Influence of molybdenum on the anodic dissolution of iron in acidic solutions

The anodic dissolution of pure iron and binary iron-molybdenum alloys (10 and 20 wt % Mo) in molar H2SO4 and HCl solutions was studied by d.c. polarization and a.c. impedance techniques. Molybdenum additive influences both the steady state polarization curves and the impedance spectra in the two media, the impact being stronger for the Fe-20 wt % Mo alloy. In general, two-slope polarization curves are obtained for the alloys; impedance spectra exhibit two time constants in addition to the charge transfer one implying the existence of two reaction intermediates. Differences between spectra measured in H2SO4 and HCl are discussed. A kinetic model advanced on the basis of literature data for pure iron and molybdenum was able to reproduce quantitatively both the steady state and a.c. impedance results for both alloys in the two media. Kinetic parameters for the anodic dissolution of Fe-Mo alloys are thereby determined.