Growth process and mechanism of hot-dip galvanised Zn-5Al coating on steel

In this paper, the growth process and mechanism of hot-dip galvanised Zn-5Al coating on steel at different bath temperatures and immersion times were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The result shows that the growth of Zn-5Al coating is approximately divided into three stages: initial incubation stage, rapid thickening stage and steady detaching stage. The thickening and outbreak of the intermetallic layer is responsible for the thickening of the coating. When immersed into the Zn-Al bath, an Fe-Al intermetallic layer forms on the interface of liquid and steel and prevents the coating growing at the initial time; with the immersion time increasing, the solid-soluble Zn in the Fe-Al intermetallic layer makes the intermetallic layer unstable, the liquid Zn is discharged outward from the intermetallic layer and many pores and cracks occur, making the coating growth enter the rapid thickening stage. As the liquid phase areas keep expanding, the intermetallic layer will start to detach outward and produce a large amount of dross, the coating with linear growth finally enters the steady detaching stage. Increasing the immersion temperature will accelerate the growing rate of the coating and shorten the initial incubation stage of the coating.