Effect of prior electro or electroless Ni plating layer in galvanizing and galvannealing behavior of high strength steel sheet

- Prior electro or electroless Ni coating leads to bare spot free galvanization on high strength steel.
- Ni or Ni-P intermediate layer promotes Ni5.225Zn41.8 phase formation in the coating.
- GI & GA coatings with prior electro or electroless Ni layers show better properties.

Galvanization (GI) and galvannealing (GA) of high strength (HS) steel surface is very prone to develop uncoated spots in coating. Sometimes, GA coating may lead to inferior mechanical performance than GI coating as different brittle (Zn-Fe) intermetallic phases are formed in the coating. In this study an innovative prior Ni plating followed by hot dip galvanization and galvannealing process was thought of to obtain good quality of alloy coating. Adherence of molten zinc on the surface of high strength steel sheet improved significantly by prior Ni coating. SEM-EDS and XRD techniques were used for depth analysis of all coatings. The presence of pure zinc phase was confirmed almost entire depth of the coating when GI coating was formed on HS steel substrate without any prior metallic coating whereas the presence of alloy phase was confirmed up to some depth of the coating when GI coating was obtained on HS steel surface with prior Ni coating. The GA coatings with prior Ni plating primarily compose of the desirable Ni5.225Zn41.8 phase. Interdiffusion of Zn and Fe is controlled by Ni layer to get desirable alloy coating. The GI coatings with prior Ni plating layer provides better resistance against chloride attack and GA coating ensured significance improvement in performance than GI coating. The best GA coating was formed on the steel substrate with prior Ni coating by electrodeposition process. This GA coating showed 3-4 times improvement in performance against aggressive chloride attack, lowest powdering and excellent phosphatability with respect to the GA coating on the steel substrate without any prior Ni plating.

Open circuit potential of all the coatings under different intervals of time.75