Permanently installed corrosion monitoring using magnetic measurement of current deflection

The detection of corrosion on insulated and/or coated pipes remains a challenge. A non-destructive evaluation method has been proposed where a low-frequency AC current is directly injected into the pipe at distant locations, and perturbations in the magnetic field induced by current deflection around defects are measured. Structural health monitoring is made possible by detecting changes in the magnetic field due to defect growth using a permanently installed array of sensitive and inexpensive magnetic sensors. The performance of current deflection structural health monitoring is evaluated using a flexible and efficient framework. Individual sensor performance was first predicted using receiver operating characteristics obtained by evaluating the stability of the magnetic field signal measured outside of a section of coated undamaged riser pipe in an environmental chamber over repeated temperature cycles. A finite element model was then used to predict the magnetic perturbation due to defect growth which allowed the potential array configurations for structural health monitoring to be explored. Results suggest that 90% probability of detection and 0.1% probability of false alarm for 3 x T (wall thickness) diameter, 30% of T depth defects are possible outside of 25- to 50-mm-thick pipe coatings/insulation using 10-50 sensors per metre of pipe and 5-10A of injected current. The structural health monitoring procedure was then demonstrated experimentally, an electrochemically grown defect being successfully monitored; this experiment also served to validate the three-dimensional finite element model. A very good agreement between the predicted and measured changes in the magnetic field due to the current deflection around the growing defect was obtained.