Research on the formation mechanism of periodic undulating structures on the surface of Hot-Dip Zn-Al-Mg alloy coating

Abstract Hot-dip Zn-Al-Mg alloy coatings are widely used in steel corrosion protection due to their excellent corrosion resistance and self-healing properties. However, periodic undulating structures often appear on the surface, affecting coating performance and process optimization. This study combines experiments and simulations to investigate the influence of air knife angle on surface morphology and element distribution. Results show that at an air knife angle of -1°, airflow disturbance causes local temperature fluctuations and solute segregation, forming alternating bright and dark stripes and layer separation (zinc-rich outer layer and aluminum-rich inner layer). In contrast, a -3° air knife angle reduces pressure fluctuations, increases temperature gradients, and results in a uniform coating structure with stable element distribution. Phase-field simulations confirm narrower isotherms at -3°, demonstrating that higher temperature gradients suppress element segregation. The optimized process (-3°, 15 kPa) successfully eliminates surface stripes and achieves a uniform eutectic structure, providing a theoretical basis for Zn-Al-Mg coating quality control.