| Premature failure of reinforced concrete structures are mostly caused by the corrosion of rebars embedded in the concrete. There are some pores exsited in the concrete, with the pore solution mainly composed of saturated Ca(OH)2, and has the pH value ranges from 12 to 13.5, the rebars is passivation. If the pore solution is carbonized by carbon dioxide via conversing Ca(OH)2 to CaCO3, the pH value will decrease, the rebars is depassivation, and the corrosion rate is increased. Hot-dip galvanizing is one of the most effective ways to protect rebars from corrosion, and has been gained much application and study home and obroad. However, detailes on the affection caused by diversification of pH value on galvanizing layer and protective calcium hydroxyzincate (CaHZn) which formed during the process of the dipping in the saturated Ca(OH)2 have not been clearly reported.In this paper, the corrosion product forming mechanism, microstructure, and transformation of corrosion product dissolving and corrosion resistance have been detected by the systemic work below: the samples of hot dip galvanizing and covered with protective calcium hydroxyzincate layer have respectively been put into the saturated Ca(OH)2 solution deploying in different pH value; after dipping, the samples were investigated with the electrochemical test analysis using a three-electrode system; the morphology of corrosion product formed under different pH conditions were observed by SEM, the composite and phase were analysed by EDS and XRD. The results have shown that:The surface of galvanizing samples dipped in the saturated Ca(OH)2 solution with pH value of 12.5 will be covered with stable protective CaHZn layer after a period, the property of which is influenced also by the pH value. As the pH value decreases from 12 to 9 and as the immersion time increases, the CaHZn decreases while Zn(OH)2 (changes to ZnO when dried and dehydrated) increases on the surface. The cover is totally composed of Zn(OH)2 when pH=9; When the pH value is in the range of 1213, there is few chip CaHZn on the zinc layer, and icorr decreases as the amount of CaHZn increases; The zinc layer is passivate under both condition above, and icorr is less than or close to 0.1μA/cm2; As the CaHZn becomes fewer and the Zn(OH)2 getting more and more, the value of polarization resistance (Rp), film resistance (Rc), penetration resistance (Rct) and the diffusion resistance (Ws) become lower, meanwhile, the corrosion current density increases; From 12.513.5, as the increase of pH value and dipping time, the CaHZn gets dissolved quickly without Zn(OH)2 existed. When pH value is 13.5, no protective corrosion products can be found on the surface, and the self-corrosion current grows ripidly to 100μA/cm2, the corrosion rate is accelerated remarkably.For the above zinc layer coverd by CaHZn immersed in the saturated solution, CaHZn decreases and Zn(OH)2 increases as the increase of pH value and dipping time. The cover is Zn(OH)2 when the pH value reachs 9; As the CaHZn becomes fewer and the Zn(OH)2 getting more and more, the value of polarization resistance (Rp), film resistance (Rc), penetration resistance (Rct) and the diffusion resistance (Ws) become lower, meanwhile, the corrosion current density increases, but is still close to the threshold value of 10-1μA/cm2. CaHZn is dissoluted gradually, and the corrosion rate is increased when pH>12.5, which is similar to that of hot-dipped steel.Carbonization can decrease the pH value, but the galvanizing layer with and without CaHZn still have better corrosion resistance to protect steel from corrosion damage; high alkalinity cement using can increase the pH value of pore solution, and the corrosion resistance of the surface cover on the steel sharply decreases which can not provide effective protection to steel anymore. Therefore, if hot-dip galvanizing steel is not used in high alkalinity cement concrete, it is worth being promoted in practical products with low cost, strong corrosion resistance.
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