Green corrosion inhibition of C38 steel in HCl by a methanolic Grewia tenax leaf extract

The methanolic leaf extract of Grewia tenax (Gt) was evaluated as a green corrosion inhibitor for C38 steel in 1 m HCl. Liquid chromatography–electrospray–high-resolution mass spectrometry tentatively assigned 26 constituents to four families of oxygen-rich metabolites: oleanane-type saponins, kaempferol and naringenin oligoglycosides, hydrolysable tannins and proanthocyanidins; attenuated total reflectance Fourier-transform infrared spectroscopy confirmed abundant hydroxyl, carbonyl and glycosidic functionalities. Weight loss, polarisation and impedance spectroscopy gave consistent efficiencies of 94.9%, 95.8% and 93.6% at 2500 ppm and 293 K. The corrosion potential shifted by at most 11 mV, far below the 85 mV threshold, establishing mixed-type behaviour. The charge-transfer resistance rose 15-fold, from 19.5 to 304.5 Ω cm2, and reached 750.8 Ω cm2 after 24 h, showing that the film consolidates rather than degrades. Adsorption follows the Langmuir isotherm (R2 = 0.9999), with ΔG°ads = −21.1 kJ mol−1 obtained on an explicit mass-based standard state, indicating predominantly physical adsorption with a chemical contribution supported by the spectrum of the recovered film, in which the ν(O–H) band shifts from 3300 to 3410 cm−1 and a carbonyl band appears at 1733 cm−1. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show the cracked corrosion-product layer replaced by a crack-free surface bearing globular organic aggregates over a restored iron signal.