The influence of trace Si on the microstructure and corrosion behavior of as-cast Mg-0.4Mn-xSi (x = 0, 0.02–0.3) alloys is studied. With the presence of trace Si in Mg-0.4Mn alloy, MnSi, Mn5Si3 and Mn3Si phases form. The MnxSiy phase significantly increases the corrosion rate of Mg-Mn-Si alloys due to strong micro-galvanic corrosion between MnxSiy phases and Mg matrix. During corrosion, the rod-like MnxSiy precipitates can lead to stronger cathodic activation behavior than the block-like MnxSiy precipitates since the rod-like MnxSiy phase is more difficult to detach from Mg matrix, which results in the more exposed area of cathodic sites. Data AvailabilityThe raw/processed data required to reproduce these findings cannot be shared at this time, because the data also forms part of an ongoing study.