Mitsui O.S.K. Lines, in conjunction with JFE Steel and Chugoku Marine Paints, have discovered that shop primer, used as anti-corrosion paint, is effective in preventing corrosion in the cargo tanks of double-hull very large crude carriers (VLCCs). Double-hull VLCCs greatly reduce the risk of oil spills, but the flat surface at the bottom may prevent formation of oil-coating, increasing the possibility of corrosion. This problem has received worldwide attention. Corrosion, if left untreated, may lead to pitting in the tank. MOL research has shown that shop primer will gain thickness and help prevent corrosion.
After inspecting the 20mm thick cargo tanks of double-hull VLCCs coated with shop primer (15ìm thickness), two and a half years after delivery, about 50 pitting corrosion spots (4 to 7 mm depth) were found. In the double-hull VLCCs not coated with shop primer, on the other hand, more than 1,000 pits were found two and a half years after delivery and 2,000 were found five years after delivery. A sample bottom plate was cut out and analysed by JFE Steel. The results showed that the shop primer had chemically transformed into a fine, hard coating of iron and zinc oxide. Results also showed that the thickness of the coating had increased from the original 15ìm to 50 ìm.
While epoxy coating may appear to provide a complete coating, once is mechanically damaged, the metal will progressively corrode. Deep pitting corrosion was observed in places where the epoxy coating has been damaged. MOL concludes that the shop primer can be considered safer than epoxy coating.