MFL testing, also known as magnetic flux leakage testing, is a type of nondestructive testing (NDT) method used to detect and evaluate defects in ferromagnetic materials, such as steel and iron. The test is performed by applying a magnetic field to the material and then using sensors to measure the magnetic flux, or the flow of magnetic energy, on the surface of the material.

Any defects in the material, such as cracks or voids, will cause a leakage of the magnetic flux, which can be detected and evaluated using specialized equipment. MFL testing is commonly used to inspect pipelines, tanks, and other large ferromagnetic structures for defects that could affect their integrity and performance.