Synergistic Effects of Multicomponent Complexing Agents on Microstructure and Corrosion Performance of Alkaline Zn–Ni Electrodeposits

In this study, a stepwise strategy was adopted to regulate and control the complexing agent system of the cyanide-free alkaline zinc–nickel (Zn-Ni) electroplating solution. A Zn-Ni coating containing a quaternary complexing agent was successfully prepared on the surface of low-carbon steel to enhance the flatness and corrosion resistance of the coating. The investigated complexing agents included triethanolamine (TEA), tetraethylenepentamine (TEPA), ethylene diaminetetraacetic acid (EDTA), and potassium sodium tartrate (PST). By fixing other parameters and testing the four complexing agents one by one, TEA was finally determined to be the best single complexing agent. This substance can effectively stabilize the metal-ion deposition process. The synergistic effects of TEPA, EDTA and PST were systematically studied, and the independent contributions of each complexing agent to performance were clarified. This design enables performance differences to be directly attributed to the inherent characteristics of each complexing agent category. The surface morphology of the sample was characterized by scanning electron microscopy (SEM). The cross-sectional morphological characteristics were analyzed by a laser scanning confocal microscope (LSCM). The chemical composition and phase composition of the samples were determined by X-ray diffraction (XRD), while the corrosion behavior of the samples was studied by electrochemical techniques, including electrochemical impedance spectroscopy (EIS) and Tafel curves. Characterization shows that when the molar ratio of EDTA to PST is 1:4 and TEA and TEPA are used in combination, the grain size of the coating is smaller, the surface is smoother, and the number of surface micropores is significantly reduced compared with coatings prepared by other complexation systems, thereby improving the flatness and corrosion resistance of the coating.