Abstract
The dense passive film on the surface of austenitic stainless steel isolates the corrosion medium from the matrix, which makes it have excellent corrosion resistance. However, it also hinders the penetration and diffusion of nitrogen atoms in the process of gas nitriding. During gas nitriding of stainless steel, we found a very interesting phenomenon. When trace water in ammonia is at a certain threshold, ammonia, water, and chromium oxide passivation film will undergo gas-solid reaction to break the passivation film on the surface of stainless steel. The results show that the trace water content in ammonia during gas nitriding is very important for in-situ breaking of passive film. When the trace water content exceeds the upper threshold, oxidation will occur on the surface of stainless steel. On the other hand, when the trace water content is lower than the lower threshold, the passivation film hinders the diffusion of nitrogen atoms, and the nitride layer cannot be obtained. The mechanism of in-situ removal of passive film by trace water during gas nitriding of stainless steel is revealed for the first time, which provides a new idea and method for the study of surface modification of stainless steel by gas nitriding.
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