Investigation of water treatment residues as corrosion inhibitors in acidic environment

The present research focuses on the valorization of residues from the treatment of colored wastewater as eco-friendly corrosion inhibitors for mild steel in a 1 M HCl medium. These residues are derived from Aleppo pine fibers treated with Maxilon Blue dye (RFPABM) and Eriochrome Black dye (RFPANE). The study evaluated their corrosion inhibition performance using weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Furthermore, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX) was employed to analyze the surface morphology of the mild steel samples. The results demonstrated significant inhibition efficiencies of 84.3% for crude Aleppo pine fiber extract (FPA), 90.5% for RFPABM, and 90.1% for RFPANE at an optimal concentration of 1 g/L. The polarization studies identified these extracts as mixed-type inhibitors, effectively reducing both anodic and cathodic reactions. EIS analysis revealed a decrease in double-layer capacitance (Cdl) with increasing inhibitor concentration, indicating robust adsorption of the inhibitors on the steel surface and the formation of a protective barrier. Adsorption isotherm analysis showed that the adsorption of FPA and RFPABM on the mild steel surface follows both Langmuir and Temkin models, suggesting monolayer adsorption with possible lateral interactions between adsorbed molecules. In contrast, RFPANE strictly adhered to the Langmuir model, indicating uniform adsorption sites without significant interactions between adsorbed molecules. SEM–EDX analysis corroborated these findings by revealing smoother and less corroded surfaces in the presence of inhibitors compared to the uninhibited sample, which exhibited severe degradation. The protective film formed by the inhibitors was rich in oxygen and carbon, confirming the presence of adsorbed organic compounds from the extracts. © The Author(s) 2025.