Multi-Absorption and Predominance Monolayer Modifies Bitter Leaf Extract on API 5L X70 Steel in 1 M HCl Environment for Corrosion Mitigation

Abstract

Internal corrosion of API 5L X70 steel remains a critical challenge affecting the integrity and service life of oil and gas pipelines, particularly in acidic environments such as those encountered during acidizing operations and hydrochloric acid (HCl) exposure. Bitter leaf (BLE) oil extract was investigated for API 5L X70 steel in 1 M HCl solution. With consideration of varying temperatures from 30 to 50°C and inhibitive response from 0, 0.2, 0.4, and 0.6 mL per 50 mL solution the thermodynamics assessment was undertaken using open circuit potential (OCP) and linear sweep voltammetry (LSV). From all indication, BLE exhibited superior inhibition performance, reducing corrosion rate from 1.32 to 0.48 mm/year at 30°C and from 1.87 to 0.78 mm/year at 50°C, while maintaining high polarization resistance values (~47 Ω) across all temperatures. It is important to established that the inhibitors followed Langmuir isotherm behavior, indicating predominantly monolayer adsorption, with Gibbs free energy of adsorption coefficient. The micrograph for BLE-treated samples confirmed the formation of compact and adherent protective films. Response surface methodology (RSM) agrees that inhibitive activities was the dominant factor influencing corrosion rate reduction within temperature changes. The developed quadratic models exhibited strong predictive capability (R² ≈ 97.8%), with optimal corrosion protection achieved at higher inhibitor concentrations and lower temperatures. Importantly, the results establish bitter leaf extract as a more effective and thermally stable green corrosion inhibitor for API 5L X70 steel in acidic environments.