The corrosion behavior of as-cast magnesium alloys (AM50, AZ91, and AZ91Si) was investigated in a 0.1 M sodium sulfate solution at the corrosion potential (E-corr) using electrochemical impedance spectroscopy. Transmission electron microscopy was used to analyze the corrosion product layer, and phase shifting interferometric microscopy was carried out to characterize the reactivity of intermetallic particles. Due to its microstructure, the AM50 alloy presented uniform corrosion during immersion, whereas corrosion of the AZ91 alloys began in the grain body and progressively spread to the eutectic areas. For the AZ91 alloys, the dissolution of the alpha -eutectic phase led to a strong aluminum enrichment of the corrosion product layer and, when a threshold was reached in the level of Al2O3 in the magnesium oxide (or hydroxide) layer, a change of phenomenology occurred in the impedance diagrams. In addition, electrochemical results revealed that an increase of silicon concentration for the AZ91 alloys decreased the corrosion resistance. This was attributed to an increase of the number of Mg2Si particles, accelerating the dissolution of eutectic areas.