Thixocasting is a new semi-solid processing route for magnesium alloys; it is claimed that finer microstructures can be produced and as a consequence, better corrosion resistance can be achieved. Therefore, it is of great interest to compare the corrosion properties of two Mg-Zn-RE alloys produced by standard squeeze casting and new semi-solid casting technique. The influence of the two different processing routes and the replacement of rare earth elements by Ca additions on the corrosion behavior were studied in NaCl aqueous solutions by (a) analyzing the corrosion morphology, (b) measuring electrochemical polarization curves, and (c) carrying immersion tests at constant pH-value. Using light microscopy, scanning electron microscopy and X-ray diffraction, the corrosion results were related to the microstructures on the specific alloys. The results indicate that Ca cannot replace rare earth elements under corrosion aspects, but they also showed that the thixocasting process resulted in better corrosion resistance.