In a method for detecting corrosion of a surface not exposed to view of a metal piece, radiant thermal energy is directed against the piece by pulsed laser beam illumination thereby causing heating of said piece. Infrared radiation emitted by the piece is detected by a thermographic camera so that corroded portions of said surface are detected due to a different thermal response as a function of time relative to non-corroded portions. The laser beam is directed against an exposed face of a wall of said piece whose opposite face is the surface on which corrosion must be detected. The thermographic camera is provided on a same side where a laser head for emitting said laser beam is provided and has a control system for performing “Lock-in” thermography. Detection of the surface corroded portions is carried out by comparing a response of different surface portions during cooling of the piece.