Is Zinc Coating The Answer For Drinking Water Pipelines?

With a new president-elect promising to focus his administration on sorely needed infrastructure development and a drinking water network consistently passing lead onto consumers, it is high time that communities across the country consider replacing their pipes. But before any major replacements can be made, a question has to be answered: What should old pipes be replaced with?

Last month, two communities joined others by answering that question with a material that could indicate a new path forward for drinking water infrastructure. They adopted zinc-coated ductile iron pipe.

“Since it is costing our ratepayers approximately $1.6 million per mile for our water main replacement program, it is important to get the most life out of our replacement pipelines as makes sense economically,” said Gary Gumm, chief engineer for the Washington Suburban Sanitary Commission (WSSC), which recently installed new pipes made from the material. “Zinc-coated ductile iron pipe is expected to last 100 years, which provides a better basis for the size of the program we are undertaking.”

Zinc coating is applied as a metallized spray or with zinc-enriched paint, going over the oxide layer that is created when the metal anneals and adding extra protection against corrosion. Engineers like Gumm are optimistic about the material because of its sustainability.

“Zinc coatings can extend the life of a pipeline by enhancing the already-present inherent corrosion resistance of ductile iron pipe,” said L. Gregg Horn, vice president of technical services at the Ductile Iron Pipe Research Association (DIPRA). “For those utilities that choose to use zinc-coated ductile iron pipe, we suspect that is the primary reason.”

However, it might be too early to say how the new pipes will meet expectations. While zinc-coated ductile iron pipes have been manufactured and shipped to Europe since the 1980s, according to the American Society of Civil Engineers, there hasn’t been much demand for domestic use to date. Kansas City may provide the oldest large-scale case study, having installed zinc-coated infrastructure in 2012. Though the list of U.S. communities served by zinc-coated ductile iron pipes continues to grow, skeptics remain.

“This is a very limited product that is essentially just being evaluated,” said Bruce Hollands, executive director of the Uni-Bell PVC Pipe Association. He said that zinc coating adds no improvements to the degradation of the cement mortar lining, which ultimately increases pumping costs as the interior wall of the pipe ages. “The zinc coating can easily be damaged and become ineffective through normal installation and operations processes such as tapping, cutting, and installing mechanical restraint devices.”

It’s also important to keep in mind that all zinc-coated ductile iron pipe is not equal. Many such pipes come with a V-Bio enhanced polyethylene encasement, an anti-corrosion safeguard offered by DIPRA and its member companies.

“The potential benefit [of zinc-coated ductile iron pipe] would come from added service life in a given soil environment,” said Horn. “Keep in mind, though, that we would not rely on zinc coatings alone to protect the pipe from an aggressive soil. In those soils, we recommend using V-Bio enhanced polyethylene encasement.”

For WSSC, installing zinc-coated ductile iron pipe costs an extra 1.25 percent per mile than its previous program. Given that Gumm expects the pipes to last twice as long, he estimates an obvious return on investment.

It will be up to each community to decide for itself whether these pipes seem like a prudent use of limited infrastructure funding. For the time being, it might be best to wait and see what happens in the places where it is already installed. However, if the time has come for replacement, the relatively new world of zinc-coated ductile iron pipe offers some promise.

“If a utility is experiencing corrosion issues that might be prematurely aging pipelines that otherwise would have an increased service life, they might want to look at this set of products and do the economic analysis and see if they might help their long-term bottom line,” Gumm said.