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An investigation into eddy current pulsed thermography for detection of corrosion blister
Corrosion Science 78 (2014) 1–6
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
An investigation into eddy current pulsed thermography for detection of corrosion blister Yunze He a,b,⇑, Gui Yun Tian b, Mengchun Pan a, Dixiang Chen a, Hong Zhang b a b
College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom
a r t i c l e
i n f o
Article history: Received 28 August 2013 Accepted 19 September 2013 Available online 27 September 2013 Keywords: A. Mild steel A. Paint coatings C. Atmospheric corrosion
a b s t r a c t This letter reports an investigation into eddy current pulsed thermography (ECPT) for detecting corrosion blister in mild steel. The detection mechanisms for various types of damage were addressed based on their interaction with eddy current distribution and heat conduction. Experimental studies showed that the blister and ruptured blister area can be easily monitored using conventional thermograms from an infrared camera. Signal processing algorithms were investigated to obtain new features. Principal components analysis (PCA), independent components analysis (ICA) and fast Fourier transform (FFT) were compared in the performance of blister detection. It was found that peak time was the best way to detect the blister and the extent of peak time increase was in the following sequence: blister intact area > ruptured blister. Several features can be specially used to separate the ruptured blister. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Blister has long been recognised as a problem with paint coatings, reducing their service life and facilitating degradation of the substrate material. Many chemical and physical methods were investigated to understand blister behaviour. Electrochemical impedance spectroscopy (EIS), scanning Kelvin probe (SKP) and scanning acoustic microscopy (SAM) were used to study blister and corrosion growth on steel coated with epoxy-modified polyester lacquer [1]. Damage to paint coatings caused by immersion was observed in situ by scanning electrochemical microscopy (SECM) [2–4]. In addition to the above methods, several non-destructive evaluation (NDE) techniques have been used for corrosion and blister characterisation. For example, acoustic emission techniques were used for monitoring corrosion under insulation and pitting corrosion [5,6]. Eddy current methods were investigated for corrosion detection and monitoring [7–10], including RFID based sensors for corrosion characterisation [11]. Infrared (IR) thermography, which can inspect large areas in a short time, has also demonstrated ability to detect corrosion under paint without paint removal [12]. For example, Han and Park used an infrared camera to detect corrosion on a ruptured blistering area, as well as blister and filiform corrosion under organic coatings [13].
⇑ Corresponding author at: College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China. Tel.: +86 13467698133. E-mail addresses: [email protected], [email protected] (Y. He). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.09.001
Jönsson et al. used pulsed thermography to detect the blister filament corrosion and compared with results from surface profile measurements [14]. Schönberger et al. studied the specific damage caused by stone chippings to coating systems including delamination, paint loss, and fracture of interlayer by using pulsed phase thermography [15]. Eddy current pulsed thermography (ECPT) is an emerging infrared thermography technique, which combines eddy current [16] and thermal effects for defect and material characterisation [17]. Compared with optical stimulation, ECPT has another advantage that the heat is generated directly inside the material in a thin skin below the surface. Therefore, the surface property like the absorption coefficient does not have an influence on the generated heat. In previous work, an ECPT system was developed and used for crack and metal loss detection in metallic alloy [18], and damage evaluation in carbon fibre reinforced plastic (CFRP) [19,20]. Combining eddy current excitation with infrared imaging and phase analysis, eddy current pulsed phase thermography (ECPPT) technique was proposed. Experimental results have shown that the phase can eliminate the non-uniform heating and improve defect detectability when compared with conventional thermograms in ECPT [21]. The aim of present work is to examine the applications of ECPT as a NDE technique to detect and monitor blisters on paint coatings. The rest of the paper is organised as follows. Firstly, the detection mechanisms for various types of corrosion are addressed in Section 2. Next, the experimental studies are presented in Section 3. Finally, conclusions are outlined in Section 4.
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Y. He et al. / Corrosion Science 78 (2014) 1–6
2. Methodology A diagram of eddy current pulsed thermography is shown in Fig. 1. The coil driving the high-frequency alternating current (AC) induces eddy currents in the conductive material. Then, an IR camera captures the radiation through reflected thermal waves from the boundaries of the interfaces. There are three physical processes involved in ECPT. The first is induction heating. The eddy currents induced in conductive material (steel and corrosion in this work) generate resistive heating then the conductive material is heated locally by Joule heating (in skin depth) [22]. The second is heat conduction. The heating will conduct from heated area in skin depth to inside of material till thermal balance is arrived. The last is infrared radiation. The Stefan–Boltzmann law states that the total energy radiated per surface area unit of a grey body per unit time is directly proportional to the fourth power of the grey body’s absolute temperature and emissivity. If there is a defect in conductive material, eddy current distribution or the heat conduction process will be obstructed. Consequently, the temperature distribution on the surface of the conductive material will show the variation. The defect’s interaction with eddy current and heat conduction was illustrated in previous work [18,19]. The detection mechanisms for various types of damage in mild steel and coating system are discussed as following.
(i) Corrosion on the surface of uncoated steel. In the case shown in Fig. 2(a), the temperature difference between the corrosion and sound area will be recorded directly by an IR camera. The variation of physical properties in corrosion must be considered in the eddy current heating process. Corrosion is a general term for a series of iron oxides and hydroxides [23]. The variation of conductivity and permeability in corrosion will lead to a change on the induction thermography response. Previous work has discussed the influence of different physical properties of corrosion and illustrated that the corrosion will generate a higher temperature than steel [24]. Therefore, the corrosion area will show a greater temperature than a non-corrosion area. In addition, because emissivity of corrosion is 0.65–0.7 and emissivity of steel is 0.2–0.3, the difference on emissivity should be taken into account during the measurement as well as the other properties. (ii) Corrosion under non-conductive paint coating. In the case shown in Fig. 2(b), the temperature on the surface of the coating is recorded. Therefore, the emissivity difference between corrosion and steel can be neglected. The heating in corrosion and steel will conduct through the coating. Due to the greater heating of corrosion, the coating area above the corrosion will display a slightly higher temperature than the sound area as a hot spot. However, the temperature difference between corrosion and steel will be weakened due to the coating. This will increase the difficulty of corrosion detection under coating. (iii) Blister on non-conductive paint coating. Fig. 2(c) shows that the corrosion or steel under blister can generate resistive heating. However, when the heating reaches the blister, which is filled by air with higher thermal resistance, the heat transfer rate is reduced or stopped. Therefore, it is concluded that the blister area has a colder temperature, as a dark spot. (iv) Crack on broken coating. In the case shown in Fig. 2(d), there is no coating to reduce the heat conduction on the crack area. Therefore, the crack area will display greater temperature than the sound area.
3. Experimental setup and studies
Fig. 1. Principle of eddy current pulsed thermography, which involves three physical processes: induction heating, heat conduction and infrared radiation.
The steel sample was prepared by International PaintÒ, United Kingdom. Firstly, a mild steel (S275) plate was cut into a sample with dimensions of 300 150 3 mm3, whose composition (wt%) was <0.22 C, 0.05–0.15 Si, <0.65 Mn, <0.3 Ni, <0.05 S, <0.04 P, <0.3 Cr, <0.012 N, and <0.3 Cu. Secondly, the mild steel sample
Fig. 2. Detection mechanisms for various types of corrosion: (a) corrosion on the surface of uncoated steel, (b) corrosion under non-conductive paint coating, (c) blister on non-conductive paint coating, and (d) crack on broken coating.
Y. He et al. / Corrosion Science 78 (2014) 1–6
Fig. 3. Photo of blister sample, which was prepared by International Paint, United Kingdom.
was covered using a non-conductive paint with a thickness of 100 lm, as shown in Fig. 3. Thirdly, the sample was immersed in the specific artificial solution (simulating marine water) until some blisters were formed. As marked in Fig. 3, blisters were dome shaped and had a little increase in height. And corrosion can be easily found at the ruptured blister area. A precision induction heating device, Easyheat 224 from Cheltenham Induction Heating, Ltd was used for induction heating, having a maximum excitation power of 2.4 kW, a maximum current of 400 Arms and an excitation frequency range of 150– 400 kHz. The excitation coil was made of 6.35 mm high-conductivity hollow copper tubing. In order to fit the sample, the excitation coil was designed as a rectangular shape in plane. The state-of-theart infrared system Flir SC7500 from was used to record the temperature change, which is a Stirling cooled camera with a 320 256 array of 1.5–5 lm InSb detectors. The pitch between detectors is 30 lm. And it has a sensitivity of <20 mK, and a maximum full frame rate of 383 Hz. The radiation of the object was sampled using the commercial thermography software Altair and the unit of radiation is digital level (DL). A non-linear transfer function after calibration can convert the radiation (unit: DL) into temperature (unit: K), which requires an operator setting several parameters (emissivity, background temperature, transmission, etc.) [25]. The key objective of this study is to demonstrate the ability to detect blister. In order to simplify the procedure, we used DL as the unit of temperature in experimental studies. In the tests, the amplitude and frequency of the excitation current in Easyheat 224 were set as 380 Arms and 256 kHz. The heating time was set as 200 ms. The sampling frequency and the whole recorded time in IR camera were set as 200 Hz and 0.5 s. Fig. 4 shows the thermograms at 30 ms, 75 ms, 200 ms and 500 ms, respectively. The units of x and y axes are pixel. The temperature
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unit is DL as mentioned before. From this figure, it was noticed that: (i) the temperature of blisters area was smaller than that of the sound area. Therefore, the blister can be found as the dark area. Some small blisters (about 1 mm diameter) can be easily detected in the thermograms at 200 ms as shown in Fig. 4(c). (ii) The ruptured blister area showed a greater temperature than the blister and intact area, as marked in Fig. 4(a). However, it showed a smaller temperature than the intact area in Fig. 4(c) and (d). (iii) The non-uniform heating along the coil affected the detectability of blister, which should be eliminated using the signal processing methods. In order to understand the temperature characteristic of different areas, the temperature responses T of three points A, B and C marked in Fig. 4(d) are shown in Fig. 5(a). The unit of T is DL. Points A, B and C were on the ruptured blister, blister and intact area, respectively. It was noticed that (i) the temperature of the blister was the smallest in the whole time and (ii) the temperature of ruptured blister was greater than that of intact area before 125 ms and smaller than that of intact area after 125 ms. Fig. 5(b) shows the normalised temperature responses Tnorm, which can be calculated by using the following equation:
T norm ¼
T maxðTÞ
ð1Þ
where normalised temperature response Tnorm is relative temperature and has no unit. Here, we defined peak time as the time when the response arrives at the maximum value. Clearly, the values of peak time for three points were different. The extent of the peak time increase was in the following sequence: blister intact area > ruptured blister. From Fig. 5(a) and (b), it was found that there was a ‘knee’ effect on the temperature response of ruptured blister, which was different from the temperature responses of blister and intact area. In order to extract this characteristic, temperature responses of three points were differentiated and the derivatives DT can be obtained using the following equation:
DT ¼ dT=dt
ð2Þ
The unit of derivatives DT is DL/s. Fig. 5(c) shows the derivatives of temperature responses DT for three points A, B and C. Here, we defined max derivative as the maximum value of the derivatives. Clearly, max derivative for the ruptured blister (point A) was the greatest among three points. The similar thing was that the minimum value of derivatives for the ruptured blister (point A) was the smallest among three points. Therefore, max derivative can be used as a specific feature to identify the ruptured blister. These new features (peak time and max derivative) were extracted from temperature response of all pixels and used to form new images. Fig. 6(a) shows the image formed by peak time.
Fig. 4. Thermograms of blister and ruptured blister at (a) 30 ms, (b) 75 ms, (c) 200 ms and (d) 500 ms.
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Fig. 5. (a) Temperature responses T, (b) normalised temperature responses Tnorm, and (c) derivatives of temperature responses DT, for three points A, B and C. A was on the ruptured blister, B was on the blister and C was on the intact area.
Fig. 6. (a) Image formed by peak time of temperature response and (b) image formed by maximum value of derivation wave of temperature response.
Clearly, the non-uniform heating along the coil was eliminated. The blister areas had much greater values than that of the intact area. On the contrary, the ruptured blister had smaller values than that of the intact area. This criterion was helpful to classify the blister and ruptured blister. When compared with Fig. 4, the image formed by peak time was better for detecting blister. Fig. 6(b) shows the image formed by maximum value of derivatives of temperature response. Clearly, just the ruptured blister can be easily found as a light spot. Principal components analysis (PCA) and independent components analysis (ICA) are widely used in IR thermography NDE. A PCA/ICA based image reconstruction method was proposed to improve the sensitivity in previous work [20]. In this work, the method was applied to detect corrosion blister. From Fig. 5(a), it can be
Fig. 7. (a) Image reconstructed by first principal component and (b) image reconstructed by third principal component after the principal components analyses based reconstruction method.
seen that the temperatures of the ruptured blister and intact area appeared in inversion at 125 ms. Therefore, the thermal sequence from 0 ms to 125 ms was used as input data for the reconstruction method. Fig. 7 shows the images reconstructed by principal components. Fig. 7(a) reconstructed by the first principal component, which carried the most of information regarding the original data, was similar to the thermogram at 200 ms in Fig. 4(c). Both blister and ruptured blister can be found, however, the non-uniform heating along the coil was still present. In Fig. 7(b) reconstructed by the third principal component, which had a relative small contribution, the non-uniform heating along the coil was eliminated and the
Y. He et al. / Corrosion Science 78 (2014) 1–6
Fig. 8. (a) Image reconstructed by first independent component and (b) images reconstructed by fourth independent component after the independent components analyses based reconstruction method.
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using conventional thermograms from an IR camera. However, non-uniform heating was a significant obstacle and the temperature variation over time by different types of blister was irregular. Therefore, signal processing algorithms were investigated in order to obtain new features. PCA, ICA and FFT were proposed and compared in the performance of blister detection. The main conclusions are as follows: (i) Peak time, third principal component from PCA, several independent components from ICA, and phase images from FFT can eliminate effectively the non-uniform heating and improve the detectability of blister and ruptured blister. (ii) Peak time was the best way to detect the blister and the extent of peak time increase was in the following sequence: blister intact area > ruptured blister. (iii) Maximum value of derivatives of temperature response, third principal component and several independent components can be used to separate the ruptured blister. To sum up, eddy current pulsed thermography has the potential application for corrosion detection and monitoring in industry. Acknowledgment This work was supported by EPSRC Grant EP/E005071, UK. The authors would like to thank Paul A Jackson with International PaintÒ for providing the experimental samples. Mr. He would like to thank the China Scholarship Council for sponsoring his visiting study to Newcastle University. References
Fig. 9. (a) Phase image at 2.34375 Hz and (b) phase image at 6.25 Hz after fast Fourier transform of temperature responses.
ruptured blister can be found. Fig. 8 shows the images reconstructed by independent components. Clearly, the non-uniform heating along the coil can be eliminated. In Fig. 8(a) reconstructed by first independent component, ruptured blister can be found. While in Fig. 8(b) reconstructed by the fourth independent component, both blister and ruptured blister can be found. Therefore, the PCA/ICA based image reconstruction method can be used to eliminate the non-uniform heating, improve the blister detectability, and separate the ruptured blister. A great deal of work in IR thermography NDE has shown that non-uniform heating and surface emissivity variations have a negligible impact on phase [15,21]. In this work, phase information was used to form the new images. The experimental data for the blister sample was processed by the fast Fourier transform (FFT). Fig. 9 shows the phase images at 2.34375 Hz and 6.25 Hz. The unit of phase is radian (rad). As expected, the non-uniform heating was eliminated. Blister can be easily found in Fig. 9(a), while just ruptured blister can be observed in Fig. 9(b). 4. Conclusion Eddy current pulsed thermography was investigated to observe corrosion blister. Both blister and ruptured blister can be detected
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[17] A.J. Yin, B. Gao, G.Y. Tian, W.L. Woo, K.J. Li, Physical interpretation and separation of eddy current pulsed thermography, Journal of Applied Physics 113 (2013) 064101. [18] Y. He, M. Pan, F. Luo, Defect characterisation based on heat diffusion using induction thermography testing, Review of Scientific Instruments 83 (2012) 104702. 104701–104710. [19] L. Cheng, G.Y. Tian, Surface crack detection for carbon fibre reinforced plastic (CFRP) materials using pulsed eddy current thermography, Sensors Journal, IEEE 11 (2011) 3261–3268. [20] M. Pan, Y. He, G.Y. Tian, D. Chen, F. Luo, Defect characterisation using pulsed eddy current thermography under transmission mode and NDT applications, NDT and E International 52 (2012) 28–36.
[21] Y. He, G.Y. Tian, M. Pan, D. Chen, Eddy current pulsed phase thermography and feature extraction, Applied Physics Letters 103 (2013) 084104. [22] J. Wilson, G.Y. Tian, I.Z. Abidin, S. Yang, D. Almond, Pulsed eddy current thermography: system development and evaluation, insight: non-destructive testing and condition monitoring 52 (2010) 87–90. [23] D. de la Fuente, I. Díaz, J. Simancas, B. Chico, M. Morcillo, Long-term atmospheric corrosion of mild steel, Corrosion Science 53 (2011) 604–617. [24] Y. He, G.Y. Tian, L. Cheng, H. Zhang, P. Jackson, Parameters Influence in steel corrosion detection using pulsed eddy current thermography, in: 17th International Conference on Automation and Computing (ICAC’11), Huddersfield, UK, 2011, pp. 255–260. [25] DL002U-D Altair Reference Guide, in: Cedip Infrared Systems.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
An investigation into eddy current pulsed thermography for detection of corrosion blister Yunze He a,b,⇑, Gui Yun Tian b, Mengchun Pan a, Dixiang Chen a, Hong Zhang b a b
College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, United Kingdom
a r t i c l e
i n f o
Article history: Received 28 August 2013 Accepted 19 September 2013 Available online 27 September 2013 Keywords: A. Mild steel A. Paint coatings C. Atmospheric corrosion
a b s t r a c t This letter reports an investigation into eddy current pulsed thermography (ECPT) for detecting corrosion blister in mild steel. The detection mechanisms for various types of damage were addressed based on their interaction with eddy current distribution and heat conduction. Experimental studies showed that the blister and ruptured blister area can be easily monitored using conventional thermograms from an infrared camera. Signal processing algorithms were investigated to obtain new features. Principal components analysis (PCA), independent components analysis (ICA) and fast Fourier transform (FFT) were compared in the performance of blister detection. It was found that peak time was the best way to detect the blister and the extent of peak time increase was in the following sequence: blister intact area > ruptured blister. Several features can be specially used to separate the ruptured blister. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Blister has long been recognised as a problem with paint coatings, reducing their service life and facilitating degradation of the substrate material. Many chemical and physical methods were investigated to understand blister behaviour. Electrochemical impedance spectroscopy (EIS), scanning Kelvin probe (SKP) and scanning acoustic microscopy (SAM) were used to study blister and corrosion growth on steel coated with epoxy-modified polyester lacquer [1]. Damage to paint coatings caused by immersion was observed in situ by scanning electrochemical microscopy (SECM) [2–4]. In addition to the above methods, several non-destructive evaluation (NDE) techniques have been used for corrosion and blister characterisation. For example, acoustic emission techniques were used for monitoring corrosion under insulation and pitting corrosion [5,6]. Eddy current methods were investigated for corrosion detection and monitoring [7–10], including RFID based sensors for corrosion characterisation [11]. Infrared (IR) thermography, which can inspect large areas in a short time, has also demonstrated ability to detect corrosion under paint without paint removal [12]. For example, Han and Park used an infrared camera to detect corrosion on a ruptured blistering area, as well as blister and filiform corrosion under organic coatings [13].
⇑ Corresponding author at: College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073, PR China. Tel.: +86 13467698133. E-mail addresses: [email protected], [email protected] (Y. He). 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.09.001
Jönsson et al. used pulsed thermography to detect the blister filament corrosion and compared with results from surface profile measurements [14]. Schönberger et al. studied the specific damage caused by stone chippings to coating systems including delamination, paint loss, and fracture of interlayer by using pulsed phase thermography [15]. Eddy current pulsed thermography (ECPT) is an emerging infrared thermography technique, which combines eddy current [16] and thermal effects for defect and material characterisation [17]. Compared with optical stimulation, ECPT has another advantage that the heat is generated directly inside the material in a thin skin below the surface. Therefore, the surface property like the absorption coefficient does not have an influence on the generated heat. In previous work, an ECPT system was developed and used for crack and metal loss detection in metallic alloy [18], and damage evaluation in carbon fibre reinforced plastic (CFRP) [19,20]. Combining eddy current excitation with infrared imaging and phase analysis, eddy current pulsed phase thermography (ECPPT) technique was proposed. Experimental results have shown that the phase can eliminate the non-uniform heating and improve defect detectability when compared with conventional thermograms in ECPT [21]. The aim of present work is to examine the applications of ECPT as a NDE technique to detect and monitor blisters on paint coatings. The rest of the paper is organised as follows. Firstly, the detection mechanisms for various types of corrosion are addressed in Section 2. Next, the experimental studies are presented in Section 3. Finally, conclusions are outlined in Section 4.
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Y. He et al. / Corrosion Science 78 (2014) 1–6
2. Methodology A diagram of eddy current pulsed thermography is shown in Fig. 1. The coil driving the high-frequency alternating current (AC) induces eddy currents in the conductive material. Then, an IR camera captures the radiation through reflected thermal waves from the boundaries of the interfaces. There are three physical processes involved in ECPT. The first is induction heating. The eddy currents induced in conductive material (steel and corrosion in this work) generate resistive heating then the conductive material is heated locally by Joule heating (in skin depth) [22]. The second is heat conduction. The heating will conduct from heated area in skin depth to inside of material till thermal balance is arrived. The last is infrared radiation. The Stefan–Boltzmann law states that the total energy radiated per surface area unit of a grey body per unit time is directly proportional to the fourth power of the grey body’s absolute temperature and emissivity. If there is a defect in conductive material, eddy current distribution or the heat conduction process will be obstructed. Consequently, the temperature distribution on the surface of the conductive material will show the variation. The defect’s interaction with eddy current and heat conduction was illustrated in previous work [18,19]. The detection mechanisms for various types of damage in mild steel and coating system are discussed as following.
(i) Corrosion on the surface of uncoated steel. In the case shown in Fig. 2(a), the temperature difference between the corrosion and sound area will be recorded directly by an IR camera. The variation of physical properties in corrosion must be considered in the eddy current heating process. Corrosion is a general term for a series of iron oxides and hydroxides [23]. The variation of conductivity and permeability in corrosion will lead to a change on the induction thermography response. Previous work has discussed the influence of different physical properties of corrosion and illustrated that the corrosion will generate a higher temperature than steel [24]. Therefore, the corrosion area will show a greater temperature than a non-corrosion area. In addition, because emissivity of corrosion is 0.65–0.7 and emissivity of steel is 0.2–0.3, the difference on emissivity should be taken into account during the measurement as well as the other properties. (ii) Corrosion under non-conductive paint coating. In the case shown in Fig. 2(b), the temperature on the surface of the coating is recorded. Therefore, the emissivity difference between corrosion and steel can be neglected. The heating in corrosion and steel will conduct through the coating. Due to the greater heating of corrosion, the coating area above the corrosion will display a slightly higher temperature than the sound area as a hot spot. However, the temperature difference between corrosion and steel will be weakened due to the coating. This will increase the difficulty of corrosion detection under coating. (iii) Blister on non-conductive paint coating. Fig. 2(c) shows that the corrosion or steel under blister can generate resistive heating. However, when the heating reaches the blister, which is filled by air with higher thermal resistance, the heat transfer rate is reduced or stopped. Therefore, it is concluded that the blister area has a colder temperature, as a dark spot. (iv) Crack on broken coating. In the case shown in Fig. 2(d), there is no coating to reduce the heat conduction on the crack area. Therefore, the crack area will display greater temperature than the sound area.
3. Experimental setup and studies
Fig. 1. Principle of eddy current pulsed thermography, which involves three physical processes: induction heating, heat conduction and infrared radiation.
The steel sample was prepared by International PaintÒ, United Kingdom. Firstly, a mild steel (S275) plate was cut into a sample with dimensions of 300 150 3 mm3, whose composition (wt%) was <0.22 C, 0.05–0.15 Si, <0.65 Mn, <0.3 Ni, <0.05 S, <0.04 P, <0.3 Cr, <0.012 N, and <0.3 Cu. Secondly, the mild steel sample
Fig. 2. Detection mechanisms for various types of corrosion: (a) corrosion on the surface of uncoated steel, (b) corrosion under non-conductive paint coating, (c) blister on non-conductive paint coating, and (d) crack on broken coating.
Y. He et al. / Corrosion Science 78 (2014) 1–6
Fig. 3. Photo of blister sample, which was prepared by International Paint, United Kingdom.
was covered using a non-conductive paint with a thickness of 100 lm, as shown in Fig. 3. Thirdly, the sample was immersed in the specific artificial solution (simulating marine water) until some blisters were formed. As marked in Fig. 3, blisters were dome shaped and had a little increase in height. And corrosion can be easily found at the ruptured blister area. A precision induction heating device, Easyheat 224 from Cheltenham Induction Heating, Ltd was used for induction heating, having a maximum excitation power of 2.4 kW, a maximum current of 400 Arms and an excitation frequency range of 150– 400 kHz. The excitation coil was made of 6.35 mm high-conductivity hollow copper tubing. In order to fit the sample, the excitation coil was designed as a rectangular shape in plane. The state-of-theart infrared system Flir SC7500 from was used to record the temperature change, which is a Stirling cooled camera with a 320 256 array of 1.5–5 lm InSb detectors. The pitch between detectors is 30 lm. And it has a sensitivity of <20 mK, and a maximum full frame rate of 383 Hz. The radiation of the object was sampled using the commercial thermography software Altair and the unit of radiation is digital level (DL). A non-linear transfer function after calibration can convert the radiation (unit: DL) into temperature (unit: K), which requires an operator setting several parameters (emissivity, background temperature, transmission, etc.) [25]. The key objective of this study is to demonstrate the ability to detect blister. In order to simplify the procedure, we used DL as the unit of temperature in experimental studies. In the tests, the amplitude and frequency of the excitation current in Easyheat 224 were set as 380 Arms and 256 kHz. The heating time was set as 200 ms. The sampling frequency and the whole recorded time in IR camera were set as 200 Hz and 0.5 s. Fig. 4 shows the thermograms at 30 ms, 75 ms, 200 ms and 500 ms, respectively. The units of x and y axes are pixel. The temperature
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unit is DL as mentioned before. From this figure, it was noticed that: (i) the temperature of blisters area was smaller than that of the sound area. Therefore, the blister can be found as the dark area. Some small blisters (about 1 mm diameter) can be easily detected in the thermograms at 200 ms as shown in Fig. 4(c). (ii) The ruptured blister area showed a greater temperature than the blister and intact area, as marked in Fig. 4(a). However, it showed a smaller temperature than the intact area in Fig. 4(c) and (d). (iii) The non-uniform heating along the coil affected the detectability of blister, which should be eliminated using the signal processing methods. In order to understand the temperature characteristic of different areas, the temperature responses T of three points A, B and C marked in Fig. 4(d) are shown in Fig. 5(a). The unit of T is DL. Points A, B and C were on the ruptured blister, blister and intact area, respectively. It was noticed that (i) the temperature of the blister was the smallest in the whole time and (ii) the temperature of ruptured blister was greater than that of intact area before 125 ms and smaller than that of intact area after 125 ms. Fig. 5(b) shows the normalised temperature responses Tnorm, which can be calculated by using the following equation:
T norm ¼
T maxðTÞ
ð1Þ
where normalised temperature response Tnorm is relative temperature and has no unit. Here, we defined peak time as the time when the response arrives at the maximum value. Clearly, the values of peak time for three points were different. The extent of the peak time increase was in the following sequence: blister intact area > ruptured blister. From Fig. 5(a) and (b), it was found that there was a ‘knee’ effect on the temperature response of ruptured blister, which was different from the temperature responses of blister and intact area. In order to extract this characteristic, temperature responses of three points were differentiated and the derivatives DT can be obtained using the following equation:
DT ¼ dT=dt
ð2Þ
The unit of derivatives DT is DL/s. Fig. 5(c) shows the derivatives of temperature responses DT for three points A, B and C. Here, we defined max derivative as the maximum value of the derivatives. Clearly, max derivative for the ruptured blister (point A) was the greatest among three points. The similar thing was that the minimum value of derivatives for the ruptured blister (point A) was the smallest among three points. Therefore, max derivative can be used as a specific feature to identify the ruptured blister. These new features (peak time and max derivative) were extracted from temperature response of all pixels and used to form new images. Fig. 6(a) shows the image formed by peak time.
Fig. 4. Thermograms of blister and ruptured blister at (a) 30 ms, (b) 75 ms, (c) 200 ms and (d) 500 ms.
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Fig. 5. (a) Temperature responses T, (b) normalised temperature responses Tnorm, and (c) derivatives of temperature responses DT, for three points A, B and C. A was on the ruptured blister, B was on the blister and C was on the intact area.
Fig. 6. (a) Image formed by peak time of temperature response and (b) image formed by maximum value of derivation wave of temperature response.
Clearly, the non-uniform heating along the coil was eliminated. The blister areas had much greater values than that of the intact area. On the contrary, the ruptured blister had smaller values than that of the intact area. This criterion was helpful to classify the blister and ruptured blister. When compared with Fig. 4, the image formed by peak time was better for detecting blister. Fig. 6(b) shows the image formed by maximum value of derivatives of temperature response. Clearly, just the ruptured blister can be easily found as a light spot. Principal components analysis (PCA) and independent components analysis (ICA) are widely used in IR thermography NDE. A PCA/ICA based image reconstruction method was proposed to improve the sensitivity in previous work [20]. In this work, the method was applied to detect corrosion blister. From Fig. 5(a), it can be
Fig. 7. (a) Image reconstructed by first principal component and (b) image reconstructed by third principal component after the principal components analyses based reconstruction method.
seen that the temperatures of the ruptured blister and intact area appeared in inversion at 125 ms. Therefore, the thermal sequence from 0 ms to 125 ms was used as input data for the reconstruction method. Fig. 7 shows the images reconstructed by principal components. Fig. 7(a) reconstructed by the first principal component, which carried the most of information regarding the original data, was similar to the thermogram at 200 ms in Fig. 4(c). Both blister and ruptured blister can be found, however, the non-uniform heating along the coil was still present. In Fig. 7(b) reconstructed by the third principal component, which had a relative small contribution, the non-uniform heating along the coil was eliminated and the
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Fig. 8. (a) Image reconstructed by first independent component and (b) images reconstructed by fourth independent component after the independent components analyses based reconstruction method.
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using conventional thermograms from an IR camera. However, non-uniform heating was a significant obstacle and the temperature variation over time by different types of blister was irregular. Therefore, signal processing algorithms were investigated in order to obtain new features. PCA, ICA and FFT were proposed and compared in the performance of blister detection. The main conclusions are as follows: (i) Peak time, third principal component from PCA, several independent components from ICA, and phase images from FFT can eliminate effectively the non-uniform heating and improve the detectability of blister and ruptured blister. (ii) Peak time was the best way to detect the blister and the extent of peak time increase was in the following sequence: blister intact area > ruptured blister. (iii) Maximum value of derivatives of temperature response, third principal component and several independent components can be used to separate the ruptured blister. To sum up, eddy current pulsed thermography has the potential application for corrosion detection and monitoring in industry. Acknowledgment This work was supported by EPSRC Grant EP/E005071, UK. The authors would like to thank Paul A Jackson with International PaintÒ for providing the experimental samples. Mr. He would like to thank the China Scholarship Council for sponsoring his visiting study to Newcastle University. References
Fig. 9. (a) Phase image at 2.34375 Hz and (b) phase image at 6.25 Hz after fast Fourier transform of temperature responses.
ruptured blister can be found. Fig. 8 shows the images reconstructed by independent components. Clearly, the non-uniform heating along the coil can be eliminated. In Fig. 8(a) reconstructed by first independent component, ruptured blister can be found. While in Fig. 8(b) reconstructed by the fourth independent component, both blister and ruptured blister can be found. Therefore, the PCA/ICA based image reconstruction method can be used to eliminate the non-uniform heating, improve the blister detectability, and separate the ruptured blister. A great deal of work in IR thermography NDE has shown that non-uniform heating and surface emissivity variations have a negligible impact on phase [15,21]. In this work, phase information was used to form the new images. The experimental data for the blister sample was processed by the fast Fourier transform (FFT). Fig. 9 shows the phase images at 2.34375 Hz and 6.25 Hz. The unit of phase is radian (rad). As expected, the non-uniform heating was eliminated. Blister can be easily found in Fig. 9(a), while just ruptured blister can be observed in Fig. 9(b). 4. Conclusion Eddy current pulsed thermography was investigated to observe corrosion blister. Both blister and ruptured blister can be detected
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