The corrosion protection of cerium-based conversion coatings (CeCC) formed on AZ91D magnesium alloy has been studied using potentiodynamic polarization measurements, electrochemical impedance spectroscopy (EIS), and salt fog testing. The effect of acid (sulfuric acid [H2SO4]) and alkaline (sodium carbonate [Na2CO3]) pretreatments on the coating morphology and corrosion resistance of the cerium conversion layer was investigated. Significant change in corrosion resistance occurred with different pretreatments. Samples with combined acid and alkaline pretreatments showed better salt fog performance than samples pretreated with an acid or alkaline solution only. The corrosion rate (mm/y) calculated from potentiodynamic polarization measurements were 0.77 for bare AZ91D alloy, while cerium coated rates were 0.16 for acid-cleaned, 0.09 for alkaline-cleaned, 0.04 for acid then alkaline, and 0.18 for alkaline then acid pretreatments. Potentiodynamic and impedance results were correlated with that of salt fog testing. Low calculated corrosion rates corresponded to few pits and tails during salt fog testing.