Pitting of Passivated Cadmium Monocrystals

Abstract

A combination of aqueous thermodynamics, anodic polarization tests and scanning electron microcopy was used to study the pitting behavior of Cd monocrystals in a phosphate buffered (pH 11.2) solution of 0.1 M NaCl. Pitting occurred in the passive region where the most stable passive film was Cd (OH)2. The initial film thickness was estimated to be ~4 x 10-9 m. Pitting potentials were not reproducible, being variable within surfaces of the same orientation and between different orientations. Variability was attributed to structural defects in the passive film playing a major role in pit site nucleation. Early stages of pitting produced crystallographic facets consistent with (0001), and planes, the morphology becoming increasingly complex with pit growth. The pH decreased in the growing pit and increased the influence of chloride species on localized dissolution behavior.