Electrochemical and surface analytical studies of synergistic effect of phosphonate, Zn2þ and ascorbate in corrosion control of carbon steel

Abstract

Synergistic inhibition of corrosion of carbon steel in low chloride environment using ascorbate as a synergist along with 2-phosphonobutane-1,2,4- tricarboxylic acid (PBTC) and Zn2þ is presented. The synergistic effect of ascorbate has been established from the present studies. In the presence of ascorbate, lower concentrations of PBTC and Zn2þ are sufficient in order to obtain good inhibition, thus making this formulation more environmentally friendly. Potentiodynamic polarisation studies inferred that this mixture functions as a mixed inhibitor, predominantly cathodic. Impedance studies revealed that an immersion period of 24 h is necessary for the formation of the protective film, with a very high charge transfer resistance. The film is stable even at 60 8C in the presence of the inhibitor in the corrosive environment. The surface analysis by X-ray photoelectron spectroscopy (XPS) showed the presence of iron, oxygen, phosphorus, carbon and zinc in the protective film.The XPS spectra inferred the presence of oxides/hydroxides of iron(III), Zn(OH)2 and [Zn(II)–PBTC–ascorbate] complex in the surface film. This inference was further supported by the reflection absorption Fourier transform infrared spectrum of the surface film. A plausible mechanism of corrosion inhibition has been proposed.