Effect of Si on Interfacial Reactions, Defect Evolution, and Mechanical Properties of Hot-Dip Galvanized Coatings

The silicon content in steel is the key to the quality control of hot-dip galvanizing. Its fluctuation will directly restrict the application of galvanized steel by altering the interface reaction and structural stability. This study systematically explored the comprehensive effects of high silicon content in the steel substrate on the microstructure, interface reactions, defect distribution, and mechanical properties of the hot-dip galvanized coating through both experimental and theoretical calculations. Results indicate significant Si segregation at the coating/interface, with co-enrichment of P, leading to uncontrolled Fe-Zn reactions, destabilization of the δ-phase, and abnormal thickening of the ζ-phase (FeZn13), manifesting as the typical “Sandelin Effect.” CT analysis shows defect volumes distributed within 10−6–10−5 mm3, predominantly concentrated at the interface, which serve as primary channels for corrosion initiation and spalling failure. Mechanical tests demonstrate that high Si content weakens coating/substrate adhesion, reduces ductility, and accelerates the transition from ductile to brittle fracture under elevated temperatures. This study reveals that high Si content significantly degrades coating reliability through the coupled effects of “abnormal phase growth–element segregation–defect enrichment.” These findings provide theoretical and practical insights into interfacial instability mechanisms and coating optimization strategies for high-Si steel hot-dip galvanizing.