A technician performs MPI on a pipeline to check for stress corrosion cracking using what is known as the "black and white" method. No indications of cracking appear in this picture; the only marks are the "footprints" of the magnetic yoke and drip marks.A close-up of the surface of a (different) pipeline showing indications of stress corrosion cracking (two clusters of small black lines) revealed by MPI. Cracks that would normally have been invisible are detectable due to the magnetic particles clustering at the crack openings. The scale at the bottom is numbered in centimetres.Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys.
