Synergistic effect of NTMP, Zn2+ and ascorbate in corrosion inhibition of carbon steel

Abstract

Nitrilotris(methylenephosphonic acid) (NTMP) inhibits corrosion of carbon steel in presence of Zn2+ . However, in order to make the inhibitor more environment friendly, it is necessary to reduce the levels of Zn2+ or phosphonate or both by introducing an environment friendly organic salt as a second synergist. In this paper, the inhibitive action of ascorbate as a second synergist in the presence of relatively low concentrations of NTMP and Zn2+, is presented. Addition of just 25 ppm of ascorbate to 20 ppm each of NTMP and Zn2+, at pH=7, increased the inhibition efficiency markedly from 35 to 95%. In the pH range of 6-4, the concentration of ascorbate required to bring up the inhibition efficiency to 94%, is only 10 ppm. However, the alkaline region demanded relatively higher concentration of ascorbate. This ternary system is quite effective in a wide pH range, 4-10. This is the uniqueness of this new synergistic formulation. Potentiostatic polarization studies indicated that this synergistic formulation acts as a mixed inhibitor, predominantly cathodic in nature. A large increase in charge transfer resistance and also a decrease in double layer capacitance of the surface film, were observed from a.c. impedance studies. X-ray photoelectron spectrum of the surface film indicated the presence of the elements, viz., P, O, N, C, Zn and Fe in the film. The shifts in binding energies of these elements indicated the presence of oxides and hydroxides of iron and zinc as well as complexes of Fe(III) and Zn(II) with phosphonate and ascorbate. The reflection absorption FTIR spectrum of the surface film also supported the formation of insoluble complexes of phosphonate and ascorbate with metal ions as well as presence of Zn(OH)2 on the metal surface. A suitable mechanism of corrosion inhibition is proposed, which corroborates all the above experimental results.