Zinc has been used to protect ferrous materials against corrosion for many years. Metallized arc-spray zinc coating is now available on ductile iron pipe produced in the United States and has been widely accepted and specified since its domestic advent several years ago.
This paper will present an overview of corrosion principles, how zinc is applied to ductile iron pipe, how zinc inhibits the corrosion of ferrous materials, and additional enhancements that, in combination with zinc, further extend the life of an already long-lasting product. A recently revised corrosion risk assessment and corrosion abatement tool will also be presented.
Finally, the paper will briefly highlight the performance characteristics of ductile iron pipe related to pressure, surge, flow characteristics, thrust restraint, and sustainability.
Overview of Corrosion Principles
Corrosion is an electrochemical process involving chemical reactions and electron transfer. Within the corrosion family, galvanic and electrolytic corrosion occur in water systems.
Galvanic corrosion involves two dissimilar metals and a conductive path between them. The less noble metal is the anode. It gives up electrons and is the place where corrosion is evident. The more noble metal is the cathode. It receives electrons and does not show evidence of corrosion. Figure 1 shows a high-level galvanic series with metals arranged in order of their nobility. The metals to the left are less noble and more likely to corrode.
In a water utility system, different metal types are used for the various components within the system. Steel, iron, brass, copper, and other metals are all likely to be in use. These dissimilar metals can be protected by providing a uniform and non-conductive environment around them that will prevent or reduce the transfer of electrons, i.e., corrosion.
If the environment around a single type of metal varies sufficiently, electron transfer can occur from one part of that single metal to another. That is why one method of corrosion protection is to create a uniform environment around the metallic mass.