A pipeline's dangerous corrosion almost always starts on the inside wall, eaten away by whatever is flowing through it. Once the pipe is buried, that inner surface is sealed off from every external method — a walking survey, a drone flyover or ground-penetrating radar can only characterise the outside of the pipe and the soil around it. None of them can see through several centimetres of steel to the internal surface, so they report a healthy pipe even while it is quietly thinning from within.
An in-line inspection "smart pig" solves this by going where the damage is: it rides the flow inside the pipe, and its ring of magnetic flux leakage sensors saturates the wall with a magnetic field and measures the field that leaks out at each point — thinner steel leaks more field. That gives a direct, continuous wall-thickness reading along the entire internal surface, catching pitting no external technique could ever detect.
- Cutaway view — toggle it off to see what an external inspection actually sees: a solid, apparently intact pipe. The corrosion is still there, just invisible.
- External readout — always reports no internal defects, regardless of what is hidden inside — a false confidence that is the whole danger of external-only inspection.
- MFL signal — spikes as the pig's sensor ring passes over each thinned patch, and the wall-thickness reading dips to match the true remaining wall.