シリコン含有鋼と亜鉛融液との反応に及ぼす無電解ニッケルめっきの影響

概要

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Steel specimens containing 0.01, 0.1 and 0.6 mass% Si were coated with 1∿10μm 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 γ_1 phases was formed on the surface of the electroless Ni deposit. Its thickness increased with dipping time according to a para0bolic rate law. After the Ni deposit was consumed by the diffusion with molten Zn, Fe-Zn alloy layer composed of δ_1 phase or δ_1 and Г phases appeared between steel substrate and Ni-Zn alloy layer. As Fe-Zn alloy layer grew, thickness of γ and γ_1 phase layers decreased and δ phase layer peeled off after disappearance of γ and γ_1 phase layers. Subsequently, in the case of the specimens containing 0.1 and 0.6 mass% Si, δ_1 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 δ_1 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.
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