New insights into the formation mechanism of a bare-spot defect in hot-dip galvanized coating

To reveal the formation mechanism of a bare-spot defect in a hot-dip galvanized coating, multiple in-depth metallurgical characterizations were conducted. The bare-spot defect sample was collected from a commercial hot-dip galvanized boron-added bake hardening steel sheet. The morphologies of the as-received coating, steel substrate, and cross-sections were observed. High-resolution lateral distributions of relevant sputtered ions and cross-sectional elemental mappings were obtained. The bare-spot defect was found to result from the poor wettability of the steel surface prior to hot dipping and the overoxidation of the molten zinc surface in the snout. Based on key observations, including nodular external Mn B oxide islands, dual-layer structured oxide films consisting of ZnO and Al 2 O 3 , thin Zn Fe IMC layers beneath the oxide films, and thick Zn Fe outburst structures at the defect border, a four-stage formation mechanism was proposed for the bare-spot defect. • A bare-spot defect in hot-dip galvanized BH steel sheet was characterized. • Mn B oxide islands, dual-layer oxide films, Zn Fe structures were observed. • Poor wettability and molten zinc surface overoxidation together cause this defect. • A four-stage formation mechanism for the bare-spot defect was proposed.