Correlation Between Microhardness and Microstructure of Anodic Film on 2024 Aluminum Alloy

The correlation between the microhardness and microstructure features of anodic films on 2024 aluminum alloy formed in the mixed sulfuric acid/oxalic acid electrolyte was studied using micro-hardness tester and scanning electron microscope (SEM). The results show that the microhardness of the anodic film is influenced by the microstructure of the anodic film such as the film porosity, and the order and continuity of the hexagon columnar cells. The film microhardness increases as the porosity of the anodic film decreases and the order and continuity of the film cells increase. With the same current density, as the anodic film thickens with anodizing time, the film microhardness increases because the film porosity decreases and the order and continuity of the cells are also improved. Under the condition of the same anodizing time, as the current density increases, the film microhardness decreases due to the higher film porosity and the poorer order and continuity of the film cells. The film porosity increases because the increased current density can accelerate the oxidation reaction, strengthen the filed-assisted dissolution and the heating effect in the anodic film, resulting in decreased film order and continuity.