Final Report - Strategic Environmental Research and Development ...

Figure 6.17. EIS spectra for sample AA7075 treated with Alodine 5700 conversion coating,
Hentzen 16708TEP primer and a polyurethane top coat (7075-AHP) after exposure to salt spray
chamber for 30 days (arrows point in the direction of increasing exposure time). Sample showed
no visual signs of corrosion (TTF>720 h) (EIS test area was 13.2 cm 2 ).
These changes in the patterns of the Bode magnitude and phase angle plot over the course of the
exposure experiment clearly differentiated these two samples. Through proper training, an ANN
can be taught to recognize these patterns and their changes and associate them to corresponding
results from an exposure experiment. When these patterns are sufficiently learned, the ANN can
be supplied with EIS data and then estimate an outcome in exposure testing based on its previous
learning experience.
5.6.2.3.2 Time to failure. The time needed for any signs of visual corrosion (TTF) to appear
after salt spray exposure was collected for all 60 coatings and is shown Table 6.5. Of the coating
systems tested, 32 did not fail after 720 hours resulting in a poorly distributed data set, known as
a right-censored data set with a surviving fraction of 0.553. The use of a neural network as a
non-linear pattern recognition tool requires that the data to be used is well distributed so as to
model the non-linearity. In our case, the data as collected by the TTF needed to be corrected.
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