Reasons of Color Difference for Spangles of Hot Dip Galvanized Sheets

Bright and dark grey spangles on the surface of hot dip galvanized sheets is a kind of surface defect. Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction were used to observe and analyze the cross-sectional microstructure and chemical composition of the coating at the position of bright and dark grey spangles. The results show that there was almost no Al in the outermost layer of bright spangles, while a large amount of Al and a small amount of Fe were detected in the outermost layer of dark grey spangles. Microelements in the galvanizing zinc preferred to accumulate in the grain boundaries and thus corroded the grain boundaries, which damaged the corrosion resistance of coating. The element Al in the coating and element Fe in the matrix would combine with O and form metallic compound Al₂O₃ and Fe₃O₄, showing macroscopic dim gray morphology. Dark grey spangles were easier to appear in hot summer or humid environment, therefore, the hot dip galvanized sheets produced in summer should be placed in low temperature and dry warehouse to ensure the surface quality.