Adsorption, kinetic, thermodynamic, thermal stability, and RSM optimization studies of Mitragyna speciosa as a green corrosion inhibitor for API 5 L X65 steel in aggressive oil and gas well treatment fluids

The present study reports, for the first time, the use of Mitragyna speciosa leaf extract (MSLE) as a sustainable corrosion inhibitor for API 5L X65 pipeline steel under high-temperature acidic conditions representative of oil and gas well treatment environments. An integrated approach combining adsorption modeling, kinetic–thermodynamic evaluation, thermal stability assessment (TGA/DTA), and response surface methodology (RSM) optimization was employed to provide comprehensive mechanistic insight into inhibitor performance at elevated temperatures (323–343 K), concentrations of 0.2–0.8 g/L, and immersion times of 2–10 h.Adsorption behavior was evaluated using Langmuir, Freundlich, and Temkin isotherms (R² = 0.942–0.995), demonstrating strong model agreement and confirming spontaneous adsorption through negative ΔG ads values. The thermodynamic parameters indicate a mixed adsorption mechanism involving both physical and chemical interactions. The presence of MSLE increased the apparent activation energy relative to the uninhibited system, confirming adsorption-mediated surface blocking. Transition state analysis revealed an endothermic activation process accompanied by reduced interfacial disorder in the inhibited system.Thermogravimetric analysis showed negligible mass loss below 120°C, corresponding to moisture and volatile removal, while major decomposition occurred between 250–380°C, confirming adequate thermal stability under corrosion-testing and oilfield acidizing conditions. RSM analysis demonstrated satisfactory predictive capability (R² = 0.8098), with inhibitor concentration identified as the dominant factor influencing inhibition efficiency. The maximum experimental inhibition efficiency of 98.40% was achieved at 0.8 g/L, 333 K, and 2 h, while numerical optimization predicted optimal operating conditions at 0.8 g/L, 343 K, and 9.13 h with a validated efficiency of 90.24%.The novelty of this work lies in the integrated thermodynamic–kinetic–thermal–statistical evaluation of MSLE under elevated temperature oilfield conditions. This comprehensive framework not only elucidates the inhibition mechanism but also establishes a systematic basis for optimizing the performance of plant-derived corrosion inhibitors in aggressive acidic environments, contributing to the advancement of sustainable corrosion mitigation strategies.