The Effect of Complexing Agent on Electroless Nickel Sulphamate Coating on Brass Alloy: Structure Characteristics & Corrosion Behavior

Abstract

This study investigated the successful performance of acidic electroless Ni–P coatings on brass alloy from the nickel sulphamate bath. Three complexing agents (i.e., sodium acetate, acetic acid, and sodium citrate) were evaluated in a practical optimized sulphamate bath. Then, the corrosion behavior of coated samples was studied by potentiodynamic polarization (DC) and electrochemical impedance spectroscopy (EIS) methods in a 3.5 wt% NaCl solution. Moreover, the structure and chemical composition of the coated samples were characterized by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), respectively. In addition, the morphology of the coatings was examined through scanning electron microscopy (SEM), as well as investigating properties such as thickness, grain size, and actual capacitance using computational methods. The results revealed that the structure and corrosion behavior of coatings were strongly influenced by both practical conditions and complexing agent factors. Low and medium phosphorus coatings with nanocrystalline and semi-crystalline structures were developed according to EDS and XRD results. Additionally, SEM images and electrochemical results demonstrated low phosphorous Ni–P coatings from baths containing sodium acetate with a cauliflower structure, as well as an acetic acid complexing agent with a dense structure and highly acceptable granulation which had the best anti-corrosion behavior (R_P = 46,369 Ω cm² & i_corr= 0.56 $\mathrm{\mu A}/{\mathrm{cm}}^{2}$ and R_P = 44,561 Ω cm² & i_corr= 0.7 $\mathrm{\mu A}/{\mathrm{cm}}^{2}$, respectively) compared to a bare sample (R_p = 3687 Ω cm² & i_corr= 2.72 $\mathrm{\mu A}/{\mathrm{cm}}^{2}$).