The development of non-hexavalent chromium conversion coating on aluminum alloy is imminent, hexavalent chromium Cr6+ compounds have serious threat to the environment. The appearances, viscosity, particle size of aluminum nitrate sol prepared by re-dissolving were tested by orthogonal experiment. The results show that a better condition for preparing sol by re-dissolving the aluminum nitrate sol is: [Al3+] of 0.3mol/L, sol at 55~85°C or 0.6mol/L, at 55°C. The pH value ranges 7~9 before re-dissolving. The salt spraying resistance for the sol-gel film prepared by sol of which viscosity ranges 4.8 ~ 6 mPa/s is better than five grade. The dry adhesion for paint film is qualified when viscosity ≤ 5.6 mPa/s and the average size of particles <920 nm for sol. In terms of sol appearance, dry adhesion and salt spray properties of the sol-gel film made by blue and white or gray sol are good. [Al3+] concentration and sol temperature for preparing sol-gel film with better corrosion resistance are equally important, secondly is pH before re-dissolved. Dry adhesion between paint film and aluminum alloy is qualified when [Al3+] ranges 0.3 ~0.6mol/L. Corrosion resistance for alumina sol-gel film of [Al3+] concentration of 0.3 and 0.77mol/L is worse than that of 0.6mol/L. The times of spray coating has no influence on corrosion resistance, while the increased frequency of coating times, and film adhesion decrease. In short, if you want that painting film adhesion and corrosion resistance are both good, Al3+ concentration should be 0.6mol/L for pure aluminum sol. The preferable prepared conditions are that aging at 55 °C, pH of sol re-dissolved with nitric acid is 4 after adding ammonia until pH reaches 8, coated one times, two or more times, the adhesion of the coating will reduce.
Effect of heat treatment of magnesium alloy substrates on corrosion resistance of a hybrid organic–inorganic sol–gel film
