Influence of the Anodic Oxide Film on Pitting of Iron

Abstract

The fundamental processes leading to the initiation of pitting on pure Fe in acetate and borate solutions containing halide ions are discussed. The results of potentiostatic step experiments show that pit initiation is associated with a critical stage in the development of the passive oxide film, likely corresponding to a certain critical oxide thickness. The critical oxide thickness depends on the identity (chloride or bromide) and the concentration of the halide ion in solution, and on the supporting electrolyte, but not on the anodic potential. Acetate is shown to inhibit pitting. The critical oxide film characteristics have been investigated using galvanostatic cathodic reduction techniques. Scanning electron microscopy with computerized image analysis has been used to study the size distribution of the pits formed in these experiments. The results can be used to support the proposed model for pitting.