The Role of Chloride in Al Oxide Surface Transformations and Pitting -an AFM Study

Abstract

In this study we used the in situ AFM technique to examine the Al surface morphological transformations during anodic polarization. In order to elucidate the possible role of chloride in the surface oxide breakdown process, we conducted in situ AFM polarization studies in sulfate (50 mM K2SO4) and chloride (50 mM NaCl) solutions where Al corrosion behavior is known to be different. The results show different changes and structures in two electrolytes: (a) for the case of polarization in sulfate solutions, uniform changes of oxide morphology are observed most likely resulting from oxide hydration and growth; (b) for the case of polarization in chloride solutions, morphological changes are observed to be surface-site energy dependant. The changes of Al-oxide are very pronounced around the regions of faceted edges and grain boundaries i.e. regions of higher local roughness and curvature.