Adsorption and thermodynamic studies of the corrosion inhibition effect of Rosmarinus officinalis L. leaves on aluminium alloy in 0.25 M HCl and effect of an external magnetic field

Abstract

The corrosion inhibition of Aluminium alloy AA8011 in 0.25 M hydrochloric acid solution by Rosmarinus officinalis L. (rosemary) leaves extract was studied using the gravimetric technique at 303, 313, 323 and 333K, and in the presence of an external magnetic field. The study reveals that the methanolic extract of rosemary leaves inhibits corrosion of Al in 0.25 M HCl. The inhibition efficiency was found to increase with increase in concentration of the extract, but decreased with increase in temperature. Thermodynamic activation parameters like the activation energy (Ea), ranged from 15.29 to 35.06 kJ/mol, thereby suggesting the mix mechanism of physichemisorption; calculated values of the standard adsorption enthalpieswere positive indicating an endothermic process, while negative values of entropiesimplied an associative interaction between the inhibitor molecules and the Al surface. The corrosion data was found to be a good fit for the Langmuir (R2 > 0.98) and Villamil (R2 > 0.99) isotherm models. Values of the adsorption free energy  obtained were negative, thereby describing a spontaneous adsorption process. The effect of an external magnetic field was found to have a mixed behaviour in the presence of the R. offinalis L. extract at room temperature. Nonetheless, the corrosion rate was generally found to decrease with increase in strength of the magnetic field. Key words: Acid corrosion, rosemary, aluminium, langmuir model, magnetic field.