Synergistic or additive corrosion inhibition of mild steel by a mixture of HEDP and metasilicate at pH 7 and 11

Electrochemical measurements (steady‐state current‐voltage curves and AC impedance) were coupled with mass‐loss measurements, SEM examinations, and EDSX analyses to investigate the inhibition of corrosion of a carbon steel by a mixture of phosphonic acid HEDP (acid 1, hydroxyethylene, 1‐1 diphosphonic) and sodium metasilicate pentahydrate Na₂SiO₃. 5H₂O in an industrial hard water containing 3.10^?3 M Ca²⁺ ions. At pH 7, HEDP and Ca²⁺ act in a synergistic manner, by formation of a HEDP and calcium containing layer. Addition of silicate at this pH value, allows to reach an efficiency of 94% due to an additive inhibition effect. At pH 11, metasilicate, HEDP, and Ca²⁺ ions reinforce the passive layer in a synergistic way. The mixture (1.7 · 10^?5 M HEDP + 2.6 · 10^‐3 M SiO^2?₃) in the Ca²⁺ containing electrolyte is shown to be able to inhibit efficaciously the corrosion of iron at room temperature, considering uniform corrosion at pH 7 or pitting corrosion at pH 11.