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

Abstract 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(CH 2 PO 3 ) 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 FeF 2 , and the resulting film is comprised of mainly iron(II) oxides and hydroxides, as well as sparingly soluble FeF 2 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 Fe 3+ 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.