On the distribution of the trace elements V and Cr in an Al–Zn–Si alloy coating on a steel substrate

Abstract

The Zn–55Al–1.6Si alloy is widely used in hot-dip galvanizing to coat steel and displays a multilayered microstructure (steel substrate/intermetallic compound (IMC) layer/coating overlay) that offers protection from corrosion. Trace level V and Cr are hypothesized to influence the corrosion performance of the coated steel via localized segregation. In this paper, the distribution of trace V and Cr is studied by using synchrotron X-ray fluorescence microscopy (XFM) and scanning transmission electron microscopy (STEM). It is revealed that V and Cr are both primarily segregated into the IMC layer and the top surface layer of the coating and are distributed continuously. The average concentrations for V and Cr are measured by XFM to be 7.7 ppm and 15.2 ppm in the IMC layer, and 4.6 ppm and 19.4 ppm on the top surface, respectively. STEM shows that the V distribution in the IMC layer is localized on the side adjacent to the coating overlayer. It is proposed that the IMC layer forms in two ways. One is the equilibrium phase transformation at 600 °C in the liquid Zn–55Al–1.6Si bath. The other way is the nonequilibrium phase transformation at the IMC/overlayer interface after V and Cr are rejected during solidification of the overlayer.