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.