AC anodizing of aluminium in phosphoric acid

Summary

AC anodizing of aluminium in phosphoric acid for adhesive bonding has been studied by high resolution scanning electron microscopy. As with DC anodizing, aluminium oxide protrusions form on the outer surface in the initial stages. The size and distribution of the protrusions depends on the pretreatment of the surface of the aluminium prior to anodizing and on the anodizing conditions. When the barrier layer has grown to its voltage determined thickness, the rate of ejection of aluminium ions in solution falls, the growth of protrusions stops, the porous layer develops and dissolution of protrusions and of the outer surface of the porous layer takes place as anodizing proceeds. The formation of the anodic oxide in AC anodizing is essentially similar to that formed in DC anodizing but the balance between oxide formation at the inner interface and oxide dissolution at the outer surface is affected by the extra time available for oxide dissolution during the cathodic part of each AC cycle.