SURFACE-TREATED, HOT-DIP ZN-AL ALLOY COATED STEEL SHEET

摘要

The invention provides a chromium-free surface treated, hot-dip Zn-Al alloy coated steel sheet which has excellent corrosion resistance and blackening resistance and is excellent in the surface appearance of plating. According to the invention, a steel sheet which has on the surface a hot-dip Zn-Al alloy plating layer containing Al: 1.0 to 10%, Mg: 0.2 to 1.0% and Ni: 0.005 to 0.1% is surface-treated with a surface treatment composition containing a specific titanium-containing aqueous solution, a nickel compound and/or a cobalt compound, and a fluorine -containing compound at a specific ratio to form a surface treatment film on the surface. Excellent blackening resistance is attained by virtue of the Ni and/or Co component contained in the surface treatment film and the optimized plating composition. Further, the surface treatment composition exhibits enhanced reactivity by virtue of the fluorine -containing compound to form a dense reaction layer on the surface of the plating and the fluorine-containing compound imparts higher barrier properties to the surface treatment film itself, which makes it possible to attain excellent corrosion resistance.