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 Na 2 SiO 3 . 5H 2 O in an industrial hard water containing 3.10 À3 M Ca 2þ ions. At pH 7, HEDP and Ca 2þ 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 Ca 2þ ions reinforce the passive layer in a synergistic way. The mixture (1.7 · 10 À5 M HEDP þ 2.6 · 10 -3 M SiO 2À3 ) in the Ca 2þ 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.