The work aims to study effects of CO2 on atmospheric corrosion as well as atmospheric corrosion behavior and corrosion mechanism of LY12 aluminum alloy with salt deposits on the surface in constant temperature and humidity with/without CO2. Different concentrations of NaCl and MgCl2 were deposited on aluminum alloy LY12 and constant temperature and humidity test with/without CO2. Effects of CO2 on salt deposition-induced atmospheric corrosion of aluminum alloy LY12 were studied by means of SEM/EDS, XRD and XPS. In the test conditions, pitting was dominant in MgCl2-induced atmospheric corrosion of aluminum alloy LY12, and MgCl2-induced weight loss was higher than that induced by NaCl. (Mg1–xAlx(OH)2)x+(Cl–, CO2–)x?mH2O (LDH) was major corrosion product in the MgCl2-induced corrosion process. However, Al2O3 or Al(OH)3 was major corrosion product under the influence of NaCl. Cu enriched in corrosion products of both mediums. Whether CO2 is present or not, Mg in MgCl2 takes part in forming corrosion products. Different effects of CO2 on corrosion behavior of pure Al and LY12 are mainly attributed to the presence or enrichment of Cu.