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Waste matter is an abundant source of recyclable materials. This study presented the corrosion inhibitor role of unripe banana peel extract (UBPE) for carbon steel in acidic environments: 1.0 M HCl, 0.5 M H2SO4, and 0.5 M HCl/0.25 M H2SO4. The aim was to demonstrate the robustness of the HCl-UBPE-steel system with respect to competing ions in the solution, and as such, generalizing its mechanism. The localized corrosion in HCl, general corrosion in H2SO4, and their theoretical midpoint were, respectively, subjected to 95.91%, 83.33%, 91.50% corrosion inhibition efficiency when UBPE was added at 3000 ppm. A protective film on the steel surface was found as the cause by electrochemical characterizations and its behavior was explained via protonation of corrosion inhibitor and rust formation on steel surface undering varying acid conditions. Therefore, the work suggests UBPE as corrosion inhibitor for steel in strongly acidic conditions containing chloride and sulfate ions.Highlights Unripe banana peel – water extract (UBPWE) acts as an eco-friendly effective inhibitor for steel in aggressively acidic environments.Carbon steel exhibits more localized corrosion in HCl solution, but more general corrosion in H2SO4 condition.UBPWE performed 95.91, 83.33, and 91.50% of inhibition efficiency for steel in HCl, H2SO4, and mixed conditions.The effective inhibition performance is assigned to the presence of a protective layer on the steel surface.