Final Report - Strategic Environmental Research and Development ...

Potential (V vs. Ag/AgCl)
and uncoated AA2024 surfaces in this medium. The OCP values, which tend to quickly stabilize
after solution exposure, are generally more noble for the TCP-coated samples by only 50-100
mV. The OCP for the Alodine TM 5200-coated samples was also slightly more noble than that for
the uncoated alloy. The OCP measurement was validated using ASTM G69-97. Measurements
of polished and uncoated AA2024 yielded values of -612 mV (± 4) and -619 mV (± 5) at the
95% confidence interval. These values are within the expected range of -600 to -620 mV vs
Ag/AgCl.
Figure 1.12b shows that R p for the TCP-coated samples is generally a factor of 8-10× greater
than the uncoated samples. The nominal values were 5.42 × 10 4 ohm-cm 2 (± 0.31 × 10 4 ) for the
uncoated samples and 4.03 × 10 5 ohm-cm 2 (± 1.15 × 10 5 ) for the coated samples (n≥3). These
values are in good agreement with those obtained from EIS data. While the TCP coating
provides some level of protection to AA2024, the 10 5 -10 6 ohm-cm 2 R p indicates the coating is
not providing full barrier protection. Such barrier coatings would be expected to yield R p values
on the order of 10 9 ohm-cm 2 . The polarization resistance of the 5200-coated samples was
nominally 1.77 × 10 5 ohm-cm 2 (± 0.65 × 10 5 ) (n=3). This value is 3-4× higher than the nominal
value for the uncoated alloys but 3-4× lower than the nominal value for the TCP-coated samples.
Therefore, assuming the general structure of the coatings is similar (i.e., hydrated ZrO 2 ), these
results suggest that the Cr(III) in the TCP coating contributes to the passivation of AA2024.
-0.20
-0.25
-0.30
-0.35
-0.40
-0.45
-0.50
-0.55
A.
Uncoated
TCP-coated
-0.60
-200 0 200 400 600 800 100012001400160018002000
Time (seconds)
Polarization Resistance (ohm-cm 2 )
4.5x10 5
4.0x10 5
3.5x10 5
3.0x10 5
2.5x10 5
2.0x10 5
1.5x10 5
1.0x10 5
5.0x10 4
0.0
B.
Uncoated
TCP
5200
Figure 1.12. (A) Plots of the open circuit potential (OCP) versus time for bare, Alodine 5200-
coated (Cr free) and TCP-coated AA 2024 samples in air-saturated 0.5 M Na 2 SO 4 . (B) Graphs of
the polarization resistance, obtained from slow scan potentiodynamic polarization scans ±30 mV
vs. OCP, for bare, Alodine TM 5200-coated and TCP-coated AA2024 samples in the same
medium.
Figure 1.13 shows typical slow-scan potentiodynamic polarization curves for bare, Alodine TM
5200-coated and TCP-coated AA2024 samples. The measurements were made in air-saturated
0.5 M Na 2 SO 4 . Similar to the data in Figure 1.12, the presence of either coating does not cause a
significant shift in the OCP value. Both coatings do, however, produce attenuated anodic and
cathodic currents around the OCP. For example, the anodic current from the OCP to about 0.4 V
vs Ag/AgCl is about a factor of 10× lower for the TCP-coated sample. Attenuated current is also
seen during the cathodic sweep from the OCP to about -0.8 V vs Ag/AgCl. The cathodic current
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