Inhibition of corrosion of API K55 steel by tannin from Acacia mearnsii bark in highly acidic medium

The corrosion inhibiting effect of the tannin from Acacia mearnsii bark on API 5CT K55 steel, used for casing in the oil and gas industry, was investigated in 1 M HCl medium with different tannin concentrations. Corrosion was monitored by electrochemical tests using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS). Complementary analyses of the corroded surfaces were performed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray diffraction (XRD). With the application of A. mearnsii tannin, the cathodic branch of the PP curves shifted to lower corrosion current density values. EIS analysis indicated that inhibitor molecules were adsorbed on the steel surface, which provided protection against corrosion. The SEM, AFM, and XRD data showed that in the presence of the inhibitor, a film and amorphous material were adsorbed on the steel surface, plausibly associated with the formation of tannates. The highest inhibition efficiency was obtained at an inhibitor concentration of 0.7 5 g L−1 (92% determined by PP and 98% by EIS), and a high degree of surface coverage was observed, compared with that obtained using the other concentrations of tannin.