Effect of Heat Treatment Process on Microstructure and Corrosion Resistance of ZnAlMg Coating
ZnAlMg coated steel plates,as the substrate for anti-corrosion and decorative coatings,have to experience thermally baking in automobile manufacturing processes,which may affect the micro-structure and corrosion resistance of the coating,but such effect was not studied yet.In this study,the ZnAlMg coated steel plates were heat treated at temperatures in the range of 150℃ to 250℃ for 10 min respectively so that to reproduce the baking effect.Then the variation of surface-and cross sectional-mi-crostructure,phase constituents and corrosion behavior of the ZnAlMg coating before and after heating were assessed by means of scanning electron microscope(SEM),and X-ray diffractometer(XRD),as well as electrochemical technology and neutral salt spray test.The results show that with the increasing heating temperature,the eutectic structure in the ZnAlMg coating gradually coarsens,and oxides rich in aluminum and magnesium are formed on the surface of the coating.When heating at 175℃,a large num-ber of oxygen is detected on the eutectic structure and surface of the coating.When heating at 250℃,the surface of the initial solidification structure of the coating becomes rough,while the gray phase in the eutectic structure gradually evolves from lamellar structure to granular structure,and a large number of oxidation products of Al,Mg and Zn appear on the surface,among which the oxides of Mg and Al are the main ones.The electrochemical test results show that with the increasing heating temperature,the polar-ization resistance and charge transfer resistance of the ZnAlMg coating increase but the corrosion cur-rent density decreases continuously.The results of neutral salt spray test show that the corrosion mass loss of the ZnAlMg coating decreases gradually with the increasing heating temperature.Compared with the as received ones,the corrosion mass loss of the ZnAlMg coating heat treated at 250℃ decreased by 17％.It was indicated that the improvement of corrosion resistance of the ZnAlMg coating may be related to the coarsening of eutectic structure and the formation of oxides rich in Al and Mg on the surface.
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