Abstract
Pure Mg films are laminated on hot-dip Zn-55Al-1.6Si-coated steel by physical vapor deposition. Ar gas pressure and deposition time are varied to control the crystal alignment and thickness of the Mg films. Potentiodynamic polarization tests, neutral salt spray tests (SST) and cyclic corrosion tests (CCT) are employed to evaluate the corrosion resistances of Mg laminated Zn-55Al-1.6Si-coated steel. The results show that lamination of Mg films enhance the corrosion resistance, especially against the cross-section by a factor of 2 to 10. The corrosion resistance is significantly affected by Mg film thickness. Polarization test results indicate that corrosion resistance of Mg films is slightly improved by crystal alignment against (101) plane. The effects of Mg films on corrosion process of Zn-55Al-1.6Si-coated steel are investigated via analysis of corrosion products formed during the SST and CCT. Lamination of Mg films seem to hinder the corrosion process by cathodic protection and formation of protective Mg corrosion products at defective areas through Mg ion migration while the underlying Zn-55Al-1.6Si coating protects steel substrate surface. The improvement in corrosion resistance under different corrosion environments of SST and CCT is explained by the transformation of Mg corrosion products from magnesium hydroxide to a more protective hydromagnesite.
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