Structure and Corrosion Resistance of Plasma Nitrided Stainless Steel

TLDR

AISI316 stainless steel was plasma nitrided at 570 °C under varied conditions, and X‑ray diffraction and TEM mapped the process parameters that produce a duplex surface compound layer of γ′ phase and austenite, whose corrosion properties were then assessed by potentiodynamic polarization. The duplex γ′/austenite surface compound layer provides superior corrosion resistance compared to the conventional austenite‑chromium nitride hardened layer, with the benefit attributed to γ′ nitride, while low‑temperature (400 °C) nitriding yields a layer whose corrosion resistance matches that of the untreated material.

Abstract

AISI316 stainless steel has been plasma nitrided at 570°C over a range of processing conditions, and the resultant corrosion properties have been investigated by a potentiodynamic polarization technique. X-ray diffraction and transmission electron microscopy studies have been used to map the process parameters under which a duplex surface compound layer of γ’ phase and austenite is formed. This surface compound layer has better corrosion resistance than a plasma nitrided stainless steel surface, where the normal hardened layer consists of austenite’ and chromium nitride precipitates. It has been found that the improvement in corrosion resistance is related to the presence of the γ’ nitride. Furthermore; low temperature plasma nitriding at 400° C produces a nitrided layer which has a corrosion resistance equivalent to that of the original material.