Study On The Protective Electroless Plating Of NdFeB Sintered Magnets

In this dissertation, corrosion mechanisms and current anticorrosion treatments of NdFeB permanent magnets were reviewed. Electroless deposition of Ni-based alloys on the surface of NdFeB magnets was employed to enhance their corrosion resistances. Utilizing electrochemical workstation, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and optical microscope (OM), the effects of compositions and processing parameters on the microstructures and properties of the coatings were studied systematically.Satisfactory adhesion between coatings and NdFeB magnets was achieved by electroless Ni-P plating in an alkaline bath with ultrasonic irradiation. With 150 W and 40 KHz ultrasonic wave, the adhesion strength was enhanced significantly from 6 MPa to 25 MPa, which can be contributed to the activation, cavitation and stirring action of the ultrasonic wave. Nd³⁺ could also improve the adhesion between coatings and NdFeB substrates, because it restrained the corrosion of Nd-rich phase.ATP was proved to be a good stabilizing agent. It enhanced the deposition rate when its concentration is below 8 mg·L^-1. ATP also acts as a complexing agent and a brightener. In the bath, ATP was easily oxidized to a bipolymer and released electrons. After accepting electrons, Ni²⁺was reduced. And then the bipolymer was reduced to ATP by H₂PO₂^-.The optimized composition of compound brightener were 20mg·L^-1 of C₆H₅SO₂Na, 80 mg·L^-1 C₅H₅N, 20 mg·L^-1 C₃H₅SO₃Na and 1 mg·L^-1 (NH₂)₂CS. The deposition rate increased with increasing C₆H₅SO₂Na and C₅H₅N in the solution when their concentrations were below 15 mg·L^-1. The coating obtained from the bath with additions of the compound brightener was compact and smooth, and possessed good corrosion resistance.Corrosion potential and corrosion current density of coatings improved with the addition of Yb³⁺. The reason was that Yb³⁺ reduced the surface energy and increased nucleation rate. Therefore, fine and cellular structures on the coating surface served as barrier layer to the electrolyte.The addition of NH₄F affected the deposition rate because F accelerated the dehydrogenation of hypophosphite complex, while hindered the adsorption of H₂PO₂^- and Ni²⁺ on the substrate surface. NH₄F significantly improved the buffering capability of plating solutions. Under the same concentration of 20 g·L^-1, the buffering capability of NH₄F was the best among the traditional buffering agents including H₃BO₃, (CH₂)₂(COOH)₂ and CH₃COONa. Refined and compact Ni-P coatings with homogeneous elemental distributions of P could be obtained with the addition of NH₄F, resulting in enhanced corrosion resistance and surface hardness of as-plated coatings.Using lactic acid and acetic acid as the complexing agents, the as-deposited Ni-P coating with high hardness and improved wear resistance was deposited. The microhardness of the as-deposited electroless Ni-P coating was determined to be 820 HV_0.1, which was the highest value reported so far. The highest microhardness and best wear resistance was obtained for the coating containing 7.8 at.% phosphorus. This can be ascribed to the fact that, at such a phosphorus content, the microstructure of the as-deposited Ni-P coating transformed from nanocrystalline to a mixture of nanocrystalline and amorphous.The activation energy of electroless Ni-P plating in the acidic solution was lower than that in the alkaline solution. The reaction kinetics of electroless Ni-P alloy in the acidic bath was discussed. Reaction kinetic parameters were obtained and an empirical equation of deposition rate was derived, which agreed well to the experimental results.For the Ni-Cu-P ternary coating, it is found that, with increasing metallic ion ratio ([Cu²⁺]/ [Ni²⁺]) from 0.01 to 0.20, the copper content of the coating increased, but the phosphorus and nickel content decreased; the structure of the coatings changed from amorphous to crystalline state. The coatings obtained from the metallic ion ratio ([Cu²⁺]/ [Ni²⁺]) 0.01 to 0.05 baths at temperature 80℃and pH 9.0 possessed excellent surface quality, low porosity and good corrosion resistance. Protection of sintered NdFeB magnets was also successfully achieved by electroless Ni-Co-P plating. With increasing metallic ion ratio ([Co²⁺]/ [Ni²⁺+Co²⁺]), the plating rate decreased and the cobalt content of the deposit increased, accompanied with simultaneous decreasing nickel and phosphorus contents. The polarization curves suggested that the corrosion rate of NdFeB magnets decreased by two orders of magnitude in the 3.5 wt% NaCl solution after electroless plating with the Ni-Co-P coating. A metallic ion ratio ([Co²⁺]/ [Ni²⁺+Co²⁺]) at 0.3 in the bath solution achieved the best corrosion resistance. When the metallic ion ratio ([Co²⁺]/ [Ni²⁺+Co²⁺]) varied from 0.1 to 0.9, loss of Br decreased from 7.5% to 3.9%, and loss of Hc decreased from 9.2% to 4.5%.