Expired chlorzoxazone/N-acetyl-para-aminophenol as corrosion inhibitor for 304 stainless steel in hydrochloric acid

Abstract

The influence of the expired drug chlorzoxazone (N-acetyl-para-aminophenol) on the corrosion behavior of 304 stainless steel (304SS) in 1.0 M hydrochloric acid was examined. Several electrochemical measurements were investigated, including open circuit potential (OCP), cyclic voltammetry (CV), potentiodynamic polarization (PD), electrochemical potentiokinetic reactivation (EPR), chronopotentiometry (CP), chronoamperometry (CA), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) using Nquist and Bode plots. These methods were analyzed to evaluate the corrosion rate, passivation characteristics, and stability of protective films with different drug concentrations (50–400 ppm). Open-circuit measurements indicated that the incorporation of the drug altered the potential to more noble levels, suggesting surface protection. CV and LSV measurements revealed a significant reduction in current density with increasing drug concentration, validating the robust adsorption of the inhibitor molecules and establishing a stable passive film. PD and EPR findings indicated substantial decreases in corrosion current density (I_corr) and increases in polarization resistance (R_p), especially at 400 ppm, where the corrosion rate was the lowest among different drug loadings. Chronopotentiometry demonstrated consistent potential responses and improved passivation with the inhibitor, whilst chronoamperometry showed a significant reduction in dissolving current relative to uncoated steel. The data collectively indicate that the drug compound functions as an effective, concentration-dependent corrosion inhibitor, establishing a protective barrier that mitigates both anodic and cathodic processes, thus improving the electrochemical stability of stainless steel in acidic environments. This study opens a new avenue for reusing expired drugs as corrosion inhibitors.