Chemistry of Zr/Cr based solutions: application to sealing of anodized aluminium alloys

摘要

The protection against corrosion of aluminium alloys 2XXX and 7XXX, extensively used in the aircraft industry, is generally achieved by an anodizing process performed in a sulphuric acid medium followed by a “sealing” step. Within the framework of developing efficient and nontoxic sealing processes, new formulations based on Zr(Ⅳ) and Cr(Ⅲ) salts are investigated to “seal” the nanoporosity.Anodized and sealed alunimium oxides were characterized by scanning electron microscopy and energy dispersive x–ray spectroscopy. The sealing process actually induces the precipitation of a thin homogeneously covering overlayer, between 200 and 500 nm. The electrochemical interface was characterized by electrochemical impedance spectroscopy carried out in K_2SO_4 0.1M.Whatever the considered alloy, the appearance of a contribution at low frequency after sealing indicates a slowing-down of the diffusion processes through the coating and induces a reinforcing of anticorrosion properties.rnIn order to basically understand the formation of the sealing products, the precipitation of Zr(Ⅳ) salts, Cr(Ⅲ) salts and mixture of both was investigated by titration.rnThe resulting precipitates were characterized by infrared spectroscopy, x-ray diffraction and thermogravimetric analysis coupled with mass spectrometry. The results show the formation of a mixed Cr/Zr hydroxide, that can be modified by Al(Ⅲ) addition.rnA sealing mechanism of anodized aluminum with Zr(Ⅳ)/Cr(Ⅲ) solutions is proposed, comparable to the formation of a chemical conversion layer: a local alkalinization of the interface induces the precipitation of Zr(Ⅳ) and Cr(Ⅲ).