NDT Magnetic (symbol MT) is a technique based on magnetic fields and/or electromagnetic. It is limited to surface defects but is very interesting inasmuch as it is a general control method, in contrast to ultrasonic or eddy current remaining local control methods.

MT consists in applying to the part to control a magnetic field so as to saturate the latter. The presence of surface defects will therefore generate field leakage flows that will be highlighted by means of magnetic developers, colored in daylight or fluorescent under ultraviolet light (UV- A).

There are several ways to magnetize a room to control: an electromagnetic field (with AC, straightened or continuous), direct current flow or also using a permanent magnet.

Parts after control need to be cleaned (possible elimination of the contrasting white background) and a demagnetization is often required.

The main advantages of this magnetic penetrant are compared to the speed of the process and the ability to detect surface defects even if they are closed. However, the limit of the magnetic is that it applies only on ferromagnetic metals.