Pitting of Passivated Cadmium Monocrystals

摘要

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 × 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), {1010} and {1120} 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.