AC-anodising of aluminium: Contribution to electrical and efficiency study

AC-anodising of AA1050 aluminium alloy in sulphuric and phosphoric acid solutions has been examined, with the contributions of the anodic and cathodic processes evaluated from the electrical signal analysis. The porous anodic film morphology and the potential-time behaviour confirm the similarity with DC-anodising, although lower anodic charge and film formation efficiencies are evident from gravimetric measurements. The anodic charge loss is mainly related to the capacitive current required to re-establish (between successive charge cycles) the electric field strength for ionic migration across the pre-existing and healed barrier layer oxide. Cathodic hydrogen gas evolution is considered to proceed at flaw sites in the barrier layer oxide, leading to high localized current densities that result in high cathodic potentials. Hydrogen gas retention at flaws and pores of the anodic film also contributes to the high cathodic potentials.