Facts About PEX Tubing and PEX Fitting Dezincification
The use of synthetic materials in residential plumbing and heating systems built in the last 20 years has increased significantly. PEX tubing has become a major component of the plumbing system and hydronic heating loops and it can also be found in the renovated plumbing systems of older homes. PEX is made from crosslinked High Density Polyethylene (HDPE) polymer. Crosslinking is a chemical reaction that allows HDPE to become stronger, more flexible, and more resistant to cracking due to cold temperatures or upon impact. PEX has several advantages over copper pipe and rigid plastic pipe such as PVC. It is resistant to scale and chlorine, doesn't corrode or develop pinholes, it is faster to install since it is flexible and has fewer connections and fittings.
With these advantages comes a hidden problem; brass fittings and valves used with PEX can corrode and fail prematurely. Brass is a copper alloy that may contain between 5 to 40% zinc. Zinc is added to brass to increase the alloy’s strength. Since the corrosion of brass increases as the percentage of zinc increases, it is generally recommended that the percentage of zinc in brass fittings be less than 15% although some experts say that number can safely go to 19%. Low quality brass fittings and valves on the market have a zinc content of 35% or more.
Zinc is a highly reactive metal and it has a weak ability to bond to other metals at the atomic level. When water flows through the fittings and valves it corrodes the metal; zinc leaches from brass and creates a powdery buildup inside the fittings. This corrosion process is known as dezincification, and it causes two main problems. First, the zinc builds up inside the walls of the fitting which decreases water flow and eventually causes a blockage. Second, the porous copper-rich structure that is left behind has little mechanical strength and is prone to seepage and structural failure.
Corroded Brass Fitting
Signs of Dezincification
A valve suffering from dezincification has a white powdery substance, which is zinc oxide, on its exterior surface. The valve may exhibit water weeping from the valve body or stem/bonnet seal. When the situation is severe, there can be signs of water damage in the floors, ceilings, and walls surrounding the faulty plumbing.
Cross-section of Fitting Dezincification
Here are some of the situations which have been present where dezincification has occurred:
• Water containing high levels of chlorine, chloride ion, oxygen and carbon dioxide
• Water high in mineral content, otherwise known as "hard water"
• Stagnant or slow moving waters
• Slightly acidic water that is low in salt content and at room temperature
• Neutral or alkaline water that is high in salt content and at or above room temperature
• Recirculating hot water loops.
Several manufacturers of brass fittings and valves used in PEX systems are susceptible to dezincification. Some manufacturers are involved in class-action lawsuits, and you should research the manufacturer of your fittings to determine if they are involved in a recall of any brass components.
Some companies now sell "dezincification-resistant" brass fittings, but check with the manufacturer to verify that the percentage of zinc is less than 15% before installing them in your home.
A complex set of conditions must be present for dezincification to occur, and the occurrence is often related to a region of the country. There is a higher probability of dezincification occurring in Galveston, TX, and Oklahoma City, and in regions using Colorado River water which has a high salt content. A number of municipalities scattered throughout the United States also have reported valve failures from dezincification attributable to an increased chloride ion content.
Water hardness has been a common occurrence in many, but not all, of the failures. Here is a map of water hardness on the U.S. with PEX failure area concentrations.
Differences in PEX Tubing
California is currently looking into alternatives for PEX tubing and fittings. They have found that some PEX leaches contaminants into the drinking water at levels higher than allowed by law. They have also found that PEX is permeable from outside chemicals like pesticides and petroleum products. California is considering mandating only PEX tubing which can meet NSF 61 and NSF P171 Cl-R, which tests chlorine recirculation.
Chlorine recirculation is important in hot water heating applications when chlorinated water (public) is used in the hydronic system and is continuously recirculated. Over time the chlorine can break down the interior of the pipe and weaken it to the point of failure.
Peroxide (Type A)
Peroxide Method (Engel Process) used to manufacture PEX pipe.
The main disadvantage of peroxide cross-linking is that certain additives (e.g. antioxidants, oils,) must be avoided because they consume free radicals. Therefore, in order to achieve the minimum required cross-link level, the use of antioxidant must be minimized. Hence, minimal stabilization for aggressive water condition (high chlorine) results.
As of January 1, 2007 no peroxide (Type A) PEX can or has passed and received NSF P171 Cl-R (chlorine-recirculation) test.
Silane (Type B)
Silane cross-linking uses a siloxane bridge to link polyethylene molecules. This is referred to as PEX-B in Europe.
After extrusion, the pipes are cross-linked by using moisture as the active reagent. The cross-linking involves a hydrolysis and condensation reaction. This process does not generate or require free radicals for cross-linking. Therefore, PEX pipe made by the silane method can be properly stabilized with the appropriate antioxidants. The antioxidants are not consumed in the cross-linking step and are available to protect the pipe in its end use application.
Only PEX pipes made by the Silane method have been able to meet the continuous re-circulation requirement of NSF P171 Cl-R Protocol.
Radiation (Type C)
This method uses high-energy electron beam or gamma ray for cross-linking. is referred to as PEX-C in Europe.
Polyethylene pipe is first extruded, and then bombarded with radiation in a second step. Hydrogen atoms are ‘knocked off’ from the carbon atom by this high-energy radiation leaving behind a free radical polymeric chain. This free radical is unstable and will seek another free radical to form a stable bond or cross-link. As irradiation progresses, more and more bonds are formed to create a cross-linked structure. However, as mentioned above, the presence of antioxidants will consume free radicals. Therefore, the use of antioxidants must be minimized in order to obtain the desired cross-link level. Hence, minimal stabilization is possible.
As of January 1, 2007 no radiation (Type C) PEX can or has passed and received NSF P171 Cl-R (chlorine-recirculation) test.
Consideration in selecting the right PEX for your home or project:
• Is there chlorine present at your water source?
• How much chlorine is present on a daily average?
• Will the pipe be exposed to UV light prior to or during installation?
• What is the expected accumulated UV exposure time?
• What is the maximum allowed UV exposure? (Type A is typically limited to 15 days and 30 days.)
Verify your PEX pipe manufacturer
To verify what each PEX manufacturer has achieved in third party testing of its product for use in traditional domestic (NSF P171 Cl-TD) and re-circulating (NSF P171 Cl-R) potable water systems that contain chlorine, please refer to the NSF web site. Site directions are:
1. Go to: www.nsf.org
2. Select "Search Listings" at the bottom of the page.
3. Select "Plumbing Systems Components" at the bottom of the page.
4. Product Standard list, select "Protocol P171".
5. Select "Search".
If you discover flexible plumbing lines in your home and you are not sure if there is cause for concern, contact a professional home inspector or licensed heating contractor or plumber trained to recognize the presence of dezincification.
Toolbase Services provides a very good design guide for PEX installations.
Copper Development Association
A Note On The Dezincification Of Brass And The Inhibiting Effect Of Elemental Additions
Glossary of Terms
Dezincification: A form of de-alloying corrosion in which the zinc-rich constituent of a brass alloy is selectively removed.
Galvanic series: Galvanic corrosion potential is a measure of how dissimilar metals will corrode when placed against each other in an assembly. Metals close to one another in the series generally do not have a strong effect on one another, but the farther apart two metals are, the stronger the corroding effect on the one higher in the list.
Zinc oxide: A compound of zinc and oxygen often observed as a white powder on the exterior surfaces of components attacked by dezincification corrosion.