early detection of corrosion damage under coatings with thermographic

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International Conference on Thermal Enerieering and Thermogrammetry THERMO OKK-OSSKI MATE

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18-20 June, 2003, Budapest, Hungary OSSKI Center (Trley Palace)

with Exhibition and Pre-Session on Thermal Energy in Hungarian

"THERMO-BRIDGE"

between East and West for technology transfer and information exchange

Scientific Society of Measurement, Automation and Informatics (MATE)

Branch of Thermal Engineering and Thermogrammetry (TE and TGM)

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E-mail: [email protected]@energia.bme.hu

PUB13 P01 S1I S2C S3M S4R S5TM S6H S7IR S8T SESSION PUB12

Thermomechanics and defectometry / Hmechanika s hibafeltrs

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Early detection of corrosion damage under coatings with thermographic

methods

G. Riegert1), G. Kunz2), R. Nothhelfer-Richter2)and G. Busse1)

1) Institute of Polymer Testing and Polymer Science (IKP), Department of Non-Destructive Testing (ZfP),

University of Stuttgart, Pfaffenwaldring 32, D-70569 Stuttgart, Germany

2) Forschungsinstitut fr Pigmente und Lacke e.V. (FPL), Allmandring 37, D-70569 Stuttgart, Germany

S5TM07

1. Introduction

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].

2. Experimental set-up

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 (21x16 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.
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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%, KNO35%, HNO3 12%,

NaOH, Parafine). Afterwards the coordinates of the resulting corrosion damages and their size were measured. Finally thesheet metals were coated and inspected with PPT. Another damaging method was shelling of the already coated sheet

metals or scratching them.

3. Results

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.

4. Quick testing

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.

Sample Sheet Metal Coating Corrosion Treatment

A

(DC0K0Z01)cold rolled steel DC 04 B Permacor 2428

scratching and salt spray

testing

Model sample with artificial damages for quick testing

Sample A

initial state 1h salt spray test 2h salt spray test

Phase image 5 Hz Phase image 2 Hz Phase image 2 Hz


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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.

5. Outdoor exposure

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.

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

C

(DC00FZ03)

Permacor 2428(2K-

epoxy-zinc phosphate

-primer)

scratching and outdoor

exposure

6. Model samples with artificial damages for outdoor exposure

Sample B

0 Weeks 11 Weeks

Amplitude image 0.15 Hz Phase image 3.5 Hz, 11 weeks Optical image, 11 weeks

e ~ 1.1 mm a ~ 2.3 mm

f ~ 1.0 mm b ~ 1.0 mm

e~ 8.7 mm a ~ 8.8 mm

f~ 17.8 mm b ~ 3.5 mm

7. Chemically contaminated, outdoor exposure

The corrosion damages increased within eleven weeks (figure 3). No corrosion is visible in the optical image, while thecorrosion 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.

Sample C

0 Weeks 11 Weeks


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Amplitude image 0.15 Hz, 0 weeks Phase image 1.9 Hz, 11 weeks

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.

9. Automotive tailgate

In order to show the applicability of PPT on automotive parts a tailgate of a passenger car with corrosion damages was

investigated.

Optical image of area Phase image at 1.9 Hz

Optical image of area Phase image at 3.0 Hz

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 variationsin coating thickness. With PPT it is also possible to detect filling under the coating.

11. Conclusions

Our results show that PPT is a very sensitive evaluation method allowing for the identification of corrosion spots down to

0.3 mm under coating thicknesses of about 60 m. By using a close-up lens, even spots of only 0.02 mm can bedetected.

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


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quality control of coating repairs as well as for the reduction of outdoor corrosion testing duration.

12. Acknowledgements

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 AssociationsOtto von Guericke (AiF).The authors are also grateful to the DEKRA for

supporting the tailgate.

13. References

[1]FOURIER, J., Thorie du mouvement de la chaleur dans les corps solides 1rePartie. In: Mmoires de lAcadmie

des Sciences 4, pp. 185-555, 1824

[2] BUSSE, G., Optoacoustic phase angle measurement for probing a metal. In: Appl.Phys.Lett. Vol. 35, pp. 759-760,

1979

[3]NORDAL, P.-E., KANSTAD, S.O., Photothermal radiometry. In: Physica Scripta Vol. 20, pp. 659-662, 1979

[4] ROSENCWAIG, A., Photoacoustic microscopy. American Lab. 11, pp.39-49, 1979

[5]LEHTO, A., JAARINEN, J., TIUSANEN, T., JOKINEN, M., LUUKKALA, M., Amplitude and phase in thermal

wave imaging. In: Electr. Lett. Vol. 17, pp. 364-365, 1981

[6] THOMAS, R.L., POUCH, J.J., WONG, Y.H., FAVRO, L.D., KUO, P.K., ROSENCWAIG, A., Subsurface flaw

detection in metals by photacoustic microscopy. In: J.Appl.Phys. Vol. 51, pp. 1152-1156; 1980

[7]MALDAGUE, X., MARINETTI, S., Pulse Phase Infrared Thermography. In: J. Appl.Phys. 79-5, pp. 2694-2698;

1996.

[8]REYNOLDS, W.N.: Quality control of composite materials by thermography, Metals and Materials, 1[2]: 100-102,

1985

[9]CIELO, P.; MALDAGUE, X.; DEOM, A.A.; LEWAK, R.: Thermographic nondestructive evaluation of industrial

materials and structures, Materials Evaluation, 45[6]: 452-460, 1987

[10] BEAUDOIN, J.L., MERIENNE, E., DANJOUX, R., EGEE, M.: Numerical system for infrared scanners and

application to the subsurface control of materials by photothermal radiometry. In: Infrared Technologyand Applications,

SPIE Vol. 590, p. 287, 1985

[11]KUO, P.K., FENG, Z.J., AHMED, T., FAVRO, L.D., THOMAS, R.L., HARTIKAINEN, J., Parallel thermal wave

imagingusing a vector lock-in video technique. In: Photoacoustic and Photothermal Phenomena, ed. P. Hess and J. Pelzl.

Heidelberg: Springer-Verlag, pp. 415-418, 1987

[12]BUSSE, G.: Nondestructive evaluation ofpolymer materials, NDT&E International, 27[5]:253-262, 1994

pdf

Contact details: Dipl.-Ing. Gernot Riegert

Institute of Polymer Testing and Polymer Science (IKP)

Department of Non-Destructive Testing (ZfP), University of Stuttgart

Pfaffenwaldring 32, D-70569 Stuttgart

Germany

Tel: +49(0)711/685-2572

Fax: +49(0)711/685-4635

Web Site: http://www.zfp.uni-stuttgart.de

E-mail: [email protected]

.

PUB13 P01 S1I S2C S3M S4R S5TM S6H S7IR S8T SESSION PUB12

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"Fodor Jzsef" National Center of Public Heath

"Frdric Joliot-Curie" National Research Institute for

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early detection of corrosion damage under coatings with thermographic

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