Influence of pH on the anodic dissolution mechanism of Fe–Mo alloys in sulphate solutions

Abstract

The anodic dissolution of Fe–Mo alloys (10 and 20 wt% Mo) in 1 M sulphate solutions with a pH ranging from 1 to 5 was studied by dc polarization and ac impedance techniques. Two types of behaviour were observed: at low pH values, small overpotentials and low Mo contents there is a qualitative similarity between the ac impedance spectra of the alloy and that of pure Fe, whereas for high pH values, overpotentials and Mo contents the impedance response is determined by the impact of the molybdenum additive. A total of four time constants was detected in the impedance spectra implying the existence of three reaction intermediates. A kinetic model is proposed representing the active dissolution of a binary alloy as a superposition of two multistep reactions which lead to the abstraction of iron and molybdenum ions from the alloy substrate into the solution. It was found to simulate adequately both the polarization curves and ac impedance response especially for the type of behaviour pertinent to the impact of molybdenum additive. Kinetic parameters of the anodic dissolution of Fe–Mo alloys are determined as depending on the solution pH and Mo content.