The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn Stainless Steel

The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn stainless steel has been investigated by potentiokinetic techniques in a 1000 ppm NaCl solution. Nitrogen additions increased the pitting resistance of the steel irrespective of structure, however, the ferritic steel was less pit resistant than the (duplex) steels containing both austenite and ferrite which, in turn, were less pit resistant than the totally austenitic steels. For steels having a duplex structure, the effect of nitrogen on the pitting resistance was observed to follow a linear function of the relative amount of austenite in these steels due to the area effects of the austenite and ferrite which are galvanically coupled in these steels. The addition of nitrogen was found to increase the amount of austenite at a rate of approximately 200 times the percent nitrogen addition from 36 pct austenite for the 0.02 pct N steel to 100 pct for the 0.40 pct nitrogen steel. The addition of nitrogen to the totally austenitic steels increased the pitting resistance at the rate of approximately 0.31 volts per pct nitrogen added, but no mechanism was found for the increased resistance. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponsorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.