Coffee Bioactives as an Efficient Mixed-Type Green Corrosion Inhibitor for Carbon Steel: Mechanism, Adsorption, and Surface Analysis

This study investigates the corrosion inhibition performance of a sustainable green extract derived from Spent Coffee Silverskin (SCS) using ultrasound-assisted extraction (UAE). High-performance liquid chromatography (HPLC) quantified Caffeine (43.9 mg/g), α-Tocopherol (36.69 mg/g), Chlorogenic acid (6.9 mg/g), and Hyperoside (4.98 mg/g) as the primary bioactive ingredients. Weight loss metrics showed a dramatic drop in corrosion rate from 4.35 mg cm−2 h−1 (blank) to 0.28 mg cm−2 h−1 at the optimum concentration of 300 mg L−1 in a highly aggressive 1.0 M HCl environment, delivering a maximum efficiency of 93.5% at 298 K. Extended immersion tests proved remarkable short-term stability of the protective phytochemical film, maintaining an inhibition efficiency of 92.1% after 48 h of continuous exposure in the aggressive acidic medium. For comparison across techniques, maximum inhibition efficiencies of 95.4% and 96.8% were obtained from potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), respectively. Importantly, these values are strictly method-dependent, reflecting the distinct physical parameters measured by each technique, and do not imply direct equivalence. Potentiodynamic polarization curves revealed mixed-type inhibitor characteristics, suppressing both anodic iron dissolution and cathodic hydrogen evolution. Notably, the corrosion current density (jcorr) dropped from 521.00 to 23.97 uA cm−2, while the shift in the corrosion potential (Ecorr) remained within 50 mV. EIS studies confirmed charge-transfer control, showing a rise in charge transfer resistance (Rct) from 23.3 to 728.0 ohm.cm2 paired with a drop in constant phase element (CPE) values from 642 to 218 uF cm−2, demonstrating the displacement of surface water molecules by a dense, protective organic film. Thermodynamic evaluation indicated a drop in efficiency at elevated temperatures (90.7% at 328 K) alongside an increase in activation energy (Ea) from 9.99 to 20.16 kJ mol−1. Adsorption behaviour strictly obeyed the Freundlich isotherm with the standard Gibbs free energy of adsorption (∆Goads) −17.35 kJ mol−1. Scanning electron microscopy (SEM) visually confirmed a remarkably smooth and uniform protected metal surface, free from the severe pitting and deep cracks seen in the blank. Energy-dispersive X-ray (EDX) spectroscopy mathematically backed this visual barrier, showing a drop in oxygen content from 28.55% to 3.35%, alongside a sharp rise in surface carbon (10.55%) and nitrogen (1.69%) originating from the adsorbed phytochemical network. Density Functional Theory (DFT) modeling supported the inhibition mechanism at the molecular level, confirming Hyperoside and Chlorogenic acid as the driving reactive variants for electron transfer and iron d-orbital coordinate bonding.