Influence of Hardness Salts on the Effectiveness of Zinc-1 Hydroxyethylidene 1,1 Diphosphonic Acid (HEDP) Mixtures in Inhibiting the Corrosion of Mild Steel in Neutral Oxygen-Containing Solutions Available to Purchase

This paper presents an electrochemical study of the effect of calcium salts on the corrosion inhibition of mild steel by zinc-1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) mixtures and compares the effect of calcium and zinc additions on the effectiveness of HEDP. The results are considered in terms of the nature, concentration, and stability of the solution species present, especially the metal–HEDP complex anions. Calcium addition showed some inhibition for the corrosion of mild steel, but it is much lower than for zinc. This is associated with the low stability of calcium complexes compared with ferrous complexes, which facilitates the displacement of calcium ions from their complexes by ferrous ions forming soluble, unprotective, ferrous complexes. The zinc–HEDP mixtures give effective inhibition at a zinc:HEDP molar ratio of 2:1 and are attributed to anodic inhibition by the 2:1 complex anion, Zn₂H_–1L^–. This inhibition is significantly affected by the presence of sufficient-free phosphonate, which is considered an aggressive anion. However, the presence of free phosphonate is not detrimental if calcium is present in the solution. Both work together to eliminate the negative effect of free phosphonate and the hardness salt, the presence of which could affect the performance of zinc–HEDP mixtures or contribute to scaling problems.