Passivity in Cu-Ni-Al Alloys—A Confirmation of the Electron Configuration Theory

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Abstract
Potentiostatic anodic polarization data show that a transition from active to passive state is observed for Ni‐Cu and Ni‐Cu‐Zn alloys only when the 3d band of energy levels in the alloy is unfilled. At and above a critical alloy composition, electrons from Cu or Zn completely fill the d band and the passive state disappears. Previous data showed that the calculated critical composition comes close to the observed value assuming that alloyed Cu donates one electron and alloyed Zn two electrons per atom. Present measurements for the 45% Ni‐Cu‐Al ternary alloys show that Al contributes three electrons per atom. Secondary passivity, which is observed for pure nickel in a potential region noble to that for primary passivity, is similarly observed in the Cu‐Ni‐Al alloys, but again only when the d band of energy levels is unfilled. Potential decay curves confirm that a passive film, either in the primary or secondary region, exists only on alloys with an unfilled d band. Decay potentials corresponding either to secondary or to primary passivity (Flade potentials) are not greatly sensitive to aluminum content nor to the absolute number of electron vacancies. However, alloyed aluminum affects the passive current density for the 45% Ni ternary alloys accounting for a minimum at 2.35percnt; Al.

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