Many studies have been conducted on the action of corrosion inhibitors on corrosion product layers. There are varying opinions: the inhibitor may preferentially interact with the metallic surface or the existence of a corrosion scale enhances inhibitor adsorption. This paper will address the effect of the type of corrosion inhibitor active on the growth of iron carbonate. Our paper1 presented at NACExpo 2008 explored the synergistic interaction a quaternized amine compound has with iron carbonate. This work is a continuation and examines how imidazoline and phosphate ester compounds interact with iron carbonate. Tests were performed using the Rotating Cylinder Electrode (RCE) technique in conjunction with LPR (Linear Polarization Resistance), Tafel studies and EIS (Electrochemical Impedance Spectroscopy). Test samples composed of 1018 carbon steel were exposed to a pH 6.5 CO2-purged brine environment for the duration of the test. SEM/EDS (Scanning Electron Microscopy/Electron Dispersive Spectroscopy) were used in the analyses of the corrosion samples in order to observe the morphology, thickness and crystal structure of the iron scale layer. This work will provide insight on how various types of corrosion inhibitor actives affect the growth of iron carbonate.
Corrosion inhibitors for the petroleum industry have been researched extensively for years; however; any key technological advances in their chemistries are going to require a fundamental understanding on the interactions between the corroding surface and the inhibitor. The composition of a commercial corrosion inhibitor usually consists of a blend of an actively adsorbing chemical (i.e., amines, imidazolines) in a solvent along with a series of supporting chemicals (i.e., emulsion breakers, salting agents, etc.) that aid in the dispersion of the active when it is applied in the field. By examining the behaviour of just the actives as a starting point, the effect of each additional component can be understood and the development process of new formulations may be simplified. Other groups2-4 have carried out extensive research on the interactions of imidazoline based compounds. They concluded imidazoline inhibitors prevents the growth of iron carbonate, which could be due to the decrease of Fe2+ at the surface or imidazoline compounds act like scale inhibitors. If an existing iron carbonate scale was present, any further growth was prevented. Work carried out by Gusmano et al5 used Electrochemical Impedance Spectroscopy (EIS) to examine the behaviour of imidazoline compounds on the corrosion of mild steel. They observed the formation of an additional peak in the phase angle plots after the steel was exposed to the test fluids for 22 hours, indicating two kinetic processes were occurring at the surface. The addition of thioglycolic acid increased the complexity of the phase angle plots, suggesting a more complicated adsorption layer. Olivares et al6 also observed the formation of two peaks in the phase plots and attributed the low frequency peak to the solution interface while the higher frequency peak is due to the film formation of oleic imidazoline inhibitor6. Phosphate ester compounds are another commonly used active component in corrosion inhibitors due to their adsorption characteristics.