Martensitic stainless steels are widely used in industries because of their high strength and corrosion resistance. However, pitting corrosion sometimes occurs in chloride environments. It is known that pitting corrosion is usually initiated at MnS inclusions. The pit initiation at the MnS inclusions in Type 304 stainless steels was found to be inhibited by interstitial carbon. However, the effect of interstitial carbon on pit initiation of martensitic stainless steels is still unknown. In this research, in order to change the amount of interstitial carbon, the specimens were heart-treated by different tempering conditions, and potentiodynamic polarization curves were measured.

A commercial Type 420J2 (13Cr-0.3C) martensitic stainless steel (low S) and re-sulfurized Type 420J2 martensitic stainless steel (high S) were used. After solution treatment at 1273 K (5 h, air-cooling), the specimens were tempered at various temperature (from 473 to 1023 K, 2h, air-cooling). After these heat-treatments, the surfaces of the specimens were successively polished with a diamond paste down to 1 µm. With the exception of the electrode area, the surfaces of the specimens were covered with a resin.

Figure 1 shows the macroscopic anodic polarization curves of low-sulfur and re-sulfurized Type 420J2 stainless steels in boric-borate buffer solution with 10 mM NaCl (pH 8.0). The electrode area was 1.0 cm². In the case of low sulfur steels, pitting potentials decreased with increasing tempering-temperature (473-773 K), but the pitting potential of the specimen tempered at 1023 K was higher than that of non-tempered specimen. It is supposed that tempering at 1023 K improves pitting corrosion resistance. For high sulfur steels, pitting potential decreased with tempering-temperature. Because the amount of interstitial carbon decreased with tempering-temperature, these results suggests that interstitial carbon inhibits pit initiation.