Final Report - Strategic Environmental Research and Development ...

C [F/cm2]
Rcorr [ohm*cm2]
7075-AHN
1.E-02
1.E-03
1.E+10
1.E-04
1.E+08
1.E-05
1.E+06
1.E-06
1.E+04
R corr C metal/coating Visual
1.E-07
1.E-08
1.E+02
C 1.E-09
coat
1.E+00
1 10 100 1000
Time [hrs]
failure
Figure 6.5. EIS modeling for sample AA7075 treated with Alodine conversion coating
and Hentzen primer (AA7075-AHN). Solid arrow represents the time needed in salt
spray chamber for the sample to present visual (un-aided eye) signs of corrosion (EIS test
area was 13.2 cm 2 ).
In this work, R corr , C dl , and the percentage of delaminated area A del were used in the coating
performance characterization. The time needed for R corr to drop below 10 7 ohm . cm 2 was used as
a degradation parameter of the coating barrier properties. Similar to R corr , the time needed for C dl
to show a marked increase was recorded and used to calculate the percentage of delaminated area
A del (equation 6.1.2). The larger the value of %A del is, the worse the adhesion between coating
and substrate.
To conclude the step-by-step analysis, sample AA7075-AHN needed only 1 hour to show an
R corr value below 10 7 ohm . cm 2 . Also, the noticeable change in C dl was after 432 hours with an
A del of 7.49. The time to failure given by ANN was 213 hours. All four coating performance
parameters (R corr , C dl , A del and ANN) were used to comment on the corrosion behavior of the
coatings (see discussion section).
5.6.1.3.3 Comparison of impedance based methods for assessing coating systems. A
comparison between the visual TTF and R corr , C dl , A del and ANN results allows an evaluation of
the effectiveness of each parameter to assess coating performance.
The total impedance response R corr for all samples tested, grouped by primer, can be seen in
Figures 6.6-6.8. R corr value among coatings showed variations from less than one order of
magnitude to almost 5 orders of magnitude in the 30-day exposure period. A compilation of the
time needed for R corr to drop below 10 7 ohm.cm 2 can be seen in Table 6.2.
Several key observations can be made. First, the Deft primer showed a constant decrease in the
R corr value except for TCP and Metallast conversion coatings. All samples showed an R corr value
lower than 10 7 ohm . cm 2 in less than 2 hours, except for TCP and Metallast that showed an
increase in the barrier properties of the coating after 48 hours exposure. For those samples
treated with Hentzen primer, TCP conversion coating showed a marked decrease from 10 10
ohm . cm 2 to 10 6 ohm . cm 2 after 30 days exposure. Alodine, Metallast, PreKote and Boegel
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