Public and private water and sewer companies have assets such as dams, culverts, tunnels, transmission/collection pipelines, water and wastewater treatment plants, pumping plants, distribution pipelines, and storage reservoirs. A considerable part of these facilities is constructed with metals and alloys such as carbon steel, cast iron, and brass, which are prone to corrosion attacks.
Our modern cities are based on a complex infrastructure network located both above and below ground. A critical component to public health and economic well-being is the municipal water which is brought to the tap through an elaborate network of pipe distribution systems. Because most of this infrastructure is below grade, it is out of sight and often neglected.
According to the American Waterworks Association (AWWA) industry database, there is approximately 1,483,000 km of municipal water piping in the United States. As discussed in Economics of Corrosion, the total annual direct cost of corrosion for the drinking water and sewer systems in the United States is estimated at $36.0 billion, which includes the costs of replacing aging infrastructure, corrosion inhibitors, internal linings, external coatings, and cathodic protection.
Loss of treated water, increased maintenance budgets, traffic, and business disruptions, and damage to private property are the main economic impacts of corrosion in the municipal water system.
The causes of corrosion in the water and wastewater systems vary based on the material used in the infrastructure and whether that infrastructure is used for water or wastewater. Moreover, the corrosion of water and sewer systems can be evaluated from different angles such as internal corrosion, external corrosion of water pipes, and corrosion control strategies. In this blog post, corrosion of water and sewer system is evaluated from different perspectives, with special attention to the municipal water system.
Municipal Water System Pipe Materials
Cast iron, ductile iron, and PVC are the main pipe material inventory for municipal water systems in North America. Water mains installed between the late 1800s and the 1920s are frequently cast iron. Ductile-iron pipe began to be installed in the 1950s. PVC pipes were introduced in the 1970s followed by high-density polyethylene in the 1990s.
Although corrosion management has improved over the past several decades, there is a significant demand to find more efficient ways to encourage, support, and implement optimal corrosion control practices in water and wastewater facilities.
Most of the ductile iron pipe and approximately 40% of the cast-iron mains in North America are mortar-lined. However, cast iron has the highest break rate among the pipe materials. About 82% of all cast iron pipe in water mains in North America is over 50 years old. The break rates of cast iron pipes have increased significantly by 43% since 2012 and are expected to continue to increase. A survey done in 2018 showed cast iron break rates of 33.2 and 48.4 breaks per 100 miles per year in the US and Canada, respectively, with corrosion causing about 28% of the total failures.
Corrosion in the Drinking Water System
In general, the corrosion of water mains can occur as internal or external corrosion. Because different corrosion forms cause internal and external corrosion, different mitigation strategies are required to minimize the risk of failure of water pipes. Dissolved gases (e.g., oxygen, carbon dioxide, etc.), mineral constituents (e.g., calcium, carbonate/bicarbonate, chloride, sulfate, and dissolved silica), organic matter (e.g., animal and vegetable origin oil), and microbiological forms (e.g., algae and slime-forming bacteria) are the most important constituents in natural water affecting internal corrosion of water pipe. Perhaps, dissolved oxygen is the most significant constituent affecting corrosion of metallic water pipes, because oxygen reduction is the dominant cathodic reaction in neutral waters. The effect of dissolved carbon dioxide on corrosion of water pipe is closely linked with the bicarbonate content of the water, which causes precipitation of a scale onto the pipe. Also, dissolved carbon dioxide lowers the water pH via the formation of carbonic acid.
The selection of the best corrosion control technique is not a one-time event. Any changes in source water, water treatment, or severe environmental conditions will require a system to revisit the corrosion control program.
Another factor that affects the corrosivity of water is the hardness, which is determined in terms of its calcium carbonate concentration. Soft waters (<50 ppm calcium carbonate) are commonly corrosive to most metals. Their corrosivity depending to some extent on pH. Very hard waters, on the other hand, are usually not very aggressive provided that they are supersaturated in calcium carbonate. Waters of intermediate hardness often contain substantial amounts of other constituents, and there is a tendency to occur corrosion under loosely attached scales formed on the metal surface in these waters. Other inorganic species, such as nitrate, phosphate, iodide, bromide, and fluoride may be found in natural waters. Because of the small quantities of these anions, their effect of corrosion of water system is negligible. Some species, like nitrate and phosphate, can act as corrosion inhibitors for steel and cast-iron pipes. The dissolved ionic species can affect the corrosivity of natural waters by increasing the ionic conductivity of water and, hence, tend to increase metallic corrosion by reducing the ohmic voltage loss between local anodes and cathodes.
Microbiologically influenced corrosion (MIC) is another important corrosion form often reported in water systems. The presence and activity of microorganisms can cause corrosion of metals and alloys by creating conditions favorable to electrochemical reactions. MIC can threaten the integrity of water mains via both internal and external corrosion.
External corrosion of water systems
The main part of the municipal water facilities is below ground. While the buried pipes suffer from soil corrosion, the above-ground facilities, such as pipes on bridge crossings, the exterior of storage tanks, valves, manholes, etc., are prone t atmospheric corrosion. Soil resistivity, pipe-to-soil potential (also referred to as pipe redox potential), soil moisture content, soil pH, temperature, presence of microorganisms and microbiological activity, and exposure to stray current from foreign structures are the key variables that determine the likelihood of external corrosion of buried water facilities. On the other hand, temperature, atmospheric salinity, pollutants (e.g., SOx), and wet/dry cycling are the main variables that determine the severity of atmospheric corrosion.
Internal corrosion of water system
Internal corrosion of distribution water systems and home plumbing can cause water quality issues, including potential health concerns, taste and odor problems, and discoloration. The health concerns are associated with the release of trace metal concentrations (e.g., lead, and copper) from corroding metal surfaces. Dissolved metal cations iron may cause the taste and odor of drinking water. For instance, the presence of iron results in a metallic taste. In addition to metallic taste, cast iron corrosion may also result in musty tastes and odors. Aluminum and zinc may contribute to an astringent mouthfeel, and zinc may also result in a sour taste. Similar to taste and odor, the release of metallic ions due to internal corrosion of water pipes causes discoloration of drinking water. Corrosion of cast iron pipe or dissolution of existing scale on cast iron pipe may result in rust-colored water or red water because of the release of ferric iron. Also, the internal corrosion of water pipes can occur in the form of uniform or localized corrosion.
Corrosion control in water and wastewater systems
Corrosion of water and sewer system can happen as internal or external corrosion, as discussed earlier. Thus, different strategies are required to mitigate the two corrosion modes. It should be borne in mind that the corrosion cannot be completely eliminated and the feasibility of corrosion control techniques is dependant on the specific water quality and piping of an individual system. Moreover, corrosion of a water utility can be minimized by properly selecting distribution system materials and having a suitable engineering design. The selection of piping material is based on water chemistry, cost, mechanical properties, and sometimes, jurisdictions that regulate the components of the distribution system.
External corrosion control
Coatings, linings, and encasements are used to reduce or mitigate corrosion of buried pipes. They act by creating a barrier between the metal and the corrosive environment (i.e., soil). Polyethylene encasement, coal-tar enamel and tape, and liquid epoxy are examples of such coatings recommended by AWWA. Cathodic protection is another common mitigation technique to decrease the risk of external corrosion. In this technique, an external anode buried in the soil is connected to the pipe and makes the entire pipeline the cathode of a corrosion cell. Although cathodic protection does not eliminate corrosion, it can reduce the corrosion rate to an acceptable level to meet the minimum service life of the buried water pipe.
Internal corrosion control
Deposition of a protective coating (i.e., scaling) onto the pipe wall and formation of a metal oxide film (i.e., passivation) are other ways to control internal corrosion of water pipes. However, both scaling and passivation can potentially promote the risk of localized corrosion, pitting for example, especially when the water flow is very low. Chemical treatment is another important method to control the internal corrosion of water mains. Chemical treatments include pH adjustment, dissolved inorganic carbon adjustment, and the application of corrosion inhibitors. Calcium carbonate, caustic soda, and phosphate blends are common examples of chemicals used for water quality and corrosion control. pH adjustment is perhaps the most common method for reducing corrosion in distribution systems. The water pH is the major factor that determines the corrosion rate of metallic pipes.
Lining the pipe wall with a protective coating is an effective technique to reduce corrosion. For new pipes, these linings are usually applied during the pipe manufacturing process or in the field when the pipe is installed. For old pipes, however, the pipe wall needs to be cleaned first. Routine maintenance of water systems can reduce corrosion and remove corrosion products. For example, metals release of old pipes can be effectively reduced by cleaning and lining. The cleaning procedure includes using acids and surfactants or mechanical scrapers to remove scale and deposit. The most common pipe linings are coal-tar enamels, epoxy paint, cement-mortar, and polyethylene.
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