Recently corrosion problem of ocean-going vessels becomes to attract strong attention. Especially, the main issue is corrosion in the ships ballast tank. Most of ships, in general, adopt the painting protection to prevent corrosion in the seawater ballast tank, but deterioration or damage of painted coating surface caused corrosion of ship's ballast tank. Thus International Maritime Organization (IMO) legislates Performance Standard for Protective Coatings (PSPC), and it will be enforced from 2008. As a result of PSPC, the protection methods on paint coating will be more complicated than before. Moreover, it will be very costly to comply with the strengthened regulation. For this reason, corrosion resistance on stainless steel thin plate cladded with steel plate was studied.
In general, the stainless steel is used as some parts of the oceanic constructions and ships to prevent the corrosion in the environment of the seawater. Because of superior characteristics against corrosion, the stainless steel is preferred as material. That is, the stainless steel has been used with stability by the passivity oxidation films (Cr2O3) made by oxygen in natural atmosphere. However, the passivity oxidation films on the surface of the stainless steel occasionally comes to be destroyed in the seawater affected by an environment with the halogen ion like Cl-. Moreover, Heat Affected Zone (HAZ) of stainless steel has poor corrosion resistance. Therefore, the localization corrosion comes to occur frequently. An application of the stainless steel in the environment of sea water requires special attention because of the severe problems like a safety-accident by the localization corrosion.
For this reason, this study exactly find out the reason of corrosion which widely occurs on the stainless steel and tries to suggest guidelines for applied developments by seeking fundamental solution for corrosion problem.
Through these experiment and study, following results was obtained -. After immersion test and other experiment, cladded stainless steels such as STS2205 and Al29-4C are known as high corrosion resistance. Thus these materials are suitable in the environment of seawater ballast tank.
-. But some of defective part or CHAZ(Cladding Heat-stress Affected Zone) is easily corroded during the immersion of long term in the seawater. Therefore cathodic protection method using the sacrificial anode is effective for protection corrosion.
-. Through the comparison and analysis of stainless steel after the immersion test which remains unprotected and applied to the cathodic protection respectively, the unprotected stainless steel specimen was partly corroded. On the other hand, the protected stainless steel specimen was not totally corroded. This result is assumed to come from the facts that anodic reaction, Fe→Fe2++2e-, has been restricted by the cathodic protection current of the sacrificial anode material.
-. Therefore, in the future, the proper application of above mentioned cathodic protection method should be recommended for the protection of cladded stainless steel in seawater environment. Whereas, studies on the atmospheric environment inside of ballast tank will have to be carried out.