Rheocast AZ91 magnesium alloy was evaluated in terms of microstructure and corrosion resistance. Rheocasting leads to a globular microstructure of alpha-Mg spheres separated by an interconnected beta-Mg17Al12 (beta-phase) network. Findings revealed that early stages of corrosion were located at the center of alpha-Mg globules and more importantly at alpha-Mg/beta-phase interfaces due to galvanic coupling as predicted from surface potential maps. Electrochemical, hydrogen evolution and weight loss measurements demonstrated the superior corrosion resistance of the rheocast alloy. This was attributed to an improved barrier effect of the beta-Mg17Al12 phase and, possibly, to a smaller area fraction of Al-(Mn,Fe) inclusions. For long immersion times, only small attacked areas were observed which corresponded to randomly corroded alpha-Mg globules. (C) 2015 The Electrochemical Society. All rights reserved.