Al-Si casting alloys with Si contents (C_si) of 0-17wt% and ADC12 die casting alloy were anodized in 10% H₂SO₄ at 20°C under a constant current density (i_a) to examine the anodizing characteristics and film properties of these alloys. The anode potential (E_a) and dissolution current of Al³⁺ (i_a) during anodizing, the average film thickness (δ) and the diffuse reflectance of films were examined as functions of the abrasion thickness (X) of specimens, anodizing time (t_a) and i_a.
The steady value of E_a, E_a*, for the casting alloys was found to increase with C_si at X=100μm, and showed a maximum at C_si=10wt% and X=1100μm. The value of E_a* for ADC12 increased steeply with i_a, and was much larger than that for the casting alloys. Dissolution current efficiency, i_d/i_a, decreased with i_a, and C_si, independent of X, for both casting alloy and ADC12 specimens.
Oxide films formed after anodizing for 40min showed thickness differences on both types of specimen, and the dispersion in the film thickness increased with C_si and i_a. The value of δ for the casting alloys decreased appreciably with increasing C_si, independent of X, at current densities of less than 100A/m², but increased with increasing X at 200A/m². The value of δ for ADC12 was considerably smaller than that for the casting alloys, suggesting higher rates of side reaction such as oxygen evolution.
The diffuse reflectance spectra showed that the reflectance of films decreased with increasing C_si, and with decreasing X.