An alternating current (AC) impedance technique has been applied to the monitoring of corrosion rate in outdoor environments and humidity chambers. A corrosion sensor used for corrosion rate monitoring consisted of a pair of ring and rod of steel electrodes embedded co-axially in epoxy resin. The instantaneous corrosion rates have been monitored by the continuous measurement of impedance at 10mHz and 10kHz. In comparing corrosion loss obtained from impedance measurement with actual corrosion loss by laser microscope, it was found that the present electrochemical technique could be applied for the atmospheric corrosion monitoring. Furthermore, from the thermodynamic analysis for drying and water absorption process of the artificial seawater, the atmospheric corrosion rate of steels showed its a maximum at an average water film thickness of 100 μ m from 10 μ m.