Effect of the molecular structure on the inhibitor properties of azoles on mild steel corrosion in 1 M hydrochloric acid

Five azoles were investigated as corrosion inhibitors of mild steel in 1 M HCl using gravimetric and polarisation techniques. The chosen azoles have similar chemical structure – their molecules have similar geometry and in an adsorption state almost the same area. The inhibiting properties were found to depend on both the concentration and the molecular structure of the compounds. The adsorption of benzimidazole and 1,3-benzothiazole is described by the Frumkin isotherm, while in the case of 1H-benzotiazole and indole the Langmuir isotherm can be applied. An attempt was made to correlate the inhibitive properties with the electronic structure of the molecules and it was found that the inhibitor properties are connected with the presence of aromatic structure which includes the heterocycle and depend on the number of the lone electron pairs. The HOMO and LUMO energies were calculated and it was found that their values can explain only some features of the observed phenomena.