Enhancing the mechanical and anticorrosion properties of 316L stainless steel via a cathodic plasma electrolytic nitriding treatment with added PEG

A cathodic plasma electrolytic nitriding (CPEN) treatment with a urea aqueous solution was performed on 316L stainless steel to rapidly improve its surface properties in this work. Test results show that the PEG2000 macromolecules increased the nitriding energy via enhancing the ability to bond the produced gas film to the metal/electrolyte interface. The cross-sectional morphologies indicate that a thick nitrided layer was obtained when the urea concentration was 543 g l⁻¹, corresponding to a Vickers hardness 450 HV_0.1, which was 3.5 times larger than that of the substrate. The nitrided layer mainly contained expanded austenite (γ_N), oxides and iron nitrides (e.g., Fe₃O₄ and FeN_0.076). In terms of its performance, coefficient of friction (COF) of the nitride layer decreased to nearly two-thirds that of the untreated layer, and the passivation current densities of the nitrided sample in a 3.5% NaCl solution decreased by an order of magnitude compared to that of the substrate. Therefore, the approach presented herein provides an attractive way to modify the effect of CPEN in a urea aqueous solution.