Final Report - Strategic Environmental Research and Development ...

components of the impedance and the phase angle at each frequency of the frequency domain.
For each variable, 14 independent measurements were acquired, each at a different exposure
time. Each measurement (p) of that input variable (x i ) was scaled to range between 0 and 1 and
the degree of membership φ i (x i ) for each measurement at each input was computed using a
Gaussian function:
ip
x
( xi
) exp
ip
x
b
i
2
(eq. 6.2.4)
where the constant b was given a value of 20% of the average of that particular input variable, p
ranged from 1 to Q where Q represent the total number of independent measurements and i is the
number of inputs [17].
To determine the extent to which an input variable affected the model output, graded or fuzzy
membership function describing each input variable was defuzzified using the centroid
defuzzification process [28]. In this method, a fuzzy parameter c i (x i ) was obtained by:
c
i
x
i
Q
( x )* y
ip
p1
Q
p1
ip
i
( x )
i
p
(eq. 6.2.5)
where y p is the output value of the p measurement. When the output y p is sensitive to a change
in the variable x i , the membership function φ ip (x i ) and output value, y p , will scale together
resulting in a large range for the product of φ ip (x i )*y p and a large value for the fuzzy parameter
c i (x i ). The range c i (x i ) over the range of the input variable x i is directly proportional to the
product φ ip (x i )*y p . Therefore as the range of the c i (x i ) increases, the significance of input variable
x i on the output y also increases. Variables with the largest c i (x i ) range this exert the greatest
influence on the value of the output. Inputs with small ranges have a small significance and are
candidates for pruning from the model.
5.6.2.3 Results
5.6.2.3.1 EIS results. Two sets of EIS spectra from a single sample type are shown in Figures
6.16 and 6.17 to illustrate the evolution in the EIS response. These spectra are for AA7075 with
Alodine 5200 conversion coating, a Hentzen 16708TEP primer, and a polyurethane topcoat
(sample AA7075-AHP), and AA7075 with PreKote pre-treament, a Sicopoxy 577-630 and a
polyurethane topcoat (sample AA7075-PSP) respectively. The AA7075-AHP sample showed no
signs of visual corrosion over the course of the exposure, while AA7075-PSP showed visual
failure after 196 h. Although both samples showed a decrease in the low frequency impedance
as a function of exposure from a value of 10 8 ohm . cm 2 to 10 4 ohm . cm 2 , the way in which this
happened is different for each sample. Sample AA7075-AHP showed a constant value of the
total impedance until 10 hours of exposure followed by a drop of two orders of magnitude. The
AA7075-PSP sample showed a more progressive decrease during the experiment. Another
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