Improvement in galvanization and galvannealing characteristics of DP 590 steel by prior Cu or Cu–Sn flash coating

• Prior Cu or Cu–Sn flash coating leads to bare spot free galvanization on DP steel.
• Cu or Cu–Sn intermediate layer promotes FeZn7 phase formation in the coating.
• GI & GA coatings with prior Cu or Cu–Sn layers show better properties.

Galvanization of a dual phase (DP) steel surface is a challenging subject as occurrence of uncoated spots is a very common attribute in coating. In this study, a novel approach was applied where a thin layer of Cu or Cu–Sn (flash coating) was deposited on the DP steel surface before galvanization to resolve this chronic zinc coatability problem. These thin layers on the steel surface were deposited by displacement processes and both flash coated layers had good surface coverage and excellent adherence to the steel surface. Both flash coated as well as without flash coated steel sheets were annealed and subsequently dipped in molten zinc. It is evident from visual appearance that the galvanized coating obtained on the DP steel surface without any flash coating consists of uncoated spots whereas the galvanized (GI) coatings obtained on the DP steel surfaces with prior flash coatings were compact and free from any defects. Galvannealing operation was done for half the number of the total galvanized samples to obtain galvannealed (GA) coating. SEM-EDS and XRD analysis reveal that galvanized iron (GI) coating for the steel without any flash coating consists of almost pure zinc throughout the depth of the coating. However, GI coatings for the steels with both flash coatings consist of alloy phases up to a certain depth of the coating near steel interface. The GA coatings with both flash coatings mainly consist of the desirable δ phase. Both flash coatings promote controlled Fe–Zn reaction. Tafel, salt spray and open circuit potential measurement tests confirmed that performance of GI coatings against chloride attack was improved for both flash coated steels and GA coating has better resistance capability than GI coating.