Eddy Current Testing with Giant Magnetoresistance (GMR) Sensors and a Pipe-Encircling Excitation for Evaluation of Corrosion under Insulation

Abstract

This work investigates an eddy current-based non-destructive testing (NDT) method to characterize corrosion of pipes under thermal insulation, one of the leading failure mechanisms for insulated pipe infrastructure. Artificial defects were machined into the pipe surface to simulate the effect of corrosion wall loss. We show that by using a giant magnetoresistance (GMR) sensor array and a high current (300 A), single sinusoidal low frequency (5–200 Hz) pipe-encircling excitation scheme it is possible to quantify wall loss defects without removing the insulation or weather shield. An analysis of the magnetic field distribution and induced currents was undertaken using the finite element method (FEM) and analytical calculations. Simple algorithms to remove spurious measured field variations not associated with defects were developed and applied. The influence of an aluminium weather shield with discontinuities and dents was ascertained and found to be small for excitation frequency values below 40 Hz. The signal dependence on the defect dimensions was analysed in detail. The excitation frequency at which the maximum field amplitude change occurred increased linearly with the depth of the defect by about 3 Hz/mm defect depth. The change in magnetic field amplitude due to defects for sensors aligned in the azimuthal and radial directions were measured and found to be linearly dependent on the defect volume between 4400–30,800 mm3 with 1.2 × 10−3−1.6 × 10−3 µT/mm3. The results show that our approach is well suited for measuring wall loss defects similar to the defects from corrosion under insulation.

A Recent Progress of Steel Bar Corrosion Diagnostic Techniques in RC Structures

“…Together with electrochemical detection, it can better diagnose the failure condition of concrete structures caused by CSB corrosion and even evaluate remaining service life, so it has great application prospects. Naasson et al [33] and Minesawa et al [34] has made great use of ECT on measurements for processes and position of corrosion in CSB, and Bailey et al [35] studied wall loss defects of pipe with ECT.…”

Multi-Frequency Magnetic Induction Tomography System and Algorithm for Imaging Metallic Objects

“…Non-destructive testing and eddy-current testing (NDT and ECT, respectively) of metal components has been extensively investigated and successfully adopted as a tool in industry. However, in using ECT to render images for detecting corrosion [ 10 , 11 , 12 , 13 , 14 ], fatigue [ 15 , 16 ], defects [ 17 , 18 , 19 ] and physical loading [ 20 , 21 , 22 ], mechanical scanners are often employed [ 23 , 24 , 25 , 26 , 27 , 28 ]. Furthermore, while ECT is generally regarded as non-contact, close proximity sensor positioning is usually required.…”

The durability of flexible eddy current array (FECA) sensors in harsh service environments

“…Current sensors for SHM include Piezoelectric transducers 5 , fiber optic sensors 6 – 8 , Comparative Vacuum Monitoring (CVM) sensors 9 , eddy current sensors 10 , 11 , acoustic emission sensors 12 , smart coating sensors 13 , 14 , ultrasonic sensors 15 , 16 and so on. FECA sensors manufactured by the flexible printed circuit board technology have the ability to monitor the structure without direct contact, and they can be used to detect the damage of components with complex geometric shapes.…”