Recent investigations of magnesium-aluminum alloys revealed a strong influence of four key microstructural parameters on corrosion: porosity, volume fraction, distribution and Al-content of the γ-Al₁₂Mg ₁₇ phase. Additionally, an improved corrosion resistance was observed by the addition of rare earth elements. The influence of cerium on the casting skin, inner microstructure, and corrosion of sand-cast and die-cast AZ91D with 0.5-2.0 wt.% cerium was investigated by SEM before and after potentiodynamic measurements and electrochemical impedance spectroscopy in pH8 and pHll. Cerium significantly improved the corrosion resistance through formation of a passive layer. Strong influences of cerium on the microstructure (average grain size, changes in morphology and/or volume fraction of Mg₁₇Al₁₂ particles, and formation of cerium-rich intermetallic phases) were observed. Microstructural influences as well as the change in chemical composition alter the local corrosion mechanism and passive layer formation in AZ91D augmented with cerium.