Effect of Coating Thickness on Stone Chip Corrosion Resistance of Hot Galvanizing Sheet and Hot-Base Zn-Al-Mg Sheet

The stone chip resistance test and neutral salt spray test were used to compare the stone chip corrosion resistance of hot-dip galvanized (GI) sheets and hot-base zinc-aluminum-magnesium (ZM) sheets with different coating thicknesses. The results show that the hardness of ZM sheet coating was greater than that of GI sheet coating, and the depth of stone chip pit of GI samples was greater than 50 μm, while the average depth of stone chip pit of ZM120 and ZM275 samples was only 22.1 μm and 23.0 μm, respectively. After 1 176 h neutral salt spray test, the red rust area on the surface of GI275 and GI1200 stone samples reached 50% and 10%, respectively, and there was no red rust on the surface of ZM120 and ZM275 stone samples. When the coating thickness was the same, the corrosion resistance of ZM sheet was obviously better than that of GI sheet, and the corrosion resistance of the same coating sheet increased with the increase of coating thickness. The Mg in the ZM sheet coating could effectively buffer the pH of the thin liquid film on the surface, reduce the content of OH^-, and then inhibit the formation of ZnO. The corrosion products of the coating were mainly dense Zn₅(OH)₈Cl₂·H₂O, which could form a physical barrier and act as an electronic insulator to prevent the further electrochemical corrosion reaction.