Theory of competitive adsorption and its application to the anodic dissolution of nickel and other iron-group metals—II. The steady state in the prepassive, passive and transpassive potential ranges

Abstract

The theory developed in Part I has been extended to include the action of chemisorbed hydroxide. Equilibrium charge transfer adsorption is able to predict the negative reaction orders with respect to hydroxide ions which have been reported for solutions high in chloride and hydrogen ions, as well as the occurrence of a maximum and its dependence on the composition of the solution. It has been concluded that it is likely that passivation is in the first instance due to the chemisorption of hydroxide. If, on the other hand, hydroxide ions are assumed to have a catalytic action in metal dissolution, albeit with a very low anodic desorption rate, then the relevant equations are in addition able to predict passive and transpassive behaviour. Thus the steady state anodic current—potential relationship in the potential range between hydrogen evolution and oxygen evolution can in principle be expressed in one equation. The remarkable agreement between predicted and observed anodic behaviour makes it likely that competitive adsorption plays an important role in the passive range, even in the presence of an oxide film. Finally it has been investigated to what degree the theory can contribute to a better understanding of the behaviour of passive metals.