Chromate conversion coating on aluminum alloys has been widely used in aerospace equipments and electronic parts to prevent the atmospheric corrosion in mild environmental conditions. Aluminum alloys were chromated with potassium dichromate by the typical method and microbial corrosions were tested according to the ASTM specification under different water activities. Chromate conversion coating on the surfaces of aluminum AA 1100 and aluminum alloy AA 6061 showed higher protective efficiencies than those of aluminum alloys AA 2014,AA 2024,and AA 7075.The chromated film on aluminum alloy AA7075 had significant corrosion under water activity 0.901,and magnesium content of the corrosion site decreased through the energy dispersive spectrometer analysis. The presence of metal oxides and biological ingredients in the corrosion site acted as the synergistic effects to enhance localized corrosion of aluminum alloys.