The development of an inhibitor film is essential for the effective performance of a
corrosion inhibitor. The use of attenuated total reflection fourier transform infrared
spectroscopy (ATR-FTIR) allows the development of inhibitor films on iron oxides to be
monitored. For two distinct corrosion inhibitor chemistries, oleic imidazoline and
phosphate ester, the film formation and corrosion processes are monitored on Fe304 in a
powdered form (a model surface). Additional data following on the physical and chemical
properties are obtained using XPS and SEM techniques, which allows for a more
complete characterization of the model inhibitor/oxide system. By the proper choice of
system and measurement techniques, the complex phenomenon of corrosion inhibition
may be analyzed directly.
Corrosion in the oilfield is a dynamic and complex chemical process occurring over a wide range of conditions. A major component of a corrosion management program is the use of corrosion inhibitors for mild steel pipes in a C02 environment. In using a corrosion inhibitor, a chemical or chemical mixture is injected into the system at a low concentration (typically parts per million). Through the preferential formation of an inhibitor film on the surface, the chemical additives prevent the corrosion of the underlying mild steel.
The properties of both the inhibitor film and that of the underlying metal are of great interest in understanding the mechanisms for corrosion inhibition. A number of previous studies have examined the corrosion products from uninhibited mild steel in an aqueous COZ environment. 1-3One frequently encountered corrosion product is Fe301, magnetite, along with hematite and carbonates. While the exact ratio and chemical composition of the corrosion product layer is variable depending upon the exact system conditions, the major components of the surface layer are the same though in different ratios. The typical corrosion product layer in an uninhibited system is on the order of microns thick and is porous.4?5 In the presence of corrosion inhibitor, such as the frequently encountered oleic imidazolines and phosphate esters3?6?7,the inhibitor film is on a molecular level while the corrosion layer is on a microscopic order. Due to the differences in length scale, the inhibitor sees the corrosion product layer as a surface. New modem surface analysis techniques are available to examine the relatively thin inhibitor film. By using a powder as the substrate which models the corrosion product layer, techniques such as XPS, SHG, and SIMS may be used to analyze the system.g-]o The exact type of substrate depends upon the corrosive environment to which the sample is to be exposed.
The following work describes the choice and analysis of a system consisting of Fe304 as the substrate powder and oleic imidazoline or a phosphate ester as the corrosion inhibitor. This system is chosen to typify an oilfield application of a corrosion inhibitor. The system is analyzed using a combination of techniques which are suitable for the system including ATR-FTIR, SEM, Auger, and XPS. This combination of techniques allows for a determination of both the physical and chemical nature of the chemical inhibitor performance in a corrosive environment.