Study of Imperfections and Hardness of Anodic Oxide Films on AA2024 Based on Ink-Impregnation Method

Abstract

Various imperfections can be formed in the anodic oxide films on aluminium alloys because of second-phase particles and they affect corrosion resistance and surface hardness of aluminium alloys. In this work, ink-impregnation method was newly employed to observe imperfections in the anodic films on AA2024 and to measure surface hardness of the anodic oxide films with different imperfections. The ink-impregnation allows clear observation of not only the imperfections within the anodic oxide films but also an indentation mark on the oxide film surface. Two different imperfections of black defects and grey defects were found in the anodic oxide films formed on AA2024 in 20% sulphuric acid solution and the surface hardness was dependent on the type, size and density of defects. Large size black defects showed the lowest hardness of 60 ~ 100 Hv. A little bit higher hardness of 100 ~ 140 Hv was measured on the large size grey defect. More increased hardness of 140 ~ 170 Hv was observed on the oxide surface region with mall size black and/or grey defects. The highest hardness of 170 ~190 Hv was obtained from the oxide surface without defects. Distortion of the pyramidal indentation marks were observed only in the regions with a large size black and grey defects.