Formation Mechanisms of Microstructures and Their Effects on Surface Appearance in Zn–11%Al–3%Mg–0.2%Si Coated Steel Sheets

Zn–11%Al–3%Mg–0.2%Si coated steel sheets exhibit high corrosion resistance. However, surface appearance issues, particularly a mottled pattern consisting of glossy and white areas, remain critical. In this study, the formation mechanism of this surface appearance was investigated by examining the relationship between surface roughness and coating microstructure. Microstructural characterization using SEM, EBSD, XRD, and GDS revealed that the contrast between the two areas originates from differences in smooth eutectic coverage over the rough preformed phases (primary Al and bulk MgZn2). Insufficient eutectic coverage, which exposes the preformed phases, increases surface roughness in the white area, whereas sufficient eutectic coverage results in a smooth surface in the glossy area. Eutectic Zn phase exhibited distinct crystallographic orientations in the two areas, leading to Zn growing in different directions between the two areas: along the coating‑thickness direction in the glossy area and along the coating‑surface direction in the white area. No remarkable differences were detected in the composition of the two areas. Further crystallographic analyses indicated that bulk MgZn2, eutectic MgZn2, and eutectic Zn formed sequentially in this order during solidification. These findings indicate that the growth behavior of bulk MgZn2 influences the eutectic coverage via the growth direction of eutectic Zn, and consequently the surface appearance. The results provide a fundamental basis for understanding surface appearance in Zn–Al–Mg–Si coatings.