Experimental and theoretical studies of carbon steel corrosion protection in phosphoric acid solution by expired lansoprazole and rabeprazole drugs

Abstract

The effects of expired lansoprazole and rabeprazole on the corrosion protection of carbon steel in phosphoric acid (3.0 ​M) solution were examined by Tafel polarization and electrochemical impedance spectroscopy (EIS). Lansoprazole and rabeprazole concentrations (0.5, 1.0, 5.0 and 10 ​mM) in acid solution were raised, which improved corrosion prevention. Both lansoprazole and rabeprazole as the mixed inhibitors retarded the anodic and cathodic processes, as indicated by polarization data. With the increasing temperature in the range of 25–55 ​°C, the inhibition efficiency drops from 92.9% to 69.3% for lansoprazole and from 94.8% to 74.2% for rabeprazole. The major decrease in the inhibition efficiency with ascending temperature proved the physisorption of the drugs. The activation energies for carbon steel corrosion in H3PO4solution were enhanced from 41.6 ​kJ ​mol−1 to 81.9 ​kJ.mol−1and 85.9 ​kJ ​mol−1 for lansoprazole and rabeprazole, respectively. The influence of temperature on the corrosion process of carbon steel in the acid medium was used to derive the thermodynamic quantities of corrosion. The adsorption of both lansoprazole and rabeprazole on carbon steel followed the Langmuir adsorption isotherm. The polarization data yielded outcomes that were consistent with the results arising from impedance measurements. The theoretical study of both lansoprazole and rabeprazole was done by a density functional theory (DFT) approach to realize the effects of molecular structure on their inhibitive action. Both lansoprazole and rabeprazole contain a higher EHOMO, a lower ELUMO and a lower energy gap than some inhibitors earlier reported as good corrosion inhibitors in the literature.