1 Drinking Water Prob lems: Corrosion tension Water Quality Coordinator Mark L. McFarland, Professor and Ex Tony L. Provin, Professor and Extension Water Testing Laboratory Director Diane E. Boellstorff, Assistant Professo r and Extension Water Resources Specialist Texas AgriLife Extension Service, Department of Soil and Crop Sciences, College Station, Texas One of the most common problems affecting domestic wa ter supplies is corrosion, a chemical process that slowly dissolves metal, resulting in deterioration a nd failure of plumbing pipes, fixtures and water-using equipment. One type of corrosion attacks and gradually thins the entire metal surface, often causing red-colored stains in iron or steel plumbing systems or blue-green stains in copper and brass plumbing systems (Figure 1). Another type of corrosion attacks small areas where deep pits can develop and penetrate pipe or tank walls. This type of corrosion may not add signi e water, but can create small holes ficant amounts of iron or copper to th ulness, cause water leaks, and result in a pipe or tank that destroy its usef in major water damage to a home or business (Figure 2). A third type of corrosion caused by the oxidation of metals i nvolves conversion of copper ilar to the rusting of steel. It often results in reduced or other base metal to an oxidized form in a process sim water flow through supply lines and her machined water flow control destruction of water valves and ot surfaces, thus resulting in internal an d external leaks at valves and faucets. This type of corrosion does not necessarily occur due to th e water chemistry, but is caused by exposure of the outside surface of the plumbing supply lines to soil or othe r corrosive environments. Especially in older installations a nd very new homes, two potentially toxi c metals, copper and lead, may occur in tap water almost entirely because of leaching caused by corrosion. Elev ated levels of copper can cause ong-term exposure result in liver and gastrointestinal problems and with l kidney damage. Elevated levels of lead can result in physical and mental development pr oblems in children, and high blood pressure and kidney problems in adults. The US Environmental Protecti on Agency has established Primary Drinking Water http://water.epa .gov/drink/contaminants/index.cfm Standards ( ) for both copper (maximum contaminant level = 1.3 milligrams/liter) and lead (maximum contaminant level = 0.015 milligrams/liter). In addition, two other metals (iron and zinc) usually present because of co rrosion can cause water to have a metallic taste. Laboratories and publications may report concentrations using units of milligrams/liter (mg/L) or parts per million (ppm), which are the same. What are the causes of corrosion? Corrosion is a natural process that occurs when metals and react to form metal are in contact with oxygen oxides. All water is corrosive to some degree as it contains some am ount of dissolved oxygen. The rate of corrosion depends on a number of factors includin g acidity or low pH, el ectrical conductivity, oxygen concentration and water temperature. Each of these fact ors is explained in more de tail in the next sections. tion of metals occurs when the water is extremely low in dissolved salts or in In addition to corrosion, dissolu the presence of certain water-borne ions . All materials have a particular le vel of solubility and in the case of corroded plumbing, the concentration of the copper or othe r plumbing material metal is lower in the water than
2 that material’s solubility. As a result, the plumbing ma d. While this process is terial is gradually dissolve with and bind the recently dissolved metal allowing more usually very slow, certain water-borne ions can react nd result is similar and so both are rapid loss. While corrosion and dissolu tion are fundamentally different, the e the general term corrosion. often discussed together under Acidity or Low pH The pH value is used to measure acidic and alkaline mate rials in water. The pH scale ranges from 0 to 14, with a pH of 7.0 representing the neutral po int where acid and alkaline materials are in balance. Water with pH values below 7.0 is dominated by acidic materials, wh ile water with pH values above 7.0 is alkaline. The terms alkalinity and pH often are confused. Total alkali nity is a measure of the total bases in water that can neutralize acid. This incl udes bicarbonates, carbonates, hydroxides, a nd even some phosphates and silicates. Alkalinity is reported in units of milligrams per liter (mg/L) of calcium carbonate. Groundwater can be acid or alkaline in pH depending on seve ral factors. Rainfall is typically acidic because it the earth, forming carbonic acid. As wa picks up carbon dioxide as it falls to ter percolates through the soil, it also can come in contact with other acid-forming ma terials such as decaying organic matter (vegetation). In areas where underground strata contai n limestone or dolomite, the acid is ne utralized and the water is usually Where limestone or dolomite is not present underground, alkaline and hard with pH values between 7 and 8. acidity and groundwater will typicall y have pH values between 6 and 7. the percolating water will retain its For ideal corrosion control, water should have modera te alkalinity (40 to 70 mg/L) and a pH between 7.0 and 8.2. Water with pH values below 6.5 will be corrosive, espe cially if alkalinity also is low. However, water with pH values above 7.5 also can be co rrosive when alkalinity is low. High dissolved solids (e lectrical conductivity) The ability of water to conduct electri tration. Dissolved soil minerals in city is affected by its ionic concen water separate into charged particle s called ions that can conduct elect ricity. Pure water does not conduct electricity. Thus, electrical conductivity is only a problem when wate r has a high mineral concentration. Water than water containing calcium containing sodium salts (soft water) is more corrosive or magnesium salts (hard water), because hard water minerals tend to coat the the potential for corrosion. inside of pipes and reduce Several different types of metals are two different metals are in contact often used in a plumbing system. When with each other and a solution that conducts electricity, a galvanic cell is created. Th e cell generates electricity and one metal dissolves or corrodes in proportion to the electricity produced. Galvan ic corrosion occurs at or very near the joint between the two metals. ss values. Likewise, copper plumbing often has solder Plumbing systems using galvanized pipe often have bra joints and valves made of a differe nt metal alloy. Any dissimilar metal connections such as these represent e water has high amounts of dissolved minerals. potential sites for galvanic corrosion if th Dissolved Oxygen and Other Gasses Oxygen dissolved in water is one of the most common a . When water is exposed nd important corrosive agents to air, some of the oxygen in the air is absorbed. Oxygen dissolved in rain a nd surface water is usually consumed as water seeps into the ground. As a result, deep wells usually are free of dissolved oxygen. In contrast, shallow wells and surface water often have a higher dissolved oxygen concentration. Dissolved
3 oxygen also may be introduced into water when a pneumatic pressure tank is used. In addition to oxygen, the sult in significant corrosion of meta ls. The presence of high levels of presence of hydrogen sulfide also can re In extreme cases, the water may dissolved gasses can be observed by di spensing water into a clear glass. initially have a pale white (milky) appearance due to the presence of very small air bubbles. Water Temperature Corrosion is more likely and more rapid at higher water temperatures. The rate of corrosion increases by a o o o factor of three to four as water temperature rises from 60 F. Above 140 F to 140 F, the rate of corrosion o doubles for every 20 F increase in water temperature. is a corrosion problem? How do I know if there corrosive water because of its ne gative effects. The most common Most people first realize they have symptoms are: • aw it in the morning, and flushing the line by running The cold water has a bitter taste when you first dr the water for a few seconds improves taste. • at the joints of copper piping. Blue-green stains in sinks and/or • Water leaks in floor, wall or ceil ing areas as a result of pin-size or larger holes in metal pipes. Many water-testing laboratories, in cluding the Texas AgriLife Extensi on Service Soil, Water and Forage Testing Laboratory in College Station, Texas, offer rout ine tests for irrigation and/ or livestock water uses. Results showing elevated levels of copper, iron or zinc can be an indi cation of on-going corrosion in a water ly indicators of a potential problem, and more detailed testing is often system. However, these tests are on needed to confirm that co rrosion is occurring and why. Special laboratory testing is needed to more specifi cally determine the causes and severity of corrosion potential. The Langelier Saturation Index (LSI) is one me thod for predicting if water is likely to be corrosive. y must measure pH, electrical conductiv In order to use the index, a laborator ity, total dissolved solids, alkalinity and total hardness. The LSI typically is either negativ e or positive and only rarely zero. An LSI value of 0 rrosive, values above 0 indi cate the water will tend to indicates that the water is “balanced” and unlikely to be co form a carbonate scale, while negative eater corrosive potential. A typical values indicate that the water has a gr reporting range for the LSI is -1=Mild Corrosion Potential to -5=Severe Corrosion Potential. Another common index is the Ryzner St ability Index (RSI). An RSI greater than about 6.5 indicates water that is probably corrosive, with higher values being increasingly corrosive. Only selected laboratories offer determination of a saturation or stability inde x for estimating corrosion potential of water. Individuals s uss costs and sample collection and hould contact the laboratory to disc handling methods before obtaini ng water samples for testing. How can corrosion be controlled? nerally not possible, but it often can be reduced to acceptable levels by Eliminating corrosion completely is ge treatment. The appropriate treatment method depe nds entirely on the type and causes of corrosion. If acidity is the only problem being experienced, a neutrali zing filter is usually the best approach. These filters contain calcium carbonate (limestone) chips, marble ch ips, magnesia (magnesium carbonate) or other alkaline
4 materials that dissolve as the The acid-neutralizing filter is usually installed after neutralization process occurs. ough the filter and as it contacts th e media, its pH is increased and the pressure tank. Raw water flows thr dness of the water, but this is necessary for proper corrosivity decreased. The process will increase the har corrosion control. Also, the resistance of the neutralizing material may lower water pressure. The tank must be routinely refilled with neutralizing material as it is dissolved. The rate of refilling can range from weeks to and the type of neutralizing material. Backwashing months depending on the water corrosivity, water use, apped particles and oxidized metals. typically is required to remove tr Another method for neutralizing acidic water is to inject a sodium hydroxide (NaOH) or sodium carbonate (soda ash - Na CO ) solution using a chemical feed pump before the pressure tank. Individuals on a low 2 3 sodium diet should investigate the use of potassium hydroxide (KOH). This treatment system is simple and hardness. Since the unit is installed ahead of the pressure tank, there is inexpensive and does not increase water times occurs with neutralizing filters. no reduction in water pressure that some The rate of injection should be There is significant maintenance including filling adjusted to produce treated water with a pH of 7.5 to 8.0. is preferred over sodium hydroxide, because it is safer solution tanks and maintaining the feed pump. Soda ash to handle. Sodium hydroxide is extremely caustic and must be handled using accepted safety practices. Generally, it should be handled only by trained individuals. quires the use of a reverse osmosis system. These Removing high levels of dissolved salts from water re house systems require large storage tanks. Reverse systems may require additional pretreatment and whole osmosis systems also will increase overall water use by 30-200%. The reverse osmosis water will be 80-95% lower in overall salts than the water entering the system. In some cases, treated water may actually be so low in total dissolved salts that rrosion of plumbing components. As a general recommendation, it too results in co reverse osmosis water should be transferred and disp ensed through non-metallic pipe and fixtures. Because there are few feasible methods for removing high levels of dissolved salts from whole house water systems, homeowners may choose to utilize f ood-grade polyphosphate or silicate co mpounds that can be fed into water a thin protective film on the inner surface of the pipe systems to control corrosiveness. These materials deposit e feeding of the materials should be which limits contact with the water. The film re-dissolves slowly, and th maintained at proper levels. Initia lly, old corrosion deposits can loosen and flush through the system making the “red water” problem appear to be worse. A higher f eed rate will clean the system and establish a protective film. The feed rate then can be lowered to maintain the protective film. In many cases, little can be done to reduce dissolved oxygen in small water systems. Using a flexible minimize exposure of the water to air. This type of “membrane” or a floating disc in the pressure tank will eliminate pressure tank water logging, a common problem with highly air- pressure tank also will nearly entrapped water. However, inject pounds may be necessary to prevent ion of polyphosphate or silicate com damage to the water system over the long term. If space is available, a large capa city, semi-open storage tank can be used to reduce super-satura tion, similar to the way air bubbles es cape from a drinking glass. This approach requires tank sizing two time s greater than the daily use rate and since the water is no longer ired prior to end use. pressurized, chlorination is requ Corrosion on the outside of supply lines? The outside of plumbing also can be subject to corrosion. The most common problem is when copper or galvanized supply lines come in contact with highly aci dic or basic soil. These conditions can occur from the oxidation of acid-producing soil materials exposed duri ng trenching or alkali created from the burning of construction materials, trees or old n use plastic-jackete d, flexible copper buildings. Modern water systems ofte tubing. Care should be used when instal ling this piping to ensure no tears or cuts occur, otherw ise the cut jacket can potentially localize/con centrate corrosion problems. Aboveground, corro sion of the exterior of plumbing is
5 often the result of a highly ude areas where hazardous materials are corrosive environment. This may incl concentrated hydrochloric acid (muriatic acid). If stored, mixed or used, for example pool systems that use allowed by local plumbing codes, supply lines compri sed of PVC, CPVC, or PEX are recommended in potentially corrosive environments. What if toxic metals are the only concern? In many cases, water corrosivity is not severe enough to cause leaks in the plumbing, but does cause undesirable e available to reduce or eliminate the increases in copper and/or lead in the water. In this case, various options ar sits in contact with the accumulate in the water as it metals from drinking water. Since copper and lead normally metal plumbing, the most simple and inexpensive soluti on is to flush the plumbing system by running the water for at least one minute before drinking the water. This dr aws fresh water from the pressure tank or well that has accumulate metals. Flushing is only necessary if the not had sufficient contact with the plumbing system to umbing for at least one hour. If this water has been in contact with the pl method is used, a water sample should the water for one minute and analyzed for be collected after running copper and lead to ensure levels are reduced to safe concentrations. To conserve water, flush the plumbing system in the morning and fill a container with drinking water for use during the day. In more recent home construction, lead within the to the brass components. Therefore, most lead contaminated water will plumbing system is restricted entirely ter to run for several seco nds, all of the dissolved be dispensed upon opening the valves. By allowing the wa lead from the brass fixtur e will bypass human contact. If excessive lead and copper persist after flushing, or if flushing is an undesirable method, there are numerous alternatives for reducing lead and copper in water. Unlik e the neutralizing filters and chemical injection units discussed above that treat all of th e water entering the home, smaller poi nt-of-use devices can be used to ivated alumina filters or distillation are all acceptable remove the metals at individual taps. Reverse osmosis, act ter that will be used for drinking and cooking. methods that can be used to treat only the wa Can corrosion be prevented? y to correct or prevent th e problem is to install Since corrosive water attacks the plumbing system, one wa to corrosion. Most often this involve s replacement of copper pipe or plumbing components that are resistant substandard plastic pipe with approved plastic pipe. The use of PEX plumbing has gained wide acceptance in recent years. Unlike PVC which can become damage d during freezing, many manufact ures of PEX claim high heat tolerance, the ability to freeze solid many times w ith no damage and single jointless supply runs from the whole-house water manifold to the end-use fixture. A pproved plastic pipe is dire ctly stamped with “NSF” (National Sanitation Foundation) and “D rinking Water” on the side. Plumbing c odes vary from city to city and ior to replacing or inst alling plumbing materials. county to county, so check your local building code pr Additional notes on plumbing system design and materials In some cases, a homeowner will experience leaks in c opper plumbing systems that are caused from excessive water velocity. If the wate r velocity is too great, the water can erode the copper pipe, espe cially when it passes through 90-degree fittings. Excessive water velocity occurs when the demand for water downstream in the plumbing system is too great for the di ameter of the supply line. Over time, the erosive nature of water wears down the copper and leaks occur, almost always in angle fi ttings. This type of leak is caused by poor system design and not by corrosion, thus the prevention met hods listed above will yield no positive benefits.
6 In rare cases, improper or faulty manufacturing of copper pi ping can result in materials that are more subject to nhole leaks that o corrosion. While uncommon, this problem can result in pi ccur relatively soon after the system is put into service. This publication was adapted from the following existing publications: Hermanson, R.E. 1991. Corrosion fro m Domestic Water. EB1581. Washi ngton State University. Pullman, WA. Oram, B. Corrosion, Saturation Index, Balanced Water in Drinking Water Systems. 2009. Wilkes University Center for Environmental Qu ality. 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.
7 Figure 1. Corrosion at a connection on a water heater indicated by the blue-green color. Figure 2. Pinhole leaks in copper tubing caused by internal corrosion.