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Lockin Thermography uses thermal waves [1] for imaging. With photo-thermal techniques the phase angle of the detected thermal wave with respect to the initial excitation wave is used for imaging of thermal features like cracks, delaminations, and other kinds of thermal boundaries [2-5]. By variation of the lockin frequency (which is the frequency of modulated excitation) the depth range can be adjusted thereby allowing for depth resolved measurement of subsurface features [6].
Pulsed Phase Thermography (PPT) uses a short light flash for sample excitation [7]. This method is the link between Pulsed Thermography (PT) [8,9] and Optical Lockin Thermography (OLT) [10,11,12]. The temperature field on the surface of the inspected object launches a thermal wave into the coating. At hidden thermal boundaries (e.g. delaminations, corrosion) the thermal wave is reflected back to the surface of the object where it is detected. Fourier transformation of the signal provides information about the temperature amplitude and the depth of the hidden boundary layers. As a light flash corresponds to a rectangular intensity pulse it provides a frequency spectrum for lock-in examination. The benefits of PPT are a short measurement duration, a low thermal load on the sample, and the possibility of analyzing at different frequencies and hence with different depth ranges. It is also possible to measure coating thickness after calibration [2].
A flash lamp with 1.5 kJ is used for sample excitation (figure 1). After flashing, the infrared camera (Cedip Jade II, MW) starts recording a sequence of temperature images at a frame rate of 110 Hz. From this sequence a frequency spectrum is calculated by Fourier transformation for the area of interest. Then a discrete Fourier transformation at a peak of the spectrum provides amplitude and phase image of the corroded region. By measuring the distance of the objective to the sample and taking into account its field of view (21°x16° at the 25 mm objective) the mapping value of the pixels (320*240) is calculated. Finally the area of the corrosion damage is evaluated by counting the pixels of the corrosion signal.
Experimental set-up of Pulse Phase Thermography (PPT)
Different model samples were produced by FPL (Research Center for Pigments and Paints) in Stuttgart. At these samples, sheet metal material, coating type, and corrosion conditioning methods were varied to find both the potential and limitations of PPT. The corrosion spots were produced by chemical contamination (NaCl 5%, KNO3 5%, HNO3 12%, NaOH, Parafine). Afterwards the coordinates of the resulting corrosion damages and their size were measured. Finally the sheet metals were coated and inspected with PPT. Another damaging method was shelling of the already coated sheet metals or scratching them.
Various investigations were performed within this paper and the corresponding DFO/AiF project. Samples with various kinds of damages were exposed to quick testing and inspected with PPT. The same kind of samples were also tested by outdoor exposure and inspected on a regular basis to monitor the corrosion progress.
In addition PPT measurements were also performed on automotive parts to demonstrate the applicability of the method on coating systems.
The presented results were obtained on a sample with scratches in the coating down to the sheet metal. There was no corrosion on this samples before corrosion treatment. After this preliminary corrosion treatment the sample was put into salt spray testing.
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Sample |
Sheet Metal |
Coating |
Corrosion Treatment |
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A (DC0K0Z01) |
cold rolled steel DC 04 B |
Permacor 2428 |
scratching and salt spray testing |
Model sample with artificial damages for quick testing
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Sample A |
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initial state |
1h salt spray test |
2h salt spray test |
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Phase image 5 Hz |
Phase image 2 Hz |
Phase image 2 Hz |
Scratched coating, quick testing
The phase image from the initial state of sample A (figure 2, left) already shows distinct black spots. They could be due to unintentionally caused corrosion on the sheet metal or to local changes in coating thickness. This is one of the limitations of PPT. Phase angle images are also very sensitive to changes in coating thickness (as visible in the vertical stripe in figure 2). From an image taken at just one frequency one cannot distinguish between local changes in coating thickness and corrosion spots. However, these effects can be separated from their different dependence on modulation frequency.
The images after one and two hours of salt spray testing still show no onset of corrosion. Time for salt spray testing has to be increased.
The reproducibility of the PPT results is very good as can be seen in figure 2.
Similar samples like in quick testing had been used in order to compare the results of both testing methods. The sheet metals have damages due to chemical contamination and scratches of the coating. After preliminary corrosion treatment the samples were put to outdoor testing. They were inspected before and after eleven weeks of outdoor exposure.
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Sample |
Sheet Metal |
Coating |
corrosion treatment |
|
B (DCK0FU00) |
cold rolled steel DC 04 B |
Permacor 1905 (1K-urethane-alkyd resin-HS- zinc phosphate -primer) |
NaCl, NaOH and outdoor exposure |
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C (DC00FZ03) |
Permacor 2428(2K-epoxy-zinc phosphate -primer) |
scratching and outdoor exposure |
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6. Model samples with artificial damages for outdoor exposure |
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Sample B |
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0 Weeks |
11 Weeks |
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 |
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Amplitude image 0.15 Hz |
Phase image 3.5 Hz, 11 weeks Optical image, 11 weeks |
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e ~ 1.1 mm² a ~ 2.3 mm²
f ~ 1.0 mm² b ~ 1.0 mm²
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e ~ 8.7 mm² a ~ 8.8 mm²
f ~ 17.8 mm² b ~ 3.5 mm²
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7. Chemically contaminated, outdoor exposure |
The corrosion damages increased within eleven weeks (figure 3). No corrosion is visible in the optical image, while the corrosion spots stand out clearly in the images obtained by PPT. It is also possible to distinguish gradations of corrosion due to the strength of the corrosion agents (a and b: NaOH, e and f: NaCl). Image quality of PPT inspection after eleven weeks is better than at the beginning of testing. This is due to improvements in the evaluation technique (phase image at higher frequency) and improved flash lamps for excitation.
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Sample C |
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0 Weeks |
11 Weeks |
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|
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Amplitude image 0.15 Hz, 0 weeks |
Phase image 1.9 Hz, 11 weeks |
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8. Scratched coating, outdoor exposure |
The phase image of sample c after eleven weeks of outdoor exposure (figure 4, right) shows beginning corrosion around the vertical and horizontal scratches while the amplitude image before corrosion treatment shows no corrosion spots. The dark spots in the horizontal scratch of the amplitude image (figure 4, left) are due to reflections. This is one of the disadvantages in using amplitude images. Phase images are less sensitive to local changes in emission coefficient or inhomogeneous illumination and are therefore more sensitive to real effects.
In order to show the applicability of PPT on automotive parts a tailgate of a passenger car with corrosion damages was investigated.
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 |
 |
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Optical image of area |
Phase image at 1.9 Hz |
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 |
 |
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Optical image of area |
Phase image at 3.0 Hz |
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10. Tailgate of passenger car with corrosion damages |
The phase images of the tailgate (figure 5, right) reveal hidden corrosion under the coating. The low resolution of the infrared camera(320*240 pix) is the main problem for inspections of such large components. Therefore the distance to the object has to be so small that the resolution is high enough to detect small corrosion spots. This causes a small field of view resulting in the need of several measurements for the inspection of the whole part. However, these results are encouraging since they indicate that PPT is interesting e.g. in the quality control of car repairs to check for hidden corrosion and local variations in coating thickness. With PPT it is also possible to detect filling under the coating.
Our results show that PPT is a very sensitive evaluation method allowing for the identification of corrosion spots down to 0.3 mm² under coating thicknesses of about 60 µm. By using a close-up lens, even spots of only 0.02 mm² can be detected.
As phase images respond sensitively to thickness changes, such local variations can affect the results as well. This effect is used for contactless measurement of coating thicknesses after calibration of the system and by measurements at a couple of different frequencies.
The time span of two hours of salt spray testing on the sample was too short to cause damages, which could be detected by PPT.
Already after eleven weeks of outdoor exposure it was possible to indicate first corrosion on the scratched model samples. After corrosion inspection, outdoor exposure can be continued to investigate corrosion propagation.
The investigation of a passenger car tailgate showed the potential of PPT in corrosion detection under coating layer systems. With that PPT could also be used for quality control in car repairs.
PPT is a promising method for remote and non-destructive corrosion detection under coatings. It could be used for quality control of coating repairs as well as for the reduction of outdoor corrosion testing duration.
The authors are grateful to the German Research Association for Surface Treatment (DFO) for supporting this work from budget resources of the Federal Ministry of Economics and Technology (BMWi) through the German Federation of Industrial Cooperative Research Associations
“Otto von Guericke“ (AiF).The authors are also grateful to the DEKRA for supporting the tailgate.
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