Considerable interest has developed in nitriding stainless steels under conditions such that the nitrogen is present only in solid solution. That is, no chromium or iron nitrides etc. are formed. This is achieved by nitriding at low temperatures (below 450°C) under which the kinetics of nucleating and growing these nitride phases are much slower than the diffusion times to achieve the desired case depth. Very high nitrogen contents are achievable: 30 atomic percent is practical! The effect of the supersaturated nitrogen is to significantly increase the hardness and develop high residual compressive stresses. Several authors have also shown improvements in fatigue life and corrosion resistance.

Recent work by Nitrex Metal Inc. has confirmed the improvements in corrosion resistance by anodic polarization tests on austenitic, martensitic and precipitation hardenable stainless steels. The nitrided corrosion currents were at least 1000× smaller than those measured on the bare metal.

This paper will expand on this topic by evaluating the relationship between the nitrogen content and the corrosion resistance. It will discuss Nano-S® nitriding processes performed at various nitriding potentials to achieve a range of nitrogen contents in an austenitic stainless steel. The corrosion resistance will be measured by anodic polarization tests. From these results, optimum nitriding conditions to achieve the desired properties may be recommended.