A sensor based on the change in the magnetic field‐induced microwave absorption due to the presence of ferrous corrosion products on steel is described. This type of sensor for detection of corrosion under structural coatings is unique in that its sensitivity is to chemical, rather than physical alterations, to steel surfaces by corrosion processes. Preliminary results suggest that this sensor may be particularly sensitive to ferrous corrosion products found in the initial phases of the corrosion process. © 2003 American Institute of Physics