Inhibition by Zinc and 1-Hydroxyethane-1,1-Diphosphonic Acid of the Corrosion of Mild Steel Available to Purchase

Inhibition of mild steel corrosion by zinc-1-hydroxyethane-1,1-diphosphonic acid (HEDP) mixtures (zinc 0 to 100 ppm; HEDP <70 ppm) was studied in the presence of low (up to 100 ppm) levels of chloride, typical of cooling waters, and was correlated with the solution complexes present. Under static conditions, it was found that the 2:1 zinc:HEDP complex can behave as an anodic passivating inhibitor at concentrations as low as 0.00015 M. This passivation was attributed to the local formation of an iron salt of the inhibitive complex anion, as with other anodic inhibitors. This passivity was prevented by the presence of sufficient chloride and by the presence of free phosphonate. Both chloride and free-phosphonate species acted as aggressive ions. With ratios >2:1 (i.e., with free Zn²⁺) there was a significant effect on the cathodic oxygen reduction reaction (probably caused by precipitation of a basic zinc salt of the 2:1 complex), and at high ratios the anodic effect was absent. Under flowing conditions, on the rotating cylinder electrode (RCE), the anodic effect persisted at high ratios and the corrosion rate was lower than under static conditions.