Imidazole-iron interaction in corrosive aqueous media.

摘要

Since Fe is known to form a "native" surface oxide when exposed to atmosphere, it was imperative to characterize this thin film. The quantitative high resolution XPS analysis involved an angle resolved study of the surface to determine the chemical composition and thickness of this native film. The film was found to be a mixture of Fe3O4 and Fe(OH) 2 with a thickness of 1.2+/-0.3 nm. This conclusion is consistent with thermodynamics as indicated by the Pourbaix diagram for the Fe-H 2O system and the phase diagram for the Fe-oxygen system. A detailed TEM study of the native surface film also supports this conclusion.;After establishing the corrosion inhibition mechanism in the neutral aqueous medium at room temperature, studies were conducted in CO2 saturated brine at 25°C, 65°C, 95°C and 130°C. The reciprocal of Rp, which is a measure of corrosion rate, was plotted at different temperatures. For the baseline case, an initial maximum occurred as the temperature was raised from 25°C to 65°C. At 65°C and above, a protective FeCO3 layer was formed which resulted in a decrease in corrosion rate up to 100°C. From 100°C to 130°C the corrosion rate again increases because of the disintegration of this FeCO 3 layer. Upon imidazole addition, the corrosion rate was reduced at all temperatures with a maximum inhibition efficiency of 86.8% at 65°C.;The corrosion behavior of Fe was studied in neutral aqueous medium saturated with Ar. Adding imidazole directly into the aqueous corrosive medium was observed to provide greater protection against corrosion, in comparison to the direct deposition of the inhibitor onto the metal surface prior to exposure to the corrosive solution. XPS investigations suggested that in the former case, the imidazole reacts through the "pyrrole type" nitrogen with the aromatic ring nearly parallel to the metal surface. In the latter situation "pyridine type" nitrogen-iron interaction is also present, with some imidazole molecules oriented normal to the surface. In addition when the initial "native" surface oxide was reduced by an application of cathodic voltage of -1.5 Volts, imidazole was found to bind better and provide greater corrosion inhibition.