Kinetic study of a novel electroless Ni P deposition on AZ91D magnesium alloy using nickel hypophosphite as the metal salt

A novel electroless nickel-phosphorus (Ni--P) process employing tetrasodium pyrophosphate and ammonium citrate as complexing agents and nickel hypophosphite as the nickel source which avoids the use of SO42- and Cl- ions was developed to improve the anti-corrosion properties of AZ91D magnesium alloy. The dependence of the electroless deposition rate on experimental parameters, the morphologies of the electroless deposit and the corrosion resistance of the electroless deposit were studied by the weight-gain method, scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS), respectively. An empirical equation was formulated to predict the Ni--P deposition rate. The results showed that tetrasodium pyrophosphate played a major role in the deposition process, followed by ammonium citrate, nickel hypophosphite, sodium hypophosphite and pH. The Ni--P coating significantly enhances the corrosion resistance of AZ91D Mg alloy.