(Invited) Formation of Self-Organized Porous Anodic Films on Iron and Stainless Steels

Abstract

High purity iron and commercial Type 304 and 430 stainless steels were anodized in ethylene glycol electrolytes containing fluoride and water. When iron was anodized in the electrolytes containing relatively high water concentrations (≥1.5 mol dm−3) at 50 V, porous anodic films with scalloped metal/film interface were developed. In contrast, the anodic films formed in the electrolytes with low water concentrations (≤0.5 mol dm−3) resulted in the flat metal/film interface. In addition, the interpore distance became smaller with respect to the thickness of barrier layer when the water concentration is reduced. These morphological changes may be associated with the change in the composition of the anodic films. Anodizing of stainless steels was dependent upon the type of stainless steel; a steady-state current continued up to ~160 V on Type 430 stainless steel, while exponential current increase occurred above ~50 V on Type 304 stainless steel during potentiodynamic anodizing. The thickness of the barrier layer and the morphology of the porous layer were largely dependent upon the type of stainless steel. Based on such findings, the mechanism of film growth was discussed.