Experimental and quantum chemical studies of synergistic enhancement of the corrosion inhibition efficiency of ethanol extract of Carica papaya peel for aluminum in solution of HCl

Abstract

The investigation of ethanol extract of Carica papaya (EECP) as an inhibitor for the corrosion of aluminum in solutions of HCl was facilitated through gravimetric, gasometric, potentiodynamic polarization and quantum chemical methods. The ccorrosion rate for the blank systems was 1.66 × 10−4 mpy but in the presence of various concentrations (0.1 to 0.5 g/dm3) of EECP, it reduces from 1.32 × 10−4 mpy to 0.96 × 10−4 mpy respectively while the inhibition efficiencies ranged from 20.93 to 42.04 % respectively. However, a synergistic combination of various concentrations of the inhibitor with 0.6 M of KBr, KI and KCl enhanced the inhibition efficiency to the following ranges: 75.44 to 87.41 %, 50.20 to 71.89 % and 77.44 to 88.73 % respectively. Potentiodynamic polarization and gasometric results were in good agreement but revealed that ethanol extract of Carica papaya peel acted as cathodic type inhibitor. Calculated activation energies indicated increasing retardation strength with an increase in inhibitor concentration and supported physisorption mechanism. The adsorption of the extract on the aluminum surface was exothermic, spontaneous, and occurred with an increasing degree of orderliness. Freundlich and Temkin's isotherms were most suitable in explaining the adsorption behaviour of the extract. Molecular structures that actively participated in the corrosion inhibition (papain, riboflavin, retinol, niacin and alpha-tocopherol) have been modeled through the calculations of global and local reactivity indices. Molecular simulation studies were also conducted to estimate the interaction between the inhibitor and the metal surface and the results indicated strong involvement of heteroatoms and aromatic rings in the adsorption process. The study concluded, that although the inhibition efficiency of EECP is low, enhancement with potassium halides can give an excellent score towards the management of waste through resource recovery.