Study on Technology Optimization and Coating Properties of Electroless Ni - P Plating on Magnesium Alloy

Abstract

In order to improve the hardness and corrosion resistance of the magnesium alloy surface, a Zn phosphating film was prepared on the surface first. And then, the optimum formula for electroless Ni - P plating on the phosphating film of AZ63 magnesium alloy was determined through orthogonal test. Subsequently, the surface appearance, composition and corrosion resistance of the coating prepared with optimum formula were analyzed by scanning electron microscope (SEM) and electrochemical methods. Results showed that the optimum bath formula for electroless plating was as follow: nickel sulfate 7.69×10-2 mol/L, sodium hypophosphite 1.70×10-1 mol/L, sodium citrate 3.40×10-2 mol/L,thiourea 1.32×10-5 mol/L and ammonium bifluoride 1.80×10-1 mol/L with a plating condition of pH=8, T=80 ℃ and t=1 h. In case of the optimal formula, the plating rate of coating was above 26 μm/h; the hardness of the coating was 648 HV, about 5 times of that of magnesium alloy; the nickel and phosphor content of the coating were 90.55% and 9.45% respectively; and the corrosion potential of the coating was up to -0.66 V in 3.5% NaCl solution with a corrosion current density of only 1.25×10-5 A/cm2 .

Keywords

• az63 magnesium alloy|phosphating|electroless ni - p plating|bath formula|corrosion resistance