Scanning Kelvin probe force microscopy (SKPFM) is used to study the initial stages of atmospheric corrosion of an AlMg alloy and of physical vapor deposition (PVD) deposited 2 mm Al dots on pure Mg. The latter system is used as a model of a two-phase AlMg alloy. The influence of CO2 was studied in situ in humid air using SKPFM. This method allows for the in situ investigation of the evolution of the Volta potential during exposure, the resolution being in the submicrometer range. The temperature was 22.0 degreesC, and the relative humidity was 85 or 95%. The concentration of CO2 was <1 or 350 ppm. The corrosion products were analyzed by gravimetry, ion chromatography, X-ray diffraction, scanning electron microscopy, scanning Kelvin probe, and Auger electron spectroscopy. We found that the initial stages of atmospheric corrosion on magnesium are influenced by the presence of cathodic PVD-deposited aluminum. A similar effect was seen in the case of AZ91D, the aluminum- rich β-phase forming the cathodic areas. The β-phase is nobler compared to the substrate because of the higher Al content. In the absence of CO2, the corrosion attack is localized in nature whereas the presence of ambient levels of CO2 results in a more general corrosion attack. The inhibitive effect of CO2 on the atmospheric corrosion of AZ91D is explained by the formation of a passivating layer of Mg-5(CO3)(4)(OH)(2) &BULL; 5H(2)O. In the absence of CO2, the increase in pH originating from the cathodic reaction results in the dissolution of aluminum in the passive layer. A corrosion mechanism is proposed explaining the behavior in the two environments. (C) 2004 The Electrochemical Society.