Method for detecting corrosion of a surface not exposed to view of a metal piece constituting a component of a motor-vehicle structure, wherein radiant thermal energy is directed by illumination against a face exposed to view of a wall of said metal piece whose opposite face is the surface on which corrosion must be detected, so as to cause heating of said metal piece, and wherein the infrared radiation emitted by the metal piece (3,30) is detected by a detection instrument comprising a thermographic camera and a laser head, so that corroded portions of said surface are located due to a different thermal response as a function of time with respect to non-corroded portions, wherein said thermographic camera is provided with a processing electronic control system, configured for performing a "Lock-in" thermography, and wherein detection of the corroded portions of said opposite surface is performed by comparing the response of the different surface portions, wherein said illumination is performed by means of said laser head emitting a pulsed laser beam against said surface exposed to view of the metal piece, and wherein said step of comparing the response of the different surface portions is performed during cooling of the piece, after that the illumination has been interrupted, said method further comprising the following steps: - providing said metal piece arranged above a bench inside a work cell, said metal piece being in the form of a hollow component, - providing said laser head and said thermographic camera carried by a robotized arm, said laser head and said thermographic camera being arranged on a same side with respect to said wall of the component to be analyzed, - driving the robotized arm by a respective electronic controller, which also includes a processing electronic control unit associated with the detection instrument, - providing a tablet for enabling an operator to control the movements of the robotized arm and the orientation of said laser head and said thermographic camera. wherein said thermographic camera is provided with a processing electronic control system, configured for performing a "Lock-in" thermography, and wherein detection of the corroded portions of said opposite surface is performed by comparing the response of the different surface portions, wherein said illumination is performed by means of said laser head emitting a pulsed laser beam against said surface exposed to view of the metal piece, and wherein said step of comparing the response of the different surface portions is performed during cooling of the piece, after that the illumination has been interrupted, said method further comprising the following steps: - providing said metal piece arranged above a bench inside a work cell, said metal piece being in the form of a hollow component, - providing said laser head and said thermographic camera carried by a robotized arm, said laser head and said thermographic camera being arranged on a same side with respect to said wall of the component to be analyzed, - driving the robotized arm by a respective electronic controller, which also includes a processing electronic control unit associated with the detection instrument, - providing a tablet for enabling an operator to control the movements of the robotized arm and the orientation of said laser head and said thermographic camera. - providing said metal piece arranged above a bench inside a work cell, said metal piece being in the form of a hollow component, - providing said laser head and said thermographic camera carried by a robotized arm, said laser head and said thermographic camera being arranged on a same side with respect to said wall of the component to be analyzed, - driving the robotized arm by a respective electronic controller, which also includes a processing electronic control unit associated with the detection instrument, - providing a tablet for enabling an operator to control the movements of the robotized arm and the orientation of said laser head and said thermographic camera.