Corrosion Inhibition and Process Optimization of AISI 4140 Alloy Steel in 15% HCl Using Newbouldia laevis (Ogirisi) Leaves: RSM-OCD Approach

The corrosion of AISI 4140 alloy steel tubing during oil and gas well acidizing with 15% HCl poses a major challenge. This study assessed the inhibitory performance of Newbouldia laevis (NBL) leaf extract on AISI 4140 alloy steel in 15% HCl by integrating mass-loss experiments, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) measurements, and scanning electron microscopy (SEM). The NBL leaf extract was prepared by Soxhlet extraction with ethanol and characterized for its phytochemical constituents. A 16-run Optimal Custom Design (OCD) was generated with immersion time (2, 4, & 6 hrs.), NBL concentration (4, 5 and 6 g/L), and temperature (303, 323 and 353 K) as factors, and corrosion rate and inhibition efficiency as responses. Inhibitor concentrations and operating conditions were optimized via Response Surface Methodology-Optimal Custom Design (RSM-OCD). Phytochemical analysis confirmed the presence of flavonoids (most abundant), phenols, tannins, steroids, terpenoids, and glycosides. RSM-OCD optimization identified the optimal conditions of 2 h, 6 g/L NBL concentration, and 303 K, predicting a corrosion rate of 1.256 mm/yr and inhibition efficiency of 94.056%. PDP results showed NBL functions as a mixed-type inhibitor with maximum inhibition efficiency of 89.42% at 7 g/L. EIS confirmed a maximum inhibition efficiency of 88.31% at 7 g/L. SEM revealed a dense heterogeneous protective film formed by 6 g/L NBL. Therefore, we recommend NBL leaf extract as a viable, sustainable, and eco-friendly corrosion inhibitor for protecting steel tubing during acidizing operations in the oil and gas industry.