Effect of carbonation process on the passivating products of zinc in Ca(OH)2 saturated solution

The effect of carbonation process on the passivating layer of zinc in Ca(OH)2 saturated solution was studied. The investigation was performed by means of corrosion potential, corrosion current density, and impedance measurements. To analyze the changes in the passivating layer, X-ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM) were utilized. The results obtained indicate that the layer of calcium hydroxyzincate (Ca[Zn(OH)3]2 2H2O) (CHZ), which determines the passivity state of zinc both in Ca(OH)2 saturated solution and in concrete, is destroyed by the carbonation process, in agreement with previous results obtained for galvanized steel embedded in concrete. X-ray diffractometry and Raman spectroscopy showed that CHZ reaction with CO2 leads to the formation of Zn5(CO3)2(OH)6 (hydrozincite) and CaCO3 (calcite). SEM observation confirms the deep transformation in the passivating layer caused by carbonation. Corrosion potential and corrosion current density measurements show that zinc maintains its passive state also after carbonation. However, impedance measurements indicate that hydrozincite has lower passivating properties than calcium hydroxyzincate in the carbonated solution.