Effect of Pb-underpotential deposition on anodic dissolution and passivation of pure Fe and Fe-Ni alloys in acidic perchlorate solution

The potentio-dynamic polarization curves of pure Fe, Fe-30 Ni, and Fe-70 Ni alloys in acidic perchlorate solutions (pH 1.9) without and with 10−3 M Pb2+ were measured to investigate the effect of Pb-underpotential deposition (Pb-UPD) on anodic dissolution and passivation in relation to Pb-induced stress corrosion cracking (Pb-SCC) of Ni base alloys. The addition of 10−3 M Pb2+ shifts the open circuit potentials of pure Fe and Fe-Ni alloys toward noble direction to inhibit the anodic dissolution and promote the passivation, which results from Pb-UPD on substrate metals. The electro-desorption of Pb proceeds with anodic potential sweep and the anodic dissolution is enhanced when the surface coverage of Pb is reduced to a critical level. Tafel slopes (b+ = 8.5∼15 mV decade−1) of anodic dissolution for pure Fe and Fe-Ni alloys in the presence of Pb2+ are significantly low as compared with those (b+ = 34∼40 mV decade−1) in the absence of Pb2+, which reflects on the rapid enhancement in surface reactivity as a result of electro-desorption of Pb. It is found that the potential region in which anodic dissolution is inhibited by Pb-UPD is located within the potential window of Pb-UPD estimated from the differences in work-function between substrate metals and Pb.