INFLUENCE OF TWO SPECIFIC INHIBITORS ON STEEL CORROSION IN A SYNTHETIC SOLUTION SIMULATING A CARBONATED CONCRETE WITH CHLORIDES

The use of corrosion inhibitors can be an efficient way to stifle steel corrosion in concrete, in the presence of aggressive conditions. In this environment, severe conditions are linked to the presence of concrete carbonation and/or relevant amounts of chloride ions. Both phenomena tend to destroy the protective iron oxide layer which spontaneously forms on steel in sound concrete and this leads to high steel corrosion rates. A common practice is to study corrosion inhibition formerly in synthetic solutions, and afterwards to test the obtained results in concrete. The present research aims at studying corrosion inhibitors in a synthetic solution, simulating the composition of the pore solution in a carbonated concrete, polluted by chlorides. The tested additives were sodium glycero-phosphate (GPH) and sodium 2-amino-benzoate (2AMB): the former was an efficient inhibitor of steel corrosion due to carbonation and the latter could inhibit corrosion in the presence of chlorides. The tests were carried out to verify if the simultaneous use of the two substances was compatible and if their addition can efficiently hinder the effects of both chlorides and carbonation. The concentration of these two substances has been varied to determine the lowest concentration necessary to reduce corrosion to an acceptable level. The synthetic solution was prepared by bubbling carbon dioxide through a saturated (and filtered) solution of Ca(OH)2, containing 0.1M NaCl, in order to reach pH 7. The electrochemical techniques adopted to evaluate the inhibition efficiency and to study the inhibition process consisted in polarization curves recording and electrochemical impedance (EIS) measurements. Usually the polarization curves were recorded on independent specimens after 45 min immersion, since this was a valid and rapid tool to screen the tested additives. Impedance measurements monitored the evolution of the corrosion inhibition process during up to 100 hours immersions In this synthetic environment, steel corrosion was markedly inhibited for at least 100 hours by the additive mixture at the highest tested concentration (0.05M). When the concentration was halved, the inhibiting effects were still high but less persistent. If the concentration was reduced to 0.01M, the inhibiting effects lasted only a few hours.