Microstructure and voltage-current characteristics of anodic films formed on magnesium in electrolytes containing fluoride

Abstract

Formation behavior of anodic oxide films on magnesium in various electrolytes including fluoride was investigated with attention to the effects of anodizing voltage, pH and aluminum content. In the range of formation voltage between 2V and 100V, porous film was formed in alkaline fluoride solution associated with high current density at around 5 V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60 V and relatively independent on substrate purity. Barrier type films or semi-barrier type films, which were composed of hydrated outer layer and inner layer, were formed at the other voltages. A peculiar phenomenon of high current density at around 5 V, which may be caused by trans-passive state, was not observed for anodizing in acidic fluoride solutions such as Dow17 and ammonium fluoride. In the case of AZ91D, the critical voltage increased to 70 V and peculiar phenomenon at 5 V was not observed, so that only barrier films were formed at less than the critical breakdown voltage. When AlO2- ion was added in the electrolytes, the critical voltage remarkably increased and current density effectively decreased with increasing AlO2- content. The passivation effect of aluminum addition in the electrolytes is more remarkable than the addition in magnesium substrates. The depth profiles of constituent elements showed that aluminum migrated into oxide film to reach near oxide/substrate interface. Atomic ratio of aluminum to magnesium increased with increasing voltage to attain 0.42 at 80 V and crystalline MgAl2O4 and MgO were found in the film.