EFFECT OF D-BAND STRUCTURE ON SECONDARY PASSIVITY IN NI-CU ALLOYS

Ni-Cu and Ni-Zn-Cu alloys are shown to exhibit a region of secondary as well as primary passivity. Potential decay curves in 1N H2SO4 show discontinuities which average 1.7V in the secondary region for both the alloy series and 0·79V for the Ni-Zn-Cu series and 0.68 to 0.78V for the Ni-Cu series in the primary region. These discontinuities, as well as observed passive c.ds. in both the primary and secondary regions, disappear when the atomic ratio Cu/Ni or (Cu + 2Zn)/Ni exceeds 1·6. The same critical atomic ratio was found previously for solid solution Ni-Al-Cu alloys. It appears plausible that at the atomic ratio 1·6, the d-band of electronic energy levels is just filled by one donor electron per Cu atom, 2 per Zn atom or 3 per Al atom. An unfilled d-band favours chemisorption of oxygen of less than monolayer to multilayer dimensions; the corresponding adsorbed film is considered to be the passive film. It functions not as a diffusion barrier, but to increase over-voltage for anodic dissolution. The film corresponding to secondary passivity is thicker than the primary film, and is also considered to have an adsorbed structure, decomposing to the primary passive film which in turn decomposes at the Flade potential. © 1969.