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The corrosion of a MgAl9Zn1 (UNS M11918, AZ91(dagger)) alloy in an atmosphere of water vapor with cyclic wet-dry conditions was studied using environmental scanning electron microscopy (ESEM). The results showed that the oxygen concentration on the surface generally increased with the wetting time. At short wetting times the corrosion was localized at the droplets and mainly appeared on the alpha-Mg areas. The corrosion rate increased with the wetting time. At long wetting times a water film covered the surface and general corrosion occurred. The rate of corrosion decreased with increased time of wetting. The magnitude of oxygen concentration at AlMn-segregated areas was similar to that on the matrix area. The segregation of the AlMn phase contributed little to the average corrosion on the surface but contributed significantly to the localized corrosion.