STEEL SHEET HOT-DIP COATED WITH COMPOSITE ZINC-ALUMINUM LAYER AS UNDERCOAT FOR COATING

摘要

PURPOSE:To obtain the titled steel sheet excellent in corrosion resistance and the adhesiveness of a coating film, by sequentially forming chromate and then resin coating films under a specified condition on the surface of a steel sheet hot- dip coated with a composite zinc-aluminum layer. CONSTITUTION:The aqueous solution of chromic-phosphoric-malonic-boric acids is applied onto the surface of a steel sheet hot-dip coated with a composite zinc-aluminum layer and then dried to form a chromate coating film having the amount of adhesion of chromium of 20-300mg/m<2>. Then, the aqueous solution of alkyl silicate-alkoxysilane compound-cationic and/or amphoteric resin-phosphoric acid is applied onto said chromate coating film and dried to form a resin coating film of 0.1-2.5mum in thickness.