Chemical conversion coatings with rare-earth metal salts are being considered as an environmentfriendly alternative to the toxic chromium-based coating to improve the corrosion resistance of aluminum alloy. However, the effects of conversion conditions on the coating performance are rarely systematically studied. In this study, we prepared the cerium-based conversion coatings on the ADC12 aluminum alloy in Ce(NO3)3 solution under different conversion conditions, including the additive (H2O2) concentration, the immersion time, the bath temperature, etc.. Then, the corrosion behavior of each cerium-coated sample was evaluated in 3%NaCl solution by the methods of potentiodynamic polarization, electrochemical impedance spectroscopy and surface observation technique techniques. The results reveal the corrosion resistance of Ce-based coatings on Aluminum alloy could be improved with increasing the concentration of additive, the immersion time and/or the bath temperature before reaching the best performance, but it will decrease if the conditions above exceed their corresponding optimal value. We believe the performance decrease after best performance is due to the cracks induced in thick coatings for higher concentration of additive or longer immersion time. As for the bath temperature, we believe the decrease is attributed to the decomposition of H2O2 in high temperature.
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Chen et al. (2015) studied this question.
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