Effect of surfactant on the growth and corrosion resistance of Ni-P coatings on sintered Nd-Fe-B magnets

摘要

Electroless deposited Ni -P coatings have been employed as protection coatings for sintered Nd -Fe-B magents in industry due to their good corrosion resistance and mechanical properties. However, direct preparation of Ni-P coating on Nd-Fe-B suffers severe gas evolution, resulting in a coating with high porosity and uncompactness. In this work, the content of surfactant (sodium dodecyl sulfate, SDS) in the solution has been regulated t o decrease the reaction rate and optimize the roughness of the Ni-P coating1. The results of morphological characterization indicate that the crystalline state and the percentage of P is closely related with the dosage of SDS in the solution. With the concentration of SDS increase from 4 mM to 32 mM, the P content exhibit a vocanical trend with the maximum percentage of P (10.2%) appearing at the concentration of 20 mM. As mentioned previously, the percentage of P decided the crystalline of Ni-P coating, which in turn affect its corrosion resistance2. Moreoever, the regulation of P content also optimizes the compactness of the coatings. As a result, the corrosion resistance of Ni -P coated NdFeB magnets has been improved greatly in 3.5% NaCl solution.