Effect of SiC particle additions on the corrosion resistance, thermodynamic stability and surface morphology of Mg–Al alloy in sulphate and chloride media

Abstract

Application of magnesium materials continues to grow due to their high specific strength and excellent castability. However, low corrosion resistance limits their application in aqueous electrolytes. Understanding their corrosion behaviour is key to the improvement of their corrosion resistance. The effect of SiC at specific weight content on the corrosion polarization behaviour, surface morphology and thermodynamic stability of Mg–Al alloy in 0.05 M H2SO4 and 3.5% NaCl electrolytes was studied through potentiodynamic polarization, optical microscopy and open circuit potential measurement techniques. Results showed SiC additions significantly and progressively decreased the corrosion rate of Mg–Al alloy in both electrolytes due to decrease in the exposed area for the formation of porous Mg(OH)2 on the alloy surface. At 0% SiC wt content the corrosion rate values are 4.930 mm y−1 and 0.259 mm y−1 while at 10% SiC wt content the corresponding values have decreased to 0.429 mm y−1 and 0.090 mm y−1. Anodic potential displacement occurred in H2SO4 at all SiC% wt contents leading to rapid destruction of the protective oxide and absence of passivation characteristics. Mg–Al alloy demonstrated sufficient passivation after anodic polarization in NaCl solution due to the localized electrochemical action of chlorides. Morphological representations showed severe general surface deterioration on Mg–Al specimens from H2SO4 and visible localized deterioration on specimens from NaCl in the absence of SiC. The extent of deterioration on the morphology of Mg–Al alloy decreased at 2.5% and 10% SiC wt Without applied potentials Mg–Al alloy in H2SO4 with SiC was relatively more electropositive due to stability of its protective oxide compare to specimens with SiC. In NaCl solution, the reactivity of chlorides hindered the formation of the protective oxide resulting in more electronegative potential plots. However, the plots of Mg–Al at 2.5% and 10% SiC wt content where thermodynamically unstable due to visible potential transients.