Improved electrochemical strategy to characterize adsorption and corrosion inhibition related to biomolecules from plant extracts: The case of Annona cherimola

Abstract

Electrochemical techniques are widely accepted to characterize performance of organic corrosion inhibitors by adsorption. Impedance experiments provide the polarization resistance Rp to estimate protection efficiency. Potentiodynamic polarization informs about the mechanisms of the anodic and cathodic reactions. However, electrochemical thermodynamics and kinetic data require validation steps and additional analysis that the literature in this field lacks. We present and discuss, four innovative points related to the electrochemical strategy, namely: a) lowest frequency in impedance experiments to derive unequivocally Rp values, b) use of Bode representations to unmistakably show the number of time constants involved in the impedance spectra, c) fastest admissible potential scan rate to obtain non-vitiated pseudo-stationary polarisation curves and d) separate calculation of percentage surface coverages by the adsorbate related to either anodic or cathodic active sites and relative to the uninhibited condition. Thus, Annona cherimola extract acts as an efficient green corrosion inhibitor for C steel in acidic medium. At low concentration corrosion prevention occurs via physical adsorption on anodic and cathodic reaction sites while at high concentration chemisorption also takes place. Gravimetric data recorded in the 298 K?328 K temperature range confirmed the type of surface adsorption. Maximum protection efficiency of 97% was achieved. SEM images unveiled mechanistic aspects of the protection process. FT-IR and UV/VIS analyses depict the protecting film developed with the participation of different acetogenins (ACGS). Quantum mechanical calculations suggest formation of Fe2+ complexes with two ACGS and higher corrosion inhibition potentiality than other extracts.