57 it has also been found that chro mate additions to

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57 It has also been found that chro- mate additions to neutral chloride solutions will ac- celerate the kinetics of repassivation and suppress reactivation of aluminum. 58 Kendig, et al., has argued that in experiments conducted under potential control, extremely acidic conditions develop on the working electrode surface because a large component of the cathodic reaction is displaced to the counter electrode. 5 Chromate be- comes increasingly protonated with decreasing pH, and as suggested by PEK measurements, less likely to adsorb on oxide-covered surfaces to promote cor- rosion resistance. For a total dissolved concentration in the millimolar range, Cr 6+ is speciated almost en- tirely as the neutral, potentially noninhibiting H 2 CrO 4 species as the pH approaches 0 (Figure 6). Figures 7(a) and (b) show current vs time behavior of Al 1100 during potentiostatic polarization at –0.500 V SCE in 1.0 M chloride solution at pH 0 and 1. At pH 0, the steady-state dissolution current is ~45 A/cm 2 . The dissolution rate is not lowered by addition of 0.01 M chromium trioxide (CrO 3 ), indicating no inhibition. At pH 1, addition of 0.01 M CrO 3 lowers the dissolu- tion rate from ~0.17 A/cm 2 to <0.05 A/cm 2 , indicat- ing an improvement in the inhibiting efficiency. These results support the notion that the ability of Cr 6+ to inhibit is associated with how it is speciated in solution, and that H 2 CrO 3 does not inhibit corro- sion because it does not adsorb on metal or oxide- covered surfaces. Inhibition by Chromate Adsorption The precise nature of anodic inhibition by chromates is not clear. As suggested by PEK mea- surements, it may involve nonreductive adsorption rather that adsorption and reduction that seems to underlie ORR inhibition. Evidence that Cr 6+ FIGURE 5. Dependence of the pitting potential of Al on the chloride: chromate ratio. 50
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 386 C ORROSION—MAY 2003 oxoanions on their own impart corrosion inhibition to oxide-covered surfaces without reduction to the Cr 3+ oxide was recognized in 1966 by Cartledge. 59 More recently, absorption of Cr 6+ oxoanions on the corrosion product in growing pits 10 and adsorption of Cr 6+ oxoanions in the porous layer of anodized alu- minum 12 have been suggested to lead to inhibition without reduction of Cr 6+ . Adsorption on anodized aluminum lowers the normally positive zeta potential of the oxide within the porous layer. Using a model similar to that described by Sato 60 for iron oxide films, the adsorption of the negatively charged chro- mate will hinder adsorption of chloride. McCafferty, Natishan, and coworkers 61-62 make similar arguments with regard to the role of chromate and other inhibit- ing oxoanions for increasing the pitting potential on aluminum alloys. Inhibition at Intermetallic Particles The interaction of the chromate inhibitor with intermetallic particles in Al alloys is essential for un- derstanding the mechanism of corrosion inhibition.
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  • pH, Chromates, Chromate conversion coating

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