Synergism between rare earth cerium(IV) ion and vanillin on the corrosion of steel in H2SO4 solution: Weight loss, electrochemical, UV-vis, FTIR, XPS, and AFM approaches

The synergism between rare earth cerium(IV) ion and vanillin (4-hydroxy-3-methoxy-benzaldehyde) on the corrosion of cold rolled steel (CRS) in 1.0 M H₂SO₄ solution at five temperatures ranging from 20 to 60 °C was first studied by weight loss and potentiodynamic polarization methods. The inhibited solutions were analyzed by ultraviolet and visible spectrophotometer (UV-vis). The adsorbed film of CRS surface containing optimum doses of the blends Ce⁴⁺-vanillin was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The results revealed that vanillin had a moderate inhibitive effect, and the inhibition efficiency (IE) increased with the vanillin concentration. The adsorption of vanillin obeyed Temkin adsorption isotherm. Polarization curves showed that vanillin was a mixed-type inhibitor in sulfuric acid, while prominently inhibited the cathodic reaction. For the cerium(IV) ion, it had a negligible effect, and the maximum IE was only about 20%. However, incorporation of Ce⁴⁺ with vanillin improved significantly the inhibition performance. The IE for Ce⁴⁺ in combination with vanillin was higher than the summation of IE for single Ce⁴⁺ and single vanillin, which was synergism in nature. A high inhibition efficiency, 98% was obtained by a mixture of 25-200 mg l⁻¹ vanillin and 300-475 mg l⁻¹ Ce⁴⁺. UV-vis showed that the new complex of Ce⁴⁺-vanillin was formed in 1.0 M H₂SO₄ for Ce⁴⁺ combination with vanillin. Polarization studies showed that the complex of Ce⁴⁺-vanillin acted as a mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. FTIR and XPS revealed that a protective film formed in the presence of both vanillin and Ce⁴⁺ was composed of cerium oxide and the complex of Ce⁴⁺-vanillin. The synergism between Ce⁴⁺ and vanillin could also be evidenced by AFM images. Depending on the results, the synergism mechanism was discussed from the viewpoint of adsorption theory.