Eco-friendly corrosion inhibition of cold-rolled steel in phosphoric acid using rapeseed meal extracts: Reflux vs....

Rapeseed meal, a byproduct generated during vegetable oil production, has attracted considerable attention as a sustainable source of anti-corrosion agents. This study investigates the corrosion inhibition properties of two eco-friendly extracts derived from rapeseed meal-reflux extract (RSMR) and ultrasound extract (RSMU)-on cold-rolled steel (CRS) in phosphoric acid (H₃PO₄) solution. At a concentration of 400 mg L^-1 and 40 °C, both extracts demonstrated remarkable inhibition efficiencies, reaching 94.53 % for RSMR and 94.94 % for RSMU. The inhibition mechanism is attributed to the spontaneous adsorption of active phytochemical constituents, including hydroxyl, carboxyl, and aromatic groups, which form a compact and adherent protective film on the CRS surface. The adsorption behavior follows the Langmuir adsorption isotherm, effectively impeding the diffusion of aggressive species such as H₂O, H₃O⁺, and acidic radicals. This film concurrently hinders both anodic and cathodic reactions, leading to a significant reduction in corrosion current density and a substantial increase in polarization resistance. Surface morphology analysis confirms the formation of a protective layer enriched with CN, CO, and CH functional groups. Molecular dynamics simulations (MD) further demonstrate strong chemisorption of RSMR and RSMU through interactions with nitrogen and oxygen atoms, while diffusion studies validate the high compactness of the protective film, effectively limiting the transport of corrosive species. This work underscores the superior efficacy and sustainability of rapeseed meal extracts as natural corrosion inhibitors in acidic environments, providing a viable alternative for environmentally friendly corrosion protection.