Effect of tungsten alloying on passivity breakdown of nickel

The addition of small amounts of tungsten (W) on the passivity breakdown of nickel (Ni) in borate buffer solutions as a function of chloride activity, pH, and voltage sweep rate has been evaluated and the data are interpreted in terms of the point defect model (PDM). The critical pitting potential of Ni–W alloys was found to become more positive (correspondingly a higher resistance to pitting) upon increasing the W content up to 3 at.% but decreases with further additions. The initial increase in the breakdown voltage with increasing [W] is attributed to changes in the diffusivity of the cation vacancy, the electric field strength, the annihilation rate of cation vacancies ( J m ), changes in the standard Gibbs energy of adsorption of Cl − into surface oxygen vacancies, and in the standard Gibbs energy of generation of Schottky pairs, as predicted by the SVIM. At high [W] (4–5 at.%), it is postulated that all of the mobile cation vacancies are complexed with immobile [W(VI) cation substituted onto the Ni(II) vacancy of the cation sublattice of the barrier layer], so that further additions of W are predicted to have little positive effect in improving the pitting resistance of the alloy. Instead, W additions above 3% are postulated to increase the population density of breakdown sites on the surface via the formation of intermetallic precipitates, which has the effect of displacing the distribution in the breakdown voltage in the negative direction.