Black-on-White Magnetic Particle Inspection

Magnetic particle testing (MPT) is a non-destructive testing (NDT) method used to find surface and slightly subsurface cracks in ferromagnetic materials like steel and iron. The part being tested is magnetized using either an electric current or a magnetic yoke. If there is a crack or defect, it disrupts the magnetic field and causes leakage fields at the surface. Fine iron particles—either dry powder or suspended in liquid—are then applied to the surface. These particles are attracted to the leakage fields and gather along the defect, making it visible to the inspector.

MPT is commonly used in industries such as manufacturing, oil and gas, and transportation to inspect welds, castings, and machined parts. It is quick, relatively inexpensive, and very effective for detecting surface-breaking defects. However, it only works on ferromagnetic materials and usually cannot detect deep internal flaws. After testing, the part is often demagnetized and cleaned to remove remaining particles.