Final Report - Strategic Environmental Research and Development ...

Pit 1 and pit 2 as shown in Figure 7.8 were calculated according to Equation 7.3 as shown in
Figure 7.11c and d.
The analysis shows that the anodic current density itself decays with time. By fitting the anodic
current density of pit 1 as a function of time, the equation is shown as follows,
0.98
i a t
(eq. 7.4).
In the same way, the anodic current density of “pit 2” in Figure 7.8 is given by,
0.74
i a t
(eq. 7.5)
Figure 7.11 d shows the aged thin film single pit growth current density. The fitting curve is,
6
i .9 1.17
t
1 (eq. 7.6)
The current densities estimated from these expressions yield values that range from 2.6 down to
0.2mA/cm 2 . However, at the same time as shown in the blue line in Figure 7.11 c and d, the
current density of aged thin films is higher than that of as-deposited condition. This is attributed
to the inhibiting effect of Cu dissolved in the solid solution of the as-deposited thin film.
Nonetheless, multiple pits nucleated at the random sits in as-deposited thin films compared with
only single pit nucleated in aged thin films, suggesting that the θ phase particles did not have an
effect on the pit initiation.
5.7.3.3 Al-Cu thin films under Deft primers exposed to 0.5M NaCl. Both the as-deposited and
aged Al-Cu thin films were coated with a Deft primer (02GN084) and exposed to aerated 0.5M
NaCl solutions. The undercoating pit morphologies are shown in Figure 7.12. For the asdeposited
thin films, round and irregular pits were observed suggesting that the growth rates
were changing at different locations around the pit perimeter with time. However, in the aged
thin film, the corrosion propagated primarily and preferentially in one direction. Both for the asdeposited
and aged thin films, large numbers of discrete pits nucleated across the whole exposure
surface. This suggested that the availability of dissolved oxygen to the substrate was greater than
with neat epoxy primer. This is consistent with high pigment loading levels in the Deft primer. In
both cases, the pit growth ended with the pit extending under the mask placed on top of the
primer.
For the case of as-deposited thin films, multiple pits nucleated at random sites in both the neat
epoxy coated samples and Deft primer. At early times the pit morphology was smooth. At later
times it became irregular suggesting a decrease in pit growth rate. Both the round pits passivated
after a burst of growth leaving an irregular pit active, which grew steadily. However, results
show that in as-deposited thin films, the pit initiated after 5 days of immersion in NaCl solution.
This was delayed for 7 days for the neat epoxy coated sample immersed to the same solution.
This indicates that the Deft primer did not delay the initiation of the pitting corrosion.
353