Cashew Nut Testa Tannin: Assessing its Effects on the Corrosion of Aluminium in HCl

Cashew nut testa tannin [CASTAN] has been found to inhibit the corrosion of aluminium in hydrochloric acid solutions using gravimetric, thermometric and UV/visible spectrophotometric techniques. CASTAN inhibition was by adsorption on aluminium following Temkin isotherm in 0.1 M HCl and Langmuir isotherm in 0.5 M and 2.0 M HCl at 303 Kelvin. Physical adsorption on aluminium has been proposed in studied HCl solutions; therefore, CASTAN is a cathodic inhibitor. Earlier reports [1] showed CASTAN to contain quercetin, azaleatin, catechin, epicatechin, cyanidin and delphinidin. However, UV/visible spectrophotometric analysis of CASTAN in ethanol reveals quercetin as its major component. This work therefore investigated the correlation between computed molecular parameters and inhibitive properties of CASTAN and adsorption sites on its components. Calculated quantum chemical parameters namely: EHOMO (highest occupied molecular orbital energy), ELUMO (lowest unoccupied molecular orbital energy), energy gap (∆E) and dipole moment ( μ ) suggest that CASTAN is a soft inhibitor and it’s components inhibited aluminium corrosion in protonated forms. Calculated Mulliken charges implicated some electron rich sites, namely: the aromatic and conjugated C=C, C=O and O-H as adsorption sites on the inhibitor molecules. Proposed kinetic model reveals complex reaction mechanism, parallel reactions, for aluminium corrosion inhibition by CASTAN.