CORROSION DETECTION BY FIBER OPTIC AE SENSORYUICHI MACHIJIMA1), MASAHIRO AZEMOTO1), TOYOKAZU TADA2),HISAKAZU MORI2)
1) Lazoc Inc., 3-40-9, Hongo, Bunkyo, Tokyo, Japan
2) Process & Production Technology Center, Sumitomo Chemical Co. & Ltd.,
5-1, Sobirakicho, Niihama, Ehime, Japan
ABSTRACTCUI (corrosion under insulation) of the piping at the industrial plant gathers more attention
than ever. Currently, plant owners need to shut down their operation, scaffold, disassemble
insulation, carry out Ultrasonic Testing mostly, and reassemble insulation for an extensive distanceof piping. In-service-inspection (ISI), or on-line monitoring is one of the keys to improve
economics. To evaluate CUI without plant shutdown, the authors group has carried out apreliminary research on AE detection occurred by corrosion progress. Fiber optic AE sensor wastested due to its anti-explosiveness, fitting to petrochemical plants. Experiment was successful, andone sensor could detect approx. 4,000mm-away corrosion. We report herein on its experimental
results and fiber-optical AE sensor with calibration data (frequency response etc).
KEY WORDS
CUI (corrosion under insulation), ISI (in-service inspection), Fiber Optic AE Sensor
INTRODUCTION
Our study is how to detect and evaluate outer piping corrosion under insulation material, whichis said to occur and accelerate by humidity and temperature correlation. To ensure that whole piping
is corrosion-free or -allowable for safe operation, plant-owners currently have to shutdown its
operation periodically. Then, in order for the precise inspection such as ultrasonic thickness
measurement, they scaffold to elevated piping level (in Sumitomo Ehime factory, majority of piping
runs 7-8 meters high in vast area), disassemble and reassemble insulation. To mitigate this work,
our research focuses on utilizing AE methods as ISI tool and on enabling to screen/choose whereamong the long distance of piping to be precisely inspected.
Another point of research is utilization of fiber optic sensor. Fiber optic sensor is advantageousof its anti-explosiveness, due to its non-electrically driven principle. Majority area of petrochemicalplant requires anti-explosiveness. Short-time AE monitoring does not need to be anti-explosive, but
assuming continual monitoring at a corrosion-fast or inflammable gaseous area, sensor needs beanti-explosiveness.
This paper reports on the outline of experiment which used piping mock-up. Corrosion was

accelerated by Sodium Chloride (NaCl). Fiber optical AE sensor was installed from 300mm to3900mm away from corroded region and obtained AE signals.
FIBER OPTICAL AE SENSOR - PRINCIPLE
When the object vibrates with some elastic motion, the attached fiber optic on the objective
surface elongates and shortens simultaneously. Light wave frequency fo is modulated by suchchange in length of fiber optic, because the number of light waves in that vibrating region isconstant in a moment. This is called “Laser Doppler effect” given as “fo - fd”. The frequency
modulation fd is proportional to the changing velocity of fiber optical length. Doppler effect isdescribed as formula (1), fd as modulated frequency, λ as light wavelength, and dL/dt as velocity 1).
fd = −

1 dL

λ dt

(1)

The frequency modulation fd is detected using the machzender/heterodyne interferometry as
shown in Fig.1. The laser with frequency fo is emitted and divided into the sensing optical path anddetecting optical path. At detecting optical path, frequency f M (80 MHz) is added by AOM (acoustooptical modulator) to create the frequency “f o + f M”. The difference in frequency given as “f M + fd” is converted into the voltage. Hereinafter, we call this sensor “Fiber Optical Doppler” sensor or“FOD” sensor.

Fig.1 Optical interferometry by machzender/heterodyne system

FIBER OPTICAL AE SENSOR - CALIBRATION DATA
For this experiment, 65m long/multi-layered FOD sensor was employed as shown in Fig.2 andTable1. This sensor was originally purposed for micro seismic monitoring at field, and embedded
into borehole near the excavated tunnel. The sensor was targeted at below 200kHz frequency. To
reconfirm acceptance to corrosion monitor, we have calibrated the frequency response of FODsensor.
Table1 Used FOD sensor spec

Fig.2 FOD sensor element

Fiber optic

Polyimide coatedFiber length (m)65Height (mm)6.00

Inner diameter (mm）8.00

Outer diameter (mm)22.00

Calibration was in accordance with NDIS 2109-1991, particularly for longitudinal wave
guidance. System and transmitting signal spec are shown in Fig.3. Three PZT sensors wereemployed to calibrate transmitting PZT sensor initially and then replaced receiving PZT sensor by
FOD sensor. Due to restricted 400mm thickness of steel cube, elimination of reflected wave (80µsec
delayed at shortest distance) was carefully examined. Accordingly, frequency response from 60kHzto 300kHz was calibrated at the same number of transmitted waves (5 waves).

Function
Generator

Oscilloscope

AmplifierInterferometer
Steel（800×800×400)
400mm

Transmit
PZT（REF-VL）

FOD

400mm
400mm

Fig.3 System and transmitted signal data for FOD Calibration
Calibrated frequency response is shown in Fig.4. As reference, 70kHz-resonant PZT (40dBamplified) sensor was also shown. 65m long/multi-layered FOD sensor has superior response toPZT sensor from 70kHz upto 140kHz. This was thought to be good for corrosion monitor 2).
140PZT̲70kH
FOD（65m)

130dB(0dB=V/m/s）

120
110
100

Bold : Fiber optic
Dotted : PZT

908070050100150200250300kHzFig.4 Frequency response data(Below 50kHz, reference only, as the number of transmitted waves is only 3)

EXPERIMENTAL SETUP
Piping mock-up is shown in Fig.5. It is carbon-steel made, 5000mm long, outer diameter
60.5mm, thickness 3.9mm (STPG-370-50A-sch.40) and has inner flow of silicon oil for heating by
circulation pump. Artificial corrosion area was located 1000mm from the right edge (in Fig.5) andwas accelerated by NaCl solution and cyclic heating (max. approx. 80 degrees C.). FOD sensors
were located 300, 2000, 3000 each on pipe, and 3900mm away both on pipe and on welded flange.
FOD sensor was installed on pipe by U-shape bolt, while directly attached on welded flange, as
shown in Fig.6.

Fig 5 Experimental mock-up

Fig.6 Sensor installation (left: on-pipe image, center: on-pipe, right: on flange)

CORROSION AE MONITOR
AE monitor was implemented with approx. 2 month interval for 3 times. 1st data was obtained
1 month after the start of continual NaCl drop-off. Till then, corrosion spread on surface, but
peeling was not observed by eyesight. At 3rd monitor, corrosion progressed aggressively, andpeeling crack was confirmed even by eyesight. Fig.7 shows corrosion of 1st and 3rd.

Fig. 7 Corrosion of pipe (left : 1st monitor, right : 3rd monitor)

DETECTED AE WAVEFORM AND FFT DATA
Fig 8 shows a sample of AE waves and FFT data at 1st experiment, which was obtained at300mm away from corroded region.
2msec

←

6000

→

3.00E+08
2.50E+08

4000

voltage(mV)

00500

1000

1500

2000

Power(a.u.)

2.00E+08

2000

1.50E+08
1.00E+08-2000

5.00E+07-4000

0.00E+00-6000

050000100000Frequency(Hz)150000200000050000100000frequency(Hz)150000200000time (usec)

6000

3.00E+08

4000

2.50E+08

voltage(mV)

00500

1000

1500

2000

Power(a.u.)

2.00E+08

2000

1.50E+08
1.00E+08-2000

5.00E+07-4000

0.00E+00-6000
time(usec)

Fig.8 AE waves and FFT data
AE DATA ANALYSIS
AE was detected by FOD sensor successfully at 3900mm away from corroded region, withenough SNR(signal to noise ratio) margin. From those data, we made a sample analysis, 1) AEactivity and corrosion status, 2) AE difference by sensor location between on-pipe andon-welded-flange, 3) AE frequency-amplitude histogram.
Fig.9 shows AE hits in half hour at 2nd monitor and at 3rd monitor at the same location(3900mm). Corrosion was largely activated clearly at 3rd, as confirmed by sight observation.

500

AE hits/30 minutes

450

3rd acquisition
2nd acquisition

400
350
300
250
200
150
100
50
0306090120 150 180 210 240 270 300 330 360time(minutes)

Fig.9 AE hits by corrosion progress at 3900mm

Fig.10 shows AE hits in half hour, describing whether AE data differs by sensor location
between on-pipe and on-welded-flange, both are approx 3900mm away. Data clarified welded
flange was relatively less in AE hits than on-pipe. This is thought to be attenuation of AE, being
unable to fly over the welded joints. However, we do not see serious affect, that can give us more
flexibility to install a sensor at anywhere location on piping structure.

AE hits by 30 minutes

500
450

on-pipe400

welded flange

350
300
250
200
150
100
50
0306090120150

180

210

240270300330360time(30 minutes)

Fig.10 AE hits by sensor location at 4000mm
Fig.11 shows histogram of AE peak frequency / peak amplitude by sensor location. As
monitoring duration of time is different from each other, the density of AE hits is not discussable
here. However, it is clear that 60-70kHz peak frequency is more observed in any location, andlarger amplitude is having lower frequency.
Sensor location at 2000mm

200200150

150
peak freq(kHz)peak freq(ｋHz)Sensor location at 300mm

1001005050

0001000

2000

3000

4000

5000

0600010003000

4000

5000

6000

5000

6000

Sensor location at 3900mm

Sensor location at 3000mm
200200150150
peak freq(kHz)peak freq(kHz)2000

max amplitude(mV)max amplitude(mV)

100

100

50

50

000

1000

2000

3000

4000

max amplitude(mV)

5000

6000

01000

2000

3000

4000

max amplitude(mV)

Fig.11 Peak frequency – amplitude histogram

CONCLUSIONS
We have successfully detected and evaluated AE, caused by corrosion progress by fiber optic
AE sensor. Assuming one pipe length is approx 10m, one sensor can cover 1 pipe in order to screen
CUI presence. Fiber optic AE sensor is naturally anti-explosive, that can be of advantage inpetro/petrochemical plants.
Next challenge is to discriminate operation noise of piping flow. This will be difficult enoughto tackle, having assumed internal steam flow and its cavitation particularly. We will continue toinvestigate and report at a next conference.
REFERENCES
1) K. Kageyama，H. Murayama，Ｋ. Uzawa，I. Ohsawa，M. Kanai，Y. Akematsu，K. Nagata andT. Ogawa : Doppler effect in flexible and expandable light waveguide and development of newfiber-optic vibration/acoustic sensor，J. of Lightwave Technology, Vol. 24，pp. 1768-1775，2006．
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