| abstract |
Painted metal sheet pretreated with a composite coating including an insoluble, inorganic inner layer and an organic outer layer. The inorganic layer is formed by rinsing the sheet with an alkaline solution containing at least 0.005M of at least one of a dissolved silicate or a dissolved aluminate. The sheet is dried to form an inorganic layer having a thickness of at least 2 ANGSTROM . The inorganically coated sheet then is rinsed with a solution containing at least 0.1 vol.-% of a hydrolyzed silane and at least 0.02 vol.-% of a hydrolyzed crosslinking agent. The silane treated sheet is dried to form a siloxane layer having an average thickness at least 5 ANGSTROM . After being painted, the siloxane layer forms an adherent bond between the paint and the metal substrate. The metal preferably is a chromium alloyed steel and may be etched prior to being rinsed with the alkaline solution to remove chromium oxide to further enhance the paint adhesion. Preferred rinsing times for the etchant, the alkaline solution and the silane solution are 2-5 seconds, 15-30 seconds and 5-10 seconds, respectively. |