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In this study the effects of low temperature plasma nitriding on the characteristics of different austenitic stainless steels, CrNi-based (AISI 304L and AISI 316L) and CrMn-based (AISI 202), were compared. Samples were nitrided at 400 and 430 C, at 1000 Pa for 5 h, and their microstructure, phase composition, microhardness and corrosion resistance were evaluated. The characteristics of modified surface layers depended on both treatment parameters and alloy composition. For all the steels modified surface layers had a double layer microstructure. In the outer modified layer, mainly consisting of S phase, deformation (or shear) bands were observed in the grains, and nitrogen induced h.c.p. martensite, ε 0 N , formed. The tendency to form shear bands and ε 0 N was higher for AISI 202 samples, and decreased for AISI 304L and then for AISI 316L ones, influencing the modified layer thickness. When nitriding was performed at 430 C, nitrides formed, and their amount was affected by steel composition. Nitriding treatments allowed to markedly increase surface microhardness and corrosion resistance, in comparison with the untreated alloys. When nitrides did not form, as for the 400-C nitrided samples, the corrosion behaviour of the considered steels was comparable. Nitride precipitation affected corrosion resistance, increasing corrosion phenomena.

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Highlights Alloy composition influences the corrosion resistance of nitrided stainless steels  S phase without nitrides can be obtained on nickel-free austenitic stainless steel  Nitrogen surface alloying increases nobility of austenitic stainless steels  Tendency to resist localized corrosion phenomena is AISI 202 < AISI 316L < P558

Austenitic stainless steels are employed in many industrial fi elds due to their very good resistance to general corrosion in several environments. However, their use is limited due to the fact that they suffer localised corrosion in specifi c environments, particularly in chloride-ion rich solutions. Low temperature nitriding treatments can improve both corrosion resistance in chloride-ion containing media and surface hardness, due to the formation of a metastable phase known as expanded austenite or S phase; this phase can be outlined as a supersaturated interstitial solid solution of nitrogen in the expanded and distorted γ -Fe f.c.c. lattice. We review the main experimental results obtained in our research on low temperature glow-discharge nitriding of austenitic stainless steels. By means of proper treatment parameters, low temperature glowdischarge nitriding is able to markedly improve the corrosion resistance of austenitic stainless steels, such as AISI 316L and AISI 202 in chloride-ion rich solutions in comparison with the untreated alloys.