The Anodic Oxidation of Cadmium

Cadmium electrodes under anodic oxidation in hydroxide and carbonate electrolytes have been examined by analysis of the decay of overpotential after current interruption. After passivation while oxygen is being evolved, except for a small instantaneous (<1 msec) ohmic drop, the potential decays logarithmically with time in the manner usually found for the decay of activation overpotential of a gas electrode. Before passivation while the metal is actively being oxidized, however, except for the small ohmic drop, the potential decays exponentially with time in two steps. This behavior is not described by activation theory. Determinations were made of the capacitance of both the oxidizing and the passivated cadmium, both from potential decay curves and from superimposed a.c. All the a‐c values correspond to those of a normal double layer, as do those obtained from decay measurements after passivation has occurred. However, the values obtained from decay curves before passivation are two orders of magnitude higher early in the oxidation but drop rapidly toward the a‐c values as oxidation proceeds. It is proposed that next to the Helmholtz double‐layer at the ‐electrolyte interface, there exists a highly polarized inner double‐layer in which the reaction takes place. When the field strength across the inner double layer becomes high enough for the reaction to take place instead, oxygen is evolved and the electrode passivates. The roles of adsorption of to form the Helmholtz layer and of interference by are discussed in terms of the results.