A corrosion sensor for detecting the action of corrosive media on a metallic component when the sensor is mounted in the vicinity of the metallic component in use is disclosed. The sensor includes an electrically conducting corrodible element mounted on a non conducting substrate, the corrodible element being covered with a protective coating such as paint adapted to protect the corrodible element from corrosive media. The protective coating defines a temporary feature such as a paint defect which extends across the corrodible element and is designed to permit attack on the corrodible element by corrosive media after a predetermined period of time. The corrodible element comprises a pair of spaced tracks extending generally in a longitudinal direction and a series of corrodible tracks, each corrodible track extending generally in a lateral direction and forming an electrical connection between the spaced tracks. The temporary feature extends longitudinally, in the space between the pair of tracks, across a number of the corrodible tracks whereby to permit a corrosive attack on a number of the corrodible tracks after the predetermined period of time.