Final Report - Strategic Environmental Research and Development ...

Figure 1.35. (A) Corrosion potentials (E corr ) and (B) polarization resistances (R p ) for TCP-coated
AA2024, 6061 and 7075 in air-saturated 0.5 M Na 2 SO 4 . E corr and R p values were also recorded
for the bare alloys (degreased and deoxidized) as controls. Each datum is an average value of no
less than 4 samples. Data shown are for the 90% confidence interval.
Figure 1.36A presents slow-scan potentiodynamic polarization curves for bare and TCP-coated
AA6061 in air-saturated 0.5 M Na 2 SO 4 . The presence of the coating causes a 500 mV negative
shift of E corr , consistent with the data in Fig. 1.36A. The coating results in significantly
attenuated cathodic currents around E corr (-0.9 V vs Ag/AgCl). Moreover, the anodic current was
also suppressed by 10x, for example, at +200 mV vs. E corr (-0.7 V for the coated sample and -0.3
V vs Ag/AgCl for the bare sample). Attenuated anodic and cathodic currents are also seen for the
TCP-coated AA7075 sample relative to the bare sample (Fig. 1.36B). The anodic current is
reduced by about 10x for the coated sample and the Cu-stripping peak at 0.2 V vs. Ag/AgCl is
suppressed by nearly 10×. The cathodic current for the coated AA7075 sample is about 100×
smaller than the current for the bare sample. The cathodic current in this potential region arises
from oxygen reduction and the inhibition by the TCP may result from the blockade of the Fe-rich
(AA6061 and 7075) and Cu-rich (AA7075) intermetallic sites. The reduced anodic current may
come from passivation of the Al matrix by the TCP coating in the vicinity of the intermetallic
compounds.
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