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Corrosion resistance of
lacquered coated metals are
tested by outdoor exposure under
various climatic conditions and
evaluated with standardized
methods. Because of the long
duration of these tests the
demand for time-savinglapse tests
raised. In such tests the
corrosion process is accelerated
by increasing the load on the
sample, i.e. salt spray testing.
Unfortunately the results of
outdoor exposure and quick
testing are not well correlated
due to differences in transport
and reaction mechanisms. Another
approach for obtaining realistic
testing results at reduced
measurement duration is to
improve the evaluation methods.
With the early detection of
corrosion damage by Pulsed Phase
Thermography (PPT) the duration
of outdoor exposure testing can
be reduced significantly. A
possible application for this
kind of thermographic testing is
the quality control of repaired
coatings spots of lacquer. This
is specially interesting for car
repairs where the optical quality
of the top lacquer coat layer
does not show the quality of the
repair itself, like hidden
corrosion on the sheet metal or
variation in coating layerlacquer
thickness.
Pulsed Phase Thermography (PPT)
uses a short light flash for
sample excitation [1].
This method is the link between
Pulsed Thermography (PT) [2,3]
and Optical Lockin
Thermography (OLT) [4,5,6].
The temperature field on the
surface of the inspected object
launches a thermal wave into
the coatinglacquer. 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 flash corresponds to
a rectangular pulse it includes
all a frequencies frequency
spectrum for lock-in examination.
The benefits of PPT isare clearly
curtateda short measurement
duration,with a low thermal load
on the sample, and the
possibility of analyzing at
different frequencies andwith
that hence with different depth
ranges [7]. It is also
possible to measure thelacquer
thickness ot the layers of the
coating after calibration [8].
The schematical experimental set-up
is shown in figure 1.
In this paper results are
presented that were obtained on
model samples with different
measurement und testing methods.
Corrosion spots down to 0.5 mm2
under coatinglacquer layers of
approx. 60 µm can be detected
with the PPT method. Outdoor
exposedure samples are compared
to results of different short
tests with corresponding duration
length. With PPT it is possible
to detect corrosion in an early
state and thuswith that to reduce
the duration of outdoor exposure
tests. Another advantage of PPT
as compared to conventional
evaluation techniques is its non
destructive (and also non-contact)
character. ThereforeWith that it
is possible to continue the
outdoor exposure testing after
evaluation in order to
investigate the corrosion
progress.
In
addition, PPT investigations were
performed on automotive parts
with lacquer partly repaireds
coatings.
Figure 1:
Schematical sSet-up of Pulsed
Phase Thermography (PPT)
[1] Maldague, X.;
Marinetti, S.: “Pulse phase
infrared thermography” Journal
of Applied Physics, 79[Mar.]:
2694-2698, 1996
[2] Reynolds, W.N.: “Quality
control of composite materials by
thermography”, Metals and
Materials, 1[2]: 100-102, 1985
[3] Cielo, P.; Maldague, X.;
Déom, A.A.; Lewak, R.: “Thermographic
nondestructive evaluation of
industrial materials and
structures”, Materials
Evaluation, 45[6]: 452-460, 1987
[4] 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 Technology and
Applications, SPIE Vol. 590, p.
287, 1985
[5] Kuo, P.K., Feng, Z.J.,
Ahmed, T., Favro, L.D., Thomas, R.L.,
Hartikainen, J., “Parallel
thermal wave imaging using a
vector lock-in video technique”.
In: Photoacoustic and
Photothermal Phenomena, ed. P.
Hess and J. Pelzl. Heidelberg:
Springer-Verlag, pp. 415-418,
1987
[6] Busse, G.: “Nondestructive
evaluation of polymer materials“,
NDT&E International, 27[5]:253-262,
1994
[7] 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
[8] Busse, G., ”Optoacoustic
phase angle measurement for
probing a metal”. In: Appl.Phys.Lett.
Vol. 35, pp. 759-760, 1979
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