This report describes a prototype dual mode corrosion monitor which is capable of measuring both instantaneous corrosion rate and cumulative corrosion loss in atmospheric environments not boldly exposed to weather. The monitor system consists of two components: a small flat sensor to be located in the area to be monitored and a battery powered electronics unit to query the sensor. The sensor element is made of steel which is far more sensitive to atmospheric corrosion than most commonly used metals; thus the sensor will react before significant damage has been done to other materials. The sensor itself is actually two discrete sensors on the same substrate and photo-etched from the same foil. Corrosion rate is measured using two-electrode Linear Polarization Resistance (LPR) technique. Cumulative corrosion loss is measured by the Electrical Resistance (ER) method. The electronics unit has been designed for operation by personnel with minimal corrosion knowledge. Its primary purpose is to alert an operator of the existence of corrosive conditions so the remedial action can be taken before extensive corrosion damage has occured.