Influence of concentration and pH of hexafluorozirconic acid on corrosion resistance of anodized AA7075-T6

Abstract

7XXX aluminum alloys show high mechanical resistance and low weight, both required properties for aircraft industry. Anodizing is an electrolytical process typically used to improve the corrosion resistance of aluminum alloys, through which a thicker and porous oxide is formed. Boiling water is used as a common sealing method to the anodic layer; however, it implies energy expenditure. In this work, a two-step coating system was performed: anodizing in tartaric-sulphuric acid and a post-treatment with a Zr-based conversion coating, obtained at room temperature by immersion in hexafluorozirconic acid (H2 ZrF6 ). To establish the best condition for coating formation on the aluminum oxide layer, different concentration and pH values of the H2 ZrF6 solution were studied. Morphological and chemical analyses were performed respectively by SEM and EDS. The corrosion resistance evaluation was carried out by EIS in 0.5 M NaCl. Heterogeneity was observed in the obtained coatings. However, the samples treated with H2 ZrF6 had a higher corrosion resistance than unsealed samples. The best concentration and pH range observed for the H2 ZrF6 solution were 1 % and 3 to 3.5, respectively. Under these conditions, a greater corrosion resistance was evidenced in comparison to that obtained with boiling water sealing.