Sudden pH and Cl− Concentration Changes during the Crevice Corrosion of Type 430 Stainless Steel

To determine whether large sudden pH and Cl − concentration changes occur during the crevice corrosion of Fe-16Cr, as in the case of Fe-18Cr-10Ni-5.4Mn, simultaneous measurement of the pH and Cl − concentration was performed in 0.01 M NaCl (pH 3.0). The corrosivity of the crevice solution was different before and after crevice corrosion initiation, and the sudden changes in the pH and Cl − concentration inside the crevice were the same as those for Fe-18Cr-10Ni-5.4Mn. The incubation period was characterized by weak acidification (pH 1.6−2.0) and a small accumulation of Cl − (0.1−0.3 M). The growth period was characterized by strong acidification (pH ≤ 1.0) and a large accumulation of Cl − (≥1.0 M). At the crevice corrosion initiation site, a metastable pit was observed and a S signal was detected from the residual in the pit. It can be concluded that the crevice corrosion of Type 430 stainless steels is initiated by Cl − , which generates metastable pitting at sulfide inclusions. The effect of the electrode potential on the pH and Cl − concentration was investigated in 0.01 M NaCl from −0.05 to 0.1 V. No effect of potential was observed during both the crevice corrosion incubation and growth periods.