Effect of NaHCO3 conversion coating on Ni-P electroless coating on AZ31 Magnesium alloy and investigating its properties

Electroless deposition of Ni-P coating on AZ31 magnesium alloy was carried out after forming conversion coating on the samples by immersing in saturated NaHCO3 solution. Effect of changing immersion time in NaHCO3 solution on surface morphology, chemical composition and phase structure of coatings were studied by SEM/EDS and XRD analyses. Hardness and corrosion resistance of the coatings were studied by microhardness test and polarization test in 3.5 wt.% NaCl solution. The results indicated significant effect of pre-treatment process on morphology and therefore corrosion resistance of Ni-P coated samples. By increasing conversion film formation time to optimum time, smooth and compact Ni-P morphology was obtained. Corrosion potential was nobler for Ni-P coatings in comparison with Magnesium alloy substrates and electroless Ni-P coating improved corrosion resistance of the substrate. In addition the microhardness of the Ni-P alloy coatings was examined and Ni-P coatings enhanced hardness, significantly. Increasing the conversion film formation time to a certain amount resulted in a better morphology of the Ni-P films with less porosity and thus better corrosion resistance. Any increase in conversion film formation time more than the optimum amount led to a lower corrosion resistance.