Influence of Fe-bearing particles and nature of electrolyte on the hard anodizing behaviour of AA 7075 extrusion products

Abstract

The hard anodizing behaviour of aluminium alloy 7075 extrusion products has been examined as functions of variation in the impurity (i.e. Si, Mn and Fe) contents in the alloy and the nature of electrolyte used for the anodizing process. A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) revealed that the chemistry, morphology and the mode of distribution of the Fe-rich constituent particles are important in determining the quality and the growth of the hard anodic oxide film. Using the conventional sulphuric acid—oxalic acid electrolyte, it was established that the Al 12(FeMn) 3Si-based particles survive the anodizing process, and this results in a steep rise in bath voltage with time as well as a non-uniform growth of the anodic oxide film. The extent of these effects are, however, greatly reduced when the Fe-bearing particles are aligned in a direction parallel to the growth direction of the anodic film. Studies employing a stronger electrolyte that is, sulphuric acid—oxalic acid—hydrochloric acid, on the other hand, revealed that this electrolyte is capable of counteracting the deleterious effects of Al 12(FeMn) 3Si-based particles by dissolving them during anodizing provided such particles are not too large in size.