Comparison of Mo–N and W–N synergism during passivation of stainless steel through x-ray photoelectron spectroscopy and electrochemical analysis

In high nitrogen stainless steels and nitrogen surface-doped Mo-containing alloys, nitrogen has been shown to enhance passivity. The mechanism of this apparent Mo–N synergism appeared to be twofold. First, the formation of stable surface nitrides affects a reduction in active dissolution, and, second, reactions between the surface nitride and the electrolyte lead to an increase in interfacial pH, a condition favorable to the formation of an enhanced bipolar passive film. In this study, we compare and contrast the effect of surface nitriding of a Fe–19Cr–9Ni–2W on passivity to the effects previously reported for analogous systems alloyed with Mo. Electrochemical potentiodynamic polarization in deaerated 0.1 M HCl is used to examine the general effects on corrosion inhibition. Qualitative and quantitative analysis of the surface of each alloy following doping with nitrogen and passivation was conducted with variable angle x-ray photoelectron spectroscopy to provide depth-dependent information on the types and amounts of chemical species present. The synergism between W and N is found to be similar to that of Mo and N in enhancing the bipolar nature of the passive film by increasing the amount of tungstate incorporated into the outer layer of the passive film. The primary differences between the structures of the passive films in the two cases result from the passive nature in acidic media of the trioxide of tungsten as opposed to that of molybdenum.