18-8ステンレス鋼の耐孔食性に及ぼすMnS介在物と定電位電解処理の影響

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The effects of MnS inclusions and of electrolytic treatment in the active and passive region on the pitting resistance of 18-8 stainless steels have been studied in 3%NaCl solution mainly by electrochemical means. The pitting resistance of the specimens depended on the electrolytic potential. In the specimens electrolyzed in the active region directly below the passivation potential, pitting potential had a tendency to become noble, and this tendency was pronounced in specimen containing a small quantity of MnS or having a small Mn/S ratio. Cr concentration on the steel surface was increased by the electrolytic treatment, and the concentration had a tendency to inrease as electrolytic potential became more noble. Although there appears to be little correlation between Cr concentration and pitting potential, there does appear to be a correlation between the morphology of corrosion and the pitting potential. That is to say, electrolyzing just below the passivation potential at which dissolution of the matrix and MnS occure, let to greatly improve pitting resistance. This was due to the dissolution of MnS, which acts as the pit initiation site, and to Cr enrichment on the steel surface, which occurrs simultaneously.
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