Effect of Nitrogen Alloying on the Pitting of Type 310 Stainless Steel.

Abstract

The effect of nitrogen alloying on the pitting behavior of type 310 stainless steel has been investigated through measurements of pitting potential (E ' pit ) as a function of temperature and concentration of NaCl (C NaCl ). Nitrogen was effective to shift E ' pit to nobler direction especially at temperatures below critical one. The critical pitting temperature was defined as the temperature below which the usual linear relationship between E ' pit and logarithm of C NaCl did not stand. Alloying the stainless steel with nitrogen increased the critical pitting temperature. Below the critical temperature where E ' pit did not follow the usual dependency on C NaCl , pitting was retarded most effectively by nitrogen except when C NaCl was so high that E ' pit lay below ca. 400 mV. Although the whole mechanism of nitrogen is not still clear, nitrogen is most likely to suppress acidification of pitting site through formation of ammonium ion. Nitrogen in a metal matrix and nitrate in a solution seemed to have a common feature with respect to the potential dependency of inhibition efficiency. The fact that nobler potentials were more favorable for both nitrogen in metal matrix and nitrate in a solution for inhibition seemed to indicate that oxidation of nitrogen to nitrate might also be involved in the inhibition mechanism.KEY WORDS: nitrogen; stainless steel; pitting; chloride; nitrate; temperature. Table 1. Chemical composition of the stainless steels investigated (mass%).

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