Surface treatment method for aluminum or an aluminum alloy and treating fluid used therefor

A method for the surface treatment of aluminum or an aluminum alloy, comprising the steps of:

• preparing a treating fluid having a pH of 2.5 or less, consisting of ammonium silicofluoride, an aluminum-free fluorine compound other than aluminum silicofluoride, a first substance, a second substance and water, by adding:
• ammonium silicofluoride in an amount of 0.01 to 10 parts by weight, the aluminum-free fluorine compound other than aluminum silicofluoride in an amount of 0.1 to 20 parts by weight, the first substance in a given amount selected from the group consisting of aluminum hydroxide in an amount of 0.02 to 0.5 part by weight, aluminum silicate in an amount of 0.04 to 1 part by weight, magnesium aluminate metasilicate in an amount of 0.04 to 1 part by weight, and powdered aluminum in an amount of 0.007 to 0.2 part by weight, per 100 parts by weight of water, and the second substance in a given amount selected from the group consisting of potassium chloride in an amount of 0.01 to 5 parts by weight, potassium hydrogen phthalate in an amount of 0.05 to 10 parts by weight, and potassium dihydrogen phosphate in an amount of 0.03 to 10 parts by weight, per 100 parts by weight of water into water;
• and
• soaking aluminum or an aluminum alloy in said treating fluid to form a film thereon.