drinking water problems corrosion

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1 E-616 7/12 Drinking Water Problems: Corrosion Mark L. McFarland, Tony L. Provin, and Diane E. Boellstorff* orrosion is one of the most common problems affecting domestic water C supplies. Chemical processes slowly dissolve metal, causing plumbing pipes, fixtures and water-using equipment to deteriorate and fail. Corrosion can cause three types of damage: • The entire metal surface gradually thins and red stains appear in iron or steel plumbing systems or blue-green stains in copper and brass plumbing systems (Fig. 1). Figure 2. Pinhole leaks in copper tubing caused by internal corrosion. • Deep pits appear that can penetrate pipe or tank walls. This type of corrosion may not add significant amounts of iron or copper to the water, but can eventually perforate a pipe or tank, and cause potentially major water damage to a home or business (Fig. 2). • Copper or other metals oxidize in a process similar to the rusting of steel. It often reduces water flow through supply lines and destroys water valves and other water Figure 1. Corrosion at a connection on a water heater control surfaces, creating leaks inside and indicated by the blue-green color. outside of valves and faucets. This type of corrosion is not necessarily caused by water Professor and Extension Water Quality Coordinator, Professor and Extension Water Testing Laboratory Director, Assistant Professor and chemistry, but by exposure to soil or other Extension Water Resources Specialist, Texas AgriLife Extension Service, corrosive environments. The Texas A&M System

2 Two common tests can determine if water is What health problems likely to be corrosive: the Langelier Saturation can corrosion cause? Index (LSI) and the Ryzner Stability Index (RSI). Copper and lead can be toxic and can leach In order to use the LSI, a laboratory must into tap water in older or in new homes. This measure pH, electrical conductivity, total leaching is caused by corrosion. dissolved solids, alkalinity, and total hardness. Copper contamination can cause The LSI is typically negative or positive and gastrointestinal problems in the short term and only rarely zero. Negative values predict that the damage the liver and kidneys over time. water is more likely to be corrosive. Potentially corrosive water typically has an LSI value –1 Lead contamination can cause physical and (mild) to –5 (severe). mental development problems in children. In adults, it can lead to high blood pressure and When the RSI is used, a value over 6.5 kidney problems. indicates that the water is probably corrosive; higher values are increasingly corrosive. What levels are harmful? Not all laboratories offer the saturation or stability index. Contact the laboratory to discuss The Environmental Protection Agency has their services, pricing, and procedures for established primary drinking water standards collecting, handling, and submitting samples. The http://water.epa.gov/drink/ for copper and lead ( Texas Commission on Environmental Quality contaminants/index.cfm ). The maximum allowable maintains a list of laboratories certified to test for copper is 1.3 milligrams/liter; the maximum drinking water: http://www.tceq.texas.gov/assets/ allowable for lead is 0.015 milligrams/liter. public/compliance/compliance_support/qa/txnelap_ Contaminants may be reported in milligrams/ lab_list.pdf liter (mg/L) or parts per million (ppm); these units are equivalent. What causes corrosion? Iron and zinc also are usually present and can cause water to have a metallic taste, but are not Corrosion is a natural process that occurs when health concerns. metals react with oxygen and form metal oxides. All water contains some dissolved oxygen How do I know if there and is therefore somewhat corrosive. The rate of corrosion depends on many factors including is a corrosion problem? the water’s pH, electrical conductivity, oxygen The most common symptoms of corrosive concentration, and temperature. water are: In addition to corrosion, metals dissolve when • Cold water tastes bitter at first use in the the water is extremely low in dissolved salts and morning, and the taste improves after in the presence of certain water-borne ions. This running the water for a few seconds. process causes the plumbing material to gradually Blue-green stains in sinks and/or at the • dissolve. Though corrosion and dissolution are joints of copper piping. fundamentally different, the result is similar and • Water leaks in floor, wall or ceiling areas they are generally discussed as corrosion. from pin-size or larger holes in metal pipes. Acidity or low pH If you suspect that your water is corrosive, have it tested by a laboratory. Water-testing The pH scale ranges from 0 to 14; a pH of 7.0 laboratories, including the Texas AgriLife represents the point where acid and alkaline Extension Service Soil, Water and Forage Testing materials are in balance. Water with a pH value Laboratory, test irrigation and/or livestock water. below 7.0 is considered acidic, while water above Elevated copper, iron, or zinc in the water can 7.0 is alkaline. indicate ongoing corrosion in a water system. Alkalinity and pH often are confused. Total However, these tests only indicate a potential alkalinity refers to the total bases in water that problem; further testing is needed to determine can neutralize acid. These include bicarbonates, the causes and severity of the corrosion potential. carbonates, hydroxides, and some phosphates and 2

3 silicates. Alkalinity is reported in milligrams per Dissolved oxygen liter of calcium carbonate. and other gases Groundwater can be acidic or alkaline in pH, Oxygen dissolved in water is a primary depending on several factors. Rainfall is typically corrosive agent. Water exposed to the air absorbs acidic because it picks up carbon dioxide as it oxygen. Oxygen in rain and surface water is falls to the earth, forming carbonic acid. As water usually removed when water seeps into the percolates through the soil, it also can come in ground; deep wells are usually oxygen free. In contact with acidic materials such as decaying contrast, shallow wells and surface water often organic matter. contain more oxygen. Water also may absorb Limestone (calcium carbonate) and dolomite oxygen when a pneumatic pressure tank is (calcium magnesium carbonate) in the soil used. Hydrogen sulfide in groundwater also can neutralize the acid and the water is usually corrode metals significantly. You can see high alkaline—pH between 7 and 8—and “hard” due levels of dissolved gases by dispensing water into to the carbonates. If there is no limestone or a clear glass. In extreme cases, the water may dolomite, the groundwater will remain acidic look milky due to very small air bubbles. with pH values between 6 and 7. Water that contains calcium or magnesium Water temperature salts (hard water) is less corrosive, because the Corrosion is more likely and more rapid at minerals that cause hard water tend to coat higher water temperatures. The rate of corrosion and protect the inside of pipes. Soft water that triples or quadruples as water temperature rises contains sodium salts does not coat the pipes and from 60°F to 140°F. Above 140°F, the rate of consequently is more corrosive. corrosion doubles for every 20°F increase. Water that is moderately alkaline (40 to 70 mg/L) with a pH between 7.0 and 8.2 is usually Design flaws not corrosive. Water with a pH below 6.5 will In some cases, leaks in copper plumbing be corrosive, especially if alkalinity also is low. systems are caused by excessive water velocity, However, water with pH values above 7.5 also especially when it passes through 90-degree can be corrosive when alkalinity is low. fittings. Water flows faster when the demand for water from the plumbing system is too great for High dissolved solids the diameter of the supply line. Over time, the and electrical conductivity water erodes the copper, causing leaks—almost always in angle fittings. This type of leak is Minerals dissolved in water separate into caused not by corrosion but by poor system charged particles (ions) that conduct electricity. design. The prevention methods listed above will Conductivity is a problem only when water has not solve this problem. a high mineral content; pure water does not conduct electricity. In rare cases, manufacturing defects can make copper piping more susceptible to corrosion. Plumbing systems use several types of metals. While uncommon, this problem can result in When different metals are in contact with each pinhole leaks that occur relatively soon after the other and a solution that conducts electricity, system is put into service. the result is a galvanic cell. The cell generates electricity, which corrodes one of the metals. Galvanic corrosion occurs at or very near the How can I reduce corrosion? joint between the two metals. Treating your water can reduce corrosion to Plumbing systems that use galvanized pipe acceptable levels, but generally will not eliminate often have brass values. Likewise, copper it. Treatment method depends on what is causing plumbing often has solder joints and valves made the corrosion. of a different alloy. Every joint where different Treating for acidity metals connect is a potential site for galvanic corrosion if the water has high amounts of If acidity is the problem, installing a neutraliz - dissolved minerals. ing filter usually works best. These filters contain 3

4 chips of calcium carbonate (limestone), marble, systems. Instead, food-grade polyphosphate or magnesia (magnesium carbonate), or other alkaline silicate compounds can be added into the water materials that dissolve as the water neutralizes. system to control corrosiveness. Acid-neutralizing filters are usually installed These materials deposit a thin coat inside the after the pressure tank. As water flows through pipe which limits contact with the water. The the filter, pH increases which decreases corrosiv - film will slowly dissolve so the material should be maintained and fed at proper levels. Initially, ity. This process makes the water harder. It also existing corrosion can loosen and flush through may decrease water pressure. the system making the red water problem seem to The neutralizing material must be replenished be worse. A higher feed rate will clean the system as it is dissolved. The chips can last from weeks and establish a protective film. Then reduce the to months, depending on the type of material, amount to maintain the protective film. how corrosive the water is, and how much water you use. The filters usually must be backwashed Reducing dissolved oxygen to remove trapped particles and oxidized metals. Often, there is little you can do to reduce Another way to neutralize acidic water is to dissolved oxygen in small water systems. add a solution of sodium hydroxide or sodium Installing a flexible membrane or a floating carbonate (soda ash). This is usually done by disc in the pressure tank will minimize the - installing a chemical feed pump before the pres water’s contact with air. This type of tank also sure tank. If you are on a low-sodium diet, con - minimizes waterlogging, which is common sider using potassium hydroxide instead of the with highly aerated water. However, it may be sodium salts. necessary to inject polyphosphate or silicate This treatment system is simple, and inexpen- compounds to protect the water system over the sive; it does not increase water hardness or long term. reduce the water pressure. The injection rate A large, semi-open storage tank can be should be adjusted to produce water with a pH of used to allow air to escape similar to the way 7.5 to 8.0. bubbles escape in a drinking glass. This requires Injection units require significant maintenance a tank twice the size of the daily-use rate - that includes filling solution tanks and main and chlorination since the water is no longer taining the feed pump. Soda ash is preferable to pressurized. sodium hydroxide, which is extremely caustic and requires special safety precautions; it should Corrosion on the outside be handled only by trained individuals. of supply lines? Reducing salts The outside of plumbing also can corrode. Removing high concentrations of dissolved This is most common when copper or galvanized salts from water requires a reverse osmosis supply lines touch highly acidic or basic soil. This system. This method can require that the water can happen when acid soil materials are exposed be pretreated, and whole-house systems require by trenching or alkali is created from burning large storage tanks. Reverse osmosis systems construction materials, trees, or old buildings. increase overall water use by 30 to 200 percent Modern water systems often use plastic- and generate wastewater with concentrated salts. jacketed copper tubing. Be careful when Reverse osmosis can remove 80 to 95 percent installing this type of piping to avoid cutting or of salts from the water entering the system. tearing the jacket as this will expose the pipe to In some cases, treated water may be so low in corrosion. total dissolved salts that it corrodes plumbing Aboveground, exterior corrosion often occurs components. Generally, reverse osmosis water where hazardous materials are stored, mixed, should be transferred and dispensed through non- or used, such as swimming pool systems that metallic pipe and fixtures. use hydrochloric acid (muriatic acid). If local It is generally not feasible to remove high plumbing codes allow it, use PVC, CPVC, or PEX levels of dissolved salts from whole-house water pipe in potentially corrosive environments. 4

5 What if toxic metals Can corrosion be prevented? One way to correct or prevent plumbing are the only concern? system corrosion is to install corrosion-resistant In many cases, the water is not corrosive components. Most often this involves replacing enough to cause leaks, but it does increase the copper pipe or substandard plastic pipe with amount of copper and/or lead in the water. Since approved plastic pipe. PEX plumbing has gained copper and lead normally accumulate when wide acceptance in recent years. Unlike PVC, water sits in the plumbing, the simplest and many PEX manufacturers claim it can tolerate cheapest solution is to run the water for at least high heat and freeze solid with no damage. 1 minute before drinking it. This draws fresh Approved plastic pipe is stamped with NSF water from the pressure tank or well that has (National Sanitation Foundation) and Drinking not had sufficient time in the plumbing system Water on the side. Local plumbing and building to accumulate metals. Flushing is only necessary codes vary; consult local regulations before when water has been in the plumbing for at least replacing or installing plumbing materials. one hour. If you use this method, collect a water sample For more information after running the water for 1 minute and have it These publications may be downloaded analyzed for copper and lead to ensure that the from the Texas AgriLife Bookstore at https:// levels are reduced to safe concentrations. agrilifebookstore.org. To conserve water, flush the plumbing system L-5451, Drinking Water Problems: Iron and • in the morning and fill a container with drinking Manganese water for the day. • L-5452, Drinking Water Problems: Lead In recent home construction, lead is restricted Drinking Water Problems: Copper L-5472, • to the brass components. By running the water for several seconds, any dissolved lead from brass This publication was adapted from the fixtures will be flushed out. following publications: If lead and copper persist after flushing, or Corrosion from Domestic Hermanson, R.E. 1991. if flushing is not possible, you can install small EB1581. Washington State University. Water. point-of-use devices to remove the metals at Pullman, WA. individual taps. Water that will be used only Oram, B. Corrosion, Saturation Index, Balanced for drinking and cooking can be treated by Water in Drinking Water Systems. 2009. Wilkes reverse osmosis, activated alumina filtration, or University Center for Environmental Quality. distillation. Wilkes-Barre, PA. Swistock, B.R., W.E. Sharpe, and P.D. Robillard. 2001. Corrosive Water Problems. F 137, Penn State University. University Park, PA. This publication was funded by the United States Department of Agriculture National Institute of Food and Agriculture under agreement 2008-51130-19537, also known as the Southern Region Water Resource Project. The Southern Region Water Resource Project is a collaborative effort of the Texas AgriLife Extension Service, 21 partner Land Grant Universities in the Southern United States, and the USDA National Institute of Food and Agriculture. 5

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