A method of detecting a locally generated corrosion of a first metal element when exposing the first metal element (2) to a corrosive environment. A first surface of the first metal element (2) is exposed to the corrosive environment. A sponge (3) is sandwiched between the first metal element (2) and a second metal element (4). The sponge (3) constituting in dry state an electrical insulator and constituting an electrical conductor when wetted in the corrosive environment. An electrical insulator together with the first metal element (2) encapsulates the second metal element (4) and the sponge (3) for preventing the second metal element (4) and the sponge (3) from being directly exposed to the corrosive environment. An impedance between the second metal element (4) and a ground electrode is measured, and a decrease in the impedance from an infinite impedance to a finite impedance caused by a pinhole being generated in the first metal element (2) is monitored.