热处理对ZnAlMg镀层组织与耐腐蚀性能的影响

Effect of Heat Treatment Process on Microstructure and Corrosion Resistance of ZnAlMg Coating

使用加热炉模拟了车身制造过程中的烘烤热处理过程,热处理温度为150~250℃,处理时间为10 min。采用扫描电子显微镜(SEM)研究了在加热前后ZnAlMg镀层表面与截面的微观组织变化,使用X射线衍射仪(XRD)分析了热处理前后镀层的物相差异,使用电化学技术和中性盐雾实验评价了镀层的耐蚀性。结果表明,随着加热温度的提高,ZnAlMg镀层中的共晶组织逐渐粗化,并且在镀层表面形成富含Al和Mg的氧化物。当加热温度为175℃时,在镀层的共晶组织中以及镀层表面均出现了大量O分布,而在镀层的共晶组织表面附近的Al和Mg分布范围更宽一些。加热温度为250℃时,镀层的初始凝固组织表面变得比较粗糙,而共晶组织中的灰色相从层片结构逐渐演变为颗粒结构,表面出现了大量Al、Mg和Zn的氧化产物,其中以Mg和Al的氧化物为主。电化学测试结果表明,随着加热温度的升高,ZnAlMg镀层的极化电阻以及电荷转移电阻不断增大,而腐蚀电流密度则不断减小。中性盐雾实验结果表明,随着加热温度的升高,ZnAlMg镀层的腐蚀失重逐渐减小。与原始样品相比,250℃热处理样品的腐蚀失重减小了17%。ZnAlMg镀层耐蚀性的提高,一方面可能与共晶组织粗化有关,另一方面则与表面形成富含Al和Mg的氧化物有关。

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 microstructure 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-microstructure,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 number 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 polarization resistance and charge transfer resistance of the ZnAlMg coating increase but the corrosion current 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.