Magnaflux is a type of inspection method which is used primarily to detect and identify cracks in ferrous metals,(metals with iron). The part is passed through a magnetic field, which ionizes, or magnetizes, the part and it is then washed with a solvent phosphoric solution containing metalic particles.The part is then washed again in clean solution and looked at through an ultra-violet black light. If a crack is present, the magnetic particles will collect at the crack and have a tendancy not to get washed away. They will be visible when closely inspected under the black light, and it will look like a little crack.
After complete inspection, the part is then washed again, and passed through the magnetic field in the other direction, and essentially, De-magnetized.
Engine blocks, crankshafts, valves, etc.
The same process is used with non-ferrous metals, such as aluminum, but it is called Zyglo, or Dye-penetrant, and there is no magnetism involved.
Cylinder heads, pistons, etc.