Internal attack during the oxidation of nitrided stainless steels

Abstract

Oxidation tests have been performed at 1123°K in CO ₂ on a 20 wt.% Cr-25 wt.% Ni stainless steel containing a dispersion of titanium nitride particles produced by prior nitriding. Two forms of internal attack are distinguished: inger-like, grain boundary penetrations of internally oxidized silicon and globular, discrete voids extending to greater depths within the alloy. The latter are the result of the dissociation of chromium-rich particles in the depleted zones formed near the specimen surface during the oxidation process. The kinetics of this globular attack are parabolic and are closely related to the growth of the depleted zone. The theoretical expectation of a linear relationship between the depth of attack and the thickness of the surface chromic oxide is confirmed. The effect of variation in the chromium and residual nitrogen contents on the depth of attack is discussed in detail.