(Today's post is the Conclusion of an article, "The science of successfully anodizing die castings substrates," which is a review of several questions received by PRODUCTS FINISHING Magazine. It was published by www.aluminumtoday.com. You can download the full article here or use this link.)
There is no ‘trick’ to anodizing complex alloys; there are ways, grounded in scientific reason, to approach challenges.
Fig 5a and 5b: Schematic of the walls of the anodic oxide at the junction of three anodic oxide cells. An entire cell is bracketed with pink dotted lines; the central pore appears gray with repulsive forces represented by a single sphere. The fcc aluminium structure is at the bottom of the schematic with the current bias represented by bold green arrows. The forming tetrahedral structure (5a and 5b) exhibits the primary oxidation reaction at the interface between the fcc and the electrolyte in the pore by the presence of oxygen atoms on the tetrahedral structure.
The schematic on the right (5b) shows non-aluminium ions (copper colored) diffusing from the fcc lattice, through the tetrahedral anodic oxide network, into the central pore where it will be carried into the electrolyte.
Note how the copper atoms ‘pile up’ at the fcc-tetrahedral interface. This is due to the structural/compositional mismatch between the aluminium and the anodic oxide