Why do we need to use chromate treatment for galvanized reinforcing steel?
The passivation of the zinc surface is often done by chromating the galvanized steel. The high corrosion resistance offered by the use of chromate films is attributed to the presence of hexavalent and trivalent chromium in the coating. The trivalent chromium is believed to be present as an insoluble hydrated oxide (chromium hydroxide or zinc chromate), whereas the soluble portion of hexavalent chromium imparts a "self-healing" character to the film during the oxidative (corrosive) attack by species, such as chloride ions.
The chromate film on the galvanized reinforcing steel is formed by the chemical reactions of hexavalent chromium with a metal surface in the presence of other components, or "activators" in acid solution. Hexavalent chromium is partially reduced to trivalent chromium during the reaction, with a concurrent rise in pH, forming a complex mixture consisting largely of hydrated basic chromium chromate (Cr203 Cr03 x H20) and hydrous oxides of both chromium and the basis metal (zinc). As the acid is consumed, the combining power of chromium in the solution changes and precipitates chromium hydroxide. A film builds until the face next to the zinc becomes nonacid and this stops the reaction. The resulting gel-like film is allowed to harden. Chromate coatings on galvanized steel are usually yellow and consist of a mixture of trivalent and hexavalent chrome solution. The chemical reaction of the chromate to the galvanized steel is shown below:
This reaction occurs during the curing of the concrete. During the curing process, hydrogen gas evolution occurs between the galvanized rebar and the concrete. The use of a passivation treatment, such as the use of chromates, can help to reduce the hydrogen gas from forming. In solutions with such a high pH, the zinc is attacked by the alkalis (calcium hydroxide) in the fresh concrete, leading to the formation of calcium hydrozincite and hydrogen. This reaction could give rise to poor adhesion. Once the concrete sets, the hydrogen evolution ceases; the coating of calcium zincate formed apparently provides a protection to the reinforcement and no further attack takes places after initial setting. Tests of the bond strength on galvanized rebars showed equal to or slightly better bond performance when compared to black rebars.