Detection and imaging of surface corrosion on steel reinforcing bars using a phase-sensitive inductive sensor intended for use with concrete

A new inductive sensor is described that exploits the principle of frequency shift and phase sensitive detection to identify and image corrosion on the surface of steel reinforcing bars intended for embedment within concrete. A search coil radiating a time-varying magnetic field experiences impedance changes when conductive and/or permeable targets are brought within its vicinity. Depending on the electrical properties of the target, the impedance changes are manifest predominantly as a shift in the Q-factor of the coil, or as a change in its inductance. It is the latter condition that is the primary effect with corrosion product. Under normal circumstances the change in inductance L is very small, but can be detected by configuring the coil as part of a free-running tuned oscillator, whose resonant frequency is governed by L. A shift in the sensor signal frequency is detected by comparing its phase to that of a signal produced by a stable reference oscillator (heterodyning), and producing a voltage proportional to the difference. Such phase sensitive detection is widely employed in metal detector instrumentation, but in this case the signal is used to produce images of corrosion. Significantly, experiments also show that the system is capable of detecting varying amounts of corrosion product, something that is beyond the capabilities of ultrasonic, X-ray or microwave sensing systems. At the present time, the sensor is capable of detecting and imaging a 2 mm thick layer of corrosion on a 20 mm diameter steel bar, located 30 mm below the surface of ceramic material whose conductivity and relative permeability is almost identical to that of concrete.