Final Report - Strategic Environmental Research and Development ...

Figure 1.39. Pit density vs. time profiles recorded during the full immersion of TCP-coated and
bare AA2024, 6061 and 7075 in air-saturated 0.5M NaCl at room temperature.
Figure 1.40. Raman spectra recorded at (a) an Fe-rich intermetallic site and (b) the terrace near
but outside the pit on the TCP-coated AA6061, (c) a Cu-rich intermetallic site and (d) the terrace
outside the pit on the coated AA7075, after full immersion in air-saturated 0.5M NaCl for 15
days. Spectra for (e) the Cr(III)/Cr(VI) mixed oxide and (f) the Al(III)/Cr(VI) mixed oxide (pH
7.40) are also shown for identification
Figure 1.41 presents the 860 cm -1 peak areas as a function of immersion time. Each datum
represents the average of spectral recordings from 5 different locations on each sample. The
Cr(VI)-O peak area increases rapidly within the first 5 days of immersion before reaching a
relatively constant level for the remainder period. The coated AA7075 sample exhibits larger
Cr(VI)-O peak areas than AA6061 during the immersion period. The stability of the peak area
after the first five days indicates chemical stability of the transiently formed Cr(VI) species on
the surface.
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