Anodic Polarization and Passivity of Ni and Ni‐Cu Alloys in Sulfuric Acid

Abstract

Potentiostatic data are reported for anodic polarization of Ni at 25° and 40°C and for Ni‐Cu alloys at 25°C in deaerated . The critical potential for passivity of Ni follows the relation: pH and the critical current density (ma/cm2) follows: . At 40° the critical potential in is the same as at 25° within experimental error; the critical current density for passivity or for maintaining the passive state is higher. A Tafel relation holds in the transpassive region with slope of 0.140 v (25°) independent of pH. The critical current density for passivity and the minimum current density to maintain passivity move to higher values as Cu is alloyed with Ni. Above about 70% Cu, vestiges of passivity are lost and the alloy behaves as does Cu. This critical alloy composition is related to a filled d band electronic structure corresponding to relatively short life of the adsorbed passive film above 70% Cu. Thermodynamic properties of the passive film are discussed.