The mechanism by which corrosion begins in sulfate anodized films, boehmite films and other typical aluminum surface films has been studied, primarily by microtome and by scanning and transmission microscopy of cross-sections of corroded portions. CASS and salt spray testing were chosen as corrosion tests.
It was found that corrosion of oxide films on aluminum did not depend on the type of film, on whether or not holes had been sealed, or on the type of material alloy, but proceeded in the aluminum underlying the film. The film on areas remaining unaffected after corrosion tests was free of dissolution and other damage, and IMA of Cl ion distribution in the film revealed no permeation to the film substrate. It is, therefore, suggested that corrosion under such conditions results from microdefects in the film, and that the corrosive materials permeate through these defects, reacting with the aluminum base material to start corrosion.
The present study also made it clear that changes in the corrosion conditions result in changes in the corrosion mechanism, indicating that even when aluminum is surface treated, it will exhibit a variety of corrosion mechanisms depending on the environment in which it is used.
