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Galvanized Steel

When galvanized steel comes in contact with wet cement (i.e., freshly placed concrete), the formation of the passivating film of CaHZn crystals is accompanied by the evolution of hydrogen gas. Though quite small quantities of hydrogen are produced, this reaction generally lasts for only a short period of time and effectively ceases once the cement starts to harden. An important consideration here is that the hydrogen evolved from a galvanized surface largely occurs where iron and zinc are in contact, but not on a pure zinc surface, suggesting that it is the zinc-iron alloy layers near the surface of the coating that initiate the formation of hydrogen. This being the case, it is to be expected that the evolution of hydrogen will not be a serious issue if the outer layer of the coating is predominantly pure zinc, which is the situation with both hot-dipped bright galvanized bars and also continuously coated bars.

While the evolution of hydrogen from a galvanized surface is possible (as occasionally observed in precasting operations where galvanized steel forms are used), the practical effects of this are often overemphasized. Concerns are sometimes expressed that the presence of hydrogen bubbles in and around the transition zone between the bar and the adjacent concrete will reduce the bond strength of galvanized bars in concrete. This matter has been the topic of extensive research that has almost universally demonstrated that there is no reduction in bond strength for galvanized bars compared with equivalent black steel (Kayali, 2004) at or beyond 14–28 day testing.

The evolution of hydrogen evolution from the zinc coating surface can largely be eliminated if the coating is passivated by another means, the most effective of which is by treatment of the freshly galvanized steel with chromate salts. This is achieved by either quenching the freshly galvanized bars in water containing 0.2% sodium dichromate or by adding chromates to the concrete mix-water at a rate of 70 ppm, expressed as CrO3 by mass of cement.

The advantage of the concrete mix-water addition approach (which has been widely used in the precasting industry) is that it ensures that the galvanized product is passivated at the actual time it is cast into concrete. However, for bar that is so-called quench passivated as the final step in the galvanizing process, there can be no guarantee that the passive film remains on the bar surface at the time of the concrete pour. This is because the chromate passivating film naturally deteriorates over time, the speed, and extent of which depends on both the nature of the storage conditions (in particular coastal or marine exposure) and the time of exposure prior to casting. For example, it is not uncommon that the chromate surface film would have been lost from galvanized product stored in moist chloride-containing conditions in just a few weeks.

Quite apart from this, however, there are serious occupation health and safety issues, and also environmental concerns, with the use of hexavalent chromium salts. As such, the use of chromates in many parts of the world has been severely restricted, and stringent environmental regulations have been put in place. As an aside, it is to be noted that in a number of countries, even the natural occurring chromate in fresh cement is required to be removed.

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