Corrosion inhibition of two tetracycline-based expired antibiotics as eco-friendly inhibitors for mild steel in 1M HCl solution

Two expired Tetracycline-based antibiotics (Oxytetracycline and Tetracycline) were utilized as eco-friendly corrosion inhibitors for mild steel in 1M HCl solution, employing weight loss analysis, electrochemical measurements, scanning electron microscopy, and so on. The effect and mechanism were discussed using thermodynamic modeling, quantum chemical calculation, molecular dynamics simulation, and so on. The corrosion inhibition rate (η) increases with higher concentration and temperature, leading to reduced heat absorption and increased order degree. The ${\eta }_{\mathrm{pmax}}$ of Oxytetracycline and Tetracycline was 96.14% and 97.92% at 323.15 K of 2 × 10⁻⁴ mol L⁻¹, respectively. The key factors affecting η in the kinetic model parameters vary at lower concentrations. Both of them belong to cathodic corrosion inhibitors and undergo Langmuir spontaneous adsorption processes involving both physisorption and chemisorption. The disparity in energy levels between the highest occupied molecular orbital and lowest unoccupied molecular orbital, global hardness, electronegativity, molecular polarizability, dipole moment, and transfer electron number are key factors that enhance a corrosion inhibitor's ability to adsorb chloride ions.