It is important to detect corrosion at the initiation stage for effective intervention to minimize the impact of corrosion. In this paper, we present a passive inductively coupled coil sensor that can detect corrosion potential with respect to a reference electrode. The sensor has a sensitivity of 0.13 kHz/mV. Several experiment results of different geometry of this sensor show that it can measure corrosion potentials with a resolution of less than 10 mV. The sensor is 25 mm in diameter and is intended to be installed by drilling a 25 mm or greater hole into existing concrete. The bottom of the hole is filled with mortar and the sensing electrodes are embedded within the mortar. Chlorides from the concrete diffuse into the mortar eventually reaching the electrodes leading to rise in the corrosion potential. This corrosion potential can be used indicate the level of chlorides within the concrete. The generated potential is a measure of the transfer of electron charge between the metal and its environment, here between the steel and the concrete. This is a property of the steel/concrete interface and not of the metal itself. Thus, it is impossible to determine the absolute value of this electrochemical potential and it is necessary to have a reference electrode to measure the potential difference of steel with respect to this reference electrode. The presented sensor can be fabricated at very low cost and is suitable for corrosion monitoring of existing structures.