Magnaflux is a fascinating nondestructive testing technique used to determine if a metal that has ferromagnetic properties contains any defects. The process can find cracks and casting issues, even those that are located inside the metal, without having to cut it open.
The magnaflux testing procedure is used in many industries including automotive, where they can test an engine block they're about to rebuild to see if there are any hidden cracks, to the petrochemical industry, where they can use it to analyze pipelines looking for cracks and bad welds even then there is oil getting pumped through the middle.
To work, a magnetic field is induced onto a surface, either directly by passing an electrical current through it or indirectly by using a tool that doesn't run electrical currents in the object, which is handy when working on pipelines containing explosive gasses.
Once the field is induced, the part has either wet (suspended) or powdered ferrous particulates spread over the surface. Defects change the flow of the magnetic field and the particles line up around those changes. The results are examined, like a radiologist examining an x-ray to see if a bone is fractured or not. It takes training to interpret the defects correctly.
Iron Noder 2017