Since the 1990s, two types of epoxy coatings have been commonly specified and used for iron valves in the waterworks industry: fusion-bonded epoxy and liquid epoxy. Both coatings are based on thermo-set epoxy systems with similar corrosion resistance and are described in the American Water Works Association’s Standard AWWA C550, “Protective Interior Coatings for Valves and Hydrants.”
Fusion-bonded epoxy is applied to preheated components in powder form in an electrostatic or fluidized bed process followed by thermal curing. Liquid epoxy is a two-component mixed material that is applied by spray, brush or other methods and chemically cures after application. The purpose of this article is to explain the typical requirements that apply to these coatings and compare some of their properties.
VALVE COATING STANDARDS
AWWA set the standard for waterworks valve interior coatings to provide long-term corrosion resistance for water, wastewater and reclaimed water service having a pH range of 4 to 9 (AWWA, 2013). The epoxy coatings are qualified for this service by subjecting test coupons to 90-day immersion tests at the full range of pH at 158°F. The coating is also tested for impact resistance by dropping a weight onto the surface in accordance with ASTM D2794. In production, coatings are visually examined for defects and randomly tested to verify coating thickness. The result has been the production of valves with highly reliable epoxy coatings for over 40 years.
Over the last decade, there has been significant debate among valve users and producers about setting standard requirements for epoxy coating thickness and holiday testing. Because of the intricate geometry of gate valves, providing uniform coating thicknesses and holiday testing on a production basis can be costly. Therefore, the associated valve standards require only a minimum coating thickness of 6 mils for gate valves and 8 mils for quarter-turn valves. Project specifications and manufacturer’s specifications typically indicate a higher coating thickness in the range of 8 to 16 mils depending on the coating and application. Holiday testing is important to guarantee the integrity of the coating, but production holiday testing has been deemed an “optional” requirement based on the purchaser’s ability to bear the cost.
Additional requirements were initiated by the U.S. Environmental Protection Agency, which in the 1990s began regulating drinking water additives including treatment chemicals and water system components including valves and coatings. A consortium of organizations produced a standard ANSI/NSF 61, “Drinking Water System Components – Health Effects,” which provides testing protocols for exposing products to test water and measuring contaminant levels extracted during a 14-day emersion test. Various state water authorities slowly adopted the NSF 61 requirements resulting in an entire industry of testing authorities with labs dedicated to certifying products to this new standard.
From the start, the waterworks valve industry found the NSF 61 approval process to be burdensome and costly. Even though all the materials in a valve, including the coating, could be independently tested to verify compliance with NSF 61, the standard required that actual production valves be tested in independent labs on a frequent interval. Ironically, NSF 61 does not consider the impact strength, adhesion strength and the corrosion resistance of the coating, only the coating’s propensity to add contaminants to the water system.
Moreover, while treatment chemicals and pipeline coatings may have a significant impact on water quality because of their immense water contact surface area, a valve and its coating comprise an insignificant percentage of a water system’s surface area. Normalization factors were developed in the standard to take this into account resulting in coating manufacturers certifying various coatings for either pipe or valve service. Finally, purchasers are warned in valve standards that specifying alternate coatings or materials will invalidate the valve’s NSF 61 certification (AWWA, 2015).
In general, the AWWA standards defer to the epoxy coating producers who publish a set of surface preparation requirements, including “surfaces to be coated must be dry, clean and free of oil, oxidation and foundry dust.” The substrate should have a minimum 1.5 mil roughness profile with no sharp edges to anchor the coating. Valve manufacturers typically employ a near-white grit blasting operation to meet these requirements and provide a good surface profile for the coating. Care is taken to prevent oxidation of the blasted surfaces before coating by starting the coating process within the same work shift as blasting.
Liquid epoxy is furnished as a two-part kit that is thoroughly mixed and applied to the valve surfaces by spray, brush or roller, taking care to vent the vapors to promote the removal of the solvents from the coating. Because of the solvents involved, there is typically a limitation to the thickness of a single coat, such as 16 mils. If a greater thickness is required, additional coats are applied within a prescribed coating window. The mixture also has a finite pot life of 3-5 hours depending on temperature and humidity conditions. Dry time for handling is typically 7 to 10 hours, but water immersion may require 5-10 days of additional cure time to assure full dispersion of the solvents.
The powder epoxy coating process involves a pre-heat process wherein the part is held in a large computer-controlled oven to 400°F for a prescribed period of time and monitored with an infrared thermometer (Figure 1) until the part reaches the desired pre-heat temperature, typically 350°F. The parts are either moved or conveyed to a spray booth where they are attached to an electrical source to achieve an electrostatic charge. The powder coating is then sprayed over the part and the parts are returned to the oven for post curing in the oven for 10-20 minutes. In some factories the heating and spraying process are controlled with a conveyor system. Once removed from the oven, the parts are allowed to cool before installation and water immersion (see headline photo). Fusion-bonded epoxy coatings do not require an additional 5-10 days of cure time as with liquid epoxy since no solvents are used.
After either coating process, all parts are visually examined to ensure adequate coverage and the dry film thickness is measured in random locations. When required by the purchaser, a holiday test is conducted to identify any voids in the coating in accordance with ASTM G62. In a holiday test, a voltage is applied over the coated surfaces and any continuity between the test wand and substrate surface will be indicated on the detector unit (Figure 2). Epoxy coatings less than 20 mils in thickness can be checked using a low-voltage (i.e., 22.5 to 80 volts) test unit while thicker coatings require the use of high-voltage test unit in the range of 500 to 10,000 volts, depending on the thickness of the coating. When voids are identified, the coating is repaired and retested.
EPOXY COATING DURABILITY
Both liquid and fusion-bonded coatings cure to a hard, smooth and glossy finish, ideal for valve interiors. Their resistance to damage due to handling can be compared by reviewing their direct impact resistance in accordance with ASTM D2794 and adhesion strength in accordance with ASTM D4541. AWWA C550 requires a minimum impact strength of 20 in-lbs, which can be met by liquid epoxies, but is not typically reported quantitatively. Fusion-bonded epoxy coatings typically have twice the impact strength and can be as high as 160 in-lbs. The adhesion strength is also rarely published for liquid epoxies but has been measured to be in the 1000 to 3000 psi range when good surface preparation practices are followed. In general, fusion-bonded epoxy coatings exhibit twice the adhesion strength in the range of 3000 to 6000 psi (Val-Matic, 2013).
Epoxy interior coatings have proven to be a reliable product for waterworks valves over the last four decades due to developments in materials and standards. These coatings prevent corrosion, tuberculation and wear in valves and promote efficient flow of fluids though piping systems.
1. American Water Works Association. AWWA C504-15 “Rubber-Seated Butterfly Valves.” Section III.A.11.a., 2015.
2. American Water Works Association. AWWA C550-13 “Protective Interior Coatings for Valves and Hydrants,” 2013.
3. Ballun, John, VALVE Magazine, “The Valve Industry’s Role in Climate Change, January 2017.
4. Val-Matic Valve, “Making Sure it Sticks” You-Tube, 2013. https://www.youtube.com/watch?v=eKJpcDZqU8I