The effect of zinc-to-HEDP molar ratio on the effectiveness of zinc-1, hydroxyethylidene-1,1 diphosphonic acid in inhibiting corrosion of carbon steel in neutral solutions

Purpose The aim of the present work was to study the synergistic effect of HEDP and zinc on inhibition of the corrosion of carbon steel in neutral oxygen-containing chloride solutions, and to investigate the effect of zinc-HEDP molar ratio on the effectiveness of the zinc-HEDP inhibitive mixtures. Design/methodology/approach The inhibition of the corrosion of carbon steel by zinc-HEDP mixtures in neutral oxygen-containing solutions was investigated in the presence of 0.003 M (106 ppm) chloride. Findings It was shown that the inhibition by these mixtures depended not only on the zinc/HEDP molar ratio but also on the concentration of both zinc and HEDP. HEDP concentration appeared to be crucial where good inhibition was not achieved at low concentrations and aggressive nature is observed at high HEDP levels. The effectiveness of the zinc-HEDP mixtures enhanced inhibition by increasing the zinc content of the mixture, but the mechanism was only effective to a certain level, above which the inhibition effect declined. The predominant corrosion control mechanism of the zinc-HEDP mixture was on the anodic (metal dissolution) reaction, but it also affected the rate and mechanism of the oxygen reduction reaction. Originality/value Demonstrates how the effectiveness of the zinc-HEDP mixtures can enhance inhibition by increasing the zinc content of the mixture.