Microstructure and formability of ZnNi alloy electrodeposited sheet steel

Abstract

ZnNi alloy electrodeposited sheet steels were made from a chloride bath using a high-speed flow cell. A Ni-rich flash coating was deposited first, upon which the ZnNi coating, with Ni contents ranging from 8 to 16 wt pct, was subsequently electrodeposited. It is demonstrated that the Ni content of the coating affects the forming properties and microstructure of the ZnNi coatings. The hardness of the ZnNi coating increased with Ni content, leading to poor formability and inferior adhesion of the coated steels, as evident from the large amount of coating loss during swift cupping and coating peel-off during low-temperature adhesion tests. On the other hand, the friction force between the coated steel and cupping die decreased with increasing Ni content. At low Ni contents of 8 wt pct, the coating had a porous equiaxed grain structure. As the Ni content increased, the coating surface changed to dense faceted morphologies. A pyramid morphology was observed for 16 wt pct ZnNi coatings. An X-ray diffraction (XRD) analysis showed that all coatings containing up to 16 wt pct Ni contained only γ phase. Transmission electron microscopy (TEM) observations showed the 8 wt pct Ni coating to have a fine-grained structure, which changed to a columnar structure at 16 wt pct Ni. The formation of the columnar structure is explained by the smaller amount of hydrogen discharge as the bath Ni ion concentration increased.