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Oxygen reduction on Cu in near-neutral 5% NaCl and its subsequent inhibition by Cr͑VI͒ and Ce͑III͒ compounds has been examined using stationary and rotating Cu electrodes. The results show that Cr͑VI͒ is a highly efficient inhibitor not matched by comparable concentrations of Ce͑III͒ in its effect. Results suggest ͑i͒ bulk reduction of Cr͑VI͒ to a three-dimensional inhibiting film is not necessary to explain the role of Cr͑VI͒; ͑ii͒ Cr͑VI͒ inhibition of oxygen reduction on Cu is irreversible as shown previously by others; and ͑iii͒ inhibition of oxygen reduction by Cr͑VI͒ at dispersed Cu cathodes in Al2024-T3 cannot explain the 100-fold increase in corrosion resistance typically observed for Al 2024-T3.

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Anodized and hexavalent chromium (chromate) "sealed" aluminum oxide films on aluminum alloys provide a corrosion-resistant surface for subsequent painting with a corrosion protective paint. This technology finds particular application for the protection of aircraft and aerospace vehicles. However, environmental considerations demand the elimination of chromate in industrial manufacturing and maintenance operations. To find suitable replacements for hexavalent chromium, a better understanding of the role played by chromate in inhibiting corrosion must be obtained.Many hypotheses have been proposed to explain the seemingly unique ability of hexavalent chromium compounds to inhibit corrosion in aluminum alloys. A detailed review of the literature on which these hypotheses are based is beyond the scope of this paper. Only a summary of the key hypotheses regarding the role of hexavalent chromium in corrosion inhibition of aluminum alloys follows.Considerable evidence shows that residual hexavalent chromium exists in sealed anodized films and conversion coatings on aluminum and its alloys. One hypothesis states that the residual chromate actively transports to defects in the coating and inhibits corrosion. The actively transported inhibitor slows the anodic and cathodic reactions that lead to corrosion and pitting. Another hypothesis states that the oxides of Cr(III) are particularly protective, inert and hydrophobic. A third hypothesis suggests that the oxyanions of hexavalent chromium (chromate or dichromate) possibly alter the physical properties of oxide films so as to discourage the adsorption of depassivating anions such as chloride. This report provides experimental evidence for this latter hypothesis.Previously reported work on hexavalent chromium-sealed anodized aluminum suggests that chromate does not actually "seal" porous anodized aluminum in the sense that it plugs pores, 1 rather, chromate seals with a reinforcement of the pore wall 2 as compared to the completely plugging hot-water sealing. One way that a small quantity of oxy-chromium(VI) anions can improve corrosion resistance without plugging the pores is by changing the surface charge of the oxide so as to discourage adsorption and transport of chloride. Such a mechanism has been shown to occur for ferrous oxide films 3,4 and has been proposed for aluminum oxide-covered surfaces. [5][6][7][8][9][10][11] According to the model of McCafferty, 6 the aluminum oxide surface exhibits the following reactions. Below the pH of zero charge (pzc pH ), protonation of the surface occurs according toand above the pzc pH two possible reactions may contribute to a fixed surface chargeSince the pzc pH for aluminum oxide is generally considered to be above 9, 8,12-14 oxide-covered aluminum surfaces in neutral-toacidic aqueous environments have a positive fixed charge. Hence, chloride ions readily adsorb electrostatically. However, a chemical transformation of the oxide film that lowers or reverses fixed positive surface charge discourages chloride adsorption and leads...

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