A Protective Film Component among Corrosion Products on Galvanized Steels Estimated by Solubility Calculations

Abstract

Galvanized steels are used extensively in atmospheric environments because of their high resistance to corrosion as well as their low cost of production. The corrosion resistance arises from the slow corrosion rate of the zinc layer that covers the steel (coating and shielding effect) and the low corrosion potential of zinc which keeps the steel under a reducing condition as long as zinc remains (sacrifice effect). However, detailed examination of the corrosion processes has shown that galvanized steels exhibit some corrosion resistance after the disappearance of metal zinc by corrosion. This effect has been ascribed to a protective nature of corrosion products from the zinc layer. It is important to establish the protective components of corrosion products for the elucidation of corrosion mechanisms and for the development and improvement of corrosion protection technologies. In this investigation, zinc oxide, zinc carbonate, basic zinc carbonate, and zinc ferrite were chosen as model corrosion products, it was noted that protective films must have very low solubilities and very low dissolution rates for them to persist, and their dissolution properties were examined by solubility calculations. The solubilities of zinc oxide, zinc carbonate, and basic zinc carbonate are similar and very large, indicating that these would be dissolved away in atmospheric environments. A farther corrosion product, zinc ferrite, is not a major component, but can be formed as a thin layer by the corrosion of the zinc-steel alloy phase. The solubility product of zinc ferrite was estimated from thermodynamic data and its solubility was calculated. The solubility is very small and it can be concluded that zinc ferrite is the most likely protective film component among the zinc compounds described above.