Study of the synergistic effect observed for the corrosion protection of a carbon steel by an association of phosphates

Abstract Surface treatments have been carried out on a carbon steel in solutions containing phosphates (tridecylphosphate (TDP) and methacryloxyethylphosphate (MOP)). Corrosion protection afforded by the compounds was evaluated by electrochemical impedance measurements. The presence of MOP in the TDP-containing solution creates a synergistic effect for the formation of a protective film on the steel surface. The optimum concentration giving the best protection is l.5% MOP. The protective film is relatively thick (10 μm) and porous. IR analysis indicates that the film was essentially the Fe(TDP)n complex (with n=1–2). It was found that the film formation occurred in two stages: first, MOP activates the dissolution of the substrate with the production of Fe2+ ions at the interface and then, the formation of the complex is favoured.