Microstructure and formability of ZnNi alloy electrodeposited sheet steel
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I. INTRODUCTION
IN light of its excellent corrosion resistance, ZnNi alloy electrodeposition technology has gained ever-increasing attention among the Zn-based coated steel technologies.[1,2,3] Researchers focusing on the improvement of corrosion resistance have found a strong dependence of corrosion resistance on the Ni content of the coating. The corrosion resistance of ZnNi-coated steels increases monotonously with Ni content up to 14 to 16 wt pct, and then a reversed trend is observed.[4–9] The Ni content is set within 10 to 14 wt pct as a compromise between formability and adhesion.[10,11] One of the major concerns with ZnNi-coated steels are their formability and adhesion during press forming and at service under subzero temperatures. Three major factors affect the formability and adhesion of Zn-based coated sheet steel: (1) the friction between the coated sheet steel and the press-forming die; (2) the deformability of the coating layer; and (3) the interfacial shear strength between the coating and steel substrate. The friction can be substantially reduced by increasing the surface hardness of the coating or by the addition of lubricant.[12,13] The deformability of the electrodeposited coating is determined by its hardness, ductility, and microstructure; particularly, the phases present, grain shape and size distribution, defects, and preferred crystallographic orientation. Shibuya et al. studied the properties of ZnNi coatings made from a sulfate bath.[4] They found that the hardness of ZnNi coatings increased with the Ni content and reached a maximum at 15 wt pct Ni, at which point the coating also exhibited its maximum density and minimum C.S. LIN, Associate Professor, is with the Department of Mechanical Engineering, DaYeh University, Taiwan 51505, Republic of China. H.B. LEE, Engineer, is with Sheng-Yung Co., Ltd., Taichung, Taiwan 403, Republic of China. S.H. HSIEH, Associate Scientist, is with the Department of New Materials Research and Development, China Steel Corporation, Taiwan 81233, Republic of China. Manuscript submitted December 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
elongation. Hsieh et al. correlated the Ni content to the formability of ZnNi-coated steels.[11] It has been shown that, as the Ni content increased, the microhardness of the ZnNi coatings increased, while the interfacial shear strength of the coating film to the steel substrate decreased. A harder coating film, together with weaker adhesion of the coating film to the steel substrate, accounted for the poor formability of ZnNi-coated steels when the average Ni content exceeded 13 wt pct. Several preplating treatments have been used to enhance the adhesion of a ZnNi coating to the steel substrate.[3,10,14–20] Among them, preplating a flash coating as an underlayer is considered to be the most effective way to enhance the adhesion of ZnNi to a steel substrate. Yau and Fountoulakis proposed that the stress-relieved sublayer formed upon acid immersion improves the overall coating adhesion.[3] Suzuki et al. speculated t
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