Metallized Zinc Coating on Ductile Iron Pipe – Results of 50 Years of Corrosion Testing in One of the Most Aggressive Soil Environments Found in the United States

Abstract Zinc metallizing with a high purity zinc wire plus a topcoat has been utilized in Europe for external corrosion control of iron pipe for over 65 years and in the United States for over 35 years. This method of external corrosion protection for ductile iron pipe has been under evaluation by the Ductile Iron Pipe Research Association and by North American ductile iron pipe manufacturers since 1975. This paper summarizes 50 years of testing and research of metallic zinc coating on ductile iron pipe (DIP) in one of the most corrosive soils found in the United States – the Florida Everglades. The soil environments in these studies are described as “swampy organic muck” and classified as “uniquely severe” by AWWA(1) Standard C105. They have a resistivity between 80 and 240 ohm-cm, a very high potential for MIC (microbiologically influenced corrosion), and a fluctuating tidal salt/brackish water table. In this severely corrosive environment unprotected DIP with a wall thickness of 0.25” experienced complete wall penetration in less than 3 years and zinc coated DIP in less than 15 years. By comparison, pipe with the combination of metallic zinc used in combination with polyethylene encasement did not experience any penetrations during the studies and indicated an average time to a single penetration of 200+ years.