To improve the corrosion resistance to salt, acid and strongly alkaline attack, a sol-gel film using zirconium tetra-n-butoxide as a starting material has been coated directly on the surface of aluminum substrates via repeating one or twice dip-coating process. The heat treatment at 573 K or the UV-irradiation at ambient temperature was performed for forming dense coating films after the dipping-withdrawing of aluminum substrate into the sol. Zirconium oxide films were prepared on aluminum surface by these sol-gel processes. The coating films formed on the smooth aluminum substrates under UV-irradiation indicated excellent corrosion resistance for salt, acid and alkaline attack tests. For example, the surface area of the coating film corroded for 3600 ks in salt attack test decreased in the order of sol-gel coating film<chromate film<boehmite film and the sol-gel coating film had the best corrosion resistance. The coating films prepared on the coarse aluminum substrate by a boiling treatment after the heat treatment at 573 K also had greater corrosion resistance and appeared water-avid surface having contact angle of approximately 3°.
