The application of surface enhanced Raman spectroscopy to the study of carbon dioxide corrosion of carbon steel in brines in the presence and absence of organic inhibitors

The mechanisms of CO{dollar}sb2{dollar} corrosion and organic inhibition were studied using surface enhanced Raman spectroscopy (SERS). Gold particles were electrodeposited onto the surfaces of carbon steel electrodes and enhanced the Raman spectra of species in their vicinity. Hence, corrosion products or organic inhibitors on the surfaces of the steel electrodes provided measurable Raman signals. SERS was shown to provide insight into the mechanisms of CO{dollar}sb2{dollar} corrosion and organic inhibition in an extremely complex environment.; CO{dollar}sb2{dollar} corrosion of steel immersed in brine at 24{dollar}spcirc{dollar}C and 65{dollar}spcirc{dollar}C was compared to corrosion of steel in N{dollar}sb2{dollar}-saturated solutions of similar pH. In the presence of CO{dollar}sb2{dollar}, a surface carbonate species (peak at 1078 cm{dollar}sp{lcub}-1{rcub}{dollar}) was identified on the steel. This species was associated with either adsorbed carbonic acid or CO{dollar}sb3sp{lcub}2-{rcub}{dollar}, a reduction product. Iron oxide/hydroxide corrosion products (peak at 512-525 cm{dollar}sp{lcub}-1{rcub}{dollar}) formed in the CO{dollar}sb2{dollar}-saturated brine. In the absence of CO{dollar}sb2{dollar}, no surface carbonate species nor iron corrosion products were present.; Three organic inhibitors were examined: oleic imidazoline/oleic amide (OI/OA), phosphate ester (PE), and octadecyldimethylbenzylammonium chloride (ODBAC). By combining polarization measurements with SERS measurements, the mechanism of inhibition of each inhibitor was postulated. Inhibitor effectiveness decreased in the order: PE {dollar}>{dollar} OI/OA {dollar}>{dollar} ODBAC. Inhibitor effectiveness was directly correlated to the inhibitor's ability to eliminate the surface carbonate species. Effects on iron oxide/hydroxide corrosion products were secondary, as elimination of these species did not necessarily result in positive inhibition behavior.; PE eliminated the surface carbonate species from the steel, and slightly diminished the formation of iron oxide/hydroxide corrosion products. PE adsorption increased as the interfacial potential was made more positive. PE reduced the ability of chloride ions to adsorb to the steel or oxide/hydroxide layer, and reduced the appearance of pits. OI/OA diminished the concentration of surface carbonate species on the steel less effectively than did PE. OI/OA adsorbed most strongly at the corrosion potential, and less so above and below this potential. OI/OA eliminated iron oxide/hydroxide corrosion products but promoted crevice corrosion. ODBAC, the least effective inhibitor, promoted the adsorption of the surface carbonate species and eliminated iron oxide/hydroxide corrosion products from the steel surface.