Graded Anodic Films for Corrosion Protection of Aerospace Alloys: Anodizing Procedure and Corrosion Behaviour

Abstract

The presence of alloying elements such as copper, magnesium and zinc in the 2xxx and 7xxx aluminium alloys increases mechanical properties but requires specific anticorrosion measures such as anodizing followed by painting. In this work, the performance of the anodic film was initially examined showing that, for a fixed anodizing electrolyte, a finer porous film morphology, generated at reduced potentials, provides better corrosion resistance than a coarser morphology generated at increased potentials. Complementing the previous, the effects of addition of tartaric acid to the anodizing electrolyte were examined, showing that tartaric acid does not affect significantly the porous film growth, but it reduces the dissolution rate of a previously formed oxide. Further, it was demonstrated that tartaric acid acts as an anodic inhibitor at concentrations in the ppm range, suggesting that the contributions to the anticorrosion performance are due to residues of tartaric acid in the pore solution.