The Study Of Electroless Ni-P/Ni-W-P Duplex Coatings On AZ91D Magnesium Alloy

Electroless deposition process experienced numerous modifications to meet the challenging needs of a variety of industrial applications since Brenner and Riddell invented the process in 1946. Among the various types of electroless plating, electroless nickel has gained immense popularity due to their ability to provide a hard, wear and corrosion resistant surface. Hypophosphite-reduced electroless nickel plating has proved its supremacy in providing improved hardness, corrosion and wear resistance.Magnesium alloy has low density, high strength-to-weight ratio and other good physical and mechanical characteristics. However, the application of magnesium alloys has been limited due to their undesirable properties including poor corrosion and low wear resistance. Thus, the formation of anticorrosion and high wear-resistance coatings on the surface of Mg or Mg alloys is necessary in practical applications. Since magnesium is one of the most electrochemically active metal, ordinary coatings, such as nickel, copper and zinc coatings, can only provide a physical barrier to corrosion attack of magnesium substrate. So, any coatings on magnesium alloys should be as uniform, adherent and porosity free as possible.In order to improve the corrosion resistance of the AZ91D magnesium alloy, electroless Ni-P/ Ni-W-P duplex coatings on AZ91D magnesium alloy directly and indirectly were investigated in this paper. SEM and XRD analysis were performed to examine the microstructure and phase composition of the coatings. The corrosion resistance of the coatings was estimated by electrochemical polarization measurement and 10%HCl solution immersion test.Direct electroless Ni-P coating of magnesium alloy was obtained from nickel sulfate plating. The magnesium alloy samples were etched first in a solution of chromate and nitric acid and than soaked in HF solution to form a conversion film before electroless nickel deposition. But chromium compounds are carcinogenic substance, which should be abstained.An available method to obtain electrless Ni-P/Ni-W-P coatings on AZ91D magnesium alloy was discussed. The three Ni-W-P coatings have been deposited from three different solutions. With the pH value increase from 5, 7 to 9, the W content increase. The different W content, and have higher corrosion potential with W content increase. When the protective Ni-P layer is about 10μm and the three Ni-W-P coating were 14μm, the porosity is zero. As for the sample 1 with the Ni-W0.65-P8.18 coating has the best corrosion resistance. The sample can withstand in 10% HCl solution for about 170 minutes with the thickness about 18μm .It was found that the ability of corrosion resistance in 10%HCl was determined by the porosity of the coatings. The Ni-P/Ni-W-P duplex coatings with the same thickness of about 30μm can withstand in10%HCl solution for 170min, 49min and 97min, respectively.The highest hardness of the as-deposited Ni-P/Ni-W-P duplex coatings was about 686 VHN, which is far higher than that of the AZ91D magnesium alloy substrate (about 100 VHN).The duplex coatings are uniform and the compatibility between the layers is good. The coatings binding force is good, which could improve the corrosion resistance of the substrate. Hence it can be concluded that electroless nickel duplex coatings will be a useful replacement for Ni–P coating.