Initiation and Growth of Corrosion Pit and Its Effect on Corrosion Fatigue Strength in 12Cr Stainless Steel

抄録

The structural components used in power generating plants sometimes fail due to the corrosion fatigue. The corrosion fatigue cracks often initiate from the pits. In order to prevent the corrosion fatigue failure, 12Cr stainless steel is employed for the components used in the corrosive environments. Even if employing 12Cr stainless steel, the components failed by the fatigue and the corrosion pits were observed at the origins of fatigue cracks. In this study, the effect of surface finishing on the nucleation of corrosion pits was investigated using 12Cr stainless steel. The corrosive environment is pure water with chlorine ion concentration. The initiation and growth of the corrosion pits are remarkably affected by the chlorine ion concentration. So-called “One-third rule” was obtained and the aspect ratio, that means the ratio of pit depth to length on the surface, was estimated as about 1/4. The surface finishing affected the nucleation and growth of corrosion pits. Especially they are reduced when the surface was finished to the mirror-like surface by the emery paper polishing. The fatigue tests were conducted in air at the ambient temperature and in de-oxygenated water at 463K. The fatigue strength decreased with the increasing the corrosion pits. The reduction of fatigue strength can be explained by the short crack theory. On the other hand, all the fatigue cracks initiated from the corrosion pits in the high temperature water. The surface observation of test specimens suggested that the corrosion pits were mainly nucleated from the manganese sulfide. This indicates that the corrosion fatigue strength could be improved by controlling the chemical compositions and the impurity atoms.