Introduction
Low-temperature nitriding of austenitic stainless steels allows for the formation of a supersaturated solid solution of nitrogen in the austenite lattice (expanded austenite or S-phase), while inhibiting the precipitation of Cr nitrides. The corrosion behavior of the nitrided layers depends on their microstructure and phase composition, as well as on the environment characteristics. The testing conditions can also play a role, as observed when the repassivation characteristics are assessed. The aim of the present study is to evaluate the corrosion behavior of low-temperature nitrided AISI 316L austenitic stainless steel using different electrochemical techniques and, in particular, to assess the repassivation capability in NaCl solution.
Methods
AISI 316L samples were glow-discharge-nitrided at 380 °C, under 130 Pa, for 5 h. Microstructure, phase composition, and surface microhardness were assessed. Corrosion behavior was evaluated in 5 wt.% NaCl aerated solution using different electrochemical techniques (electrochemical impedance spectroscopy (EIS), cyclic potentodynamic, and galvanostatic techniques).
Results
The hardened nitrided layers mainly consisted of expanded austenite. EIS analysis showed that nitrided samples had higher impedance values than the untreated steel. The cyclic potentiodynamic test results were affected by testing conditions. Corrosion potential and pitting potential values of the nitrided samples were higher than those of the untreated ones, but the nitrided samples were not capable of repassivating. On the contrary, when the galvanostatic technique was employed, the potential value, below which localized corrosion phenomena did not occur, was usually higher than the corrosion potential of the untreated alloy, suggesting that repassivation could occur.
Conclusions
The experimental results suggested that the repassivation capability of nitrided AISI 316L austenitic stainless steel was particularly sensitive to the extent of damage. When minor damage occurred, from, for example, using the galvanostatic technique, high corrosion resistance was maintained. However, fairly significant corrosion damage, such as that occurring in cyclic potentiodynamic tests, hindered repassivation.
