Corrosion inhibition of mild steel in binary acid mixture

The influence of mercaptobenzimidazole (MBI), mercaptobenzoxazole (NBO) and mercaptobenzoic acid (MBA) on adsorption and corrosion inhibition of mild steel in binary acid mixture (12% HCl + 3% HF) has been studied using weight loss, gasometric and FTIR methods. All the tested mercaptans showed very good inhibition efficiency. The adsorption of MBI and MBO on the mild steel surface from binary acid mixture has been found to obey Temkin's adsorption isotherm while MBA obeys Langmuir's adsorption isotherm. The lower value of Ea in inhibited acid suggests that the inhibitors do not change the temperature coefficient of the reaction rate but are firmly held on the metal surface. Values of the high heat of adsorption at 10 mM concentration of MBI (95.7 kJ mol(-1)) and MBO (24.9 kJ mol(-1)) indicate chemisorptions while MBA (7.6 kJ mol(-1)) may follow physical adsorption. Values of Delta S* and Delta H* indicate that the corrosion inhibition process is entropic and enthalpic controlled. The analysis of the FTIR spectra shows that adsorption of MBI molecules on the metal surface takes place both through the sulphur of the -SH group and the nitrogen atom in the heterocyclic ring of the inhibitor molecule. In the case of MBO, linkage of the inhibitor molecule may take place through its oxygen atom and the sulphur of the -SH group on the metal surface while MBA shifts the band position of -SH and -CO which may indicate attachment of the inhibitor molecule on the metal surface through S and O atoms.