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Concrete reinforced by galvanized reinforcing bars requires the addition of inhibitor anions to passivate the zinc; otherwise evolution of hydrogen reduces the bond strength. In the present study, zinc in saturated calcium hydroxide solution, which is the principal electrolyte formed on initial hydration of cement, was found to passivate in the presence of either sodium chromate or chromic oxide. In both cases chromate ions were reduced instead of water so that there was no evolution of hydrogen. The passive film consisted of zinc chromate and chromic oxide, but calcium hydroxo-zincate was also produced, which assisted in passivating the zinc surface. In the case of chromic oxide a smaller concentration of chromate ions was present and passivated the zinc to a lesser extent. While 70 ppm of sodium chromate was sufficient to passivate the zinc, at least 300 ppm of chromic oxide was necessary to achieve the same degree of passivation. Chloride ions in the presence of chromate ions compete for the zinc surface and a critical concentration of chromate is necessary. This observation has been correlated with the passivation of galvanized reinforcing bars in concrete and it is considered that passivation is best achieved by the addition of 70-ppm sodium chromate to the concrete mix.
corrosion, concrete, mortar, reinforcing steels, galvanized reinforcement, chlorides, passivation, inhibitor anions
Senior lecturer, School of Metallurgy, The University of New South Wales, Kensington,
Materials engineer, Technisearch Limited, Collilngwood,