Effect of prior electro or electroless Ni plating layer in galvanizing and galvannealing behavior of high strength steel sheet
By Manna, M.; Dutta, M.
Published in Surface and Coatings Technology
2016
Abstract
Abstract 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.