Abstract: Apparatus and method to detect corrosion in metal junctions. Corroded metal junctions are usually discovered by visual inspection. The present invention detects corrosion in metal junctions when it is not visually apparent. A corroded metal junction acts as a nonlinear device. It generates harmonics and other nonlinear products (such as intermodulation) of any signals applied to the junction. The presence of relatively high level harmonics and/or intermodulation products indicates directly that corrosion has occurred. To detect corrosion in a metal junction, one couples a fundamental frequency signal (f0) into the junction and tests for harmonics of that frequency, especially the third harmonic. Harmonic frequency signals that are relatively large (i.e., above the harmonics generated by the testing system) indicate the presence of corrosion. Measurements to determine if a metal junction is corroded are performed without disturbing the junction.