Magnetic Flux Leakage (MFL) is a method used to nondestructively inspect ferromagnetic materials like steel. A magnetic field is applied to steel to magnetically saturate it, so that it can't hold any additional field. In the presence of a flaw, some of the magnetic flux escapes or "leaks" into the surrounding environment, where magnetic sensors like Hall probes detect it and report a flaw signal. An example of flux leaking into the surrounding environment in the presence of a flaw in steel is shown below.

MFL is most commonly used for inspecting oil and gas pipelines, as the inspection technology behind inline inspection tools, commonly known as "pigs." A schematic of a pig based on MFL inspection is shown below. As an inspection technology MFL is attractive for pipeline pigging because of reduced power requirements: by using permanent magnets to create the magnetic field, the pig can reserve power for other operations. Ultrasonic and other tools require much more power, meaning a larger pig as it must now carry a larger battery.

MFL signals carry a tremendous amount of information about the pipe they inspect, including local stress conditions, presence or absence of cracks, corrosion or pitting, and the like. An illustration of two MFL signals from pipeline steel is shown below: the flaw in profile is a rounded rectangle about 1.6" long and 0.5" wide, with depths of 0.125" and 0.25" for the left and right signals, respectively. Actually, the flaw is of the same dimensions as the signals you see: the MFL signals almost exactly match the size and shape of the actual flaw. Part of my graduate school studies confirmed that MFL signals also carry information about the stress conditions at the time of flaw formation: a flaw that forms when the pipe is pressurized has a slightly different MFL signal than that of a flaw that formed when the pipe wasn't pressurized. In effect, this allows the pipeline operator to determine when a flaw occurred: if it occurred when the pipe wasn't under pressure, the flaw was most likely created during manufacture of the pipe or during installation. In contrast, if it occurred while the pipe was pressurized, the flaw was created during regular operation of the pipe. In effect, if the pipe fails, we can use MFL to determine who is most likely liable for damages.

More On MFL

• Effects Of Stress On MFL
• Introduction To MFL Data Analysis
• Reducing MFL Background Fields With Median Filtration