The measurement of perturbations in the magnetic field resulting from the deflection of an injected electric current around a defect has been shown to have many potential applications in NDE. Characteristics of such a technique include through-wall sensitivity to defects at tens of millimetres of lift-off between sensor and sample, which makes the technique suitable to a number of traditionally difficult-to-inspect scenarios where extensive insulation and/or coating removal is undesirable.
This paper presents a study into the feasibility of using Magnetoresistive (MR) sensors to measure current deflection from corrosion-like defects via an internal or external scan of the pipe. Prediction of the signal due to different defect geometries have been facilitated by a validated FE model, and current deflection measurements using Anisotropic MR sensors have allowed prediction of the sensitivity of the technique. The various practical challenges are explored which include the misalignment of the sensors with the target components of the magnetic field, the influence of nearby ferromagnetic objects, and the effect of pipe features such as bends and crossings. The results suggest that low-powered, inexpensive MR sensors in a gradiometric configuration can be used to measure current deflection from defects occurring on the inner or outer wall of pipes carrying a few amps of current while the coating, insulation and (non-ferromagnetic) cladding remain intact. This has many potential applications, particularly in the oil and gas and power industries
