Corrosion inhibition of mild steel by sodium tungstate in neutral solution Part 3: Coinhibitors and synergism

Abstract

The corrosion inhibiting effects of three additives, sodium benzoate, calcium gluconate, and sodium glycerophosphate, as coinhibitors with Na₂WO₄ were investigated by measuring the corrosion weight loss of mild steel in distilled water and in 10⁻³M NaCl. The specimens were examined and photographed under a metallographic microscope. In distilled water each of the substances completely inhibits the corrosion of steel at a definite concentration. In mixtures with Na₂WO₄, complete inhibition was achieved when the concentration of each component was considerably less than the minimum required to prevent corrosion when present alone (synergism). An inverse, non-linear relationship governed the concentrations of the inhibitor and the coadditive. In 10⁻³M NaCl the Cl⁻ ion competed with the two additives for adsorption sites; corrosion retardation required higher concentrations of WO²⁻ ₄ and its coinhibitor. In the presence of lower inhibitor concentrations the test coupons suffered pitting attack, the form of which depended on the absolute, as well as the relative, amounts of the two additives. The results indicated further that organic additives could retard pitting corrosion.