Localized Corrosion due to the Reverse of Electrical Potential between the Zinc and the Steel of Galvanized Steel Pipes

Abstract

The standard electrode potential of zinc is 323 mV lessnoble compared with iron （steel）. However, zinc shows high corrosion resistance with the oxide film formed on the surface in pH neutral-weak alkali area compared with iron. Therefore, galvanized steel is used all over the world. However, the potential of zinc might rise while using it, and the potential reversal of zinc and steel be caused. In that case, the corrosion of steel is accelerated because there is an effect with cathode/anode area ratio in the zinc plating layer defect part by the localized corrosion etc., too. As a result, it might cause the water leak accident. Actually, the localized corrosion that originated in the potential reversal of steel/zinc frequently occurred in the cold and hot water piping and the sprinkler piping system. The potential reversal of steel/zinc was verified by the sample measurement. In this report, it introduces the corrosion case with building piping systems by the potential reversal of steel/zinc. Moreover, the mechanism and measures are described.