Properties of Galvanized-steel bars

The proper execution of the galvanization process should guarantee that the temperature and the time of galvanization do not negatively affect the mechanical properties of the steel bars [34,35]. 

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

Since the pH of the concrete pore solution may vary in the interval where remarkable changes in the behaviour of zinc occur (Section 2.1.1), the behaviour of galvanized steel may be influenced by the composition of the concrete and, especially by the cement type and its alkali content. In practice, however, the pH of the pore solution in concrete usually is below 13.3 during the first hours after mixing, due to the presence of sulfate ions from the gypsum added to the Portland cement as a set regulator. A protective layer thus can be formed on galvanized bars. 

The passive film that forms on zinc not only reduces the rate of the anodic process (zinc dissolution), but even hinders cathodic reactions of oxygen reduction and hydrogen development. In conditions of passivity, the corrosion potential of galvanized steel is therefore much lower than that of carbon steel. Values typically measured are between -600 and -500 mV SCE compared to values above -200 mV usually found for passive carbon-steel reinforcement. 

Often galvanized bars are chromate treated in order to inhibit zinc corrosion and to control hydrogen evolution (the presence of small amounts of Cr in the cement also has a similar effect) [42]. 

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