Throughout most the world in developed countries, the public relies on access to a safe drinking water supply; something we frequently take for granted until it is compromised. Potable water frequently traverses through steel pipe to deliver it both safely and economically. However, because water in contact with steel will cause it to corrode, the interior of the pipe is coated or lined. These coatings or linings come in contact with the potable water. This article discusses testing procedures and the types of coatings that are frequently used to line water pipe, including their inherent advantages and limitations.
Potable water, also known as drinking water, is water that is safe to drink or to use for food preparation, without risk of health problems. Typically, in developed countries, tap water meets drinking water quality standards, even though only a small proportion is actually consumed or used in food preparation. Other typical uses of potable water include laundering, toilet supply, and irrigation. Examples of potable water would be tap water from treated municipal water systems, or that has been UV filtered, water distilled, or purified by reverse osmosis.
Differences between an interior coating and a lining
The McGraw-Hill Dictionary of Scientific and Technical Terms generally defines a coating as any material that will form a continuous film over a surface. A lining is described as a material used to protect inner surfaces as of tunnels, pipe, or process equipment. In the pipe industry vernacular, the term, “coating” is often referenced for the exterior surface of the pipe and “lining” is the term used to distinguish material on the internal pipe surface. However, an interior coating may suggest a thinner applied material compared to a lining which is often a thicker applied material. Therefore, an interior coating can be applied as a lining, and a lining is a type of coating.
Industry standards for interior coatings for potable water
Water treatment and distribution products in North America are required to comply with NSF International – The Public Health and Safety Organization (NSF) standards (NSF was an abbreviation for the National Sanitation Foundation). Forty-eight U.S. states have legislation, regulations, or policies requiring potable water system components to comply to NSF/ANSI 61. Eleven Canadian provinces/territories have similar compliance regulations. NSF/ANSI 61 sets health effects criteria for many water system components including:
- Protective barrier materials (cements, paints, and coatings)
- Joining and sealing materials (adhesives, caulks, lubricants)
- Pipes and related products (pipe, hose, fittings)
- Mechanical devices (water meters, valves, filters)
- Plumbing devices (faucets, drinking fountains)
- Process media (filter media, ion exchange resins)
- Non-metallic potable water materials
NSF SE 9857 is a specification for special engineered products and a requirement for internal epoxy pipe coatings produced specifically for use on the interior of metallic potable water pipe as applied by a mechanical means. The specification establishes the minimum testing, marking, and in-plant Quality Control (QC) requirements for epoxy coatings to be used on the internal surfaces of potable water pipe.
Specifications written to protect owner assets often include requirements of material conformance to American Water Works Association (AWWA) standards for coating systems to be used on the interior of water transmission pipe. Currently, there are 23 approved standards under the auspices of AWWA’s Steel Pipe Committee. Of these standards, 14 deal with coatings and linings that are available for the protection of metallic pipe, and five are applicable to linings of water transmission pipe. Distinct AWWA testing standards exist for each type of lining:
C203 – Coal-Tar Enamel Lining for Steel Water Pipelines
C205 – Cement–Mortar Protective Lining and Coating for Steel Water Pipe 4 In. (100 mm) and Larger—Shop Applied.
C210 – Liquid-Epoxy Coating System for the Interior and Exterior of Steel Water Pipelines.
C213 – Fusion-Bonded Epoxy Coating for the Interior and Exterior of Steel Water Pipelines.
C222 – Polyurethane Coatings for the Interior and Exterior of Steel Water Pipe and Fittings.
All AWWA steel pipe coating standards are based on the maximum service temperature of potable water. The purpose of the standards is to provide the minimum requirements for coating systems for the interior of steel water pipelines, including material, application, inspection, testing, handling, and packaging requirements.
Testing required to determine suitability for contact with potable water
NSF/ANSI 61 is a health effects standard that evaluates the level of contaminants that leach from the products into drinking water, rather than setting prescriptive limits on content. This differs from U.S. Food and Drug Administration (FDA) requirements that are based on prescriptive content requirements. NSF/ANSI 61 requires analysis for any chemicals that leach from a material into drinking water and it requires a toxicological evaluation of chemical concentrations to ensure that they are below levels that may cause potential adverse human health effects. The toxicological evaluation criteria are based on lifetime exposure to the concentration of contaminants in drinking water.
NSF toxicologists perform a formulation review of each water contact material to determine any possible ingredients, contaminants or reaction by-products that may potentially leach from the material into drinking water. This formulation review then determines the battery of chemical analyses that will be performed on a particular material. The testing of the finished product is performed by exposing the product to pH 5 and pH 10 waters and analyzing for regulated metals such as antimony, arsenic, barium, cadmium, chromium (including chromium VI), copper, lead, mercury, selenium, thallium and nickel.
The product is also exposed to pH 8 water which is tested for organic chemical contaminants that could leach out into the water. The certification process covers two separate applications: ambient/cold water use, which is tested at 23°C, and domestic or commercial hot water use, which is tested at 60°C or 82°C.
Common interior coating types
There are five common types of interior coating or lining material for steel potable water transmission pipe. The list includes coal tar enamel, cement mortar, liquid-applied epoxy, fusion bonded epoxy (FBE), and polyurethane (PU). Each of these lining technologies have inherent advantages and limitations. Additionally, each interior coating type has installation requirements for surface preparation and application.
Coal Tar Enamel – The coal tar pitch, which forms the basis for the coal tar enamel (CTE), consists of stable molecules that are formed during coking operations at about 1,300°C. The fillers and coal add flexibility and strength to the product. The strong molecular arrangement provides CTE with the characteristics necessary to produce pipeline corrosion protection. These can be summarized as follows:
- Water resistant — negligible water absorption and vapor transmission.
- Stable chemical structure — resistant to acid and alkali
- Resistant to cathodic disbonding — most pipelines are protected using impressed current or sacrificial metal anodes; CTE is resistant to the alkaline environment formed at exposed metal surfaces.
- High electrical resistance — even after two years’ immersion in water, the electrical resistivity remains 1014-ohm cm3.
- Adhesion — forms a strong permanent bond to the steel surface.
- Resistant to attack by bacteria, marine organisms and root penetration.
Shortages in the availability of qualified applicators, increasing costs, challenges of worker exposure during application, concerns of leachability of trace contaminants into the potable water, and effects on human health have contributed to the decline in usage of coal tar enamel, specifically in the United States.
Application considerations: Surface preparation is critical to CTE performance, and application of the CTE requires an understanding of its limitations during cold-weather and high humidity. Coal tar enamels generally require heating and continuous agitation of the material during application, and the enamel is required to be maintained moisture and dirt free during application.
Cement Mortar – Cement mortar linings provide long-term protection at a low cost, and remain one of the standard linings for potable water pipes.
A major benefit with cement mortar is the ease of application. The mixing and application of mortar is straight forward, leading to low risks in application. Cement mortar linings provide active protection of the steel pipe by creating a stable hydroxide film at the steel-mortar interface. The corrosion protection is referred to as active, because it provides protection even where there are discontinuities in the lining. Cement mortar linings have a track record of conveying water for extended periods to required water quality standards, and currently meets all applicable standards throughout the world. Cement mortar does not support microbiological growth.
The actual cement application of cement-mortar linings is performed by pumping or pouring a high slump cement mixture onto a slowly rotating length of pipe. The rotating speed is then increased, creating centrifugal forces that level out the wet mortar to a uniform thickness. Continued spinning removes the excess water and compacts the mixture to a dense and solid surface. After the spinning process, the lining is cured either by moist air at ambient temperature or by an accelerated process using steam. Like concrete, cement-mortar linings can develop drying cracks, but these cracks will self-heal when the lining is wet. Wetting the cement lining also causes the lining to swell, which increases strength and adherence. Cement-mortar linings can add significant stiffness for resistance to deflection forces. The strength of the mortar lining may be added to the strength of the steel when calculating stiffness.
Soft, aggressive waters, as well as prolonged contact with heavily chlorinated water, may be detrimental to cement-mortar linings. Cement-mortar linings perform best when flow velocity is 20 feet per second or less. In situations where the conveyed water is aggressive and the flow rate is low (resulting in a long residence time), a high pH can develop with cement mortar lined pipe. Cement-mortar linings add considerable weight and reduce the available flow volume of a transmission pipe.
Liquid Applied Epoxy – The liquid applied epoxy lining systems may consist of any of the following three types:
- A two-part, chemically cured epoxy primer and one or more coats of a different two-part, chemically cured epoxy topcoat;
- Two or more coats of the same two-part, chemically cured epoxy coating; or,
- a single coat of a two-part, chemically cured epoxy coating.
Epoxy linings have excellent water and chemical resistance properties. They can be applied at various thicknesses and are factory applied to provide a dielectric lining. Bonded dielectric lining systems can be applied as either a single or a multiple-coat process. They are tough, resilient, and extremely abrasion resistant, making them a lining choice for high internal velocity service environments.
Epoxy linings do have some limitations that must be considered prior to application. A critical performance factor to all linings is the surface preparation of the metal. In most cases, SSPC-SP 10/NACE No. 2 near-white blast with a nominal surface profile of 2-3 mils) is required for proper adhesion. Minimum curing times and temperatures must also be closely followed, and can range from hours to days depending on the formulation. Epoxies are typically applied by airless spray or brushed on to the pipe. They are considered barrier linings, requiring 100% continuity to achieve corrosion protection. Any discontinuity can result in corrosion; unless a cathodic protection system is employed on the pipeline.
With proper surface preparation, controlled application, and conformance to strict curing procedures, thin-film epoxies can provide a strong, resistant, durable lining.
Fusion Bonded Epoxies – Fusion bonded epoxies are a one part, heat curable, thermosetting epoxy. FBEs are applied to heated parts in a powder form (10-40 mils) that rapidly gels from liquid to a solid, have excellent adhesion to the steel surface, and are very resilient coatings that resist damage during handling. FBEs are considered environmentally friendly since they contain no volatile organic compounds (VOCs).
FBE should be applied immediately following the heating process to avoid excess pipe cool down (if the pipe cools below 450° F the FBE may not fully cure). The powder is generally applied using semi-automated application rings, electrostatic guns or flocking units to a minimum thickness of 14 to 16 mils. The FBE material is generally applied in several passes of 2 to 5 mils and should always be completed in an expedient manner to avoid lamination. The FBE will usually be dry to the touch in less than a minute and be fully cured within three minutes or less depending on the formulation of the material. Handling and testing commences once the applied coating cools to approximately 200°F.
Polyurethane – The aromatic polyurethanes are 100% solids materials that contain no VOCs. Polyurethane linings are typically applied at 20 mils minimum thickness; however, thicker lining applications are possible. Specific to the internal surface of potable water pipes, polyurethane materials have following advantages:
- Fast curing – ensures economy of high production rates and efficiency.
- Excellent adhesion to ferrous and properly prepared steel surfaces.
- High impact resistance
- Effectively protects pipe from corrosion
- Lower lining thickness is required compared to other technologies, and hence pipe design can be more efficient, reliable and economical as the wastage factor will be reduced, and pipeline capacity is higher for the same size of pipe.
- Reduced head loss and pumping losses due to a smoother internal surface of the pipes.
- Longer economic life as deterioration due to erosion cavitation is low.
There are some limitations associated with polyurethane material when used for lining steel water transmission pipe. Polyurethanes require heated, plural-component equipment and qualified, experienced applicators. Polyurethane coatings require that the host pipe be thoroughly cleaned and for in-service pipe that includes removal of hard deposits, nodules, scale, corrosion and other debris and be substantially dry prior to application of the coating to ensure good adhesion between the liner and the pipe wall. Voids and blisters may form if the pipe is not properly prepared and there is a potential for uneven liner thickness due to inconsistencies in dual material component pumps associated with the application equipment.
In an effort to provide safe drinking water, coating materials intended for contact with potable water are evaluated for of the level of contaminants and chemicals that leach from the products into drinking water. The coatings products typically are coal tar enamel, cement mortar, liquid-applied epoxy, fusion bonded epoxy (FBE), or polyurethane (PU) and all are initially required to have a toxicological evaluation of chemical concentrations. NSF toxicologists then perform a formulation review which determines the battery of chemical analyses that will be performed. Lastly, the finished product is tested by exposing the product to pH 5 and pH 10 waters and analyzing for regulated metals. Each of the coating types have inherent performance and application advantages and challenges which should be considered when selecting a coating for the intended service environment.
Cindy O’Malley is a Vice President and the Professional Services Group Manager. Under Ms. O’Malley’s oversight, consultants, engineers, and laboratory chemists provide independent analyses of coating problems and advance the industry’s understanding of the performance characteristics of protective coatings. She is an SSPC Certified Protective Coatings Specialist, Past President of the Pittsburgh Society for Coatings Technology, and chair of ASTM D01.21 for Chemical Analysis of Paints.