Model of anodized layers formation in Zn–Al (Zamak) aiming to corrosion resistance

Zamak are zinc-aluminum alloys that, when processed by squeeze casting (injection), presents porosity defects on its surface, compromising its corrosion resistance. Thus, with the purpose to minimize these defects and increase corrosion resistance of Zamak, this work proposes formation of layers by anodizing, containing oxalates and oxides based on Zn and Al. Anodizing parameters used were 100 V, 10 mA.cm−2, room temperature, oxalic acid 0.3 mol.L−1 as electrolyte and 5 or 60 min. Samples were analyzed by infrared spectroscopy (ATR-FTIR), x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electrochemical impedance spectroscopy (EIS). The model suggested in this work shows that Zamak anodization occurs in three stages. In the first stage, zinc oxalate is formed as crystallites. In the second stage, zinc oxalate is solubilized and the formation of Al2O3 and ZnO (with smooth aspect) takes place. In the third stage, increasing thickness of the oxide layers promotes cracks on the anodized surface. Anodization of Zamak promotes better performance against corrosion, when compared to non-anodized samples, and this effect is attributed to the barrier effect of the oxides and the formation of corrosion products that fill the cracks generated during anodizing.