Pitting corrosion on the bottom plates of cargo oil tanks (COT) in very large crude carriers (VLCC) is very serious problem. Each tank may suffer up to 1,000 pits, with some reaching a depth of as much as 10 mm. As a result, the workload of repairing such pitting corrosions in periodical dock inspection is extremely heavy. Many studies have already been conducted to clarify the mechanism of pitting corrosion, but it has yet to be fully understood.
We have clarified the pitting corrosion mechanism through onboard research on some VLCCs and various corrosion tests, in addition to the findings obtained by previous studies. Based on our understanding of the mechanism, we developed a corrosion test method to simulate the corrosive environment within the pits. Furthermore, we developed a new corrosion resistant steel (CRS) with trace amounts of alloying elements. The corrosion rate for CRS is less than one-fifth of conventional steels. Due to its very low alloy content, the weldability and mechanical properties of CRS remain similar to conventional steels. This CRS has already been applied to several VLCCs and we have examined its corrosion resistance through onboard investigations of the two VLCCs. One was for all its uncoated COT bottom plates, which were built of CRS, at the first docking (after 2.5 years). No pits deeper than 4 mm were found in the bottom plates of any COTs. Also, only about twenty pits of 2∼4-mm depth were found. The other was for the bottom plates of six unpainted COTs built of CRS at her first and second dockings (after 2.3 and 5 years). At five years, tens of pits deeper than 4 mm were found in all, but the pit count was much lower than that of VLCCs constructed of conventional steel. Thus, the good corrosion resistance of CRS was confirmed. In addition, it was also revealed through onboard research during a dock inspection that pit growth halted on VLCCs with more than five years service.
SOLAS II-1 Cargo Oil Tank Corrosion Protection, which adopts the test method developed by us as the qualification test for bottom plates, comes into effect in 2013. In addition, CRS has already been certified by Class NK as corrosion resistant steel for COT bottom plates.
CRS is set to play its part in the safe navigation of oil tankers. Furthermore, CRS does not require a protective coating of paint, which also benefits the global environment by reducing the use of volatile organic compounds.
Corrosion mechanism of corrosion-resistant steel developed for bottom plate of cargo oil tanks