Glow-discharge nitriding of AISI 316L austenitic stainless steel: influence of treatment time

In the present paper, the results relative to low-temperature glow-discharge nitriding treatments of AISI 316L austenitic stainless steel are reported. This process is able to produce surface-modified layers essentially composed of a metastable phase called S-phase. Nitriding treatments were carried out at 703 K for times ranging from 0 to 5 h. Treated samples were characterized by means of morphological analysis, surface and profile microhardness measurements and electrochemical tests in NaCl aerated solutions in order to investigate the influence of treatment time on the microstructure, the hardness and the corrosion resistance properties; moreover, the results were compared with those observed for untreated samples. The thickness of the modified layers increases as the nitriding time increases. The surface hardness of treated samples is higher than the untreated AISI 316L stainless steel samples and increases as the treatment time increases up to a value of about 1450 HK0.1. The pitting corrosion resistance of nitrided samples increases as the treatment time increases; after a 5-h nitriding treatment, the anodic current values are reduced of about 3 magnitude orders in comparison with the untreated samples. Low-temperature glow-discharge nitriding treatment is an effective technique to increase the pitting corrosion resistance and the surface microhardness of AISI 316L austenitic stainless steel.