Investigation of oxide growth during hard anodizing using in-situ and ex-situ EIS measurements

In this work, the oxide growth during soft and hard anodizing was investigated by a combination of in-situ and ex-situ EIS measurements. In-situ electrochemical impedance spectroscopy (EIS) measurements were found to provide accurate estimates of barrier layer thickness during soft anodizing but became inaccurate during the later stages of growth under hard anodizing conditions. This was attributed to the formation of a gradient in electrolyte composition, from the pore mouth to the pore base, caused by diffusion limitation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that, in conditions where such a gradient was established, a rough internal pore morphology was present. The reduced availability of sulphate anions at the pore base, hinders the ejection of aluminium ions into the electrolyte, resulting in the formation of additional oxide or hydroxide at the pore base and the rough internal pore morphology.