MPI is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).
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 magnetic particle inspection. 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 centimeters.