Final Report - Strategic Environmental Research and Development ...

In other words, no Cr(VI)-O peak was expected in the Raman spectrum for the sample aged in
vacuo.
Multiple spectra were acquired across the TCP-coated surfaces and two representative curves are
shown in Figure 1.23. The Cr(VI)-O peak, centered at 853 cm -1 was often detected in and around
the pits on the sample aged in air while no such peak was found in the coating aged in vacuo. For
the latter, only the Cr(III)-O peak at 538 cm -1 was observed. The 438 and 680 cm -1 peaks are
associated with Cu-O bonds of the Cu-rich intermetallic compound. No evidence of any Cr(VI)-
O species was found when probing the coated sample aged in vacuum. This strongly implies the
involvement of dissolved O 2 in the transient formation of Cr(VI), possibly through the
production of H 2 O 2 . Recall that the scattering cross section for Cr(VI)-O is considerably larger
than the value for Cr(III)-O so the technique is sensitive to the presence of the former.
Figure 1.23. Raman spectra for the TCP-coated samples aged (a) in air and (b) in vacuum at
room temperature overnight. Spectra were acquired within the pitted areas.
To further confirm the importance of dissolved O 2 in the conversion of Cr(III)-O to Cr(VI)-O,
we coated another two samples and immersed them for 3 days in 0.5 M NaCl, with and without
dissolved O 2 . After immersion and prior to Raman analysis, the samples were rinsed for 10 s in
ultrapure water. The sample immersed in the air-saturated solution was then dried in the
laboratory air (room temperature) for 30 min. The sample immersed in the O 2 -free solution was
dried for the same period of time in vacuo at room temperature. Figure 1.24a shows that the 860
cm -1 Cr(VI)-O band forms during immersion in the air-saturated solution but does not in the
deaerated solution. Deaeration was accomplished under vacuum. For the sample immersed in the
air-saturated electrolyte, both the Cr(VI)-O peak and the Cr(III)-O peak (520 cm -1 ) are present,
consistent with the formation of a mixed oxide. In contrast, the spectrum for the sample
immersed in the deaerated electrolyte shows only the Cr(III)-O peak at 520 cm -1 . These results
further confirm the necessary role of dissolved oxygen in the transient formation of Cr(VI) in the
TCP coating.
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