Study on the formation mechanism of black spot defects on hot-dip Zn-Al-Mg coated steel

Hot-dip galvanized aluminum-magnesium (Zn-Al-Mg) coated steel sheets are widely used in the fields of automobiles, construction, and home appliances due to their excellent corrosion resistance and self-healing performance. However, the existence of surface black spot defects seriously affects the appearance and anti-corrosion performance of the products. This study combines macroscopic and microscopic analysis methods to reveal the formation mechanism of black spot defects. The experimental results show that the black spots are mainly caused by the heterogeneous nucleation of Al/ Si-enriched scum (such as Mg 2 Si cubes) in the plating solution and their deposition at the substrate defects. Further analysis indicates that there are delamination and pore structures on the substrate surface below the black dots, which have not been completely reduced after the hot-rolled iron oxide scale is pressed in. Due to the high surface energy in this area, it is easy to capture scum, forming local protrusions. During the subsequent finishing rolling process, the scum particles are crushed and oxidized, eventually forming black oxide. In addition, EDS analysis confirmed that the contents of Al and Si in the black spot area significantly increased, while the content of Fe decreased, indicating that this defect was not a uniform solidification product but the result of the coupling effect of scum and substrate defects. This study clarifies the multi-factor collaborative mechanism of black spot formation and proposes that control should be carried out from aspects such as substrate cleanliness, uniformity of plating solution composition, and optimization of finishing process, providing a theoretical basis and engineering guidance for improving the surface quality of zinc-aluminum-magnesium coatings.