Passivation and Depassivation Behavior of Zinc in Concrete

Abstract

Steel rebar in concrete structure is in a passive state due to high pH in sound concrete. However, depassivation takes place by neutralization of concrete due to CO2 gas and penetration of chloride through the concrete. To extend the life time of concrete structure, it is of great importance to examine the degradation of concrete and corrosion of steel rebar. Recently, hot-dip galvanized steel rebar and epoxy-coated rebar are being used. They are expected for long durability. For the galvanized steel rebar, it has been reported [1] that zinc becomes passive by formation of Ca(Zn(OH)3)2・2H2O in simulated concrete solutions, although zinc generally corrodes in high alkaline solutions. In this study, passivation and depassivation behavior of zinc has been investigated in simulated concrete solutions, and fresh concrete. Special attention has been paid to effect of mass transfer in the concrete on the passivation and depassivation. Difference from carbon steel in passivation and depassivation was also discussed. A pure zinc plate and carbon steel plate were used as the specimens, and they were buried in epoxy resin as working electrodes. Two test solutions of a saturated Ca(OH)2 solution (pH12.5) and fresh mortar (water: cement: sand = 0.6:1:2) simulating concrete environment were employed. To determine a critical chloride concentration for the depassivation, NaCl was added into the test solutions and mortar. EIS was measured at certain intervals of the test, and Ecorr was continuously measured. To characterize the corrosion products, the surface and the cross-section were observed by SEM, and elemental analysis was conducted by EDX. The corrosion products were identified by XRD. The corrosion potential of zinc gradually shifted in positive direction, and reached around -0.5 V vs. SSE after 400 h in a saturated Ca(OH)2 solution. On the other hand, in a fresh mortar, it jumped up to about -0.5 V after 20 h. The charge transfer resistance (R ct) of zinc in the Ca(OH)2 solution and mortar was in the other of 106 Ω∙cm2 after the positive shift to -0.5 V in the both solutions. Since the R ct value was comparable to that of carbon steel in the same solutions, the zinc surface was passivated in the both solutions. The different time to passivation could be attributed to the difference of mass transfer of dissolved zinc ions. A compact Ca(Zn(OH)3)2∙2H2O film was observed in the whole surface of the passivated zinc. The passivation might occur due to highly protective Ca(Zn(OH)3)2∙2H2O film. On the other hand, in Ca(OH)2 solution at pH10.5, the R ct of zinc was about 104 Ω∙cm2, where porous ZnO was observed on the surface. In the presence of chloride, zinc and carbon steel were depassivated in the saturated Ca(OH)2 solution at pH12.5, when chloride was increased by 0.05 M. On the other hand, zinc and carbon steel were passivated in the fresh mortar containing 0.05 M NaCl. The film of Ca(Zn(OH)3)2∙2H2O was detected on the zinc surface in the mortar containing 0.05 M NaCl. They were depassivated in the fresh mortar when NaCl was higher than 0.5 M. A critical chloride concentration for the depassivation in the mortar was much higher than in mortar. This might be attributed to the difference of mass transfer. The threshold of chloride concentration for the depassivation of zinc in mortar (pH12.5) was almost same as that of carbon steel. Reference[1] Z.Q.Tan et al.,Corr. Sci., 50(2008),2512