Abstract
Development of the sol-gel films for painting pretreatment of aluminium alloy is to replace bichromate conversion films such as Alodine. Corrosion resistance of Alodine film and sol-gel film were evaluated through potentiodynamic polarization curves, electrochemical impedance spectroscopy, salt spraying and acidic dropping solution. Sol-gel film is almost the same as Alodine film at the film surface density, salt spraying resistance and adhesion with painting coating. Changing color times of dropping solution on sol-gel film is shorter than Alodine film. But the corrosion current of sol-gel film is lower than Alodine and the impedance value is higher than Alodine in 35g/L NaCl solution. Mechanism of corrosion resistance of alumina sol-gel film is that the cathode reaction and anodic reactions are restrained by sol-gel film in the Cl- corrosive medium. The EIS of sol-gel film consisted of only a single capacitive arc with one time constant. Sol-gel coating can prevent or delay the corrosive solution from infiltrating the substrate for its better isolation function, thus protecting 2024 aluminium alloy from corrosion. Sol-gel films can improve corrosion resistance of aluminum alloy and have the same adhesion as Alodine film. It will be a promising alternative pretreatment for aluminum alloys prior to painting.
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