Improved saline corrosion and hydrogen embrittlement resistances of superaustenitic stainless steel by PIII nitriding

Abstract

PIII nitriding was applied to superaustenitic steel to improve wear resistance without compromising pitting and hydrogen embrittlement corrosion resistance. Nitriding at temperatures between 300 °C and 400 °C produced modified layers rich in γN phase and with thicknesses ranging from 1.2 ± 0.1 μm to 4.2 ± 0.1 μm. Nanoindentation tests indicated an increase in hardness up to ∼4 times compared to the untreated sample. The potentiodynamic polarization curves indicated an increase in pitting and crevice corrosion resistance after nitriding. In the PIII 350 °C nitrided sample, Ecorr and Epit increased 50% and 14%, respectively, while current densities icorr and ip decreased by 80% and 77% and crevice volume decreased by 93% when compared to the untreated sample. However, in the nitriding condition at 400 °C, the improvement the resistance to saline corrosion is not so marked was observed as a result of the increased passive current density due to formation of CrN The micrographs of surfaces after cathodic hydrogenation also indicated reduction in hydrogen embrittlement in the PIII 300 °C sample. In the PIII 400 °C sample, the hydrogen embrittlement was identified by the delamination and cracking of the modified layer. Thus, the formation of chromium nitrides must be avoided and only the γN phase must be formed to improve the resistance to hydrogen embrittlement and saline corrosion.