Circumferential and axial magnetizers for a magnetic flux leakage pig. A circumferential magnetizer module for a smart pig comprises a central shaft and at least one magnet bar for inducing a magnetic field transverse to a longitudinal axis of the shaft. The magnet bar comprises at least one magnet, and may collapse radially inward to the shaft. A sensor head disposed between circuit poles at each polar end of the magnet monitors magnetic flux. An axial magnetizer module comprises a central shaft and at least one magnet bar to induce a magnetic field coaxially to a longitudinal axis of the shaft. Magnets of opposite polarity are circumferentially disposed around ends of the central shaft. A sensor head disposed between circuit poles at each polar end monitors magnetic flux. The central shaft of a circumferential magnetizer or axial magnetizer may comprise a joint linking an additional smart pig module.