Experimental Measurements and Kinetic Investigation of Fe-Al Alloy Layer During the Galvanizing Process

Lack of coating adhesion and bare spot defects of hot-dip galvanized steel plates were experimentally investigated, and the Fe-Al alloy layer was analyzed by both experimental methods and kinetic calculations. Scanning electron microscopy, electron probe micro-analyzer and glow discharge spectrometer method were utilized to determine the microstructures and the distribution of elements of the galvanized steel plates. It is discovered that lack of coating adhesion and bare spot defects depend on the quality of the Fe-Al alloy layer. Fe-Al alloy layer (alias inhibition layer) affects the adhesiveness of the alloy coating to the steel plate, and prevents the interdiffusion process of elements between the zinc pot and the coated steel. The kinetic analysis was carried out by means of CALPHAD (CALculation of PHAse Diagrams) method. Because the main constitutuent of Fe-Al alloy layer is Fe₂Al₅ phase when the Al content in the liquid Zn is kept below 1 wt.%, diffusion models with Fe₂Al₅ were constructed to investigate the element distribution during the galvanizing process, and the calculated results can reasonably predict the element distribution trend in both the alloy coating and the steel plate. It is shown that besides the wetting reactions of Fe and Al during the galvanizing process, the diffusion behavior of elements also influences the coating quality. In summary, the appropriate control of Fe-Al alloy layer can prominently benefit the surface quality of the galvanized coatings, and the manufacturing parameters, such as strip speed, galvanizing time and zinc pot temperature, act as important factors on the formation of the Fe-Al alloy layer. The optimum combination of the aforementioned parameters may contribute to the improvement of the coating quality.

Graphical Abstract