The degradation mechanism of a marine coating under service conditions of water ballast tank

Abstract An epoxy amine-based seawater ballast tank coating was characterized after 18 years of exposure to service environment. Coating flake specimens were taken from a flat region of the ballast tank, which appeared intact and well attached to the steel substrate. The flakes were examined using analytical electron microscopy and Fourier transform infrared (FTIR) spectroscopy. Significant morphological changes in the pigments and binder in the near surface region were observed. These were related to the hygrothermal cycles due to filling and discharge of sea water in the ballast tank, accompanied by cyclic temperature variations, generating expansion and contraction stresses within the coating. The analyses also reveal for the first time differences in the damage to barrier pigments (i.e. talc and kaolin) compared to the degradation of aluminium flake metal pigment. The former were fractured internally while the latter showed failure at the pigment/binder interface. Since pigments play a significant role on the degradation of the coating, their orientation towards the surface was also found to be of high importance. Regions of coating with pigments orientated nearly vertically to the surface showed a more severe degradation than regions with pigments orientated nearly parallel to the surface.