Inhibition efficiency and mechanism of nitrilo-tris(methylenephosphonato)zinc on mild steel corrosion in neutral fluoride-containing aqueous media

Corrosion-electrochemical behaviour of steel E 235 in borate buffer solution (pH = 7.4) containing F- ions with and with no added ZnNTP as an inhibitor, where NTP = N(CH2PO3)(3), was studied by the potentiodynamic polarization and depth-profiling XPS analysis of specimens polarized at different potentials applied. Depending on the potential applied, F- ion was shown to influence differently on the formation, composition and structure of passive film. At E < 0.1-0.2 V/SSCE, F- ion promotes the passivation via forming sparingly soluble FeF2, and the resulting film is comprised of mainly iron(II) oxides and hydroxides, as well as sparingly soluble FeF2 and FeZnNTP. At E > 0.1-0.2 V/SSCE, F- ion works for the destruction of the passive film by forming soluble compounds with Fe3+ ions, which leads to its thinning. ZnNTP inhibitor forms FeZnNTP heterometallic complex with iron ions, which is the most stable constituent of the passive film. When F- ion concentration does not exceed 1.4 mmol/L, ZnNTP inhibitor is optimal to be added in amount of 0.5-1.0 g/L, whereas concentrations of 5.6 mmol/L F- ion and higher require 5 g/L ZnNTP or even more to be added.

Automated summary

The corrosion-electrochemical behavior of steel E235 in borate buffer solution was studied, where the influence of F- ions and the addition of ZnNTP inhibitor on passive film formation and destruction were investigated. It was found that F- ions can promote passivation at lower potentials but lead to passive film thinning at higher potentials. The addition of ZnNTP inhibitor can enhance the corrosion resistance of the steel.