Evaluation of alternative environment-friendly buffers for Ni–B alloy electroplating

Abstract

Sodium citrate (SC) and sodium propionate (SP) were evaluated as candidates for environmentally friendly alternatives to replace boric acid (BA), which causes wastewater problems, as a buffering agent in the plating bath for electrodeposition of high-strength Ni–B alloy. The overall material characteristics and performances (corrosion resistance and mechanical properties) of the Ni–B coatings electrodeposited with the three different buffers were compared. The Ni–B coating electrodeposited with SC had the smallest crystallite size and the lowest B content, and showed a smooth surface and a dense crack-free cross section, while the coating prepared with SP had the largest crystallite size and showed a nodular surface morphology with obvious micro-cracks as a result of the higher internal stress caused by its highest B content. Their corrosion resistance and mechanical properties were closely related to the crystallite sizes and morphological properties. The Ni–B coating electrodeposited with SC exhibited the best corrosion resistance, with an icorr of 2 × 10−6 A/cm2, the lowest wear rate of 26.2 × 10−6 mm3/Nm, and the highest hardness of 971 Hv. Additionally, a heat treatment at 400 °C revealed that this coating also has the best high-temperature endurance, which is desirable for applications such as gun barrel coatings. In comparison, the Ni–B coating prepared with the SP buffer exhibited properties slightly lower than, but satisfactorily close to, those of the conventional coating prepared with BA. Therefore, both SC and SP could replace BA in any industrial applications, but SC is the best choice since it offers improved performances in all aspects. The results offer a greener method for preparing high-strength Ni–B coatings suitable for applications involving high temperature operation.