Tech Forum: Protecting Ductile Iron Pipe
Gaston
Like hand-in-glove, zinc coating and V-Bio enhanced polyethylene encasement protect ductile iron pipe against the most adverse environments. This was recently outlined and explained within the Design Decision Model.
First revealed in 2003 and recently updated, the model is a collaborative development of Corrpro, the world’s largest corrosion consulting firm, and the Ductile Iron Pipe Research Association (DIPRA).
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The Design Decision Model is a two-dimensional matrix that uses points to assess likelihood and consequence of corrosion. The resulting recommendations are commensurate actions to halt corrosion before it begins, thereby assuring sustainability and resiliency of ductile iron pipeline systems.
As most readers know, corrosion is an electrochemical process involving chemical reactions and electron transfer. Corrosion affects all infrastructure and all materials. Although many materials, such as plastic, are perceived by the public not to corrode, they do but in a different way than more familiar metals. For example, ultraviolet degradation and fatigue are areas where plastic is particularly vulnerable to corrosion.
Corrosion is simply a manifestation of entropy, the second law of thermodynamics, and a universal principle. Some types of corrosion more common to water systems are concentration cells, pitting, impingement, microbially influenced corrosion, stress and fatigue, dezincification, and atmospheric. Zinc coating and V-Bio enhanced polyethylene encasement protect against any of these types of corrosion that are caused by the surrounding environment.
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In 2003, DIPRA and Corrpro introduced this Design Decision Model – a collaborative multi-year project using 75 years of DIPRA soil surveys, test data, dig-ups and hands-on experience, as well as Corrpro’s renowned corrosion science and expertise. In 2013, following a decade of research and testing, DIPRA introduced V-Bio enhanced polyethylene encasement – an innovative infusion within the protective encasement that prevents the development and expansion of galvanic corrosion cells and depolarizing bacteria.
In 2015, a second significant development occurred within the industry, as ductile iron pipe producers made metallized arc-spray zinc coating commercially available. This meant the Design Decision Model needed to be reviewed and updated. DIPRA and Corrpro also chose at this time to make public the values assigned to the likelihood and consequence axes of the model. This intersection of likelihood and consequence determines one of five recommendations for corrosion abatement. Four of these include V-Bio enhanced polyethylene encasement, and two include the use of metallized arc-sprayed zinc coating.
But how does zinc work with ductile iron pipe to prevent corrosion? Zinc provides galvanic protection to iron. The topcoat over zinc provides a protective layer against exposed zinc and traps zinc oxides to form a protective shell. If scratched or damaged, a galvanic couple is created, resulting in continued corrosion protection.
Zinc as a protective coating for iron pipe has been used since 1958, and an international organization for standardization protocol (ISO 8179-1) was developed in 1985. Every major worldwide producer of ductile iron pipe is capable of applying zinc, and many offer it as standard. The delayed embrace of zinc coatings in the U.S. ductile iron pipe market was related to the emphasis on polyethylene encasement as an effective and preferred protective system.
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Used together, zinc protects V-Bio, V-Bio protects zinc and both protect iron. V-Bio polyethylene encasement isolates iron from a fresh supply of oxygen thus inhibiting corrosion. It eliminates microbial cells that can deplete the zinc and attack the iron. Not all conditions require the use of polywrap, but V-Bio provides another layer of protection.
Data and photographs from specimens of zinc-coated ductile iron pipe buried within the intensely corrosive soils of the Florida Everglades were recently presented at the NACE 2019 Corrosion conference in Nashville, Tennessee. In these harsh-environment demonstrations, V-Bio enhanced polyethylene encasement was intentionally damaged, and the protective zinc coating was intentionally damaged. The results showed how V-Bio protects against not only corrosive soils and bacteria, but also against damaged zinc coating. They also showed how zinc coating protects against damaged V-Bio enhanced polyethylene encasement. Like hand-in-glove, each supports the other.
These developments and test results show how the ductile iron pipe industry is continuing to innovate and further strengthen an already sustainable and resilient product, adding long-term asset value to every water system that specifies it. With more than 600 U.S. water utilities having iron pipe in continuous service for more than 100 years, the service record of iron pipe supports this. The addition of zinc to the corrosion prevention arsenal will ensure the continuity of iron pipe’s exceptional record of resilience and sustainability.
Maury Gaston, manager of marketing services for AMERICAN Cast Iron Pipe.
