Zinc-Coated Ductile Iron Pipe FAQS

What is zinc-coated ductile iron pipe?

It is ductile iron pipe that is sprayed with a zinc coating on the outside of the pipe and covered with an asphalt topcoat.

How does zinc protect ductile iron pipe?

Zinc serves as corrosion control or as a sacrificial anode. (Sacrificial anodes are highly active metals that are used to prevent a less active material surface from corroding.) When ductile iron pipe is coated with zinc, the zinc is consumed in place of the iron, which is why it is referred to as a sacrificial anode.

How long will zinc-coated ductile iron pipe last?

It is expected to last at least 100 years. Traditional ductile iron pipe has a life expectancy of 50-75 years.

Does zinc-coated ductile iron cost more than traditional ductile iron?

No, the overall construction cost of pipe replacement with zinc-coated ductile iron is the same as the replacement cost with traditional ductile iron.

Why is the zinc-coated ductile iron pipe wrapped before it goes in the ground?

WSSC has been wrapping its traditional ductile iron pipe with a polyethylene encasement (polywrap) since 2005. The polywrap prevents contact between the pipe and surrounding soil for an extra layer of corrosion protection, however it is not airtight or watertight. Now that WSSC has changed its specifications from traditional ductile iron to zinc-coated pipe, we also started wrapping the pipe with V-Bio® Enhanced Polyethylene Encasement. The V-Bio® encasement is an additional level of active protection that works in concert with the zinc coating. It targets anaerobic bacteria activity and inhibits the formation of corrosion cells under the wrap.

How long has WSSC being using traditional ductile iron pipe?

WSSC, like most water utilities used cast iron pipe until the mid to late 1960s. By the early 1970s, WSSC was using ductile iron pipe. Some classes of ductile iron pipe manufactured in the 1970s were notorious for being very thin, resulting in a life expectancy of 40-50 years. The industry changed the specifications by the mid to late 1970s to a thicker ductile iron pipe that became the standard. While some water utilities in the United States are starting to use zinc-coated ductile iron, WSSC is one of the few in the country that has changed its specifications to use zinc-coated ductile iron for all replacement (planned and emergency).

How long has zinc-coated pipe been around?

The water industry in Europe has been using zinc coatings on iron pipes since the 1950s, however zinc-coated ductile iron pipe is a relatively new product in the United States. In the past few years, water utilities in the United States have started to use  zinc-coated ductile iron pipe in pilot projects but WSSC is believed to be the first water utility in the country to make the combination of zinc-coated ductile iron pipe and V-Bio® Enhanced Polyethylene Encasement standard materials. That happened in February 2016. KC Water in Missouri is believed to be the first to make zinc-coated ductile iron pipe a standard specification, which was done in 2011.


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