Role of complexes in inhibition of mild steel by zinc-1-hydroxyethylidene-1, 1-diphosphonic acid mixtures

The action of 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP) in combination with zinc ions as a corrosion inhibitor for mild steel in oxygen containing aqueous solutions has been investigated in relation to the presence of different zinc levels, the presence of aggressive anions, and different pH values. The highest inhibition effect was obtained at a 2.7:1 zince/HEDP molar ratio, corresponding to 60 ppm zinc, and was associated with a significant anodic effect. In the absence of chloride and with no free forms of the inhibitor components, the Zinc-HEDP inixture exhibited good protection over a wide range of pH (6.5-9.5). However, in a weakly acidic medium, protection could not be achieved. Calculations based on the dissociation constants of the HEDP and the stability constants of the possible zinc-HEDP complexes showed that this inhibition call be attributed to the presence of the 2:1: zinc-HEDP complex (Zn2H-1L-) at a sufficient level. It is found that this anionic complex call behave as an anodic, passivating, inhibitor at a concentration as low as 0.00015M (20 ppm zinc-32 ppm HEDP mixture). This passivity is prevented by the presence of sufficient chloride. The critical chloride concentration ill these studies is approximately tell times higher than the complex concentration, in molar terms. It is also found that free phosphonate is aggressive and can prevent passivity due to the preferential formation of soluble iron-HEDP complex. The tolerance of the 2:1 complex to free phosphonate is substantially less than to chloride, approximately equimolar.