Eco-friendly corrosion inhibition of API 5L X42 steel in acidic media using Sclerocarya birrea leaf extract: Experimental, thermodynamic and computational insights

The persistent degradation of Pipeline steels in acidic environments demands effective, low-toxicity inhibitors that are deployable at industrial scale. Plant-based extracts have emerged as promising candidates, but their mechanistic understanding, especially when applied to high-grade pipeline steels under realistic service conditions, is limited. This study aims to evaluate the corrosion inhibition performance and adsorption mechanism of Sclerocarya birrea (marula) ethanolic leaf extract on API 5 L X42 pipeline steel in 1 M H₂SO₄. A combined experimental-computational approach was adopted, integrating gravimetric analysis, Open Circuit Potential, Electrochemical Impedance Spectroscopy, and Potentiodynamic Polarization at 303, 313, and 323 K with inhibitor concentrations of 0, 10, and 20 g·L^-1, applying rigorous error analysis to ensure reproducibility. Spectroscopic and chromatographic analyses (FTIR, UV–Vis, GC–MS, XRD, Raman) revealed oxygen-bearing and π-rich phytoconstituents as active adsorption centers. Maximum inhibition efficiency of 93% (weight-loss) was achieved at 20 g·L⁻¹ and 303 K, with electrochemical data showing increased charge-transfer resistance (1489 → 6359 Ω·cm²) and decreased corrosion current density, consistent with mixed-type inhibition dominated by anodic suppression. Thermodynamic analysis revealed ΔG ads values of -9.2 to -10.2 kJ·mol⁻¹, indicating predominantly physisorption, and positive ΔH (up to +47.99 kJ·mol^-1), suggesting endothermic adsorption. Adsorption followed the Temkin model (R² up to 0.985), implying lateral molecular interactions. Density Functional Theory and Monte Carlo simulations confirmed favorable adsorption geometries on Fe(110) surfaces and highlighted phytol derivatives as the strongest adsorbates. The study demonstrates that S. birrea extract forms a robust, eco-friendly protective layer capable of mitigating acid-induced corrosion of pipeline steel.