Comparison of the synergistic inhibition mechanism of two eco-friendly amino acids combined corrosion inhibitors for carbon steel pipelines in oil and gas production

• Amino acids (L-histidine/L-cysteine) and thiourea were tested as compounded corrosion inhibitors for carbon steel. • Electrochemical tests indicate that L-histidine/L-cysteine and thiourea exhibit prominently synergetic inhibition effect. • The adsorption of L-histidine/L-cysteine and thiourea on carbon steel surface is significantly strengthened after mixing. • The co-adsorbed inhibitors film can effectively prevent the diffusion of corrosive particles to carbon steel surface. • More and shorter bonds are formed between inhibitors and carbon steel surface with the co-adsorptions of mixed inhibitors. The combination of corrosion inhibitors is a significant method to enhance the inhibitive effects of inhibitors owing to their synergism. In this work, the inhibition synergism of two eco-friendly amino acids (L-histidine, L-cysteine) and thiourea on the corrosion of N80 carbon steel in CO 2 -containing environment was studied by electrochemical tests and surface analysis. The results indicate that the combination of L-histidine/L-cysteine and thiourea could synergistically boost the inhibition performance of L-histidine/L-cysteine and thiourea, i.e., present a remarkable inhibition synergism. Moreover, the synergism mechanism is in-depth revealed by theoretical calculations. The stronger interaction between L-histidine and thiourea accounts for the better synergistic inhibition effect.