Influence of Pb++ ions on the dissolution and passivation of nickel and Ni–21Cr in acidic solutions

The deleterious effect of dissolved Pb++ on passivity of Ni and Ni–21Cr has been explored at 90 °C using two solutions: 0.1 M perchloric acid, and a pH 3.5 acetic acid/sodium acetate buffer. Pb++ strongly inhibits true (low-potential) active dissolution of pure Ni; this is consistent with underpotential deposition (UPD) of elemental Pb, apparently a new observation. The range of potentials where this occurs is greater for the more acidic solution. By inoculating Pb++ at different potentials, the maximum underpotential can be shown to be at least 300 mV. In the less-aggressive acetic acid environment, a ‘pre-passive’ state of Ni is more noticeable than in the perchloric acid, and in contrast to the true active region, this region is subject to a strong activating effect of Pb. Dissolution of Ni proceeds rapidly on a surface that is trying to form its pre-passive film but is partly covered by Pb. The time-dependence of the passive current in the acetic solution is altered subtly by incorporation of Pb into the metal-film interface, such that the normal decay with time is truncated, leading to a higher long-term dissolution or film thickening rate. The presence of 21% Cr suppresses active dissolution, greatly ennobles the open-circuit potential, and eliminates (for these particular conditions) both the inhibiting and the activating effects of Pb. The relevance of these results to corrosion and SCC caused by lead in nuclear power systems is briefly discussed.