Study On RE, Mg And Si In The Modification Of ZAM And Galvalume Coatings

Steel is the main construction material for automobile, building and appliance industries, but the corrosion resistance of it in most environments is very low. Hot dip zinc coating can extend the lifetime of steel constructions, and it has the advantage of the low manufacturing cost. Many research results showed the corrosion resistance and mechanical properties of zinc coating can be enhanced by alloying elements like Al, Mg, Si and Sn, and several kinds of commercial Zn alloy coatings were developed in the 20th century, such as Zn-55Al-1.6Si, Zn-5Al-0.1RE, Zn-4.5Al-0.1Mg, Zn-11Al-3Mg-0.2Si, Zn-0.5Mg and Zn-6Al-3Mg, etc.ZAM zinc-coated steel plate was developed by Nisshin Steel Corporation in the 1990s, and it showed superior corrosion resistance in both atmospheric and accelerated corrosion environments. Due to its superior properties, many researchers have done extensive studies on its corrosion mechanism via different methods. In this study, the effects of Si and RE on the microstructure, corrosion resistance and formability of ZAM coating were studied via SEM, XRD, OM, the neutral salt spray test, the electrochemistry test, immersion test and drawing test. The detection of the microstructure of coating surface showed that the additions of Si (0.1 wt.%) and RE (0.1 wt.%) can improve the uniformity of ZAM coating, and RE can refine the microstructure of coating. The observation of cross-sectional microstructure showed that Si can suppress the reaction between Al and Fe, and the thickness of coating can be decreased by Si and RE. The thickness of ZAM was 33 ?m, while that of Zn-6Al-3Mg-0.1Si-0.25RE coating was only 11.2 ?m. The micromorphology of the corrosion product showed that Si and RE can enhance the stability of Zn5(OH)8Cl2·H2O, thereby improving the protection ability of the corrosion product. The detection of the formability showed that Si and RE can improve the formability of ZAM coating, the duration of ZAM cup in the neutral salt spray test was 1128 h, while that of Zn-6Al-3Mg-0.1Si-0.1RE cup was 2136 h. The corrosion type detections showed that Si and RE can change the corrosion types of ZAM coating, and intergranular corrosion, pitting and exfoliation corrosion occurred in the ZAM coating during the neutral salt spray test, while only uniform corrosion occurred in the Zn-6Al-3Mg-0.1Si-0.1RE coating. The results of the neutral salt spray test showed that Si and RE can obviously improve the corrosion resistance of ZAM coating, and the corrosion rates of ZAM, Zn-6Al-3Mg-0.1Si and Zn-6Al-3Mg-0.1Si-0.1RE coatings were 0.056,0.045 and 0.033 g·m-2-h-1, respectively, so the corrosion rate of Zn-6Al-3Mg-0.1Si-0.1RE coating was only 58.9%of that of ZAM coating. Therefore, Zn-6Al-3Mg-0.1Si-0.1RE coating has a wider industrial application prospect when compared with ZAM coating.Galvalume (Zn-55Al-1.6Si) zinc-coated steel plate was developed by Bethlehem Steel Co. in the 1970s, and it has shown a tremendous increase in use as a substitute for hot dip galvanized steel sheet during the past several decades, because of its high corrosion resistance under severe conditions such as acid rain or when deicing salts are present. The detection of the microstructure of coating surface showed that the additions of Mg (1.5 wt.%) and RE (0.15 wt.%) can improve the uniformity of Galvalume coating, and RE can refine the microstructure of coating. Galvalume coating composed of bluky aluminium-rich dendrites and zinc-rich interdendritic, while Zn-55Al-1.6Si-1.5Mg-0.15RE coating composed of the uniform equiaxed grains. The observation of cross-sectional microstructure showed that Si and RE can decrease the thickness of coating. The thickness of Galvalume was 28.6 ?m, while that of Zn-55Al-1.6Si-1.5Mg-0.25RE coating was only 13.2 ?m. The micromorphology the corrosion product showed that Mg and RE can enhance the stability of Zn5(OH)gCl2·H2O and the compactness of the corrosion product, thereby improving the protection ability of the corrosion product. The detection of the formability showed that Mg and RE can improve the formability of Galvalume coating, the duration of Galvalume cup in the neutral salt spray test was 1248 h, while that of Zn-55Al-1.6Si-1.5Mg-0.15RE cup was 1968 h. The corrosion type detection results showed that Mg and RE can change the corrosion types of Galvalume coating. The zinc-rich interdendritic was seriously corroded, and there were many etch pits with certain depth in the zinc-rich interdendritic. The addition of 1.5wt.% Mg can strengthen the zinc-rich interdendritic of the Zn-55Al-1.6Si-1.5Mg coating, and there were only some cracks in the interdendritic of the corroded Zn-55Al-1.6Si-1.5Mg coating, while the corroded Zn-55Al-1.6Si-1.5Mg-0.15RE was still smooth, and the corrosion type of this coating was the uniform corrosion. The results of the neutral salt spray test showed that Mg and RE can obviously improve the corrosion resistance of Galvalume coating, and the corrosion rates of Galvalume, Zn-55Al-1.6Si-1.5Mg, Zn-55Al-1.6Si-1.5Mg-0.15RE coatings were 0.0356, 0.0223 and 0.0164 g-m-2·h-1, and the corrosion rate of Zn-55Al-1.6Si-1.5Mg-0.15RE coating was only 46.1% of that of Galvalume coating. Therefore, Zn-55Al-1.6Si-1.5Mg-0.15RE coating has a wider industrial application prospect compared with Galvalume coating.During this experiment, an electrolyte and an electroplating aiding technology for multicomponent alloys hot-dipping was developed. The hot-dipping progress can be finished in one step with this electroplating aiding technology, and there were no defects in the coating such as leak of coating, and the coatings were smooth and had a good adhesive strength with the substrate (steel). In addition, an experimental hot-dip galvanizing simulator was developed, and it has many advantages with respect to convenient for operation, high automation, easy control, low investment, etc.