Protection against corrosion of aluminium alloys 2XXX and 7XXX, extensively used in the aircraft industry, is traditionally ensured by a coating system containing carcinogen chromate Cr(+VI). Within the framework of developing new efficient and nontoxic surface treatments, a Cr~(3+) and ZrF_6~(2-) salts sealing is investigated on porous anodized layer formed on 2618 and 7175 alloys by sulphuric acid anodizing. This study focuses on the sealing mechanism in a Cr~(3+) / ZrF_6~(2-) solution. In situ interfacial pH measurements perfomed by microelectrochemistry and in situ global electrochemical measurements (open-circuit, electrochemical impedance spectroscopy) have revealed a three-step sealing mechanism. A first quick step of activation consists of an interfacial alkalinization due to a fluoride exchange between ZrF_6~(2-) and alumina of the anodized layer. The precipitation of Cr~(3+) and ZrF_6~(2-) salts then leads to a surface film made of Cr(OH)_(2.5)(SO_4)_(0.25) and Zr(OH)_(4-x)F_x as revealed by DTA, EDS, and XPS analyses. Finally, the sealing film stabilizes the anodized layer and greatly enhances the corrosion resistance of the alloys.
展开▼
