Corrosion-resistant surface-coated steel sheet

Abstract

A surface-coated steel sheet having improved corrosion resistance and suitable for use as automobile inner and outer panels comprises a steel sheet having on at least one surface thereof a composite coating which comprises the following layers (a) to (d) from the bottom to the top of the coating: (a) a first zinc alloy plating layer with a coating weight of 10-100 g/m.sup.2 which contains at least one of Ni and Co in an amount satisfying the following inequality: 0.05.ltoreq.(5.times.Co)+Ni.ltoreq.10 (in weight percent), (b) a second zinc alloy plating layer with a coating weight of 0.05-10 g/m.sup.2 which contains at least one of Ni and Co in an amount satisfying the following inequality: 10<(5.times.Co)+Ni.ltoreq.40 (in weight percent), (c) a chromate film layer with a coating weight of 20-300 mg/m.sup.2 as Cr, and (d) an organic coating layer with a thickness of 0.2-5 .mu.m.

Dependencies are inferred from the claim text.