Figure 3 also suggest that hydrogen and water reduc

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Figure 3 also suggest that hydrogen and water reduc- tion are inhibited at the lowest measured potentials. It is usually the case that the cathodic polarization
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CRITICAL REVIEW OF CORROSION SCIENCE AND ENGINEERING 384 C ORROSION—MAY 2003 response of Al alloys is affected by cathodic corro- sion. In Cu-bearing alloys, this leads to Cu surface enrichment and an increase in cathodic kinetics. However, the data presented in Figure 3 suggest that dichromate is potent enough to inhibit cathodic ki- netics and to suppress cathodic corrosion and atten- dant Cu surface enrichment, even at very large cathodic overpotentials. In high-strength Al alloys, Cu-rich intermetallic particles support the ORR in aerated solutions. Re- duction of Cr 6+ to Cr 3+ and subsequent adsorption at these sites inhibits the ORR. In neutral sulfate solu- tions, a Cu rotating disk electrode (RDE) shows a ro- tation rate-dependent cathodic current. 44,51-52 In the presence of chromate, the rotation rate dependence disappears as a result of the inhibition of the ORR by chromate. Furthermore, the inhibition is nearly irre- versible. Upon removal of chromate from solution, the ORR remains inhibited. This supports the idea of a strongly and irreversibly adsorbed species from the chromate-containing electrolyte that subsequently inhibits the ORR. This result has also been reproduced in neutral NaCl solutions, a detailed analysis of which shows that hexavalent chromium inhibits both the ORR and further Cr 6+ reduction. 48,53 The current-voltage behavior for a rotating Cu electrode (2,000 rpm) in Figure 4 illustrates this point for a 5% NaCl solution at pH 6. 53 With the addition of chromate, the current in the –0.60-V SCE to –0.85-V SCE region decreases as a result of the inhibition of oxygen reduction. However, at potentials < –0.85 V SCE , the chromate contributes to a current plateau as its reduction proceeds under diffusion control. The extremely large overvoltage re- quired for the diffusion-limited Cr 6+ reduction attests to the self-inhibition of Cr 6+ reduction. A more detailed study 48 shows that the inhibition of the ORR involves the adsorption and rapid reduc- tion of Cr 6+ oxoanions to form an inhibiting Cr 3+ hy- droxide, which is probably bound strongly to the surface. The Cr 3+ hydroxide surface may also contain co-adsorbed Cr 6+ oxoanions. It has been suggested that such a surface film would form a bipolar barrier that would inhibit electron transfer in a manner not explained by a monolayer barrier alone. 48 The reduc- tion of Cr 6+ may also have significance to the corro- sion process since it appears to be coupled to an induced anion adsorption. 54 (a) (b) FIGURE 3. Cathodic polarization curves on Al 2024-T3 in actively oxygen-sparged 1 M NaCl solution with: (a) dichromate additions and (b) 0.3 vol% peroxide and dichromate additions. Reprinted with permission from The Electrochemical Society.
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