Investigation of the effect of ni-b based composites onthe corrosion resistance of Ni-B based composites on AZ31magnesium alloy

The present work investigates the electroless plating of nickel-boron and nickel boron composites coatings on AZ31B magnesium alloy. Pickling is particularly critical before Ni-B electroless plating to create a uniform and catalytically active surface. Pickling treatment on AZ31-B-Mg alloy was investigated in phosphoric acid followed by immersion in 40% HF. Pickling treatment was also performed in an one solution containing both H3PO4 and HF. In either case, the effect of concentration of both H3PO4 and HF as well as the effect of time were investigated. The pickled samples were then electroless plated with a Ni-B composite. The thickness of the plating layer was affected by temperature, time and bath composition. The performance of the Ni-B composite coating was investigated under the SEM. It is shown that the weight loss of the pickled samples is increased with the increase of H3PO4 concentration and reaches a maximum in 400 ml/L acid solution. Further increase in H3PO4 concentration results in a higher concentration of PO4 3−, leading to a higher tendency to produce insoluble films (mainly Mg3(PO4)2 and Al3PO4) on the substrate surface slowing down the rate of Mg alloy oxidization and dissolution, leading to a decrease in weight loss. The highest thickness of the plating layer was obtained at 6 g/L of NaHB4 at 85 oC. SEM investigation predicted the electroless deposition of an adherent compact layer of Ni-B composite coating on the H3PO4/HF pickled AZ31 samples. Electrochemical impedance spectroscopy measurements in 3.5%wt. sodium chloride aqueous solution suggest that the plating can protect the magnesium alloy substrate from corrosion attack. The present study has also presented the synthesis and a comparison of properties (structural and mechanical) of Ni-B and Ni-B-Al2O3 and Ni-B-WO3 composite coatings to elucidate the beneficial role of Al2O3 or WO3 addition in binary Ni-B coatings. The Ni-B-Al2O3 and Ni-B-WO3 coatings were synthesized by adding Al2O3 or NaWO3 powdered particles into the above mentioned Ni-B coating solution. A significant improvement in mechanical properties has been observed by the addition of Al2O3 or WO3.