Density functional theory and molecular dynamics simulation of the corrosive particle diffusion in pyrimidine and its derivatives films

The search and discovery of corrosion inhibitors to mitigate sweet corrosion in the oil and gas industry has been carried out mainly using tedious and costly experimental approaches. In this study, theoretical approach using density functional theory (DFT) and molecular dynamics (MD) simulation were utilized to investigate the adsorption and diffusion properties of pyrimidine and six (6) of its derivatives on carbon steel corrosion in simulated CO2-containing environment. The DFT calculations revealed that the electrons can easily flow from the HOMO orbitals of all the pyrimidine molecules to the empty substrate orbitals until reaching the equilibrium state as the values of EHOMO (-7.63 to −5.55 eV) are much higher than the Fermi level energy of Fe (-13.16 eV). Besides, the binding energy of molecules on the Fe (1 1 0) surface obtained using MD simulation was found to be in the range of 60.67 to 99.28 kcal/mol, indicating the pyrimidine molecules can strongly interact with the iron atoms of the metal surface. In the sweet medium, the diffusion coefficient (D) of HCO3– was in the range of 0.81 to 1.41 m2 s−1, which is about three times less than the diffusion coefficient of HCO3– in water. Self-diffusion coefficient (D'), and fractional free volume (FFV) were used to further investigate the diffusion mechanism of the pyrimidine molecules in CO2 corrosive medium. Pyrimidine containing carboxylic acid and thioamide groups gave the best result as CO2 corrosion inhibitor theoretically. The DFT and MD results provided useful insights and a theoretical basis for the screening and design of new sweet corrosion inhibitors for carbon steel.