Internal Corrosion Control of Water Supply Systems is deliberately brief in its presentation of a wide array of complex information, in order to provide direction to practitioners that can be more easily related to their specific circumstances. The book also provides a series of check-lists and criteria to be used in risk assessment. Part C. Part A. Part B.
Galvanic properties between dissimilar metals
Colin Hayes. Other types of corrosion in the waterworks industry that are not found as commonly as those discussed previously include 1 stray current corrosion and 2 dealloying or selective leaching. Stray current corrosion is a type of localized corrosion usually caused by the grounding of home appliances or electrical circuits to the water pipes. Corrosion takes place at the anode, the point where the current leaves the metal to return to the power source or to ground.
Stray current corro- sion is difficult to diagnose since the point of corrosion does not necessarily occur near the current source. It occurs more often on the outside of pipes, but does show up in house faucets or other valves. Dealloying or selective leaching is the preferential removal of one or more metals from an alloy in a corrosive medium, such as the removal of zinc from brass dezincification.
This type of corro- sion weakens the metals and can lead to pipe failure in severe cases. An exam- ple of this is shown in Fig. Pitting of steel pipe. Tuberculation in a cast iron pipe. Galvanized steel pipe from a domestic hot-water system showing almost complete clog- ging by corrosion products. Extreme example of stray current corrosion in an outside water faucet caused by lightning leaving the pipe.
The purpose of this and the following sections is to point out some of the easiest, as well as the most effective, methods of identifying, monitoring, and correcting corrosion-related problems. In other words, these sections answer the questions how do you know if your utility has a corrosion problem, and what can you do to control or reduce the effects of the corrosion.
The effects of corrosion, which may not be evident without monitoring, can be expensive and may even affect human health. Monitoring methods most useful to the small water utility are emphasized; that is, those methods which are the least expensive and the simplest to implement in terms of manpower and technical requirements.
Methods for control- ling or reducing corrosion are covered in the following section. Just as there is no one cause of corrosion, there is no one way to measure or "cure" corrosion. Since corrosion in a system depends on a specific water and the reaction of that water with specific pipe materials, each utility is faced with a unique set of problems. There are, however, general1 methods of measuring and monitoring for corrosion that can provide a basis for a sound corrosion control program for any utility. Although no one method may provide an absolute or quantitative measure of corrosivity, several methods used together over a period of time will indicate if corrosion is occurring and will point out any undesirable effects on the system.
The indirect methods do not measure corrosion rates. Rather, the data obtained from these methods must be compared and interpreted to determine trends or changes in the system. The indirect methods dis- cussed here are 1 customer complaint logs, 2 corrosion indices, and 3 water sampling and chemical analyses.
The direct corrosion measurements call for the actual examination of a corroded surface or the measurement of corrosion rates, particularly actual metal loss. The direct methods discussed here are 1 examination of pipe sections and 2 rate measurements. The most common symptoms are listed in Table 6. The Table 6. For example, red water may also be caused by iron in the raw water that is not removed in treatment. Therefore, in some cases, further investigation is necessary before attributing the complaint to corrosion in the system.
Complaints can be a valuable corrosion monitoring tool if records of the complaints are organ- ized. The complaint record should include the customer's name and address, date the complaint was made, and nature of the complaint. The following information should also be recorded: 1.
Type of material copper, galvanized iron, plastic, etc. Whether the customer uses home treatment devices prior to consumption softening, carbon filters, etc. Whether the complaint is related to the hot water system and, if so, what type of material is used in the hot water tank and its associated appurtenances; and 4. Any follow-up action taken by the utility or customer.
These records can be used to monitor changes in water quality due to system or treatment changes. The development of a complaint map is useful in pinpointing problem areas. The complaint map would be most useful when combined with the materials map discussed in Sect. If complaints are recorded on the same map, the utility can determine if there is a relationship between complaints and the materials used.
To supplement the customer complaint records, it might be useful to send questionnaires to a random sampling of customers. These questionnaires should be short but thor- ough. A sample questionnaire used by the city of Seattle is shown in Fig. Customer complaint records and questionnaires are useful monitoring tools that can be used as part of any corrosion monitoring and control program.
The low costs associated with keeping a good record of complaints can be well worth the time. The resulting information would indicate the real effect of water quality at the customer's tap and would show the effect of any process changes made as part of a corrosion control program. Corrosion Indices Many attempts have been made to develop an index that would predict whether or not a water is corrosive; unfortunately, none of these attempts has been entirely successful. However, several of Do you ever have rusty water?
Yes If so, how often? Yes No Where? L Sample questionnaire. These indices can be calculated by all small utili- ties and can be used in an overall corrosion control program.
Since the LSI and AI are the two most commonly used corrosion indices in the waterworks industry, they are the only indices discussed in detail in the following paragraphs. However, several of the less frequently used indices are briefly described to acquaint the reader with their usefulness and method of calculation.
A thin layer of CaCO3 is desirable, as it keeps the water from contacting the pipe and reduces the chance of corrosion. Although the pipe is protected from corrosion, excessive scaling can result in loss of carrying capacity in the system, as is shown in Fig. If the reaction proceeds to the left, the scale is dissolved, leaving the surfaces that had been protected exposed to corrosion.
Langelier Saturation Index. The LSI is the most widely used and misused index in the water treatment and distribution field. At pHs, a protective scale will neither be deposited nor dissolved. The LSI is defined by the following equation:. Excessive CaCOj scaling resulting in loss of carrying capacity. Values for A and B are tabulated in Tables 6. The log of the calcium and alkalinity is obtained from Table 6.
Now, let's take as an example Chicago's tap water, which has the following characteristics: Calcium as CaCO3 , The step-by-step calculation of the LSI, using Tables 6. Source: Federal Register, Table 6. The above examples show two important factors.
First, they show the effect of the change in temperature and pH on the calculated LSI value. This demonstrates the need for accurate, onsite pH and temperature measurements. There are several limitations to the LSI.
First, it is generally agreed that the LSI may only be used to estimate corrosive tendencies of waters within a pH range of 6. More importantly, the LSI only indicates the tendency for corrosion to occur. It is not a measurement of corrosivity.
Internal Corrosion Control in Water Distribution Systems (M58) : Awwa Staff :
Pipe sections were physically examined to establish whether or not the water was corrosive. The results confirm that the LSI, by itself, does not indicate corrosiveness. It is, however, a valuable monitoring tool where a protective CaCO3 film is being used or when used in conjunction with other indirect or direct corrosion monitoring methods. A useful procedure for estimating the pHs is an experimental method commonly called the Mar- ble Test. In this test, duplicate samples of the water are collected. After a time interval usually 1 h or longer , aliquots from both samples are filtered and analyzed for alkalinity or pH.
If the alkalinity or pH in the untreated sam- ple is greater than that of the sample with CaCO3, the water is supersaturated with CaCO3 and may be scale forming. If the alkalinities or pHs of the two samples are equal, the water is just saturated with CaCO3. Aggressive Index AI.
The AI was developed at the request of consulting engineers to govern the selection of the proper type I or II A-C pipe and to ensure long-term structural integrity. However, it can be a useful tool in select- ing materials or treatment options for corrosion control. A sample calculation for the AI follows. Other corrosion indices commonly seen in the literature are 1. This curve is shown in Fig. The values obtained apply to the soft waters of the eastern seaboard of the United States, but not to the harder waters of the middle part of the country.
The major contribution of this index is that it introduces factors other than CaCO3 solubility, such as dissolved oxygen, chloride ion, and noncarbonate hardness, as well as the useful effect of silica. It can be useful in estimating the amount of precipitate that may be formed. There have been attempts to use other water quality parameters to predict the tendency of a water to attack metal pipes. The classic studies of the Illinois State Water Survey by Larson, Sollo, and their co-workers have shown that other factors, such as the ratios of various anions, velocity, pH, and calcium ion concentration, affect the rates of corrosion of mild steel and cast iron.
M58 Internal Corrosion Control in Water Distribution Systems, Second Edition
It was shown that increasing the Cl" to HCOs" ratio, particularly above 0. Graphic representation of the various degrees of corrosion and encrustation. These stu- dies have led to a much better understanding of corrosion but have not resulted in a corrosion index. Sampling and Chemical Analysis Since corrosion is affected by the chemical composition of a water, sampling and chemical anal- ysis of the water can provide valuable corrosion-related information. Some waters tend to be more aggressive or corrosive than others because of the quality of the water.
It is generally desirable to collect water samples at the following locations within the system: 1. Water entering the distribution system i. Water at various locations in the distribution system prior to household service lines, 3. Water in several household service lines throughout the system, and 4. Water at the customer's taps. Water entering the distribution system at the plant can be conveniently sampled from the clearwell, the storage tank, or a sample tap on a pipe before or after the high-service pump.
To represent conditions at the customer's tap, "standing" samples should be taken from an inte- rior faucet in which the water has remained for several hours i. The sample should be collected as soon as the tap is opened. A representative sample from the household service line between the distribution system and the house itself can be obtained by collecting a "running" sample from the customer's faucet after letting the tap run for a few minutes to flush the household lines. Frequently, the water tempera- ture noticeably decreases when water in the service line reaches the tap.
By letting the same faucet run for several minutes following the initial temperature change, the running water sample at the tap is representative of the water recently in the distribution main itself. If a comparison of the sampling results shows a change in the water quality, corrosion may be occurring between the sampling locations.
Analysis of Corrosion By-product Material. Valuable information about probable corrosion causes can be found by chemically analyzing the corrosion by-product material. Scraping off a por- tion of the corrosion by-products, dissolving the material in acid, and qualitatively analyzing the solution for the presence of suspected metals or compounds can indicate the type or cause of corro- sion. These analyses are relatively quick and inexpensive.
Internal Corrosion Control of Water Supply Systems
If a utility does not have its own labora- tory, samples of the pipe sections can be sent to an outside laboratory for analysis. The numerical results of these analyses cannot be quantitatively related to the amount of corrosion occurring since only a portion of the pipe is being analyzed.
However, such analyses can give the utility a good' overview of the type of corrosion that is taking place. The compounds for which the samples should be analyzed depend on the type of pipe material in the system and the appearance of the corrosion products. For example, brown or reddish-brown scales should be analyzed for iron and for trace amounts of copper.
Greenish mineral deposits should be analyzed for copper. Black scales should be analyzed for iron and copper. Sampling Technique. Since many important decisions are likely to be made based on the sam- pling and chemical analyses performed by a utility, it is important that care be taken during the sampling and analysis to obtain the best data.
Samples should be collected without adding air, as air tends to remove CO2 and also affects the oxygen content in the sample. To collect a sample without additional air, fill the same container to the top so that a meniscus is formed at the opening and no bubbles are present. The sample bottle should be filled below the surface of the water. To do this, slowly run water down the side of a larger container and immerse the sample bottle in the larger container.
Cap the sample bottle as soon as possible. Recommended Analyses for Additional Corrosion Monitoring. The parameters which should be analyzed for in a thorough corrosion monitoring program depend to a large extent on the materials present in the system's distribution, service, and household plumbing lines. In all cases, temperature and pH should be measured in situ in the field. Dissolved gases, such as hydrogen sulfide H2S , oxygen, CO2, and chlorine residual, also should be measured as part of a corrosion monitoring pro- gram. These parameters can be measured in situ or fixed for laboratory measurement.
Total hard- ness, calcium, alkalinity, and TDS or conductivity must be measured if a protective coating of CaCO3 is used for corrosion control or if cement-lined or A-C pipe is present in the system. These analyses are also necessary to calculate the CaCO3-based corrosion indices. Measurement of anions, such as chloride and sulfate, may also indicate corrosion poten- tial. Frequency of analysis depends on the extent of the corrosion problems experienced in the sys- tem, the degree of variability in raw and finished water quality, the type of treatment and corrosion control practiced by the water utility and cost considerations.
Interpretation of Sampling and Analysis Data. Comparing sampling data from various locations within the distribution system can isolate sections of pipe that may be corroding. Increases in levels of metals such as iron or zinc, for instance, indicate potential corrosion occurring in sections of iron and galvanized iron pipe, respectively. The presence of cadmium, a minute contaminant in the zinc alloy used for galvanized pipe, also indicates the probable corrosion of a galvanized iron pipe. Corrosion of cement-lined or A-C pipe is generally accompanied by an increase in both pH and calcium throughout the system, sometimes in conjunction with an elevated asbestos fiber count.
The following example illustrates the changes that can take place between a distribution system and a customer's tap. The analytical results in Table 6. In this case, A-C pipe is used throughout the distribution system. The home plumbing systems are mostly copper.
The water in the distribution system had no traces of copper or lead, and the LSI, calculated from the data as the water entered the distribution system, was slightly positive or potentially non- corrosive. Data in Table 6. Further investigation of the household plumbing showed that the customer's hot water system was corroding. Another example of the importance of data interpretation to an overall corrosion monitoring program is discussed below for A-C pipe. Recommended analyses for a thorough corrosion monitoring program In situ measurements pH, temperature Dissolved gases Oxygen, hydrogen sulfide, carbon dioxide, free chlorine Parameters required to calculate CaCOa-based Calcium, total hardness, alkalinity, total dis- indices, or required for cement-lined or solved solids, fiber count A-C pipe only A-C pipe Heavy Metals Iron or steel pipe Iron Lead pipe or lead-based solder Lead Copper pipe Copper, lead Galvanized iron pipe Zinc, iron, cadmium, lead Anions Chloride, sulfate Source: Environmental Science and Engineering, Inc.
Water quality data from a Florida water utility 0 , , ,. Cu Pb Samp e ocation , ,, , , ,. The following conditions indicate situations in which the water may not attack A-C pipe; 1. An initial AI above about 11; 2. No significant change in the pH or the concentration of calcium at different locations in the system; 3.
No asbestos fibers consistently found in representative water samples after passage through A- C pipe; a. Significant asbestos fiber counts being found in representative water samples at one time but not another at a location where water flow is sufficient to clean the pipe of tapping debris recent tapping can cause high fiber counts not related to pipe attack and b.
Significant asbestos fiber counts being found only in water samples collected from low- flow dead ends or from fire hydrants nonrepresentative samples and nowhere else in the system. The following conditions indicate situations in which the water may be attacking A-C pipe: 1. An initial AI below about 11, 2.
A significant increase in pH and the concentration of calcium at different locations in the sys- tem, 3. Significant asbestos fiber counts being found consistently in representative water samples col- lected from locations where a the flow is sufficient to clean the pipe of debris and b the pipe has been neither drilled nor tapped near or during the sampling period, and 4, Inlet water screens at coin-operated laundries become plugged with fibers. The data obtained by sampling for corrosive characteristics can be used as a guide to water quality changes that might be required to reduce or control corrosion, such as pH adjustment or the addition of silicates or phosphates.
Results of additional sampling, conducted after starting a corro- sion control program, can indicate the success of any water quality changes. For example, a high concentration of calcium in a scale may shield the pipe wall from DO diffusion and thereby reduce the corrosion rate. Methods used to examine scale on pipe walls include physical inspection [both macroscopic human eye and microscopic], X-ray diffraction, and Raman spcctroscopy.
Physical inspection is the only method of practical use to utility personnel, as X-ray diffraction and Raman spectroscopy require expensive, complicated instruments and experienced personnel to interpret the results. Physical Inspection. Physical inspection is usually the most useful inspection tool to a utility because of the low cost. Both macroscopic human eye and microscopic observations of scale on the inside of the pipe are valuable tools in diagnosing the type and extent of corrosion. Macroscopic studies can be used to determine the amount of tuberculation and pitting and the number of crev- ices.
The sample should be examined also for the presence of foreign materials and for corrosion at joints. Utility personnel should try to obtain pipe sections from the distribution or customer plumbing systems whenever possible, such as when old lines and equipment are replaced. If a scale is not found in the pipe, an examination of the pipe wall can yield valuable information about the type and extent of corrosion and corrosion-product formation, such as tubercles , though it may not indicate the most probable cause.
Examination under a microscope can yield even more information, such as hairline cracks and local corrosion too small to be seen by the unaided eye. Such an examination may provide addi- tional clues to the underlying cause of corrosion by relating the type of corrosion to the metallurgi- cal structure of the pipe.
Photographs of specimens should be taken for comparison with future visual examinations. High magnification photographs should be taken, if possible. X-ray Diffraction. The diffraction patterns of X-rays of scale material can be used to identify scale constituents. The diffraction of the X-rays will produce a pattern on a film strip which can be compared with X-ray diffraction patterns of known materials. It is possible to identify complex chemical structures by their X-ray "fingerprint. Raman spectroscopy is a technique for identifying compounds present in corrosion scale and films without removing a metal sample.
In Raman spectroscopy, an infrared beam is reflected off the surface to be analyzed, and the change in frequency of the beam is recorded as the Raman spectrum. This spectrum, which is different for all compounds, is compared with Raman spectra of known materials to identify the constituents of the corrosion film. Raman spectroscopy and X-ray diffraction are useful in corrosion research and in corrosion stu- dies where the nature of the scale is unknown. However, the cost of the analyses makes them too expensive to be used in solving most corrosion problems.
Nearly all corrosion problems can be solved without the detailed information provided by these techniques. Rate Measremcnts Rate measurements are another method frequently used to identify and monitor corrosion. The corrosion rate of a material is commonly expressed in mils 0. Common methods used to measure corrosion rates include 1 weight-loss methods coupon testing and loop studies and 2 electrochemical methods. Weight-loss methods measure corrosion over a period of time. Electrochemical methods measure either instantaneous corrosion rates or rates over a period of time, depending on the method used.
Coupon Weight-Loss Method. This method uses "coupons" or pipe sections as test specimens. It is used for field, pilot-, and bench-scale studies, provided the samples are cleaned and installed in the corrosive environment in such a way that the attack is not influenced by the pipe or container. The coupons usually are placed in the middle of the pipe section. The weight of the specimen or coupon is measured on an analytical balance before and after immersion in the test water. Coupon weight-loss test results do not measure localized corrosion but are an excellent method for measuring general or uniform corrosion.
Coupons are most useful when corrosion rates are high so that weight loss data can be obtained in a reasonable time. The ASTM method above should be followed. Following are lists of the advantages and disadvantages of the coupon method: Advantages 1. Disadvantages 1. Loop System Weight-Loss Method. Another method for determining water quality effects on materials in the distribution system is the use of a pipe loop or sections of pipe.
Either the loop or sections can be used to measure the extent of corrosion and the effect of corrosion control methods. Pipe loop sections can be used also to determine the effects of different water qualities on a specific pipe material. The advantage is that actual pipe is used as the corrosion specimen. The loop may be made from long or short sections of pipe. Water flow through the loop may be either continuous or shut off with a timer part of the time to duplicate the flow pattern of a household.
Pipe sections can be removed for weight-loss measure- ments and then opened for visual examination. Following are lists of the advantages and disadvantages of a loop system; Advantages 1. Electrochemical Rate Measurements. These methods are based on the electrochemical nature of corrosion of metals in water. An increasing number of these instruments are now on the market. However, they are relatively expensive and probably not widely used by smaller utilities. They are discussed here for completeness. One type of electrochemical rate instrument has probes with two or three metal electrodes that are connected to an instrument meter to read corrosion in mpy.
The electrode materials can be made of the material to be studied and inserted into the pipe or corrosive environment. For the other type, the loss of material over time is detected by an increase in the resistance of an electrode made of the metal of interest.
Measurements made over a period of time can be used to estimate corrosion rates. Following are lists of the advantages and disadvantages of electrical resistance measurements: Advantages 1. A schematic representation of a general approach to solving corrosion problems is shown in Fig, 7. To completely eliminate corrosion is difficult if not impossible. There are, however, several ways to reduce or inhibit corrosion that are within the capability of most water utilities. This sec- tion describes several methods most commonly used to control corrosion.
The utility operator should use common sense in selecting the best and most economical method for successful corrosion control in a particular system. Because corrosion depends on both the specific water quality and pipe mate- rial in a system, a particular method may be successful in one system and not in another. Corrosion is caused by a reaction between the pipe material and the water in direct contact with each other. Consequently, there are three basic approaches to corrosion control: 1, modify the water quality so that it is less corrosive to the pipe material, 2.
The most common ways of achieving corrosion control are to 1. As discussed in Sect. In general, the less reactive the material is with its environ- ment, the more resistant the material is to corrosion. When selecting materials for replacing old lines or putting new lines in service, the utility should select a material that will not corrode in the water it contacts. Admittedly, this provides a limited solution since few utilities can select materials based on corrosion resistance alone.
Usually several alternative materials must be compared and evaluated based on cost, availability, use, ease of installation, and maintenance, as well as resistance to corrosion. In addition, the utility owner may not have control over the selection and installation of the materials for household plumbing. There are, however, several guidelines that can be used in selecting materials.
First, some materials are known to be more corrosion resistant than others in a given environ- ment. For, example, a low pH water that contains high DO levels will cause more corrosion damage in a copper pipe than in a concrete or cement-lined cast iron pipe. Other guidelines relating water quality to material selection are given in Table 4. Second, compatible materials should be used throughout the system. Two metal pipes having different activities, such as copper and galvanized iron, that come in direct contact with others can set up a galvanic cell and cause corrosion.
The causes and mechanisms of galvanic corrosion are discussed in Sect. As much as possible, systems should be designed to use the same metal throughout or to use metals having a similar position in" the galvanic series Table 3. Galvanic corrosion can be avoided by placing dielectric insulating couplings between dissimilar metals. Steps toward solving corrosion problems. A faulty design may cause severe corrosion, even in materials that may be highly corrosion resistant. Some of the important design considerations include 1.
Many plumbing codes are outdated and allow undesirable situations to exist. Such codes may even create problems, for example, by requiring lead joints in some piping. Where such problems exist, it may be helpful for the utility to work with the responsible government agency to modify outdated codes. Because of the differences among raw water sources, the effec- tiveness of any water quality modification technique will vary widely from one water source to another. However, where applicable, water quality modification can often result in an economical method of corrosion control.
Acid waters are generally corrosive because of their high concentration of hydrogen ions. When corrosion takes place below pH 6. In the range between pH 6. Most materials used in water distribution systems copper, zinc, iron, lead, and cement dissolve more readily at a lower pH. Increasing the pH increases the hydroxide ion OH" concentration, which, in turn, decreases the solubility of metals that have insoluble hydroxides, including copper, zinc, iron, and lead. When carbonate alkalinity is present, increasing the pH, up to a point, increases the amount of carbonate ion in solution.
The cement matrix of A-C pipe or cement-lined pipe is also more soluble at a low pH. Increasing the pH is a major factor in limiting the disso- lution of the cement binder and thus controlling corrosion in these types of pipes, 3. The relationship between pH and other water quality parameters, such as alkalinity, carbon dioxide CO2 , and TDS, governs the solubility of calcium carbonate CaCO3 , which is com- monly used to provide a protective scale on interior pipe surfaces. To deposit this protective scale, the pH of the water must be slightly above the pH of saturation for CaCO3, provided sufficient alkalinity and calcium are present.
A protective coating of CaCO3, for instance, will not form unless a sufficient number of carbonate and calcium ions are in the water. Some metals, notably lead and copper, form a layer of insoluble carbonate, which minimizes corrosion rates and the dissolution of these metals. In low alkalinity waters, carbonate ion must be added to form these insoluble carbonates. The number of carbonate ions available is a complex function of pH, temperature, and other water quality parameters. Bicarbonate alkalinity can be converted to carbonate alkalinity by increasing the pH. If carbonate supplementing is neces- sary to control corrosion in a water system, pH also must be carefully adjusted to ensure that the desired result is obtained.
The proper pH for any given water distribution system is so specific to its water quality and sys- tem materials that a manual of this type can provide only general guidance. Other indices can be used to check this value. To start, the pH of the water should be adjusted such that the LSI is slightly positive, no more than 0.
Keeping the pH above the pHs should cause a protective coating to develop. If no coating forms, then the pH should be increased another 0. It is important to watch the pressure in the system carefully as too much scale build-up near the plant could seriously clog the transmission lines. Soft, low alkalinity waters cannot become supersaturated with CaCOj regardless of how high the pH is raised. In fact, raising the pH to values greater than about Excess hydroxide alkalinity is of no value since it does not aid in CaCO3 precipitation. For systems that do not rely on CaCO3 deposition for corrosion control, it is more difficult to estimate the optimum pH.
Practical minimum lead solubility occurs at a pH of about 8. Phosphates and other corrosion inhibitors often require a narrow pH range for maximum effec- tiveness. If such an inhibitor is used, consideration must be given to adjusting the pH to within the recommended range. Schematics of typical chemical feed systems are shown in Fig. The pH should be adjusted after filtration since waters having higher pHs need larger doses of alum for optimum coagulation. It is recommended that a corrosion monitoring program, such as that described in Sect.
Evaluating the performance of chemi- cal feed systems for pH adjustment is the key to an effective corrosion control program. Addition of lime, soda ash, or other chemicals for pH control can be evaluated by continuous readout pH recorders. The recorders monitor the pH of the water as it leaves the utility and can be wired to send a signal to the feed mechanism to add more or fewer chemicals as necessary.
The pH levels at the outer reaches of the distribution system should be checked periodically for indications of any changes occurring within the system that might be due to corrosion. Keep in mind that although pH adjustment can aid in reducing corrosion, it cannot eliminate corrosion in every case. However, pH adjustment is the least costly and most easily implemented method of achieving some corrosion control, and utilities should use it if at all possible.
Reduction of Oxygen As explained in Sect. If oxygen could be removed from water economically, the chances of corrosion starting, and also the corrosion rate once it had started, would be reduced. Unfortunately, oxygen removal is too expensive for municipal water systems and is not a practical control method.
However, there are ways to minimize the addition of oxygen to the raw water, particularly to groundwaters. Often, aeration is the first step in treating groundwaters having high iron, hydrogen sulfide H2S or CO2 content. Though aeration helps remove these substances from raw water, it can also cause more serious corrosion problems by saturating the water with oxygen.
In lime-soda softening plants for treating groundwater, the water is often aerated first to save on the cost of lime by elimi- nating free CO2. The actual result is that DO increases to near saturation, and corrosion problems are increased. Thus, the attempt to save on lime addition may actually end up costing a great deal more in corrosion damage. Measures that help keep the DO levels as low as possible include 1 sizing well pumps and dis- tribution pumps so as to avoid air entrainment and 2 using as little aeration as possible when aerating for H2S or CO2 removal.
This can be achieved by by-passing the aerators with part of the raw water. It has even been possible to completely eliminate the use of aerators if enough detention time is available in the reservoir so that enough oxygen can be absorbed at the surface to oxidize the H2S or to let the CO2 escape. DO levels can be kept as low as 0. This is low enough, in many cases, to reduce corrosion rates considerably.
These chemicals, called inhibitors, reduce corrosion but do not totally prevent it. The three types of chemical inhibitors commonly approved for use in potable water systems are chemicals which cause CaCO3 scale formation, inorganic phosphates, and sodium silicate. There are currently several hundred commercially available products listed with various state and federal agencies for this use see Sect.
The success of any inhibitor in controlling corrosion depends upon three basic requirements. First, it is best to start the treatment at two or three times the normal inhibitor concentration to build up the protective film as fast as possible. This minimizes the opportunity for pitting to start before the entire metal surface has been covered by a protective film. Usually it takes several weeks for the coating to develop.
Second, the inhibitor may be fed continuously and at a sufficiently high concentration. Interrup- tions in the feed can cause loss of the protective film by re-dissolving it, and too low concentrations may prevent the formation of a protective film on all parts of the surface. Both interrupted feeding and low dosages can lead to pitting.
On the other hand, excessive use of some alkaline inhibitors over a period of time can cause an undesirable build-up of scale, particularly in harder waters. The key to good corrosion inhibitor treatment is feed control. Third, flow rates must be sufficient to continuously transport the inhibitor to all parts of the metal surface, otherwise an effective protective film will not be formed and maintained. Corrosion will then be free to take place. For example, corrosion inhibitors often can not reduce corrosion in storage tanks because the water is not flowing, and the inhibitor is not fed continuously.
To avoid corrosion of the tanks, it is necessary to use a protective coating, cathodic protection, or both. Simi- larly, corrosion inhibitors are not as effective in protecting dead ends as they are in those sections of mains which have a reasonably continuous flow. CaCO3 Deposition Under certain conditions, a layer of CaCOa will deposit on the surface of the pipe and serve as a protective barrier between the pipe wall and the water. This process is discussed in Sect. It is mentioned again here because the addition of lime or alkalinity is a kind of inhibitor treatment. Inorganic Phosphates Phosphates are used to control corrosion in two ways: to prevent scale or excess CaCOs build-up and to prevent corrosive attack of a metal by forming a protective film on the surface of the pipe wall.
Phosphates inhibit the deposition of a CaC03 scale on the pipe walls, which is an advantage only in the waters in which excessive scaling occurs. The mechanism by which phosphates form a protective film and inhibit corrosive attack, though not completely understood, is known to depend on flow velocity, phosphate concentration, temperature, pH, calcium, and carbonate levels.
Recent develop- ments in corrosion control include the use of zinc along with a polyphosphate or orthophosphate. In such cases, the addition of glassy phosphates masks the color, and the water appears clear because the iron is tied up as a complex ion. The cor- rosive symptoms are removed, but the corrosion rates are not reduced. Other glassy phosphates which contain calcium as well as sodium are more effective as corrosion inhibitors. The zinc phosphate treatment has also been used to elimi- nate rusty water, blue-green staining, lead pickup, and to reduce measured corrosion rates of metals.
The choice of a particular type of phosphate to use in a corrosion control program depends on the specific water quality. Some phosphates work better than others in a given environment. It is usually advisable to conduct laboratory or field tests of one or more phosphate inhibitors before long-term use is initiated. The case histories in Sect. For smaller water utility plants [up to 1 million gallons per day MOD ], phosphate feed solu- tions can be made up easily by batch as needed.
A maximum phosphate solution concentration of 10 wt. Sodium Silicate Sodium silicate water glass has been used for over 50 years to reduce corrosivity. The way in which sodium silicate acts to form a protective film is still not completely understood. However, it can effectively reduce corrosion and red water complaints in galvanized iron, yellow brass, and cop- per plumbing systems in both hot and cold water.
The effectiveness of sodium silicate as a corrosion inhibitor depends on water quality properties such as pH and bicarbonate concentration. Silicate has been found to be particularly useful in waters having very low hardness and alkalinity and a pH of less than 8,4. It is also more effective under higher velocity flow conditions. The equipment needed to feed sodium silicate is the same as that needed to add phosphate.
The application of sodium silicate requires the use of solution feeders and small positive displacement pumps that deliver a specific volume of chemical solution for each piston stroke or impeller rotation. Figure 7. Monitoring Inhibitor Systems When phosphates or silicates are added to the water, samples should be collected at the far reaches of the system and analyzed for polyphosphates, orthophosphates, and sodium silicate, as appropriate. If no residual phosphate or silicate is found, the feed rate should be increased. Commercially available phosphate or silicate feed system.
If the concentration at the far reaches of the sys- tem is the same as that applied at the utility e. As previously discussed, initial inhibitor feed rates for the first 2 weeks should be 5 to 10 times higher than normal. During this time, water from the far reaches of the system should be sampled about twice a week to determine if corrosion products are leaching from the pipe wall. If the pipes are heavily tubercled, the tubercles are frequently broken loose by the inhibiting chemical.
Where pitting has occurred, the system may be suddenly plagued with leaks as a result, and other corrective action must be initiated. After the system has stabilized, sampling frequency can be reduced to about once a month or quarterly, depending on the resources available to the utility. Most metering pumps used to add phosphate or silicate are positive displace- ment pumps. Pumping action for this type of pump is achieved by means of a piston, plunger, or diaphragm in which movement in one direction draws in a liquid through a valve, and movement in the opposite direction forces the liquid out through a second valve, causing a positive displacement of the liquid during each stroke of the unit.
These types of pumps are generally used for chemical feeding when liquids heavier than water are being added. Chemical feed rates can be adjusted by changing the length and speed of the piston or diaphragm stroke. Usually, the water is pumped from a well or storage tank by centrifugal pumps throughout the distribution system. A signal can be wired from the centrifugal pump to the feed pump so that the feed pump is activated only when water is being pumped to the distribution system.
Chemical feed pumps can be single or dual headed so that one or two chemicals can be added at the same time. The advantage of these pumps is that they are both accurate and reliable in feeding a specified amount of chemical to the system. The feed pumps should be calibrated about once a week to ensure that the desired amount of chem- ical is added. Cathodic protection stops the current by overpowering it with a stronger current from some outside source.
This forces the metal that is being protected to become a cathode; that is, it has a large excess of electrons and cannot release any of its own. There are two basic methods of applying cathodic protection. One method uses inert electrodes, such as high-silicon cast iron or graphite, that are powered by an external source of direct current.
The current impressed on the inert elec- trodes forces them to act as anodes, thus minimizing the possibility that the metal surface being protected will become an anode and corrode. The second method uses a sacrificial galvanic anode. Magnesium or zinc anodes produce a galvanic action with iron such that they are sacrificed or cor- rode while the iron structure they are connected to is protected from corrosion.
This type of system is common to small hot water heaters. Another form of sacrificial anode is galvanizing where zinc is used to coat iron or steel. The zinc becomes the anode and corrodes, protecting the steel, which is forced to be the cathode. The primary reason for applying cathodic protection in water utilities is to prevent internal cor- rosion in water storage tanks. Because of the high cost, cathodic protection is not a practical corro- sion control method for use throughout a distribution piping system.
Another limitation of cathodic protection is that it is almost impossible for cathodic protection to reach down into holes, crevices, or internal corners. These linings are usually mechanically applied, either when the pipe is manufactured or in the field before it is installed. Some linings can be applied even after the pipe is in service, though this method is much more expensive. The most common pipe linings are coal-tar enamels, epoxy paint, cement mortar, and polyethylene. Water storage tanks are most commonly lined to protect the inner tank walls from corrosion.
Common water tank linings include coal-tar enamels and paints, vinyls, and epoxy. Although coal-tar-based products have been widely used in the past for contact with drinking water, currently there is concern at EPA about their use because of the presence of polynuclear aromatic hydrocarbons and other hazardous compounds in coal tar and the potential for their migration in water. Table 7.
Common water tank linings are summarized in Table 7. Concerns about the public health risks focus on the residual amounts of water treatment chemicals in drinking water and the impurities found in them and on the poten- tially hazardous chemicals which could leach from materials and substances in contact with the water. The EPA, operating in cooperation with the States and under the authority of the Safe Drinking Water Act, is charged with assuring that the public is provided with safe drinking water.
Under the auspices of that charge, EPA assists the States and the public by providing scientific advice on the health safety of chemicals and other substances in and in contact with drinking water. However, in practice, many state health departments have relied heavily on EPA's opinions in their approval of products and equipment for use in treatment and distribution systems of public utilities.
These opinions on product safety are handled through a voluntary product safety evaluation program at EPA, Additionally, the National Academy of Sciences NAS , under contract to ODW, recently pub- lished the first edition of the "Water Chemicals Codex," which sets recommended maximum impur- ity concentrations RMICs for harmful substances found in many common direct additives bulk treatment chemicals.
Methods used to monitor and control corrosion in the distribution systems are presented. The case histories are as follows: Case 1. Mandarin Utilities, Jacksonville, Florida; Case 3. Each case presents a corrosion problem unique to that utility or complex because of a specific water quality in a given environment. In each case, the source and the effects of the corrosion are differ- ent, and the control methods implemented also are unique to each system. However, the approaches to the problems are similar and relevant to most utilities, regardless of size or the nature of the cor- rosion problem.
Each case is presented in some detail to emphasize the different steps used in corro- sion control, such as investigating the extent and cause of the problem, sampling and analyzing to further evaluate the problem, testing different control alternatives, and implementing the corrective actions. In addition to the case histories discused here, another excellent case history is the corrosion monitoring and control program implemented by Seattle, Washington.
The Seattle experience has been described in several journals but is not included here because of the complexity and length of the study. Interested readers are referred to the report written by J. Courthene and G. Many corrosion problems can be solved by the water utility itself. Sometimes, however, in-house diagnosis may lead to wrong conclusions and ineffective treatment. There is often no substitute for consulting with experienced corrosion engineers, the local health department, or state water treat- ment personnel for assistance in solving corrosion problems.
Nelson and F. Water production averages about 40 MOD. The water source is wells averaging ft in. Water treatment origi- nally involved aeration to remove H2S, chlorination to give a free chlorine residual to 2. Table 8. Reports of leaking copper pipes in numerous homes and apartment complexes alerted PCWS personnel to its copper corrosion problem.
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To determine the cause and extent of the corrosion and correct deficiencies, the PCWS initiated an investigative monitoring program. Initial Investigation and Monitoring Program Procedure. To determine the extent of copper corrosion and acquire background information for evaluating future treatment modifications, the following investigation and monitoring program was instituted before any changes in plant operation were made: 1.
Approximately 25 random samples were collected from customers' residences. Twenty residents' homes were monitored weekly beginning in September for copper, pH, DO, and chlorine residual. Weekly sampling continued through May of Drinking fountains throughout Pinellas County were monitored for copper content and found to average 1.
The results of the investigation indicated that not only was there a pitting problem, but also that copper levels averaged 1. In some isolated points, 5. It became evident that it was necessary to reduce the pitting action and to reduce the copper level to under 1. To determine the degree of copper corrosion caused by low pH and thus high CO2, the pH was increased to 7. Raising the pH reduced the CO2 level from about 8.
The average copper content was reduced by 0. This demonstrates that in an effort to control an existing problem, one frequently creates another, possibly worse, problem. Especially when using pH adjustment as a means of controlling corrosion, CaCO3 solubility must be kept in mind. By adjusting to slightly positive in the dis- tribution system, the utility frequently runs the risk of scaling consumer water heaters or other equipment in the system. Alternative 2: Reduction of DO Procedure. To determine the degree of copper corrosion caused by DO, the Plant 1 aerators were by-passed.
Plant 1 supplies one area of distribution exclusively before blending with water from Plant 2 about 10 miles away at a million gallon storage and booster station. The service area fed by Plant 1 consisted of 5 of the original 20 distribution sample points and provided an excellent opportunity to compare results of further treatment changes. After by-passing the Plant 1 aerators, the DO of the finished water was reduced from 7. Daily samples were taken of both plant effluents and within the distribution system.
The copper level in the Plant 1 effluent at the ft copper tubing dropped from 2. Oxygen levels averaged 1. Figure 8. A micropump was used to feed a stock solution of SZP at the rate of 1. An untreated section of copper pipe was used as a control. Water dosed with SZP was allowed to flow through one section of copper tubing for 8 h. Both the untreated and dosed water were then turned off and allowed to stand in the copper pipe for up to 24 h before testing. The CO2 content was 9. Samples were taken from each tap and analyzed for their copper content.
Sequestering with 2.
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