Composition for surface treatment of mg-containing galvanized steel sheet and mg-containing galvanized steel sheet surface-treated using same

Abstract

Provided is a composition for surface treatment of an Mg-containing galvanized steel sheet comprising a trivalent chromium compound, a carboxylated vinylidene copolymer, and a cross-linking agent. The composition for surface treatment forms a chromium (III) film and is also cross-linked in coating and drying of the surface of a galvanized steel sheet, thereby forming a three-dimensional compact molecular structure. Thus, a galvanized steel sheet coated with the composition has a coating film with high ductility, and can reduce moisture and/or oxygen permeability such that, not only does a worked section have excellent corrosion resistance, but also the occurrence of darkening and/or black dots can be prevented even when exposed to high-temperature and high-humidity environment for a long period of time.