Properties of inorganic corrosion inhibition admixtures in steel-containing OPC mortars Part 2: Electrochemical properties

The inhibiting action of molybdate, borate and nitrite ions against Cl-induced corrosion of steel embedded in OPC mortar has been investigated by impedance spectroscopy and linear polarization techniques. Analytical electron microscopy has also been carried out on pure iron powder dispersed in the same mix formulations. All specimens were cured under conditions which mildly accelerate corrosion: 40°C and 100% relative humidity (RH). Anion inhibition efficiency is found to vary with cure duration, but after 180 days their inhibition is in the order nitrite < borate < molybdate. Both impedance spectroscopy and electron microscopy analyses indicate that molybdate inhibition is achieved through surface layer formation involving molybdenum participation during the normal thickening of the passivating iron oxide film. Similarities between the modes of inhibition of molybdate and borate are inferred and overall mechanisms are discussed; borate is, however, a less satisfactory additive because it inhibits cement hydration.