| As the most widely used construction materials, the reinforced concrete structure and its failure has attracted more and more attention. The main reason is the attack from the surroundings suffered by the rebars embedded in. One of the most serious attack is brought by the chloride ion. For the protection of rebars, there are already many ways have been taken into use, the galvanized steel ,as an efficient protective measure, also have a range of applications. In this paper, the corrosion behaviour of the galvanized steel in the saturated Ca(OH)2 solution with different chloride ion has been investigated under the background of the application in the reinforced concrete structure of the galvanized steel.In the experiment, two aspects ,of both the protection about the galvanizing layer for the steel matrix and the attack about the chloride ion for the protective layer which has grown on the surface of the galvanizing layer immersed in the saturated Ca(OH)2 solution ,have been investigated. During the process of experiments, high magnification optical microscope, scanning electron microscopy(SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) have been used to observe the micro surface appearance under different magnification, study the corrosion products composites as well as various phases respectively. The electrochemical experiments were also employed to detect the protective behaviour of the surface corrosion product on the galvanizing layer via Tafel polarization curves and electrochemical impedance spectroscopy (EIS).The results of both the observation and electrochemical test data have helped to analyse the corrosion mechanism of the two aspects mentioned in the front parts of this passage. The analysed results have summarized as follows:The saturated Ca(OH)2 solution's pH value is 12.5, immersed in which ,the galvanizing layer will be corroded that firstly the corrosion pit hole appears on the surface with a priority of the grain boundary ,then the corrosion product calcium hydroxyzincate (CaHZn) grows from the hole as a way of dense ,each sheet calcium hydroxyzincate connects with the adjacent one, and lastly all of the sheets cover the entire surface of the sample. As a physical barriers of chemical bond with matrix, has good adhesion to the zinc layer, and the percent of coverage area decides the protective property; With the growth of calcium hydroxyzincate ,the coverage area enlarges ,and then the self-corrosion potential shifts towards the positive direction , EIS of the low frequency impedance modulus value increases while the corrosion current decrease; After 100h-immersing, the self-corrosion current is reduced to the vicinity of the threshold value 10-1μA/cm2 ,the galvanizing layer is passivated then.If the reinforced concrete structure works in corrosive environment, the existed Cl- will drive the zinc layer covered with CaHZn to destruction. The way of Independent growing and connected with each other in which CaHZn protect the surface leaves some tiny region on it ,this region facilitated Cl- to attack the coating. [Cl-]≤0.5mol/L, the passive film is attacked a little by Cl- in the weaker protective area with the corrosion product of alkali zinc chloride(ZnCl2·4Zn(OH)2·H2O), while in stronger protective area , the passive film will not be corroded, besides , CaHZn in homeostasis state can release the surrounding Ca2+ which will react with the dissolved CO2 gas to CaCO3 deposit. The partial continuous ZnCl2·4Zn(OH)2·H2O, surface deposit CaCO3 ,as well as large area dense CaHZn ,can still provide good protection to the Zinc layer.[Cl-]≥0.5mol/L, the self-corrosion current will exceed the threshold value 10-1μA/cm2 , the corrosion of the surface become more severe, the pitting damage has converted to localized destruction: plenty of ZnCl2·4Zn(OH)2·H2O has appeared because of much more Cl-, the loose ZnCl2·4Zn(OH)2·H2O don't have close combination to the matrix, and easy to crack off that can not play the protective role as in small region. After the Spalling of ZnCl2·4Zn(OH)2·H2O from the matrix .The ZnO layer has exposed out. ZnO layer is adjacent to the CaHZn nearby, however, the junction is not smooth but have gradation ,which can provide a guiding fast path to Cl- underneath the CaHZn that is collapsed gradually, so the corrosion area is enlarged, and then the metal matrix has destructed.In conclusion, as the dense CaHZn covered on the Zinc coating can efficiently delay the corrosion of Cl- to the matrix, in addition ,the Zinc coating can still help to protect the steel as sacrificial anode, the galvanized steel deserve to be endowed as an efficient anti-corrosion methods. |
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