Examples of ferromagnetic materials include iron, nickel, cobalt, and some of their alloys.
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.
An AC electromagnet is the preferred method for find surface breaking indication.
The current or magnetic field is then slowly reduced to zero, leaving the part demagnetized.
[2] A common particle used to detect cracks is iron oxide, for both dry and wet systems.
It is common industry practice to use specifically designed oil and water-based carriers for magnetic particles.
A technician performs MPI on a
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.
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.
An automatic wet horizontal MPI machine with an external power supply, conveyor, and demagnetizing system. It is used to inspect engine cranks.
A pull through AC demagnetizing unit
After applying wet magnetic particles, a U.S. navy technician examines a bolt for cracks under ultraviolet light.
