Environmental factors affecting the corrosion behavior of reinforcing steel. IV. Variation in the pitting corrosion current in relation to the concentration of the aggressive and the inhibitive anions

The pitting corrosion current of reinforcing steel is measured under natural corrosion conditions in Ca(OH)(2) solutions in presence of Cl- as aggressive ions and CrO42-, HPO42-, NO2-, WO42- and MoO42- as inhibiting anions. The corrosion current starts to flow after an induction period which depends on solution composition (concentration, pH and presence or absence of the aggressive and the inhibiting anions). The limiting corrosion currents increase with increasing the Cl- ion concentration and decrease with increasing the pH and inhibiting ions concentration. The inhibition efficiency of the studied inhibiting ions increases in the following order: CrO42- < HPO42- < NO2- < WO42- < MoO42-, and depends on the way by which the inhibitor is added to the solution. Injection of the inhibiting anions in solution causes repassivation of the pre-formed pits through competition with Cl- ions for adsorption sites on metal oxide surface. The adsorbability constant and the free energy of repassivation of the inhibiting anions are calculated. (C) 2010 Elsevier Ltd. All rights reserved.