A risk-based analysis of the components underneath army trucks indicated that the frame should be given the highest inspection and maintenance priority. The frames for this truck series in the Canadian army are sets of overlapping steel C-channels held together by bolts and rivets. Crevice corrosion or occluded cell corrosion occurs between the plates and causes them to separate due to the higher volume associated with corrosion products. Knowing when a critical gap was actively corroding or held in check by naturally dry conditions, controlled dehumidification or corrosion prevention compound (CPC) application would be of use to maintainers. An impedance sensor showed promise in distinguishing between three simulated cases corresponding to these states (completely dry, water-filled and CPC-filled) by differences in impedance modulus and particularly impedance phase angle. The optimal window for a single-frequency measurement that can distinguish between these states is probably between 10 and 50 Hz.