Iranian Journal of Oil and Gas Science and Technology

Abstract The effect of using Conocarpus extract as a green inhibitor on the corrosion behavior of mild steel in a 1 M HCL environment is investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR). The impedance tests show that the polarization resistance increases from 29 for the blank solution to 299 for the solution containing 2500 ppm of Conocarpus extract. The polarization test results show that at room temperature, the corrosion current density for the blank solution decreases from 3.5 × 10^–4 to 2.6× 10^–5 for the solution containing 2500 ppm of Conocarpus extract, and the potential is shifted to negative values. The polarization test is performed at three temperatures of 25, 55, and 85 °C. The results show that the efficiency of 1925 ppm decreases from 93% at room temperature to 86% at 85 °C. The high-temperature efficiency does not decline significantly, indicating the effectiveness of Conocarpus extract at high temperatures. The FTIR tests also prove that the corrosion inhibitory effect of Conocarpus extract is due to the presence of heteroatoms such as N, S, and O. The adsorption isotherm results show that the adsorption of the extract as a single layer on the surface is consistent with the Langmuir isotherm.

Abstract Corrosion results in hazardous and expensive damage to pipelines, vehicles, water and wastewater systems, and even home appliances. One of the most extensively practical methods for protecting metals and alloys against corrosion is to use organic inhibitors. The inhibition capability of 2-Mercaptobenzothiazole (2-MBT) against the corrosion of carbon steel in a 2 M NaCl solution was examined by Tafel polarization. By using 2-Mercaptobenzothiazole both the cathodic and anodic reactions are delayed through chemical and physical adsorption and blocking the active corrosion sites. Based on the polarization curves, it was indicated that by increasing the inhibitor concentration, the inhibition efficiency increases up to 70% at room temperature, and it improves at higher temperatures. The adsorption of 2-Mercaptobenzothiazole was based on the Langmuir adsorption isotherm. The enthalpies of activation were determined to be around +50 kJ.mol^-1. The endothermic nature of the steel dissolution procedure is reflected by the positive symbols of the enthalpies (ΔH) of activation process. The determined  values range from -32.69 to -35.81 kJ.mol^-1, which shows both electrostatic adsorption and the chemisorption of the adsorption mechanism. The calculated entropy of adsorption was 78 J.mol^-1_.K^-1 indicating the increment in the solvent entropy and a more positive water desorption entropy.