Incorporation of alloying elements into porous anodic films on aluminium alloys: the role of cell diameter

The presence of alloying elements in aluminium alloys has a significant impact on the anodizing behaviour and results in the formation of porous anodic films with different chemical composition and morphology compared with those generated on pure aluminium. In this work, the effect of alloy element enrichment at the alloy/film interface and of cell diameter on the incorporation and distribution of alloying element species in porous anodic films is considered. It is proposed that above a critical cell diameter, Dcrit, the critical alloy enrichment sufficient for oxidation of the alloying element and its incorporation into the film can be maintained across the alloy/film interface. Below Dcrit, only a sub-critical enrichment can be maintained and the alloying element is then incorporated into the film at the cell boundaries. Dcrit depends on the concentration of the alloying element in solid solution and on the critical enrichment. The proposed role of Dcrit is supported by alloying element distributions from literature data for model Al-Au and Al-W alloys and new results for anodic films on AA 2024-T3 alloy.