Aqueous extract of cumaru (Dipteryx odorata) seeds as corrosion inhibitor for mild steel in hydrochloric acid solution

The corrosion inhibition of mild steel by aqueous extract of cumaru (Dipteryx odorata) seeds and its high molecular weight fraction (HMWF) was investigated in 1 mol L^-1 HCl solution. The study was carried out through gravimetric essays, open circuit potential measurements, anodic and cathodic polarization curves and electrochemical impedance analyses, as well as morphological analysis of the mild steel by scanning electron microscopy. Gravimetric tests showed that the inhibition efficiency (IE) increased with immersion time and inhibitor concentration, reaching 98% of IE using 100 mg L^-1 of the HMWF, after 6 h immersion time. The activation energy (E_a) increased with the addition of both inhibitors which characterizes a physical adsorption of the constituents of the aqueous extract of cumaru on the metal surface. Polarization curves indicated that both aqueous cumaru seeds extract and its HMWF act as mixed-type inhibitor. Impedance results showed a decrease in double layer capacitance (C_dl) and an increase in charge transfer resistance (R_ct) evidencing a typically screening effect. The adsorption of molecules present in the aqueous extract of cumaru followed the Langmuir isotherm. The aqueous cumaru seeds extract was characterized by Fourier transform infrared spectroscopy (FTIR), ¹H, ¹³C and 2D nuclear magnetic resonance (NMR). (author)