An electrochemical sensor for monitoring atmospheric corrosion of metallic materials was constructed. Two metallic materials with different corrosion potentials, i.e. low alloy steel and copper conductive paint, were chosen to form the two electrodes of the sensor based on the principle of galvanic corrosion. The two electrodes are separated by a thin silicone insulating film, and thus an electrochemical corrosion system may be established by the two electrodes while which was covered by a thin liquid film came from the atmosphere, correspondingly, galvanic corrosion current is generated. This galvanic current is monitored to reflect the corrosion state of the material in the environment. A new type of electrochemical sensor was used to study the influence of factors such as insulation film thickness, liquid film thickness, temperature, relative humidity, and cathode to anode area ratio on the galvanic current with the assistance of an electrochemical workstation and a high-precision electronic balance. It is confirmed that the new electrochemical sensor has high sensitivity and can be used for monitoring the atmospheric corrosion process under thin liquid film.