Corrosion sensors

A corrosion sensor for detecting the action of corrosive media on a metallic material when mounted in situ adjacent a location in the metallic material is disclosed. The corrosion sensor comprises a patterned conductive thin film formed on a substrate. The thin filmdefinesfirst and second terminals, a first conducting track running between the first and the second terminals, and conducting regions separated from theterminals and the first conducting track, and disposed adjacent either side of the first conducting track.In preferred embodiments, paint substantially covers the substrate and the thin film. A defect is formed in the paint at the location of the firstconducting track. This defect extends over at least a part of the first conducting region. In one disclosed embodiment, there are a plurality of conducting tracks and conducting regions defined by the patterned conductive thin film.