Performance Variances of Galvanized Steel in Mortar and Concrete

Mild steel is used as reinforcement in concrete structures because it is passivated by the highly alkaline cement paste system, preventing typical corrosion. Two processes can corrode the initially passivated steel: air carbonation and chloride (Cl⁻) attack. To eliminate those destructive processes, steel often is galvanized by coating with zinc (Zn). Zn protection usually is attributable to two mechanisms. First, Zn is “sacrificial.” All Zn must corrode before any of the underlying steel corrodes. Secondly, the usual corrosion product of Zn, zinc oxide (ZnO), occupies about 50% more space than the metal, in contrast to the 100% or higher increase for rust. Despite those mechanisms, instances have been reported where galvanized steel performed as poorly as ordinary steel when embedded in mortar or concrete. Several masonry structures made with mortar that contained large concentrations of Cl⁻ have cracked at the location of corroded galvanized steel. Examinations have shown only the Zn corroded. No rusting of the underlying steel occurred. This experience was duplicated in the laboratory with reinforced concrete. Analysis of the corrosion products of galvanized steel in such Cl⁻-containing mortar and concrete showed the Zn corrosion product under some conditions was zinc hydroxychloride II (Zn₅ [OH]₈Cl₂·H₂O). This product has been found by others, but its significance has not been recognized. X-ray diffraction analyses determined the unit cell of this corrosion product. The corrosion product occupied 260% more space than Zn and produced a much greater expansive force than most forms of rust. It was theorized that differences in performance of galvanized steel were the result of the corrosion product’s form.