Electrochemical Properties of Fe-Cr-Mo Alloys and Fe₂O₃-Cr₂O₃-MoO₂ Artificial Passivation Films in 1 M HCl

This chapter presents a study to analyze the composition of passive films formed on Fe-Cr-Mo alloys for the model of artificial passivation films, and to clear the effect of Mo content on the reductive dissolution of passive films on Fe-Cr-Mo alloys. It examines the effects of MoO₂ content on the reductive dissolution of Fe₂O₃-Cr₂O₃-MoO₂ films, which simulate passive films on Fe-Cr-Mo alloys, and also investigates the pitting inhibition mechanism of alloyed Mo in 1 M HCl. Anodic polarization curves of Fe-18Cr-xMo alloys showed that the alloys containing 5-10% Mo suffered from no pitting in 1 M HCl. The depassivation pH of the alloy depended on the Mo content, and indicated that the reductive dissolution of the passive film was suppressed by Mo. The thinning rate of Fe₂O₃-Cr₂O₃-MoO₂ films as a function of potential showed that the reductive dissolution occurred on the films with low Cr and Mo cationic fractions in the potential range of pitting of the alloys. The film with the cationic fractions X_cr = 0.5 and X_MO = 0.1, however, suffered from no reductive dissolution in 1 M HCl, and these cationic fractions coincided well with the composition of passive film on the Fe-18Cr-10Mo alloy.