Mechanistic study of thin film sulfuric acid anodizing rate difference between Al2024 T3 and Al6061 T6

Why different aluminum alloys anodizing at very different rate has been a puzzle for more than half a century. In this study, two high strength aluminum alloys (Al2024 and Al6061) were quantitatively compared for thin film sulfuric acid anodizing (TFSAA) process, on anodizing current density, anodized coating thickness, coating weight, and anodizing efficiency, to understand the underlying mechanism for the different anodizing rates. In-situ electrochemical impedance (EIS) measurement was carried out to deconvolute the different factors that affect the anodizing process. Anodized coating thickness and weight growth were correlated to different charge transfer resistances from the equivalent circuit model. The higher Cu content in Al2024 led to slower anodizing rate due to enhanced charge transfer resistance, and to lower anodizing efficiency through its catalytic effect for O2 evolution side reaction. A new schematic mechanism of the anodizing process was proposed taking into consideration of the electrolyte chemistry. O2 generation from water oxidation in the porous anodic oxide layer during anodizing was elaborated as the main factor for anodizing efficiency loss.