Corrosion detection by fiber optic AE sensor.
CUI (corrosion under insulation) of the piping at industrial plants gathers more attention than ever. Currently, plant owners need to shut down their operation, scaffold, disassemble insulation, carry out non-destructive test and reassemble insulation of extensive piping installation. On-stream inspection (OSI), or on-line monitoring is a key to improve economics. To evaluate CUI without plant shutdown, we have carried out a preliminary research on detecting AE produced by corrosion. Fiber optic AE sensor is explosion proof, and is suitable for applications in petrochemical plants. Evaluation testing was successful, and one sensor can detect corrosion 3.9 m away. We report experimental results and subsequent field test, using fiber optic AE sensor.

Keywords: CUI (corrosion under insulation), OSI (on-stream inspection), Fiber optic AE sensor

Article Type:
Corrosion and anti-corrosives (Identification and classification)
Optical detectors (Usage)
Acoustic emission testing (Equipment and supplies)
Machijima, Yuichi
Azemoto, Masahiro
Tada, Toyokazu
Mori, Hisakazu
Pub Date:
Name: Journal of Acoustic Emission Publisher: Acoustic Emission Group Audience: Academic Format: Magazine/Journal Subject: Physics; Science and technology Copyright: COPYRIGHT 2009 Acoustic Emission Group ISSN: 0730-0050
Date: Annual, 2009 Source Volume: 27
Event Code: 440 Facilities & equipment
Geographic Scope: Japan Geographic Code: 9JAPA Japan
Accession Number:
Full Text:

Our study attempts to detect and evaluate outer piping corrosion under insulation material. Such corrosion is accelerated by humidity and correlates to temperature. To ensure that whole piping is corrosion-free or within corrosion-allowable for safe operation, plant-owners currently have to shutdown the operation periodically for precise inspection using methods like ultrasonic thickness gage. This requires scaffolding to elevated piping level (in a typical Japanese petrochemical plant, the majority of piping runs at 7-8 m high), disassembling and reinstalling insulation. To mitigate this work, our research focuses on utilizing AE methods as an OSI tool and on enabling to screen/choose where among the long distance of piping to be precisely inspected.

Another point of research is the utilization of fiber optic sensor. Fiber optic sensor is explosion proof due to its non-electric principle. Most location of a petrochemical plant requires explosion proof devices. Short-time AE monitoring does not need to be explosion proof, but assuming continual monitoring at a corrosion-critical or inflammable gas areas, sensors need be explosion proof.

This paper reports on the outline of experiment, which used piping mock-up. Corrosion was accelerated by sodium chloride (NaCl). Fiber optic AE sensor was installed from 0.3 m to 3.9 m away from corroded region and obtained AE signals.

Fiber Optic AE Sensor--Principle

When an object vibrates in elastic manner, the attached fiber optic element on the object surface elongates and shortens simultaneously. Light-wave frequency [f.sub.o] is modulated by such changes in length of fiber optic element, because the number of light waves in that vibrating region is constant at a moment. This is called "laser Doppler effect", given as "[f.sub.o] - [f.sub.D]". The frequency modulation [f.sub.D] is proportional to the changing velocity of fiber optical length. Doppler effect is described as equation (1), with [f.sub.D] as modulated frequency, [lambda] as light wavelength, and dL/dt as velocity [1].

[f.sub.D] = -1/[lambda]dL/dt (1)

The frequency modulation [f.sub.D] is detected using Mach-Zender/heterodyne interferometry as shown in Fig. 1. The laser with frequency [f.sub.o] is emitted and divided with a half-mirror (HM) into the sensing optical path and detecting optical path. At the detecting optical path, frequency [f.sub.M] (80 MHz) is added by AOM (acousto-optical modulator) to create the frequency "[f.sub.o] + [f.sub.M]". This is combined with [f.sub.o] + [f.sub.D] from the detecting optical path, again using an HM. The difference in frequency given as "[f.sub.M] + [f.sub.d]" is converted into the voltage output using a detector. Hereinafter, we call this sensor "fiber optical Doppler" sensor or "FOD" sensor.


Fiber Optic AE Sensor--Calibration Data

For this experiment, a 65-m long, multi-layered FOD sensor was employed as shown in Fig. 2 and Table 1. This sensor was originally developed for field micro-seismic monitoring, and embedded into a borehole near an excavated tunnel. The sensor was targeted for sensing below 200-kJTz frequency. To reconfirm acceptance to corrosion monitor, we have calibrated the frequency response of FOD sensor.



Calibration was in accordance with NDIS 2109-1991, particularly for longitudinal wave detection. System and transmitting signal specifications are shown in Fig. 3. Three PZT sensors were employed to calibrate transmitting PZT sensor initially and then replaced a receiving PZT sensor by an FOD sensor. Due to the 400-mm thickness of steel cube, the elimination of reflected wave (80 us delayed at the shortest distance) was carefully examined. Accordingly, frequency response from 60 kHz to 300 kHz was calibrated at the same number of transmitted waves (5 waves). Calibrated frequency response is shown in Fig. 4 (Below 50 kHz, data is for reference only, as the number of transmitted waves is only 3). Results for a 70-kHz-resonant PZT (40-dB amplified) sensor was also shown. The 65-m long/multi-layered FOD sensor has superior response to PZT sensor from 70 kHz to 140 kHz. This was adequate for corrosion monitor [2].



Experimental Setup

Piping mock-up is shown in Fig. 5. It is made of a carbon steel, 5-m long, outer diameter 60.5 mm, thickness 3.9 mm (STPG-370-50A-sch.40) and has inner flow of silicone oil for heating by a circulation pump. Artificial corrosion area was located at 1 m from the right edge (Fig. 5) and was accelerated by NaCl solution and cyclic heating (maximum ~80[degrees]C). FOD sensors were located at 300, 2000, 3000 mm away on the pipe, and 3900 mm away both on the pipe and on a welded flange. Each FOD sensor was installed on the pipe via a U-shape bolt, while directly attached on welded flange with a C-clamp, as shown in Fig. 6.



Corrosion AE Monitoring

AE monitoring was implemented 3 times with approximately 2-month interval. First data was obtained 1 month after the start of continual NaCl dripping. Until then, corrosion spread on the surface, but no peeling was observed visually. At the third monitoring, corrosion progressed aggressively, and peeling crack was confirmed even visually. Figure 7 shows corrosion at the first and third monitoring.


Detected AE Waveform and Data Analysis

AE was detected by FOD sensors successfully even at 3.9 m away from the corroded region, with enough SNR (signal-to-noise ratio) margin. From those data, we made a sample analysis, 1) AE activity and corrosion status, 2) AE difference by sensor location between on-pipe and on-welded-flange, and 3) AE frequency-amplitude histogram.

Figure 8 shows a sample of AE waves and FFT data from the first experiment. It was obtained by an FOD sensor 300 mm away from the corroded region.

Figure 9 shows AE hits per 30 min during the second and third monitoring at the same location (FOD at 3.9 m away). Corrosion was clearly severe during the third monitoring, as confirmed by visual observation. Figure 10 shows AE hits per 30 min, describing whether AE data differs by sensor location between on-pipe and on-welded-flange, both being at 3.9 m away from the corroded area. Data indicates the welded flange location was slightly lower in detected AE hits than the on-pipe sensor at the same distance. The attenuation of AE on the flange location was not serious, giving us more flexibility to install an FOD sensor on piping structures.

Figure 11 shows the correlation of AE peak frequency and peak amplitude by sensor location. As monitoring duration is different from each other, the density of AE hits is indicated qualitatively. However, it is clear that 60-70 kHz peak frequency is more prominent at any location, and larger amplitude events have lower frequency.



Field Test

Following in-house experiment, we have conducted field test on an insulated reactor vessel at the owner's plant (3.5-m outer diameter, 25.5-m height). The vessel was partially corroded due to ingress of rain water to the extent of 0.3 to 7 mm depth, as confirmed by visual test. Four FOD sensors were installed at 90[degrees] interval at the same height near corroded region. Figure 12 is a water-proof FOD sensor, which was bonded to the vessel by epoxy resin. Figures 13 and 14 show AE hits per 30 min before and after de-rusting work. Some AE hits still remained after the de-rusting work, which seemed to be from internal liquid flow, because this vessel was being operated at the normal condition. Separation of operation noise is under study, installing the applicable equipments on site.







We have successfully detected and evaluated AE signals, caused by corrosion progression using fiber optic AE sensor both in laboratory and at plant. Assuming a pipe is roughly 10-m length, one sensor can cover the pipe in order to screen CUI presence. Fiber optic AE sensor is naturally explosion proof and this is especially advantageous in petrochemical plants.


[1] K. Kageyama, H. Murayama, K. Uzawa, I. Ohsawa, M. Kanai, Y. Akematsu, K. Nagata and T. Ogawa: Doppler effect in flexible and expandable light waveguide and development of new fiber-optic vibration/acoustic sensor, J. of Lightwave Technology, 24, 2006 1768-1775.

[2] High Pressure Institute of Japan: Recommended Practice for Acoustic Emission of Corrosion Damage in Bottom Plate of Oil Storage Tanks, HPIS G 110 TR 2005.


(1) Lazoc Inc., Hongo 3-40-9, Bunkyo, Tokyo 113-0033, Japan; (2) Process & Production Technology Center, Sumitomo Chemical Co. & Ltd., Sobiraki 5-1, Niihama, Ehime, Japan
Table 1 FOD sensor specification.

Fiber optic           Polyimide coated

Fiber length (m)      65
Height (mm)           6.00
Inner diameter (mm)   8.00
Outer diameter (mm)   22.00
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