Hence consideration of the relative role of anodic

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Hence, consideration of the relative role of anodic and cathodic inhibition by chromate must take into account the electrode processes taking place on the intermetallic phases. Arguments can be made that inhibition of ORR at Cu-rich cathodes alone cannot explain the inhibition of corrosion of Al 2024-T3 by chromate in near-neutral solutions, 44,52-53,63-64 and that it is necessary to account for other components of corrosion protection. Passivation of the Cu-rich intermetallic phases and other cathodically active sites by chromate con- version films appears likely, although details of the interaction are still being studied. Initiation of CCC formation on 99.99% Al by reductive deposition of Cr 3+ oxide at cathodic sub-grain boundaries has been shown by transmission electron microscopy (TEM) of microtomed sections. 65 Atomic force microscopy (AFM) of the nucleation of chromate films from di- lute, cold (5 ² C), commercial CCC on Al 2024-T3 shows preferential deposition of a film of undeter- mined composition on the cathodic Al-Fe-Cu-Mn par- ticle (Figure 8 and Reference 66). The use of a cold solution was necessary to catch the very earliest stages of nucleation. Even at this temperature, com- plete coverage of the surface occurred within 10 s of exposure. Shorter exposure times allowed for the evaluation of the partial coverage of the surface. Several groups have shown the Al-Cu-Fe-Mn phase (a) (b) FIGURE 6. Relative concentration of the oxo-Cr 6+ species as a function of pH in the pH 0 to 3 range for a total Cr 6+ concentration of 0.01 M. FIGURE 7. Current vs time for 600-grit polished Al 1100 polarized at –0.500 V SCE in 1 M chloride solution at: (a) pH ~1 and (b) pH ~0.
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING C ORROSION—Vol. 59, No. 5 387 to be relatively noble, 67-69 which would account for enhanced chromate reduction. Whether the mechanism of chromate inhibition of the Cu-rich intermetallic phases in 2XXX alloys is as simple as the rapid absorption-reduction passi- vation process observed on pure Al remains to be determined. Guillaumin, et al., 70 have shown that nucleation of chromate films on the various interme- tallics of Al 2024-T3 from dilute dichromate in 0.5 M NaCl depends on the initial conditions of the surface. A surface that is abraded with an AFM Si tip shows selective deposition of chromate on the Al-Cu-Fe-Mn phase, but an “as-polished” surface (1200-grit silicon carbide [SiC], diamond paste, final ethanol [C 2 H 5 OH] rinse) of the more active Al 2 CuMg phase showed a thicker coating deposit. In studies of CCC formation from ferricyanide-accelerated solutions, it has been observed that the relative intensity of the 860-cm –1 Raman band associated with a Cr 3+ /Cr 6+ complex unique to CCC virtually disappears over Cu-rich in- termetallic phases. 71 These authors hypothesize that Fe(CN) 6 –3 is preferentially adsorbed on Cu-rich inter- metallic particles, which passivates these regions with respect to the film-forming Cr 6+ to Cr 3+ reduction reaction. This results in a locally thinner coating.
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  • Fall '19
  • pH, Chromates, Chromate conversion coating

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