Abstract
Background: Corrosion in industrial structures has generated much concern with regard to material loss, especially in the oil production. Organicmolecules can act as inhibitors because they can be adsorbed at the metal-solution interface, replacing the water molecules and thereby inhibiting the metal dissolution. In general the imidazoline derivatives have unique structure that make them efficient corrosion inhibitors in CO2 environments. Castor oil is the only unsaturated fatty acid occurring in natural vegetable oils with a functional hydroxyl group in the 12C. The presence of OH groups in the fatty acid chains makes the oil unusually polar. Therefore, based on its molecular structure, castor oil or its derivatives can be used as an effective corrosion inhibitor.
Methods: Inhibitor used was a hydroxyethyl-imidazoline derivate based on castor oil. Concentrations of inhibitor used were 5, 10, 25, 50 and 100 ppm. Corrosivesolutionusedwasa CO2 saturated mixture (90:10, of 3% NaCl solution and diesel) at 50°C. Corrosion tests were carried out by real-time monitoring and EIS measurements.
Results: Real-time monitoring showed that the castor oil-based imidazoline has an inhibition efficiency greater than 99%. From EIS measurementstwodifferentbehaviors were obtained. Evolution of the EIS spectra was similar for additions of 5, 10 and 25 ppm, and for 50 and 100 ppm it was different. For 5, 10 and 25 ppm the evolution of the spectrum in the high frequency region was the characteristic fingerprint of the self-assembled of the oil-based imidazolines. However, for 50 and 100 ppm after 9 hours, the time constant decreased as time passed. This behavior could be due to the presence of the functional groups in the oily-tail which tend to interact with the metallic surface forming a denser inhibitor film. The molecular reactivity (HOMO and LUMO) of the optimized molecule of inhibitor showed that LUMO is moved to unusual location the middle of the molecule due to the presence of double bond in 39C-41C and the hydroxyl group in 45C.
Conclusion: Real-time corrosion measurements showed that the inhibition efficiency was greater than 99%. From EIS measurements two different behaviors are observed. Presence of the functional groups in the alkyl chain tend to interact with the metal surface. Quantum chemical calculations showed that LUMO is located in the alkyl chain, favoring a flat-adsorption process.
Keywords: Castor oil, green inhibitor, imidazoline, electrochemical sweet corrosion.
Graphical Abstract
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