Passivation Mechanism of Galvanized Steel Rebar in Fresh Concrete

We studied the passivation behavior of hot-dip galvanized steel (HDG) rebar and ordinary steel (black) rebar in fresh concrete using electrochemical techniques. Although the passivation behavior was considerably different, both types of rebar were fully passivated. The black rebar was immediately passivated after exposure to fresh concrete; however, the HDG rebar was passivated after being kept active for tens of hours. The corrosion rates of both types of rebars after passivation seemed comparable. To further investigate the difference, we monitored the passivation processes by electrochemical impedance spectroscopy using carbon steel and zinc electrodes in fresh mortar and saturated Ca(OH)2 solution, which simulated the water in the concrete pores; furthermore, we observed similar trends for fresh concrete. Initially, the Zn surface was partially covered with calcium hydroxy zincate (CHZ) whose coverage increased with exposure time. Finally, the surface was fully covered with a CHZ film after passivation.