Corrosion inhibition by phosphonic acid –Zn²⁺ systems for mild steel in chloride medium

The inhibition efficiencies of 2‐carboxyethyl phosphonic acid (2 CEPA) – Zn²⁺ system and ethyl phosphonic acid (EPA) – Zn²⁺ system in controlling corrosion of mild steel in a neutral aqueous environment containing 60ppm CI^– have been compared. Polarization study reveals that both the systems function as mixed inhibitors. The inhibition efficiencies of these systems decrease with period of immersion due to the dissolution of the protective film as the period of immersion increases. The 2 CEPA – Zn²⁺ system has a better inhibition efficiency than the EPA–Zn²⁺ system. This is due to the fact that the amount of Fe²⁺– 2 CEPA complex present on the metal surface is higher than that of Fe²⁺– EPA complex, as revealed by fluorescence spectra. The molecular size of the iron phosphonate complex exercises a greater influence than the electronic (inductive) effect of the substituent in controlling corrosion of mild steel in chloride environment.