Effect of Nitrogen Addition on Localized Corrosion Behavior in SUS316L Stainless Steels

Abstract

The effect of nitrogen addition up to 0.556 mass% on corrosion behavior in SUS316L type austenitic stainless steels was examined, where a particular emphasis was put on chemical analysis of an anolyte by the macro-cell method. It is found that the pitting potential becomes higher with an increase in nitrogen content. The critical pitting temperature is elevated by nitrogen addition and well summarized by using the pitting index of Cr+3Mo+26N (mass %). From the analysis of the anolyte cell solution by the galvanostatic electrolysis with macro-cell instrumentation, NO₃^- is detected only in nitrogen bearing steels when the potential is kept at a higher level, while NH₄⁺ is detected in nitrogen free steels and in nitrogen bearing steels when the potential is lowered. The potential-pH equilibrium diagram for nitrogen-water system shows that NH₄⁺ is stabilized at lower potentials and NO₃^- is done at higher potentials. It is believed that NO₃^- acts as a powerful inhibitor against CΓ attack to the passive film while NH₄⁺ raises pH in a corrosion pit so as to suppress the localized corrosion.