Unraveling the Structure-Property Relationship of Pyrimidine-Derivatives as CO2 Corrosion Inhibitors: Experimental and Periodic DFT Investigations

CO2-containing environments commonly cause corrosion of carbon steel (CS) pipelines used in the oil and gas industry. In this work, the inhibition of corrosion by three derivatives of pyrimidine, namely 2-mercaptopyrimidine (MPY), 4-hydroxy-2-mercapto-6-methylpyrimidine (HMMP), and 4,5-Diamino-2,6-dimercaptopyrimidine (DDMP) for C1018 CS in CO2 saturated 3.5% NaCl brine was investigated. The corrosion rates and inhibition efficiency were investigated at 25 and 70 oC using open circuit potential, linear polarization resistance, potentiodynamic polarization and electrochemical impedance measurements. The post-immersion surface characterization of the exposed C1018 CS with and without the three molecules was carried out using SEM and 3D surface optical profilometer. Theoretical computation using DFT was further utilized to examine the role of the molecular structures and the mechanism of inhibition of the three molecules on steel surface at the atomic level. The experimental results indicate the following order of inhibition efficiency: MPY > DDMP > HMMP. Our experimental work proved that MPY molecule (the smaller one with the least number of heteroatoms) as the most efficient corrosion inhibitor. Surface characterization analysis confirmed the order of inhibition efficiency. The theoretical results revealed that the stability of molecules adsorbed on Fe (110) surface in the parallel orientation and on FeCO3 (104) is identical on both surfaces, which is completely consistent with the experiment. Our study provides a feasible strategy for developing new corrosion inhibitors by understanding how similar organic inhibitors work.