2005 Volume 91 Issue 9 Pages 706-711
Detailed examinations of the corrosion processes of galvanized steels have shown that the steels exhibit some corrosion resistance after the zinc has corroded away. This corrosion resistance has been ascribed to a protective nature of the corrosion products from the zinc layer. Rusts exposed to environments containing air and water are subject to dissolution, and the protective components in the rust must be resistant to dissolution to persist and play a protective role in steels. In this investigation, zinc oxide, zinc carbonate, basic zinc carbonate, zinc ferrite, magnetite, and hydrated iron(III) oxide were chosen as model corrosion products, and their solubilities in water were examined as a function of pH. 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. The values of the solubility products of zinc ferrite and magnetite have not been reported and were estimated from thermodynamic data for the solubility calculations. The solubilities of zinc ferrite, magnetite, and hydrated iron(III) oxide are very much smaller than those for zinc oxide and carbonates. At pH lower than 7 the solubility order is: zinc carbonate-zinc oxide-basic zinc carbonate>>zinc ferrite>magnetite>hydrated iron(III) oxide. The compounds containing iron(III) ions are likely to show the adequate resistance to dissolution and to form the protective film. It is known that protective rusts are composed of fine particles of low crystallinity.The protective nature of these compounds can be explained by the effect of zinc ions on the crystalinity, particle size, and other properties of the compounds during their formation and growth.
