Final Report - Strategic Environmental Research and Development ...

additional pitting. The TCP coating protects the samples from corrosion. The R p values are
inversely proportional to the pit density, as expected.
Figure 1.38. (A) Polarization resistance (R p ) and (B) pit density vs. time profiles recorded during
the full immersion of TCP-coated AA2024, 6061 and 7075 in air-saturated 0.5M Na 2 SO 4 at room
temperature.
The TCP-coated and bare alloys were also fully immersed in the more aggressive air-saturated
0.5 M NaCl electrolyte solution for 14 days. The pit density as a function of immersion time is
shown in Figure 1.39. The pit density, particularly for the uncoated AA2024 and 6061
progressively increase with immersion time. In contrast, all the coated samples exhibit relatively
constant pit density throughout the immersion period. No significant pitting or cracking were
observed on the coated samples.
5.1.4.20 Transient formation of Cr(VI) during immersion. Previously, we demonstrated that
Cr(III) can transiently form on TCP-coated AA2024 during immersion. The same holds true for
TCP-coated AA6061 and 7075. Three samples of each alloy were immersed in the air-saturated
0.5M NaCl for 30 days. Spectra were collected at five spots on each sample. Figure 1.40 presents
typical spectra acquired at the intermetallic sites and on the terrace outside pits of coated
AA6061 and 7075 after a 15-day immersion. The spectra acquired near an intermetallic and on
the terrace away from any intermetallic on AA6061 exhibits a symmetric 860 cm -1 peak (Fig.
1.40-a, b), which indicates the presence of the Cr(III)/Cr(VI) mixed oxide (Fig. 1.40-e). The
same applies for the spectrum acquired at a terrace site on AA7075 (Fig. 1.40-d). In contrast, a
spectrum acquired at the intermetallic site on AA7075 contains an asymmetric peak centered at
850 cm -1 (Fig. 1.40-c). The 437 and 678 cm -1 peaks that are attributed to Cu-O bonds associated
with the Cu-rich intermetallic compound. The variation of the Cr(VI)-O peak position is due to
the interaction of the Cr(VI)-O species with Al oxides. 1 The 850 cm -1 Cr(VI) peak in Fig. 1.40-c
is identified as the Al(III)/Cr(VI) mixed oxide at pH 7.40 (Fig. 1.40-f).
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