Optimization of Electroplated Zn-Ni Alloy Coatings on SAE 1020 Steel Using Response Surface Methodology for Enhanced Corrosion Control in Chloride Environments
摘要
Zinc–nickel coatings combine sacrificial protection with barrier behavior associated with Ni-rich intermetallic phases, making them promising candidates for corrosion protection strategies that reduce reliance on cadmium- and Cr(VI)-based systems. In this study, Zn-Ni coatings were electrodeposited on SAE 1020 steel at bath temperatures of 30-60 °C and current densities of 25-55 mA cm⁻2, and the process was evaluated using a 22 full factorial design combined with response surface methodology-assisted screening, allowing identification of the most favorable condition within the investigated experimental domain. Top-view SEM-EDS indicated Zn-rich near-surface compositions, with apparent Ni contents ranging from approximately 19 to 49 wt.%, while cathodic current efficiencies varied from 12.5 to 52.9%. X-ray diffraction confirmed the coexistence of γ-Ni2Zn11 and δ-NiZn3 phases, with Scherrer-derived crystallite sizes in the 7-12 nm range. Electrochemical testing in 3.5 wt.% NaCl solution showed that the highest corrosion resistance was observed for Experiment 3 (60 °C, 25 mA cm⁻2), corresponding to a coating with a near-surface Zn69Ni31 (wt.%) composition, which exhibited Rp = 33.75 kΩ cm2, Ecorr = − 0.54 V (vs. SCE), and icorr = 1.90 µA cm⁻2, resulting in a corrosion rate of 0.0262 mm year⁻1. Electrochemical impedance spectroscopy further yielded a high interfacial resistance for this condition (R1 = 49.1 kΩ cm2), consistent with a more protective interfacial response. Within the investigated parameter window, variations in bath temperature and current density were accompanied by changes in near-surface Zn/Ni composition, phase constitution, microstructure, and corrosion behavior of the Zn-Ni coatings in chloride-containing environments.