On the corrosion behaviour of as-cast and heat-treated Mg-RE alloys in 0.9% NaCl solution

Abstract

The microstructure and corrosion behaviour of as-cast and heat-treated Mg-1.44Nd and Mg-1.43Ce (wt.%) alloys in 0.9% NaCl (wt.%) solution were investigated using electrochemical tests, X-ray diffraction (XRD) and scanning electron microscopy, combined with energy-dispersive X-ray spectroscopy (SEM-EDS). The as-cast microstructure of both alloys revealed the presence of second phases. Heat treatment at 535°C for 6 h led to a more uniform distribution of the second phases in Mg-1.44Nd alloy and their dissolution along the grains boundaries in the Mg-1.43Ce alloy. As a result, the corrosion resistance was improved in the heat-treated alloys. Accordingly, the corrosion resistance values for the heat-treated alloys were much higher than those of the as-cast alloys, indicating that the heat-treated alloys were less susceptible to the corrosion. Also, the heat-treated Mg-1.43Ce alloy seems to have very good corrosion resistance (26890 Ω cm2) compared to the Mg-1.44Nd alloy (6156 Ω·cm2) by preventing pitting corrosion along the grains boundaries. The corrosion product was made up mainly of magnesium hydroxide Mg(OH)2 and magnesium oxide MgO and more uniform corrosion morphology were found in the heat-treated alloys.