Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces

Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute 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 at the near monolayer level, and the spectra were compared to those from oxyhydroxide species and mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to solution, the characteristic Cu oxide Raman bands disappeared, and a band increased in intensity over a period of ∼20 h. The intensity of the band on Cu became self-limiting after the formation of several monolayers, as supported by chronoamperometry experiments. This spectrum was stable after was removed from the solution provided the potential remained negative of −200 mV vs. Ag/AgCl. The results support the conclusion that is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of formation is the substitution lability of compared to The bond can be broken much more rapidly than the substitution inert bond, making formation of mixed oxide kinetically favorable. Once reduced to however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile form to an insoluble, substitution inert oxyhydroxide. Furthermore, reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable film with a thickness of a few monolayers.