Thermodynamic, Adsorption,thermal stability and optimization studies of expired valsartan drug for corrosion protection of N80 steel in a simulated acidified oil well system

Abstract

The use of expired pharmaceutical compounds as corrosion inhibitors represents a sustainable approach to both corrosion mitigation and pharmaceutical waste valorization. In this study, the corrosion inhibition performance of expired valsartan (VST) for N80 steel in a simulated acidized oil-well environment (1.0 M HCl) was investi gated using gravimetric measurements, adsorption and thermodynamic analyses, thermal stability evaluation, and Response Surface Methodology (RSM) optimization. The novelty of this work lies in the utilization of expired valsartan as a low-cost, environmentally sustainable corrosion inhibitor while simultaneously providing an effective strategy for pharmaceutical waste reutilization.The results showed that the inhibition efficiency increased with increasing inhibitor concentration but decreased with increasing temperature and immersion time. Adsorption studies revealed that VST molecules adsorbed spontaneously onto the steel surface, predomi nantly through a physisorption mechanism, forming a protective barrier that reduced metal dissolution. Ther modynamic analysis further confirmed the spontaneous and protective nature of the adsorption process, while thermal analysis demonstrated that expired valsartan remained structurally stable up to approximately 260 ◦C, indicating adequate thermal stability for application under acidized oil-well conditions.A Box–Behnken design based on Response Surface Methodology was successfully developed to optimize the corrosion inhibition performance. The optimum conditions were identified as an inhibitor concentration of 125 ppm, a temperature of 328 K, and an immersion time of 12 h, under which an experimental inhibition efficiency of 76.8% was achieved. The findings demonstrate that expired valsartan is an effective, environmentally benign, and economically viable corrosion inhibitor for N80 steel in acidic media. This study provides a sustainable approach to corrosion control while offering a practical solution for the reutilization of expired pharmaceutical waste in petroleum industry applications.