Role of sodium hypophosphite on morphology and corrosion resistance of Ni-P coated Nd-Fe-B

Abstract

Uniform and dense electroless nickel (EN) was recognized as a promising protective coating for Nd-Fe-B magnets. However, large potential difference between intergranular phase and main phase of Nd-Fe-B made the deposition of EN coating uncontrollable. Sodium hypophosphite was used to regulate the microstructure of as-deposited EN coating. It was evidenced that the nodular size of as-deposited EN coating decreased obviously with the raised concentration of reducing agent due to increased nucleation rate of EN coating. However, overdose reducing agent produces excess volume of hydrogen gas, resulting in pinholes in the EN coating, breaking compactness of EN coating. As a result, the EN coating deposited with 22 g L−1 sodium hypophosphite exhibits a dense and compact amorphous structure with a high phosphorus content of 8.86 wt.%, delivering the best hardness, adhesion strength and the lowest coercivity loss among the three EN coatings (R15, R22 and R30). In addition, the corrosion potential of coated Nd-Fe-B magnets improved from −0.75 V for R30 to −0.47 V for R22 in 3.5% NaCl solution. This work unraveled the critical role of reducing agent on the microstructure and composition evolution, which is significant for constructing high corrosion resistive EN coating on Nd-Fe-B magnets.