Effect of Electroless Nickel Plating on Reaction between Silicon-containing Steels and Molten Zinc

Abstract

Steel specimens containing 0.01, 0.1 and 0.6 mass% Si were coated with 110μm nickel layer by means of electroless plating and then galvanized in molten Zn at 733K for various periods of time. Formation and growth kinetics of alloy layer on the surface of the specimens was examined. Ni-Zn alloy layer composed of δ, γ and γ₁, phases was formed on the surface of the electroless Ni deposit. Its thickness increased with dipping time according to a paraObolic rate law. After the Ni deposit was consumed by the diffusion with molten Zn, Fe-Zn alloy layer composed of δ₁, phase or δ₁ and γ phases appeared between steel substrate and Ni-Zn alloy layer. As Fe-Zn alloy layer grew, thickness of γ and γ₁ phase layers decreased and δ phase layer peeled off after disappearance of γ and γ₁ phase layers. Subsequently, in the case of the specimens containing 0.1 and 0.6 mass% Si, δ₁ phase layer cracked along the boundaries of palisade structure and molten Zn penetrated into the layer near the substrate, while ζ phase layer appeared on the surface of δ₁ phase layer of the specimen containing 0.01 mass% Si. The elctroless Ni plating was found to be effective to prevent the abnormal structure during hot dip galvanizing of silicon-containing steels.