Anodic Oxidation Behavior of Aluminum Die Casting Alloy ADC 12 in 13M and 1.5M Sulfuric Acid Solutions

Abstract

ADC 12 specimens were anodically oxidized in 13M and 1.5M sulfuric acid solutions at constant current densities (i_a), and the amounts of oxide film (W_oxide), Al³⁺ions dissolved in the solutions (W_d) and the volume of gas evolved from the anode (V_g) were examined as functions of the anodizing conditions. There was more W_d, less V_g, and more W_oxide in the 13M solution than in the 1.5M solution. This suggests that the Al³⁺ions dissolution current (i_d) is larger, O₂ gas evolution current (i_g) is smaller, and oxide formation current (i_ox=i_a-(i_d+i_g)) is larger in the 13M solution than in the 1.5M solution. Since the films formed in the 13M solution had larger values of i_ox and larger amounts of SO₄^2-ions than the films formed in the 1.5M solution, the film weight, coating ratio, Si content, and thickness were larger for the 13M-films than for the 1.5M-films. Because microvoids were present in both the barrier layer and the porous walls, the hardness of the 13M-films was lower than that of the 1.5M-films.