Why Do We Have Cracks in Epoxy Coatings for Water Ballast Tanks?

摘要

Crackings of the epoxy coatings applied on ship's water ballast tanks can lead to damage of ship's hulls. To avoid this, it’s critical to have clear understanding of the underlying mechanism and primary factors of the coating crack. In this study, the efforts were made to clarify the integrated effects of critical factors, I.e., initial coating shrinkage, thermally induced strain, steel-structural strain and the intrinsic coating flexibility at the initial and after aging, to the premature cracking of epoxy coating for the water ballast tank. A new screening test method was proposed to predict the long-term resistance of epoxy-based paints to cracking. These results indicated that crack was caused by combination of thermal stress, structural mechanical stress, and internal stresses, closely related to chemical structures of the coatings. On the other hand, for WBT coatings having rather large flexibility, thermal stresses and dimensional stabilities would rarely play a major role in coating crack. Crack resistance of the coatings at early service life can be estimated roughly by measuring internal stress, FT-IR and Tg value of the coatings.