Nature of anodic films on cadmium in alkaline electrolytes

Abstract

Smooth cadmium electrodes with a surface essentially free of cadmium oxide and hydroxide were oxidized at constant potential between 0.05 and 1.0 V in various alkaline electrolytes. Layer formation was followed by recording the current as a function of time. Anodic films formed after different oxidation times at various potentials were investigated by electron microscopy, infra-red spectroscopy, and X-ray diffraction. The films grow non-uniformly on the surface. The number of nuclei increases with oxidation potential. Infra-red spectra demonstrate the presence of β-Cd(OH)₂ or γ-Cd(OH)₂, or a mixture of both types of hydroxide in agreement with the X-ray diffraction results. The total amount of hydroxide decreases with potential and is, in general, smaller than the amount corresponding to 100 % efficiency of hydroxide formation. This deficit is attributed to the simultaneous formation of amorphous CdO. During the reduction at constant cathodic potential cadmium oxide and γ-Cd(OH)₂ disappear first, and then β-Cd(OH)₂. The conversion of anodically formed CdO into β-Cd(OH)₂ is a slow process.