Surface-spectroscopic and microscopic evidence for distinct protection of active carbon and passive stainless steels by green biomaterials

The assumptions that polar plant extracts inherently outperform less polar counterparts as green corrosion inhibitors, and that single-alloy evaluations suffice, are critically re-examined. Pterocarpus santalinoides extracts obtained using five solvents spanning methanol to n-hexane were systematically compared on actively corroding CS1018 carbon steel and passivating SS316L stainless steel in 1 M HCl. Electrochemical results reveal that moderately polar isopropanol (IPA) and butanol, along with non-polar n-hexane, consistently outperform polar extracts. IPA achieves ∼ 96 % efficiency on CS1018 and ∼ 58 % on SS316L, with PDP efficiency reaching 98.7 % and 77.5 %, respectively. Mechanistically, IPA protects CS1018 through compact heteroatom-assisted adsorption and active-site blocking (C_dl < 20 μF cm⁻²), whereas on SS316L it reinforces passive film integrity at defect sites. Surface characterization, including SEM/EDS, AFM, contact angle measurements, XPS, and ToF-SIMS, confirms reduced degradation, suppression of chloride ion penetration, enhanced hydrophobicity, and formation of organic–inorganic interfacial species. ToF-SIMS provides direct molecular evidence of N-/O-bearing hydrocarbon fragments and organic–inorganic complexes on the inhibited surface, validating the adsorption mechanism at the outermost interface. This study establishes that solvent polarity alone is an unreliable predictor of inhibitor performance and demonstrates that moderate-polarity plant extracts provide broad-spectrum, mechanism-tunable protection across ferrous alloys, advancing the rational design of sustainable corrosion inhibitors.