Carbon steel is among the most widely used engineering materials in the world, serving in automotive components, machinery, construction structures, offshore platforms, and ship hulls. Yet it corrodes readily in the presence of chloride and sulfate ions, the chief aggressive species in coastal and industrial atmospheres. Chloride ions drive localized pitting corrosion, while sulfate species raise the conductivity of the aqueous film on the metal surface and participate in the formation of iron sulfates and oxides. Traditional protection strategies, including coatings, cathodic protection, and synthetic inhibitors, often rely on compounds containing toxic heavy metals such as chromium and lead, which environmental legislation like the United States Toxic Substances Control Act and the European Union Restriction of Hazardous Substances Directive increasingly restrict.

That regulatory pressure has pushed researchers toward greener alternatives, including plant extracts and expired pharmaceuticals. Expired drugs are attractive because their active molecules typically contain unsaturated bonds, aromatic rings, and electronegative heteroatoms such as oxygen, nitrogen, and sulfur, all of which can anchor to metal surfaces. Omeprazole, a benzimidazole-based proton pump inhibitor, is particularly well suited: its structure packs nitrogen and oxygen atoms into heterocyclic rings, a sulfoxide group, and a delocalized pi-electron system capable of donating electron density to iron atoms. Previous studies of expired omeprazole in strong acids such as hydrochloric, phosphoric, and sulfuric acid have reported inhibition efficiencies around 90 percent, and an earlier electrochemical noise study by the same group found efficiencies above 90 percent for AISI 1018 steel in the same chloride-sulfate electrolyte.

The new work goes further by asking how the protection evolves with time. The team prepared AISI 1018 steel electrodes, polished them with silicon carbide papers, and immersed them in a neutral solution of 0.1 M sodium sulfate plus 3 weight percent sodium chloride. The inhibitor was the powdered content of expired commercial 20 mg omeprazole capsules, dissolved in methanol and added at nominal concentrations of 25, 50, 75, and 100 ppm. Rather than relying on a single measurement, the researchers combined open-circuit potential, potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy, following each specimen over 72 hours and extending the best condition to 96 hours.

The electrochemical data told a clear story. Polarization curves showed that the inhibitor acts mainly as an anodic-type inhibitor, suppressing iron dissolution and producing passivation regions in the anodic branch, while leaving the cathodic oxygen-reduction kinetics largely unchanged. Linear polarization resistance measurements revealed that 75 ppm delivered the best sustained performance: polarization resistance climbed steadily from about 2,298 ohm-square centimeters at the start of immersion to 5,198 ohm-square centimeters after 72 hours, corresponding to an inhibition efficiency of roughly 70 percent. Impedance spectroscopy confirmed the trend, with charge-transfer resistance rising to a maximum and the inhibition efficiency reaching 66.54 percent at 72 hours for the 75 ppm condition.

Notably, the results were not uniformly positive. Several concentration-time combinations produced negative inhibition efficiencies, meaning the inhibitor-containing solutions actually corroded faster than the uninhibited blank. At 50 ppm, for example, the polarization curves showed an apparent acceleration of corrosion. The authors retained these values deliberately, arguing that they reflect genuine electrochemical behavior rather than artifacts, and that the transition from negative to positive efficiency with immersion time demonstrates how strongly inhibition depends on both dose and exposure duration. At 100 ppm, the team suggests that excess inhibitor may cause electrostatic repulsion between adsorbed molecules, preventing efficient packing and leaving active sites exposed.

Time also proved to be a double-edged sword. When the optimal 75 ppm condition was followed to 96 hours, the impedance response collapsed, with charge-transfer resistance falling back toward blank levels. The phase-angle maxima in the Bode plots, which had risen steadily to 69.15 degrees at 72 hours, dropped to 62.47 degrees, and the fitted parameters indicated an increase in the electrochemically active area. The researchers interpret this as partial desorption, defect formation, or electrolyte penetration through the protective layer, marking the point at which the film begins to deteriorate.

Surface analysis and quantum chemistry filled in the molecular picture. Scanning electron microscopy showed far less deterioration on steel exposed to 75 and 100 ppm than on the blank, and energy-dispersive spectroscopy recorded higher relative iron content with lower oxygen and chlorine on the protected surfaces. Adsorption modeling showed that the Frumkin isotherm best described the data, with a positive interaction parameter of 2.52 indicating attractive lateral interactions between neighboring adsorbed molecules, and a negative adsorption free energy of -24.4 kJ per mole confirming that adsorption is thermodynamically favorable. Density functional theory calculations using the B3LYP functional with a water continuum model identified specific atoms as the most reactive adsorption centers: one oxygen atom, one nitrogen atom, and the sulfur atom of the sulfoxide group showed the highest dual-descriptor values and local electrophilicity, pinpointing them as the sites that donate electron density into vacant iron d-orbitals.

The proposed mechanism is therefore a sequence: omeprazole molecules displace water at the steel-solution interface, anchor through their oxygen, nitrogen, and sulfur atoms and aromatic pi-electrons, and progressively occupy electrochemically active sites, choking off charge transfer. At the optimum concentration, attractive lateral interactions help organize a compact monolayer whose stability peaks at 72 hours before slow degradation sets in. The authors caution that the material tested was a commercial formulation containing excipients rather than analytical-grade omeprazole, and that spectroscopy alone cannot prove molecular bonding to the surface.

The broader implications are twofold. Scientifically, the study demonstrates that inhibition efficiency numbers cannot be compared across techniques or time points without care, since electrochemical noise, impedance, and polarization probe different aspects of the corrosion process. Practically, repurposing expired pharmaceuticals as corrosion inhibitors could divert pharmaceutical waste from disposal streams, though the researchers stress that toxicity, biodegradation, leaching, and life-cycle impacts must be assessed before any environmental benefit can be claimed. For now, the image of a discarded heartburn capsule quietly defending a steel plate against salt and sulfate attack is a vivid reminder that valuable chemistry can outlive its expiration date.

Subject of Research: Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 carbon steel in chloride-sulfate solution

Article Title: Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 steel: adsorption, interfacial evolution, and film stability

Article References: Noriega, O. A. G., Nicolás, A. F., Pérez, C. A. G., Cárdenas, M. Y. D., Larios, A. K. G., Valis, A. S. M., Chavarín, J. U., & Campos, E. C. M. (2026). Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 steel: adsorption, interfacial evolution, and film stability. Results in Chemistry, 31, Article 103904. https://doi.org/10.1016/j.rechem.2026.103904

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103904

Keywords: corrosion inhibition, expired omeprazole, AISI 1018 steel, electrochemical impedance spectroscopy, adsorption isotherm, Frumkin model, density functional theory, chloride-sulfate electrolyte, green corrosion inhibitors, pharmaceutical waste repurposing, anodic passivation, protective film stability