Electrodeposited Zinc-Nickel/Chromium Binary Coating and Its High Temperature Protection Properties for Direct Hot Stamping Steels
Date Issued
2014
Date
2014
Author(s)
Wu, Chun-Hung
Abstract
Electrodeposited cadmium layer had provided excellent cathodic protection under serious marine conditions and expressed bright metallic color. Nowadays, the electrodeposited zinc and zinc-based coatings have been extensively applied in general auto industries as replacements for cadmium, which was found carcinogenic in many previous literatures. Chromium is the main alloy element to be added in stainless steel to form a compact chromium oxide above the surface, which retards Fe2+ ions diffusing through the oxide, thus slows down the oxidation rate. In this work, the electroplated bilayer (Zn-Ni/Cr) was used to prevent the Liquid Metal Induced Embrittlement (LMIE) phenomenon and serious oxidation that typically occur in hot stamping process degrading the mechanical properties and consuming the available coatings. The microstructure of Zn-Ni and Zn-Ni/Cr coatings was observed by SEM and TEM, and the thermal stability was examined by EPMA and Line scan. The mechanical properties of steels with and without zinc-based coating were illustrated by the strain-stress curve of high temperature tensile test and the broken specimens are examined by Metallographic analysis. Moreover, the surface oxides are characterized by X-ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), which help to further understand the oxidation situation in high temperature. By combining the results mentioned above with open circuit test and dynamic polarization curves, it is able to evaluate the performance of Zn-Ni/Cr bilayer coating in simulated hot stamping process.
The results show that Zn-Ni coating’s morphology and current efficiency both strongly depend on the ion ratio in the bath and bath temperature. The normal co-deposition of Zn-Ni always occurs at high nickel ion ratio in the electrolyte and high deposition temperature. EPMA mapping results revealed that there’s a serious separation of compositions in the Zn-Ni coating after heat treatment due to the effect of oxidation and inter-diffusion with the substrate. With the addition of chromium layer on top the situation of separation of compositions in the Zn-Ni coating was apparently reduced. The thermal stability is becoming higher when nickel content increases in the Zn-Ni coating; however, the overall reduction potential will also move close to the substrate comparatively. In addition, spinel structure (ZnCr2O4) formed by ZnO and Cr2O3 at high temperatures was identified by XRD and XPS measurements, which indicated zinc diffused through the chromium layer to form zinc oxide at the surface. The phenomenon can be explained by the great solubility of zinc in chromium and high oxygen affinity of zinc. ZnCr2O4 layer was also observed in TEM for detailed information. In conclusion, the Zn-based coatings exhibit higher thermal stability and lower corrosion rate the electro-galvanized coating. On the other hand, Zn-Ni/Cr bilayer provides better oxidation resistance than Zn-Ni coating that reduces the loss of coating through serious oxidation at high temperatures. With the adjustment of suitable nickel contents in the Zn-Ni coating and achieve equilibrium between the prevention of LMIE and enough ability of cathodic protection, Zn-Ni/Cr bilayer might be a solution to meet the requirements of hot stamping process.
Subjects
熱沖壓鋼板、液態金屬誘發脆化
抗高溫氧化
陰極保護
電鍍複合層
SDGs
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-103-R01527024-1.pdf
Size
23.54 KB
Format
Adobe PDF
Checksum
(MD5):02b02c888924151d20965d4b51317845