Corrosion Resistance Of Electroless Plating On Sintered NdFeB Magnets

Sintered NdFeB permanent magnets possess outstanding magnetic properties such as high saturation magnetization, high coercive force and high energy products. It was used broadly as important functional materials in high technology sector. However, sintered NdFeB permanent magnets were corroded in humid environment easily because of the molding technology and its constitution and structure defects. To resolve the poor corrosion resistance and increase the service life of sintered NdFeB permanent magnets, electroless plating methods were used to provide an inoxidizing coating. Electroless plating nickel-phosphorus alloy or nickel-copper-phosphorus ternary alloy were deposited on the surface as an anticorrosive coating.The pretreatment processes were necessary before electroless plating on sintered NdFeB surface. These processes include plug process, oil removing process, pickling and activation process. The function of plug process was increasing the binding force of electroless coatings and substrate. However, the experiment result showed that the effect of plug process can't be exhibited until a period of time. Saponification and emulsification in the oil removing process can bring a clean surface for subsequent processes without decompose the magnets after ultrasonic cleaning 2 to 3 minuter in the pH range of 9 to 10 and at a temperature of 65℃. The pickling and activation process of the sintered NdFeB should not use the strong acid because it will corrode the magnets, so the weak acid was adopted. The processing parameter of alkaline electroless plating for sintered NdFeB permanent magnets affects the deposition process and pre-deposit coating's performance. The pre-deposit coatings which have good binding force with substrate can be obtained at temperature 85 to 90℃, and pH 8.5 to 10.5.The complexing agent concentration, the ratio of lactic acid to sodium acetate, the type and use level of stabilizer affect the electroless deposition process and the coating's performance during electroless plating nickel-phosphous alloy. A compound nickel-phosphorus brightener agent was used in the electroless plating bath and the bright coatings can be obtained. The additions of brightener improved the phosphoruscontent, decreased the porosity and reinforced the corrosion resistance of the nickel-phosphorus coating, but the microhardness of the coating was reduced. The optimized composition of compound brightener were 20mg/L of sodium benzene sulphinate, 80mg/L pyridine, 20mg/L sodium allylsulphonate and 1mg/L thiourea.Using sodium citrate as complexing agent can realize the co-deposition of copper, nickel and phosphorus. The complexing agent concentration, metallic ion ratio ([Cu~2+]/ [Ni~2+]) and processing parameters such as temperature and pH affect the deposition rate, the component and performance of the ternary alloy coatings. As the increase of the metallic ion ratio ([Cu~2+]/ [Ni~2+]) from 0.01 to 0.20, the copper content of the coating was increased, but the phosphorus and nickel content were decreased;the structure of the coatings changes from amorphous to crystalline. The coatings obtained from the metallic ion ratio ([Cu~2+]/ [Ni~2+]) 0.02 to 0.05 baths at temperature 80℃ and pH 9.0 has the excellent surface quality, lowest porosity and the best corrosion resistance.