THE EFFECT OF LEAD IONS ON THE DISSOLUTION AND PASSIVATION OF NICKEL BASE ALLOYS

摘要

The anodic behaviour of a range of nickel-base materials including pure Ni, Alloy 600, and Ni-21Cr has been examined at 90℃ using strongly and mildly acidic solutions. Lead oxide was injected at potentials above the Pb~(++)/Pb equilibrium so that no bulk Pb deposition was possible. Underpotential deposition (UPD) of Pb on Ni was inferred from the strong inhibiting effect of PbO addition in the true active dissolution region of pure Ni in strong acid. In the prepassive region, where a Ni(OH)_2 type of film is thought to be present, competition of Pb and OH for surface sites led to a strong activation of dissolution (delay of true passivation) in the mildly acidic environment. In the passive region, some impairment of the long-term passive current decay was observed. Cr alloying generally suppressed all three effects of lead, but the Cr content of Alloy 600 was insufficient to do this reliably, and evidence of nickel-like behaviour was present for this alloy.