Nd-Fe-B permanent magnet is widely applied with excellent magnetic properties and low price. The major drawback is its low corrosion resistance in humid and chloride environment. To overcome this problem, it is critical to protect it from corrosive environment with surface treatment. The major surface treatments are electroplating Zn, Ni, Zn-Ni and electroless plating Ni. Surface treatments are not sufficient which levels of corrosion protection and adhesion are relatively low. Taking into account its characteristic, this work brings forward a technique to protect Nd-Fe-B which is to alkaline electroless plate Ni-P to enhance adhesion, to electroplate Cu to inhibit H and to electroplate Zn-Ni as anode. Through this method, we can enhance adhesion and protection at the same time.
Pretreatment is the key to have good coating. To enhance the adhesion of Nd-Fe-B with electroless Ni plating, the effect of pretreatments on the adhesion of electroless Ni coating is studied. The result reveals that with ultrasonic rinsing，the optimized pretreatment process was that the concentration of nitric acid was 60 ml/L with an acidic cleaning time 30-40s. By this process，the adhesion strength of the electroless Ni coating with the substrate of sintered Nd-Fe-B permanent magnet may reach over 28 MPa.
Electroless Ni plating is usually achieved in acidic solutions. To reduce the corrosion of electroless Ni solutions, alkaline solution is developed. The key drawback of alkaline solution is its low stabilization. The effect of stabilizer thiourea is studied. Thiourea can stabilize the solution, but it reduces the deposition rate at the same time. Deposition rate is very important. If the deposition rate is high, Ni layer can grow fast and Nd-Fe-B can be protected from electroless plating solution. To enhance the deposition rate, concentration of NH4F, pH of solution and temperature are studied. The results reveal that the solution is stable, deposition rate reach 21 µm/h and the adhesion is very well. Ni coating can relatively protect Nd-Fe-B.
Alkaline Zn-Ni solution is low corrosive and its coating is uniform which excels acidic solution. But the current efficiency of alkaline Zn-Ni solution is very low and lots of H is produced which is deleterious to Nd-Fe-B. With extremely low hydrogen permeability, Cu coating is chose to inhibit H. The effect of composition and electroplating parameters (concentration of Ni in solution, current density and temperature etc) of Zn-Ni plating are studied. Basing on the results, the electroplating process is optimized. Nd-Fe-B with Ni/Cu/Zn-Ni layers can stand 880 hours and can stand 1500 hours in salt spray test after it passivated.
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