Final Report - Strategic Environmental Research and Development ...

Z” [ohm]
Z''
Z” [ohm]
Z''
-20000
-750000
-500000
-15000
-10000
-5000
7075_AHN_02.dta
7075_AHN_03.dta
7075_AHN_05.dta
7075_AHN_07.dta
7075_ahn_11(edited).dta
7075_AHN_12.DTA
7075_AHN_13
AA7075_AHN_14.dta
7075_AHN_02.dta
7075_AHN_03.dta
7075_AHN_05.dta
7075_AHN_07.dta
7075_ahn_11(edited).dta
7075_AHN_12.DTA
7075_AHN_13
AA7075_AHN_14.dta
-250000
0
0 5000 10000 15000 20000
Z’ [ohm]
Z'
0
0 250000 500000 750000
Z’ [ohm]
Z'
R pore
R pore
Figure 6.4. Nyquist plot for sample 7075-AHN after exposure to salt spray chamber for
30 days (720 hours). The intersection of the semi-circle with the x-axis can be used as a
graphic representation of the R pore value (EIS test area was 13.2 cm 2 ).
The regression of process model method (K-K transforms) was used in conjunction with the EC
shown in Figure 6.1 to determine a valid frequency range where EIS data could be modeled [27].
After the modeling was done, true capacitance values for C coat and C dl were extracted from the
constant phase element using the Brug method (equation 6.1.1) [24]. The evolution of the system
was then evaluated by plotting the passive elements C coat , C dl , and R corr as a function of exposure
time.
In Figure 6.5 both capacitance responses (C coat and C dl ) had a decrease at earlier exposure times
reaching a plateau at 10 -8 F after 2 h of exposure while the total impedance, R corr , remained at 10 6
ohm . cm 2 . After 288 h exposure, C dl increased more than 3 orders of magnitude together with a
decrease of 1 order of magnitude in R corr . The variation of C coat was less than one order of
magnitude throughout the duration of the experiment and is not discussed further.
266