should identify spots with very high strain, give the local geometry of the steel below the vulnerable coating a perfect shape for the benefit of a sustainable coating.