Improvement in the detection performance of extremely low-frequency eddy current testing for application in underground steel corrosion detection

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1 Improvement in the detection performance of extremely low-frequency eddy current testing for application in underground steel corrosion detection More info about this article: Shunki Wakabayashi a*, Takuya Tomioka, Kenji Sakai, Toshihiko Kiwa, Keiji Tsukada b* Graduate School of Interdisciplinary Science and Engineering in Health Systems Okayama University, Japan KEY WORDS: Nondestructive testing; Extremely low-frequency; Underground steel corrosion; Magnetic spectrum curve ABSTRACT The corrosion of steel structures, such as light and road sign poles, often occurs near the ground due to rainwater. The subsequent collapse of the corresponding steel structure can cause traffic accidents and potentially danger people. Therefore, a highly accurate and easy nondestructive inspection technique is required to detect corrosion near the ground, especially in hidden locations. Recently, we reported a novel inspection method using extremely low-frequency eddy current testing (ELECT) to detect the reduced steel thickness that was caused by corrosion. Wide magnetic-field exposure was achieved underground using a tilted magnetic sensor probe. Further, the fundamental detection performance and signal analysis of the test samples were also reported. In this study, we attempted to improve the detection performance and applied the measurement to actual steel structures as a field test. The ELECT system to detect corrosion near the ground comprised a magnetic sensor probe, a sensor circuit, a multiple-frequency alternative current (AC) current source for the induction coil, a lock-in detector, and a personal computer. The magnetic probe consisted of two anisotropic magnetic resistance sensors, induction coils to generate eddy currents in the steel structures, and the cancellation coil to reduce the directly coupled magnetic field exposure. The magnetic spectrum curve, which can be used to trace the obtained magnetic field vectors (intensity and phase), was acquired by multiple-frequency exposure. The reduction in steel thickness was further estimated based on the changes in magnetic curve. The optimization of certain parameters, such as the exposure coil shape and lift-off between the sensor probe and the steel, led to improvements in the magnetic curve, which enabled sensitive detection; further, 1-mm thinning was successfully detected at a depth of 60 mm. Furthermore, the results obtained by the ELECT system at the ground surface were in good agreement with the results of the thickness measurements that were directly obtained from the structure after digging into the ground. 1. Introduction Several steel pipes and other such infrastructure were built five decades ago during the construction boom that was observed during the high economic growth era. Recently, the aging of steel pipes, such as light and road sign poles, has become a major problem, and their corrosion is expected to accelerate in the subsequent decade. The corrosion often occurs near the ground due to rainwater and leads to a reduction in the thickness of the steel pipe over time; the subsequent collapse of the structure can cause traffic accidents and potentially danger normal people. Therefore, a highly accurate and easy nondestructive inspection technique is required to detect corrosion near the ground. Conventionally, an ultrasonic flaw detection method has been used. However, to ensure the usage of this method in an effective manner, the paint and rust on the surface of the structure must be removed prior to performing the measurement, which is expensive and time consuming. Furthermore, the conventional approach still requires digging into the ground to observe the buried corrosion. We previously reported a method to detect the thickness of steel pipes using extremely low-frequency eddy current testing (ELECT) [1], [2]. Spectrum analysis of the magnetic field was performed by Creative Commons CC-BY-NC licence

2 measuring magnetic field vectors at multiple frequencies [3], [4], [5]. We also described a measurement method to detect corrosion at hidden locations near the ground using a tilted magnetic sensor probe [6], [7]. We observed good correlation between signal attenuation and steel thinning, both of which could be attributed to corrosion [7]. In this study, we developed an integrated magnetic sensor probe with two tilted sensors to estimate the location of corrosion defects and the thinning rate. We also optimized the corrosion detection at locations hidden beneath the ground by changing the lift-off. Finally, we adapted the developed magnetic sensor probe to inspect the corrosion of the buried sections of the actual light poles. 2. Measurement The configuration diagram of the integrated probe featuring two tilted magnetic sensors that is developed to estimate the location of corrosion defects and the thinning rate is depicted in Fig. 1. The magnetic sensor probe comprised two anisotropic magnetoresistance (AMR) sensors, an induction coil, and a cancellation coil. To prevent the applied magnetic field from entering the AMR sensors, a cancellation coil that created a magnetic field in the opposite direction to that of the applied magnetic field was installed. The tilt angle of the magnetic sensor probe was fixed at 30 to detect corrosion beneath the ground surface. The distance between the two AMR sensors was 40 mm. The induction and cancellation coils comprised 50 and 40 turns, respectively. Fig 1. Configuration of the integrated magnetic sensor probe with two tilted sensors The schematic of the ELECT system that exhibits the sample configuration is presented in Fig. 2. The system comprised an AC power supply, an oscillator, an induction coil, a cancellation coil, two AMR sensors, an amplifier, a lock-in detector, and a personal computer. Stainless steel plates (350 mm long, 200 mm wide, and 4 mm thick) were used as test samples, and three thinning zones, each 60 mm wide and 1.0, 2.0, or 3.0 mm deep, were present on each sample. Measurements were performed at 10-mm intervals (apart from the thinning zone) using an induction coil driving current of 0.15 A. The applied magnetic field frequency was swept from 3 to 50 Hz. To evaluate the variation in magnetic spectrum, differential magnetic vectors were used. The difference between magnetic vectors that were measured at two different frequencies was computed; the intensity of the magnetic vector that was measured at 3 Hz was subtracted from that measured at 50 Hz. To optimize corrosion detection, three lift-off values of 0, 5, and 10 mm were tested.

3 Fig 2. Schematic of the ELECT system and sample configuration 3. Result In Fig. 3, the dependence of differential intensity on the distance from each thinning zone per lift-off depth is depicted. The differential intensity gradually decreased near the thinning zone; further, its change was dependent on the thinning zone depth. Overall, the differential intensity decreased as the thinning zone depth increased. Incrementally altering the lift-off made it possible to determine the thinning zone depths, even in case of increasing measurement distances. When the lift-off was 10 mm, the thinning depth could be determined up to 60 mm away from the thinning zone [Fig 3 (e)]. To obtain the lift-off at 0 mm, the detection distance was nearly doubled. It was possible to estimate the corrosion depth and distance from the thinning zone using the distance dependence obtained from sensors 1 and 2, which were separated by a constant distance of 40 mm. (a) Distance dependence of sensor 1 at 0-mm lift-off (b) Distance dependence of sensor 2 at 0-mm lift-off

4 (c) Distance dependence of sensor 1 at 5-mm lift-off (d) Distance dependence of sensor 2 at 5-mm lift-off (e) Distance dependence of sensor 1 at 10-mm lift-off (f) Distance dependence of sensor 2 at 10-mm lift-off Fig 3. Dependence of differential intensity of the test samples on the distance from the thinning zones at different corrosion depths for each lift-off 4. Light pole application Based on the success of the laboratory experiment using test samples, the magnetic sensor probe was applied to corroded light poles, specifically to areas that were hidden beneath the ground surface. Fig. 4 depicts corrosion sections 1 and 2 of the light pole, the measurement range, and the ground line. Measurements were performed at 30 different positions (6 at 10-mm intervals in the y direction and 5 at 15-mm intervals in the x direction) with an induction coil current of 0.15 A and an applied magnetic field frequency ranging from 3 to 50 Hz. The intensity of differential magnetic vector was obtained via subtraction of the magnetic vector at 3 Hz from that at 50 Hz. Additionally, standardization was performed by determining the intensity in the uncorroded sections of the light pole. The thickness of the uncorroded sections of the light pole was 4.3 mm. The differential intensity of the signal that is calculated at each measurement position is depicted in Fig. 5. The change in differential intensity was clearly observed near the corrosion sections based on the data obtained using sensor 1. The differential intensity in corrosion section 1 changed significantly between 30 and 60 mm in the x direction and between 0 and 10 mm in the y direction [Fig 5. (c)]. In corrosion section 2, the change occurred from 0 to 20 mm in the x direction and from 0 to 10 mm in the y direction [Fig 5. (d)]. Thus, the change in signal intensity due to corrosion of the buried steel pole was obtained using both sensors.

5 (a) Corrosion section 1 (b) Corrosion section 2 Fig 4. Corrosion sections (a) 1 and (b) 2 of the light pole (a) Intensity of the sensor 2 in corrosion section 1 (b) Intensity of the sensor 2 in corrosion section 2 (c) Intensity of the sensor 1 in corrosion section 1 (d) Intensity of the sensor 1 in corrosion section 2 Fig 5. Two-dimensional mapping of differential intensity in corrosion sections 1 and 2 using the integrated magnetic sensor probe with two tilted magnetic sensors.

6 The thickness of corrosion sections was also directly measured by digging around the pole just above the corrosion sections and using a perpendicular magnetic sensor probe with a single sensor. The measurement was performed at 30 different positions (6 at 10-mm intervals from 30 to 20 mm in the y direction and 5 at 15-mm intervals from 0 to 60 mm in the x direction). The two-dimensional mapping of the corrosion section thickness, as calculated from the spectrum analysis of the magnetic field, is presented in Fig. 6. In corrosion section 1, a reduction in thickness was observed from 30 to 60 mm in the x direction and from 0 to 30 mm in the y direction [Fig 6. (a)]. In corrosion section 2, thickness reduction was observed from 0 to 30 mm in the x direction and from 15 to 10 mm in the y direction [Fig 6. (b)]. We obtained a good agreement between the thickness measurements from the integrated sensor probe that was obtained at the ground surface and from the thickness that was directly measured after digging into the ground. Therefore, these results demonstrate that the magnetic sensor probe with two tilted sensors can be potentially used to estimate the thickness of buried steel. (a) Corrosion section 1 (b) Corrosion section 2 Fig 6. Two-dimensional mapping of thickness measured directly using a perpendicular magnetic sensor probe 5. Conclusion An integrated magnetic sensor probe with two tilted magnetic sensors was developed to detect corrosion defects beneath the ground surface. Furthermore, lift-off was optimized to enable inspection at long distances from the corroded defect, resulting in successful determination of the thinning zone depth up to a distance of 60 mm. When the probe was applied to corrosion on light poles, changes were clearly observed in the thickness of the steel plate because of corrosion. The rapid inspection of steel corrosion of the structures that were buried underground will be facilitated by the developed measuring method because time-consuming and costly digging into ground can be avoided. Acknowledgments This research was conducted by the Cross-ministerial Strategic Innovation Creation Promotion Program (SIP).

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