Characterization of anodized and bare 7075-T6 aluminum alloy treated with Zr-based conversion coating

Abstract

AA7075-T6 is an aluminum alloy that has a high mechanical resistance; however, it shows corrosion vulnerability. A usual method to improve the corrosion resistance is anodization in acidic medium, which forms a thin barrier layer and a thicker porous oxide layer over the surface. This ultimate layer must be sealed to avoid electrolyte penetration. In the present work, a Zr-based conversion coating was investigated as a novel method of cold sealing and compared to the precipitation over bare AA7075-T6. The samples were characterized by Scan Electronic Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) to elucidate the mechanism of reaction on both natural aluminum oxide and anodic oxide layer. The hydrophobicity properties were evaluated by contact angle measurements. The mechanism of deposition suggested for the coating over the anodic oxide was analogue to the one that takes place over the bare alloy: an initial attack by fluoride ions, local pH increase and Zr oxide precipitation. It was also possible to precipitate Zr oxide inside the pores. An increase of 125% on the contact angle was observed for the Zr coating over anodized surfaces, while the increase over the bare alloy was of 32%. Therefore, a robust coating system can be proposed involving the anodic layer and the nanometric Zr oxide.