Evaluation of Desmodium adscendens (Sw.) DC. extract as an eco-friendly corrosion inhibitor for carbon steel in hydrochloric acid: Gravimetric with statistical modeling, electrochemical, and surface analysis

The development of sustainable corrosion mitigation strategies is essential for protecting metallic materials in aggressive environments. In this study, Desmodium adscendens (Sw.) DC. extract (DAPE) was investigated as a bio-based corrosion inhibitor for carbon steel in hydrochloric acid. A central composite design integrated with response surface methodology (CCD–RSM) was employed to quantify and optimize the combined effects of acid concentration, temperature, inhibitor concentration, and immersion time on weight loss (WL), corrosion rate (CR), and inhibition efficiency (IE). The resulting quadratic models exhibited high statistical reliability and predictive accuracy (R² = 0.9881 for WL, 0.9743 for CR, and 0.9685 for IE). Analysis of variance identified acid concentration and temperature as the dominant factors accelerating corrosion, whereas increasing DAPE concentration significantly reduced WL and CR while enhancing IE. Optimal inhibition performance was achieved at 0.80 g/L DAPE in 2 M HCl at 323 K, yielding WL = 0.027 g, CR = 0.0011 g·cm⁻²·h⁻¹ , and IE = 85.02 %. Numerical optimization predicted comparable values (WL = 0.027 g; CR = 0.0010 g·cm⁻²·h⁻¹; IE = 85.32 %), which were experimentally validated within a 95 % confidence interval. Electrochemical impedance spectroscopy and potentiodynamic polarization analyses confirmed that corrosion inhibition occurs through adsorption of DAPE phytochemicals, forming a compact, adherent, and electrically resistive interfacial film that suppresses both anodic dissolution and cathodic hydrogen evolution. The inhibitor exhibited mixed-type behavior without altering the intrinsic corrosion mechanism. Surface morphology analysis further corroborated the formation of a smooth and protective layer on the steel surface. These findings demonstrate the potential of DAPE extract as an effective, sustainable, and environmentally benign corrosion inhibitor for carbon steel in acidic media.