Environmentally Compliant Silica Conversion Coatings Prepared by Sol-Gel Method for Aluminum Alloys

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Chromate conversion coatings have been widely applied for the corrosion protection of aluminum alloys. However, the waste containing Cr6+ has many limitations due to environmental considerations and health hazards. Silicates are among the proposed alternatives to chromating. A series of specimens were prepared under the following conditions: (a) as polished, (b) directly treated with silica, (c) etched, (d) oxide thickened and, (e) etching followed by oxide thickening. After surface preparation, the specimens were dipped in two different silica solution prepared via the sol–gel method. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 3.5% NaCl. The optimum conditions under which silica treatments can provide good corrosion protection to the aluminum substrate were determined. The surface preparation prior to silica treatment was found to have a marked effect on the corrosion protection of AA6061 T6. Generally, silica treatments improve the corrosion resistance due to formation of protective oxides that act as a barrier to oxygen diffusion to the metal surface. According to the EIS and polarization measurements, a combination between etching and oxide thickening prior to silica treatment plays an important role in the corrosion protection mechanism.