Remanent magnetization is the magnetization level left after a tool passes. Remanent magnetization can either increase or decrease the applied field, and so, it influences the detection and characterization of metal-loss regions.

Pipeline steels exhibit a hysteresis effect when magnetized; specifically, when the applied field is removed, a flux density is left in the pipe. When the magnetic field is reapplied, the magnetization curve starts at an applied magnetic field equal to zero and a flux density equal to the remanent flux density. Hence, a new magnetization curve is generated. In other words, magnetization curves are not only nonlinear, they are also different for repeated magnetizations.

Remanent magnetization and the resultant magnetization curves are complex. The figure schematically shows the magnetic field and flux density for a magnetizer passing a point in the pipe. The magnetization curve starts at the origin, which means the pipe has no previous magnetic inspections or that it has been demagnetized. The curve moves through the following points:

• Position A: As the magnetizing assembly approaches, a negative magnetic field and flux density are produced due to air-coupled fields.
• Position B: As the forward leg of the magnet passes the sensor location, the applied field changes orientation and increases significantly.
• Position C: The maximum field intensity is achieved when the sensor is between the pole pieces, although not necessarily at the halfway point.
• Position D: As the trailing leg of the magnet passes the sensor location, the field decreases significantly and changes orientation.
• Position E: The applied field reaches a maximum negative value before the magnetizing assembly leaves the sensor region.
• Position F: The applied field now is zero but some flux density remains in the pipe. This remaining flux density is the remanent magnetization.

The effects of multiple passes on the applied field can be seen in the above figure. These data are from a measurement program using an instrumented pipe without metal-loss regions ^[Eiber91]. Subsequent passages of a magnetizing assembly created different magnetization curves.

Two important changes can be observed. First, the maximum field and flux density are less for each passage. Second, the remanent magnetization changes between each passage. These effects become less significant after the first few passages. For this case, even though the magnetization levels varied widely, the applied flux density levels in pipe did not vary significantly because the pipe wall was saturated.

For lower applied field levels, the effect on flux density is greater. The above figure schematically illustrates these effects. For tools that operate at low or medium magnetization levels, remanent magnetization effects can be significant.