Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces
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Publisher:The Electrochemical Society
Citation:Belinda L. Hurley and Richard L. McCreery, "Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces," Journal of the Electrochemical Society 150, no. 8 (2003), doi:10.1149/1.1586923
Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute Cr^VI solutions with silver and copper surfaces in situ. Using silver as a model surface which supports strong SERS with a 514.5 nm laser, it was possible to observe Cr^III at the near monolayer level, and the spectra were compared to those from Cr^III oxyhydroxide species and Cr^III/Cr^VI mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to Cr^VI solution, the characteristic Cu oxide Raman bands disappeared, and a Cr^III band increased in intensity over a period of ~20 h. The intensity of the Cr^III band on Cu became self-limiting after the formation of several Cr^III monolayers, as supported by chronoamperometry experiments. This Cr^III spectrum was stable after Cr^VI was removed from the solution provided the potential remained negative of –200 mV vs. Ag/AgCl. The results support the conclusion that Cr^VI is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The Cr^III film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of Cr^III formation is the substitution lability of Cr^VI compared to Cr^III. The Cr^VI-O bond can be broken much more rapidly than the substitution inert Cr^III-O bond, making formation of Cr^III/Cr^VI mixed oxide kinetically favorable. Once reduced to Cr^III, however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of Cr^VI as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile Cr^VI form to an insoluble, substitution inert Cr^III oxyhydroxide. Furthermore, Cr^VI reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable Cr^III film with a thickness of a few monolayers.
Rights:© The Electrochemical Society, Inc. 2003. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of The Electrochemical Society, 150 (8) B367-B373.
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