Corrosion of low-temperature nitrided molybdenum-bearing stainless steels

Research highlights: → S phase exhibited improved pitting resistance but deteriorated passivity. → Initially high anodic currents for S phase were followed by faster current decay. → Surface films after corrosion contained CrN and Cr2N and oxides of Fe, Mo and Cr. → Pitting resistance can be due to fast accumulation of oxides and CrN. - Abstract: Austenitic stainless steels with up to 6.1 wt.% Mo were nitrided at 425 oC and examined in 0.1 M Na2SO4 without and with chlorides at pH 3.0 and 6.5. Nitrided steels exhibited an increased resistance to pitting, but at pH 3.0 they had a decreased resistance to general corrosion. After corrosion at pH 3.0 surface films contained chromium nitrides and oxides of Mo, Cr and Fe. It is proposed that the improved pitting resistance of nitrided steels is associated with the initially accelerated dissolution which leads to the accumulation of corrosion resistant CrN and of oxidised steel components.