Green Inhibition of Mild Steel Corrosion in a CO2 Saturated Saline Solution

Abstract

Mild steel is extensively used in equipment such as pipelines and machinery by many industries, where it is exposed to corrosive environments. Dissolved carbon dioxide (CO2) can be found in produced water and results in severe corrosion due to the formation of carbonic acid. CO2 corrosion presents not only an economic loss but also an environmental and safety risk. Most industries use synthetic inhibitors, which are effective in reducing corrosion, but are also toxic and persistent. This has led to stricter regulations and thus, there is a need for alternative inhibitors which can replace them but not exhibit their undesired characteristics. The aqueous extracts of Fig leaves (FLE), Calotropis procera (CPLE) and Eggplant peels (EPPE) were investigated as novel corrosion inhibitors for mild steel in a CO2-saturated 3.5wt% NaCl solution using various electrochemical techniques such as linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and cyclic sweep (CS). The results showed that the corrosion rate is decreasing and inhibition efficiency is increasing as the concentration of inhibitor increased. Corrosion inhibition efficiencies of 90-95% were obtained using low dosage of the green inhibitors. FLE, CPLE and EPPE proved to be effective inhibitors and are compared to the performance of two commercially available inhibitors, A (green inhibitor) and B (synthetic inhibitor). Polarization studies show that FLE, CPLE and EPPE act as mixed inhibitors. The adsorption data was analyzed using various adsorption isotherm models and the results at temperatures of 25, 40, 50 and 70°C have shown that the adsorption behavior of FLE, CPLE and EPPE is best described by the Langmuir adsorption isotherm. The conclusion drawn from this work is that the FLE, CLE and ELE inhibitors proved to be effective inhibitors when compared to Inhibitor A. Inhibitor B proved to be a much more effective inhibitor than the others due to its synergistic mix of active ingredients.