Non-destructive detection of the surface properties of ferromagnetic materials

Claim 1

A non-destructive method of inspecting the form of the internal surface of a pipeline of ferromagnetic material, comprising: positioning a source of a magnetic field adjacent to, but spaced apart from, said surface to irradiate said internal surface with magnetic flux; maintaining the field emanating from said source at a low magnetic field strength to avoid the generation of leakage flux and such that said internal surface is approximately a magnetic equipotential surface, whereby the lines of flux of the field enter said surface in a direction perpendicular thereto at normal profiles thereof; causing said system to move axially along said pipeline; measuring the component of the magnetic field parallel to the pipeline axis caused by imperfections in the internal pipeline surface at a plurality of locations adjacent the internal pipeline surface during the course of said axial movement with a magnetic field detector located between the magnetic field source and the internal pipeline surface to obtain a corresponding plurality of output measuring signals representative of the profile of the surface of the pipeline in the vicinity of said locations.