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

摘要

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{sub}2SiO{sub}3. 5H{sub}2O in an industrial hard water containing 3 × 10{sup}(-3) M Ca{sup}(2+) ions. At pH7, HEDP and Ca{sup}(2+) 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{sup}(2+) ions reinforce the passive layer in a synergistic way. The mixture (1.7 × 10{sup}(-5) M HEDP + 2.6 × 10{sup}(-3) M (SiO{sub}3){sup}(-2) in the Ca{sup}(2+) 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.