Effect of concrete carbonation process on the passivating products of galvanized steel reinforcements

The composition of passivating products on galvanized steel reinforcement in concrete during carbonation was studied. Cube-shaped concrete specimens were manufactured with Portland 52.5 R cement and reinforced with hot-dip galvanized steel sheets obtained from pure Zn and Zn-Sn-Bi-Ni alloy bath. The concrete specimens were exposed to air curing for 28 days and then to the carbonation chamber. Corrosion rate and potential measurements were performed both during the curing in air and exposure in a carbonation chamber. At defined periods of time, some concrete specimens were broken and the galvanized steel sheets were submitted to XRD, SEM observations and EDX analysis. The growth of the passivation products was evaluated by integrating the diffraction peaks. XRD analysis and SEM observations show that the layer of calcium hydroxyzincate (CaHZn), formed on the galvanized sheets during the air curing, is destroyed by the concrete carbonation process. However, potential measurements indicate that the galvanized steel always remains in the passive state. X-ray diffractometry was not able to identify the new passivating product; EDX maps suggest the presence of zinc carbonates.