Final Report - Strategic Environmental Research and Development ...

AD
BD
BH
TD
TH
AH
PH
PS
AS
MD
PD
BS
MH
TS
MS
WORST BEST
1000
1000 1000 1000 1000 1000
900
800
700
600
600
500
400
300
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100
0
196 196
96 96 96
24 24 24 10
Coating System
Figure 6.2. Visual time to failure TTF for all 15 samples. Any sign of corrosion
(blistering, localized or uniform) was considered failure.
Figure 6.2 also shows that of the 15 coating systems tested, samples either survived 1000 hours
with no evidence of coating damage, or showed such evidence in less than 196 hours of
exposure. Using this strongly segregated data set enables a comparison to be made among the
various types of impedance-based methods for assessing coating system performance.
5.6.1.3.2 Impedance-based analyses of coating systems. The analysis of coating systems was
done using the corrosion resistance R corr , the metal-coating interface capacitance C dl , and the
delaminated area A del . Also, a new approach using ANN fed with only phase angle at low
frequencies was used. The ANN output parameter was time to failure in hours and the
calculation process is described in section 6.2 of this report.
A step-by-step analysis for a single sample is shown first, followed by the analysis for all the
samples. This analysis was performed as an example to show the calculations of how R corr , C dl
and A del were obtained. The sample chosen for the single analysis was Alodine 5700 conversion
coating and Hentzen 16708TEP primer (7075-AH).
The Bode plot for sample 7075-AH (Figure 6.3) shows a shift of the total impedance response in
all of the frequency ranges to lower values as a function of exposure (solid arrow). In particular,
the magnitude of the impedance at low frequency (0.01Hz) shows a decrease of 2 orders of
magnitude in the total length of exposure, where the larger decrease is seen before and after 288
h. Also, the change in the slope shifts to higher frequencies as the exposure time increases
(dashed arrow).
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