Coupled experimental and theoretical investigation of an antidiabetic drug as a green corrosion inhibitor for low-carbon steel in acidic media

Pharmaceutical waste valorization offers a promising strategy for sustainable corrosion protection within a circular economy framework. In this study, the inhibition performance of expired metformin on low-carbon steel in 1 M HCl, simulating industrial acid pickling conditions, was systematically investigated. A comprehensive multi-technique approach was employed, combining weight loss measurements, electrochemical methods, and both qualitative and quantitative surface analyses, including scanning electron microscopy (SEM) and digital image analysis using MIPAR software, to evaluate inhibition efficiency over a concentration range of 100–600 ppm. The results reveal that metformin exhibits optimal inhibitory performance at 400 ppm, achieving maximum inhibition efficiencies of 84.9% (weight loss), 86.3% (LPR), 87.0% (EIS), and 89.4% (PDP). Quantitative surface analysis further confirms this trend, indicating 92.8% uncorroded surface area at the optimal concentration. Potentiodynamic polarization measurements demonstrate that metformin acts as a mixed-type inhibitor, suppressing both anodic metal dissolution and cathodic hydrogen evolution reactions. Furthermore, spin-polarized density functional theory (DFT) calculations demonstrate that the inhibitor-metal interaction is governed by chemisorption, driven by charge transfer from nitrogen lone pairs in metformin to vacant Fe 3d orbitals, leading to Fe N coordinate bond formation. These findings demonstrate the effectiveness of expired metformin as a sustainable corrosion inhibitor and highlight the advantage of integrating quantitative surface analysis with electrochemical and computational approaches for a more rigorous and objective evaluation. This work contributes to the development of environmentally friendly corrosion protection strategies and supports the principles of green chemistry and circular economy.