Corrosion Control in Drinking Water Distribution Using Simple Methods — Properties and Corrosion of Cast Iron and Steel

Metals are valued for their excellent mechanical properties and are often used in the distribution of drinking water. Ferrous materials include unalloyed and low-alloy steels, as well as cast iron. However, iron is thermodynamically unstable in water and corrodes through hydrogen corrosion, whereby hydrogen ions are reduced to hydrogen. In the presence of oxygen, oxygen corrosion occurs, in which dissolved oxygen is reduced to hydroxide ions. In both processes, the metal oxidizes and either passes into the corrosion layer or dissolves in the water. However, low oxygen levels can also lead to quality issues, such as instationary corrosion, which causes rusty water. Many older pipes in the supply network are not protected against corrosion, so corrosion problems are still expected, especially if the water properties change. For unalloyed and low-alloy steels, as well as cast iron pipes without internal protection, the corrosion types "surface corrosion" and "pitting corrosion" are particularly significant. Cast iron must also consider the selective corrosion type "spongiosis," which can occur in older cast iron pipes with lamellar graphite ("gray cast iron") or in inadequately protected cast iron pipes with spheroidal graphite ("ductile cast iron"). Currently, cast iron pipes used for drinking water are well-protected with cement mortar on the inside and a zinc and polymer coating on the outside, which allows them to have a long service life.