Effect of additive ethylene glycol on morphology and mechanical hardness of anodic oxide film formed on AA7075

This paper reports the effect of additive ethylene glycol (EG) on the morphology and mechanical hardness of anodic oxide films formed on AA7075 by hard anodizing technique. Anodizing was conducted in 2 M H2SO4 solution with the addition of 0, 10, 20, and 30% EG at a constant current 300 A/m² at 10°C. The cross-sectional morphology was studied by FE-SEM and the elemental composition was analyzed by EDX. The mechanical hardness was measured by a Vickers Hardness machine. The results indicated that the addition of EG improved the films adherent on the substrates by preventing the formation of cracks and the enrichment of the intermetallic phases at the film-substrate interface. The presence of EG accelerated oxidation reaction of the intermetallic phase. The competing oxidation reactions led to the significant decrease in film thickness from 38 μm down to below 20 μm with the addition of EG. The film formed in EG containing electrolyte composed not only aluminum oxide but also copper and zinc oxides. The film formed in EG containing electrolytes exhibited microvoids as a result of oxygen gas evolution which accompanied the oxidation reaction of the intermetallic phase. As a consequence, the mechanical hardness decreased with increasing EG concentration.