Final Report - Strategic Environmental Research and Development ...

5.1.4.8 Effects of Aging on the Film Properties. Chromate conversion coatings (CCCs) can be
viewed as having a biphasic structure of an interpenetrating solid Cr(III) matrix and aqueous
channels [4-8]. The aqueous channels are likely the pathways for Cr(VI) transport [4-8]. We
hypothesized that like the CCCs, the TCP coating may also consist of imperfections and
hydrated channels, and that aging or dehydration might improve the barrier properties by
condensing the film and collapsing these channels. This hypothesis was tested by aging TCPcoated
samples in air from 24-96 h (1-4 days). Daily electrochemical testing was performed to
assess the passivation properties. For some, but not all samples, aging produced a significant
improvement in the coating’s barrier properties. Figure 1.15A and B show plots of the total
impedance and phase shift as a function of the frequency for TCP-coated AA2024 samples. This
particular sample was aged in air for 3 days at room temperature, while the other was aged only
overnight. Impedance analysis was performed at the OCP in 0.5 M Na 2 SO 4 (air saturated). It can
be seen that the low frequency impedance, R p , increased to 10 9 ohm-cm 2 for the 3-day aged
sample (Fig. 1.15A). This is three orders of magnitude greater than the low frequency impedance
of 10 5 -10 6 ohm-cm 2 seen for typical samples. The phase shift-frequency plot shows the
capacitive-like behavior of the 3-day aged coating, especially at high frequencies (Fig. 1.15B).
Clearly, at least in this case, aging can have a beneficial effect by producing significantly
improved barrier properties. At present, only some 30%, but not all, of the TCP-coated samples
tested have exhibited improved barrier properties after 1-4 day aging. We are presently carrying
out a systematic study of aging effects and will report on this in a future publication.
A.
10 10
B.
100
Aged for 3 days in air
Log Impedance (ohm-cm 2 )
10 9
TCP-AA2024 aged for 3 days in air
10 8
10 7
10 6
10 5
10 4
TCP-AA2024 after typical 1 h dry
10 3
10 2
10 1
Phase Shift (-)
80
60
Dried overnight
40
20
0
10 0
10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6
Log Frequency (Hz)
10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6
Log Frequency (Hz)
Figure 1.15. (A) Total impedance-frequency and (B) phase shift-frequency plots for a TCPcoated
AA2024 sample aged for 3 days in air. The electrochemical measurements were made at
the OCP in 0.5 M Na 2 SO 4 (air saturated).
5.1.4.9 Artificial Scratch Tests. Artificial scratch cells were opened after various exposure
times (7, 14 and 21 days) and examined for evidence of corrosion. Figure 1.16 shows the two
sides of a cell exposed to 0.5 M NaCl for 21 days. The TCP-treated AA2024-T3 sheet still
exhibited the pale tan color characteristic of TCP, Figure 1.16a. The non-TCP side of the cell,
Figure 1.16b, exhibited attack, but considerably less than the two sides of the control cell
comprising two non-TCP samples, Figures 1.16c and 1.16d. The crevice corrosion evident on
both sides of the control cell is absent on both sides of the TCP/non-TCP cell.
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