Editing Magnetic particle inspection (section)

{{Short description|Non-destructive method used to detect defects in ferrous materials}}
''Magnaflux'' redirects here
{{Refimprove|date=April 2010}}
[[file:Wet magnetic particle testing on a pipeline.jpg|thumb|A technician performs MPI on a [[pipeline transport|pipeline]] to check for [[stress corrosion cracking]] using what is known as the "black on white" method. No indications of cracking appear in this picture; the only marks are the "footprints" of the magnetic yoke and drip marks.]]
[[file:Stress corrosion cracking revealed by magnetic particles.JPG|thumb|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 centimeters.]]

'''Magnetic particle inspection''' ('''MPI''') is a [[nondestructive testing]] process where a [[magnetic field]] is used for detecting surface, and shallow subsurface, discontinuities in [[ferromagnetic materials]]. Examples of ferromagnetic materials include [[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. 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). Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source.

The presence of a surface or subsurface discontinuity in the material allows the [[magnetic flux]] to leak, since air cannot support as much magnetic field per unit volume as metals. 

To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause,  and course of action, if any.