FORMATION OF DOUBLE LAYER FILM FOR ELECTRODEPOSITION COATING SUBSTRATE TO ALUMINUM OR ALUMINUM ALLOY

摘要

PURPOSE:To obtain the electrodeposition coating substrate having excellent resistance to elution of Al and Cr by applying a specific amt. of a specific silica sol-contg. an org. high-polymer resin compsn. on a specific deposition of a chromate film formed on an Al or Al alloy surface, then drying the coating. CONSTITUTION:The chromate treating liquid contg. 5 to 90g/l total chromium ions is applied on the surface of the Al or Al alloy and is dried to form the chromate film layer of 10 to 150mg/m2 in terms of metal chromium. The above- mentioned treating liquid consists preferably of 3 to 50g/l Cr6+, 2 to 40g/l Cr3+, and 0.25 to 1.5 Cr3/Cr6+ and further, a specific ratio of PO + or silica sol may be incorporated therein. The org. high-polymer resin compsn. contg., by solid content, 0.2 to 1.2 silica sol having 10 to 500mum average grain size is applied on the above-mentioned chromate film and is dried to form 0.2 to 5g/m2 film. The double layer film for the electrodeposition coating substrate having the excellent resistance to the elution of the Al and Cr and the excellent corrosion resistance, coating adhesive property an coating appearance is formed in this way.