Improvement of Corrosion Resistance of AZ91D Magnesium Alloy by Nickel Plating

AbstractThe demand of lightweight structural materials is growing in automobile industry for energy saving. Magnesium alloy is one of the best candidates due to its low density. However, its low thermal stability and poor corrosion-resistant property against wear and environment limit applications of magnesium alloy. The main object of this investigation is to improve corrosion resistance of magnesium alloy, AZ91D, by nickel coatings. The nickel coatings protect the magnesium substrate by providing a barrier layer between the substrate and its environment. After suitable pretreatment procedures and copper strike on AZ91D surface, nickel coatings were obtained by successive electroless and electrical nickel platings with thickness of 18μm and 10μm, respectively. The microstructural analyses were performed by field-emission scanning electron microscope (FE-SEM) with elemental mapping. The uniform deposition of nickel was observed from the cross-sectional SEM micrograph. Local compositions were studied with energy dispersive analysis using X-ray unit attached to the SEM. Linear polarization curves were measured by potentiostat and neutral 3.5% NaCl aqueous solution was adopted as electrolyte for electrochemical study at room temperature. The measured corrosion potentials were –1.49V and –0.79V (vs. Ag/AgCl) for AZ91D and nickel-coated samples, respectively. The measured corrosion current densities were 5.03μA/cm2 and 0.67 nA/cm2 for AZ91D and nickel-coated samples, respectively. Pitting corrosion of AZ91D was found from the anodic polarization curve. The results of present study are promising for the improvement of corrosion prevention of AZ91D material by nickel plating methods