In this lesson we will answer the following questions:
- What problems are associated with corrosive and scale-forming water?
- How does the electrochemical reaction of corrosion work?
- What are the types of corrosion?
- What factors influence the stability of water?
- How does stabilization fit into the water treatment process?
Along with the online lesson, read Chapter 8: Corrosion Control, in your textbook Operation of Water Treatment Plants Volume I .
What is Stabilization?
Stable water is water which neither tends to be corrosive nor scale-forming. Corrosive, also known as aggressive or unstable, water will tend to corrode (rust) metal in the pipes or tanks it passes through. Scale-forming water will tend to deposit calcium carbonate scale on the surfaces of these pipes or tanks.
Corrosive and scale-forming waters are at the opposite ends of a spectrum. A variety of water characteristics (which we will discuss in a later section) combine to influence water's location along this spectrum. The goal of the treatment plant operator is to find the point along the stability spectrum at which the water is stable and will neither corrode pipes or form scale.
Unstable water causes problems mainly in the distribution system, though it can also harm the treatment plant equipment and fixtures in the customers' homes. Scaling is problematic because it forms on the insides of pipes and reduces the area available to carry water. In addition, scaling can form on equipment and on hot water heaters and cause other problems.
Despite these problems caused by scaling, we should be aware that a small amount of scale is beneficial because it coats the insides of pipes and retards corrosion. Typically, the water treatment plant operator will strive to produce water which is slightly scale-forming.
Corrosive water, in contrast, is never beneficial. Corrosion, like that shown in the pictures above, can cause economic, health, and aesthetic problems.
Economic problems result from damage to pipes, storage tanks, valves, and meters. Damage to pipes is the most prevalent, consisting of leaks and reduced carrying capacity. These pipe corrosion problems often result from tuberculation, which is the production of mounds of rust on the inside of the pipe, as shown in the picture below.
These mounds reduce the space in the pipe available to carry water, just as scaling does. In addition, tubercles are usually associated with pits in the pipe wall, which may go all the way through the pipe and cause leaks.
Corrosion in the distribution system can also cause health hazards. When pipes are corroded, some of the metal from the pipes enters the drinking water and is consumed by the customer. If the pipes contain lead or copper - and brass pipes, for example, are made up of about 7-11% lead and a much higher percentage of copper - then the metals in the water are hazardous to the customer's health. Lead causes a variety of problems in children and increases blood pressure in adults while copper causes stomach and intestinal problems and Wilson's Disease. As a result of these health hazards, the EPA passed a Lead and Copper Rule in 1991 which limits the amount of lead and copper that can be found in drinking water.Finally, corrosion can cause aesthetic problems. When metal pipes corrode, the rust can break free and be carried to the customer in the water. This phenomenon, known as red water, can stain laundry and plumbing fixtures. In addition, corrosion in the distribution system can result in taste problems.
We have already discussed scaling, so we will be primarily concerned with corrosion in the rest of this lesson. Corrosion is an electrochemical reaction involving the movement of electrons. Let's first consider a more familiar electrochemical reaction - that which occurs when electricity comes out of a battery.
In a battery, electrons build up in the negative end, also known as the anode. The positive end, known as the cathode, is attractive to electrons due to its positive charge. If the two ends of the battery are connected with a conductive object, such as a metal wire through which electrons can flow, the electrons will flow from the anode to the cathode as an electric current. The battery and the wire make up what is known as an electrolytic cell, which is a device which causes an electric current to flow.
Corrosion in a metal object, such as a pipe, acts in the same manner. A negative area of metal (the anode) is connected to a positive area (the cathode) by the pipe wall itself. As a result, electrons can flow from the anode to the cathode.
In addition to the anode, the cathode, and the connecting conductive material, the electrochemical reaction requires one more element - the electrolyte. The electrolyte is a conducting solution, which in the case of a pipe is the water within the pipe with its dissolved salts. (In a battery, the electrolyte is found within the battery - the "battery acid".) The electrolyte accepts the electrons from the cathode, making the cathode maintain a positive charge which draws more electrons to it.
So, in summary, any electrochemical reaction requires four elements, all of which must be in contact - the anode, the cathode, the conductive material, and the electrolyte. In the battery, the anode and cathode are the two ends of the battery, the conductive material is a wire or other object touching both ends, and the electrolyte is found inside the battery. In the case of corrosion of a pipe, the anode, cathode, and conductive material are all found in the pipe wall while the electrolyte is the water within the pipe. If any of these four elements, which make up the corrosion cell, are absent or are not touching each other, then corrosion cannot occur.
In the last section, we discussed the electrical side of the electrochemical reaction occurring during corrosion. In order for the flow of electrons to occur, however, chemical reactions must also be happening. In this lesson, we will consider the chemical reactions which occur in an iron pipe as it corrodes. Other types of pipes will have different, but homologous, chemical reactions driving their corrosion.
The main force behind corrosion is the tendency of iron to break down into its natural state. The iron found in pipe is elemental iron (Fe0) which is unstable and tends to oxidize, to join with oxygen or other elements. In nature, this oxidation produces an iron ore such as hematite (Fe2O3), magnetite (Fe3O4), iron pyrite (FeS2), or siderite (FeCO3). In corrosion, the result of this oxidation is rust, Fe(OH)2 or Fe(OH)3.
Oxidation of the elemental iron occurs at the anode. First, the elemental iron breaks down as shown below. In this reaction, elemental iron leaves the pipe, so pits form in the pipe's surface at the anode.
Elemental Iron → Ferrous iron + Electrons
Fe0 → Fe2+ + 2e-
The reaction produces ferrous iron and two electrons. The electrons are then able to flow through the pipe wall to the cathode. Meanwhile, the ferrous iron reacts with the water (the electrolyte) in the pipe to produce rust and hydrogen ions.
Ferrous iron + Water ↔ Ferrous hydroxide + Hydrogen ions
Fe2+ + 2H2O ↔ Fe(OH)2 + 2H+
The rust builds up a coating over the anode's surface. Ferrous hydroxide may then react with more water to produce another form of rust called ferric hydroxide (Fe(OH)3). These layers of rust are what creates the tubercles we mentioned earlier.
Tubercles can become problematic because they decrease the carrying capacity of the pipe and can be dislodged during high water flows, resulting in red water complaints. But in the corrosion process, the tubercle actually slows the rate of corrosion by cutting the anode off from the electrolyte. When the tubercle becomes dislodged and the anode comes in contact with water again, the corrosion rate increases.
The electrons from the breakdown of elemental iron flow through the pipe wall to the cathode. There, they leave the metal and enter the water by reacting with hydrogen ions and forming hydrogen gas:
Hydrogen ions + Electrons ↔ Hydrogen gas
2H+ + 2e- ↔ H2
Hydrogen gas will coat the cathode and separate it from the water in a process called polarization. Just as the buildup of a tubercle breaks the connection between the anode and the electrolyte and slows the corrosion process, polarization breaks the connection between the cathode and the electrolyte and slows corrosion.
Dissolved oxygen in the water is able to react with the hydrogen gas surrounding the cathode:
Hydrogen gas + Oxygen ↔ Water
2H2 + O2 ↔ 2H2O
This reaction is called depolarization. Depolarization removes the hydrogen gas surrounding the cathode and speeds up the corrosion process. So, you can see why water high in dissolved oxygen is more corrosive.
The Electrochemical Reaction
By combining the electrical and chemical reactions discussed above, we can see what is really happening during corrosion of a pipe.
Types of Corrosion
Internal vs. External Corrosion
Corrosion can occur on the outside of a pipe (due to corrosive soil) or on the inside of a pipe (due to corrosive water.) We will be most concerned with internal corrosion, although external corrosion is a similar process and can also cause problems in the distribution system.
Either outside or inside a pipe, corrosion can have one of several causes. Each cause somehow sets up an anode and a cathode so that corrosion can occur. The creation of the corrosion cell can be through electrolysis, oxygen concentration cells, or through galvanic action.
In electrolysis, a D.C. electric current enters a metal pipe and causes flow of electrons through the pipe and to the ground. The pipe, fueled by the electric current, becomes the anode while the soil becomes the cathode. The outside of the pipe corrodes, with the metal from the pipe plating out in the surrounding soil.
Electrolysis can occur when D.C. electric currents are grounded onto pipes. Nearby electric transit systems can also cause electrolysis.
Oxygen Concentration Cell
More commonly, the water and its constituents may set up a corrosion cell within the pipe. These corrosion cells, known as oxygen concentration cells, result from varying oxygen concentration in the water. The portion of the pipe touching water with a low oxygen concentration becomes the anode while the part of the pipe in contact with a high oxygen concentration becomes the cathode.
Oxygen concentration cells are probably the primary cause of corrosion in the distribution system. They may occur at dead ends in the distribution system where water is stagnant and loses its dissolved oxygen. Alternatively, oxygen concentration cells may begin in annular spaces, which are ring-shaped spaces between two pipes or between a pipe and a pipe lining. In every case, oxygen becomes depleted in these regions since they are cut off from the normal flow of water, so a difference in oxygen concentration is set up between the dead end or annular space and the main flow of water.
Oxygen concentration cells can also be caused by bits of dirt or bacteria. Both of these can become attached to the pipe walls, shielding the metal from dissolved oxygen in the water and setting up an anode.
Metals themselves can also set up corrosion cells. When a pipe consists of only one type of metal, impurities in the pipe wall can develop into anodes and cathodes. Alternatively, when two dissimilar metals come into contact, galvanic corrosion will occur. Galvanic corrosion is often set up in the distribution system in meter installations and at service connections and couplings.
The galvanic series, shown below, arranges metals according to their tendency to corrode. This series can be used to determine whether galvanic corrosion is likely to occur and how strong the corrosion reaction will be.
As you can see on the series, some metals (such as gold and silver) are very inactive and unlikely to corrode. Many of these metals have been traditionally used as jewelry because of their low tendency to corrode even when in the presence of salts (in sweat) and oils found on the human body. Although these inactive metals would make non-corrosive pipes, they are usually too expensive to use in the distribution system.
At the other end of the galvanic series are metals which are very active and have a high tendency to corrode. These metals can be used as sacrificial anodes, which we will discuss later. They should not be used for distribution system pipes.
Most of the metals used in piping - iron, steel, and copper - are found in the middle of the galvanic series. These metals have some tendency to corrode, with those higher on the galvanic series (such as iron and steel) tending more toward corrosion.
The distance on the galvanic series between two metals will also influence the likelihood of galvanic corrosion when the two metals are placed in conjunction with each other. For example, if aluminum is brought in contact with a steel pipe, the likelihood of corrosion is low since aluminum and steel are close together on the galvanic series. However, if a stainless steel fitting is used on an iron pipe, the likelihood of corrosion is much higher.
When galvanic corrosion occurs, the more active metals always become the anodes. This means that they are corroded, and in extreme cases can begin to leak. The less active metal becomes the cathode and is not damaged.
Characteristics Influencing Corrosion
Corrosion in the distribution system is a very complex situation which is influenced by many water characteristics, by the metals used, and by any stray electrical current. We will briefly describe the influence of each characteristic in the following sections. You may want to refer back to the explanation of the chemistry behind corrosion in order to understand some of these factors better.
Primary Water Characteristics
The chemical characteristics of the water flowing through a pipe will influence whether the water is stable and will also affect the extent of any corrosive reaction. Primary factors include alkalinity, hardness, and pH, but oxidizing agents, carbon dioxide, and dissolved solids can also influence corrosion and will be discussed in the next section.
Alkalinity, hardness, and pH interact to determine whether the water will produce scale or corrosion or will be stable. The table below summarizes characteristics of corrosive water and of scale-forming water.
- low pH
- soft or with primarily noncarbonate hardness
- low alkalinity
- high pH
- hard with primarily carbonate hardness
- high alkalinity
In general, corrosion is the result of water with a low pH. Acidic waters have lots of H+ ions in the water to react with the electrons at the cathode, so corrosion is enhanced. In contrast, water with a higher pH (basic water) lowers the solubility of calcium carbonate so that the calcium carbonate is more likely to precipitate out as scale.
Scaling, as mentioned in the last lesson, tends to be the result of water with a high hardness. Hard water typically contains a lot of calcium compounds which can precipitate out as calcium carbonate. However, if the hardness in the water is primarily noncarbonate, the chlorate and sulfate ions will tend to keep the calcium in solution and will prevent scale formation.
Alkalinity is a measure of how easily the pH of the water can be changed, so it can be considered to be a mitigating influence with regards to pH. Water with a high alkalinity is more likely to be scale-forming even at a relatively low pH. In contrast, low alkalinity waters lack the buffering capacity to deal with acids, so they can easily become acidic and corrosive.
The graph above is known as the Baylis Curve. It shows the relationship between pH, alkalinity, and water stability. Water above the lines is scale-forming while water below the lines is corrosive. Stable water is found in the white area between the lines.
Secondary Water Characteristics
Other chemicals and compounds found in water also influence the corrosion process. The most common of these are oxygen, carbon dioxide, and dissolved solids.
Oxygen, as you will remember, reacts with hydrogen gas at the cathode, causing depolarization and speeding up the corrosion. As a result, water with a high D.O. (dissolved oxygen) will tend to be corrosive. Other oxidizing agents can perform the same function, although they are less common. Nitrates and chlorine are two other oxidizing agents found in water.
Carbon dioxide in water also tends to cause corrosion. The carbon dioxide gas can combine with water to form carbonic acid, which lowers the pH of the water. As mentioned in the last section, a low pH promotes corrosion.
Dissolved solids are typically present in water as ions. These ions increase the electrical conductivity of the water, making the electrolyte more effective. Thus, they will increase the rate of corrosion.
Physical Water Characteristics
In addition to the chemical properties of water, physical characteristics will influence corrosion. The most important of these physical characteristics are temperature and velocity of flow.
Temperature speeds up the rate of corrosion just as it does most other reactions. However, the effect of temperature on corrosion can be more complex. A high water temperature reduces the solubility of calcium carbonate in water, which promotes scale formation and slows corrosion. Temperature also alters the form of corrosion. Pits and tubercles tend to form in cold water while hot water promotes uniform corrosion. Uniform corrosion spreading across the entire surface of a pipe is far less problematic than tuberculation, so high temperatures can actually seem to slow the corrosive process.
The influence of flow velocity on corrosion is also rather complex. Moderate flow rates are the most beneficial since they promote the formation of scale without breaking loose tubercles. At low flow velocities, corrosion is increased and tends to be in the form of tuberculation due to the prevalence of oxygen concentration cell corrosion. At very high flow velocities, abrasion of the water against the pipe tends to wear the pipe away in a very different form of corrosion. High flow velocities also remove protective scale and tubercles and increase the contact of the pipe with oxygen, all of which will increase the rate of corrosion.
Bacteria can both cause and accelerate the rate of corrosion. In general, bacterial colonies on pipe walls accelerate corrosion below them due to oxygen cell concentration, causing increased pitting and tuberculation. Like humans, some bacteria produce carbon dioxide, which can combine with water to become carbonic acid and accelerate corrosion. The bacterial colonies also block the deposition of calcium carbonate scale on the pipe walls.
A colony of iron bacteria.
There are two main types of corrosion-related bacteria, each of which causes its own set of additional corrosion problems. Iron bacteria use the ferrous iron created at the anode, converting it into rust which they deposit in the slime around their cells. Since they use up the ferrous iron, this increases the rate of corrosion. Their slime can also come loose during high flow velocities, causing red water complaints and a bad smell.
Sulfate-reducing bacteria use up sulfate in the water to produce hydrogen sulfide. Hydrogen sulfide is an acid which can react with metals, causing corrosion. In addition, the sulfides produce a distinctive rotten egg smell.
Factors other than water characteristics and bacteria can also influence corrosion. Characteristics of the metal pipe and electrical currents are common causes of corrosion.
We have already discussed many corrosion-related characteristics of metal in the section on galvanic corrosion. To summarize, metals higher on the galvanic series tend to be more corrosive while metals further apart on the series are more likely to cause galvanic corrosion. In galvanic corrosion, the size of the cathode in relation to the anode has a large influence on corrosion as well. Larger cathodes tend to promote corrosion by speeding the electrical current's flow. When a system has very small anodes and large cathodes, corrosion is so rapid at the anodes that pinholes tend to form all the way through the metal.
Stray electrical current can cause electrolytic corrosion. Electrolysis usually causes problems on the outsides of pipes.
Every treatment plant should have a corrosion control plan for its distribution system. This system may be as simple as long-term monitoring of the water to determine if water is corrosive, or it can include a complex array of chemicals or equipment. Here, we will consider methods used to monitor the stability of water.
The most common indicators of corrosion in the distribution system are red water complaints and leaks. If the incidence of these problems increases in a certain area of the distribution system, then some sort of corrosion control may need to be undertaken. Red water is usually caused by tuberculation and iron bacteria while leaks are caused by the pitting below tubercles. However, the operator should be aware of other possible causes of these problems. High iron concentrations in the source water can cause red water problems while leaks can be caused by corrosive soil acting on the outside of the pipes as well as by corrosive water acting on the inside of the pipes.
During routine maintenance of the distribution system, the operator should watch out for signs of corrosion and scale. When pipes are removed and replaced, the old pipes should be visually examined for signs of tubercles, pitting, or uniform corrosion, and for excessive scaling.
More active forms of corrosion monitoring include coupons and tests for flow, dissolved oxygen, and heavy metals. These tests will determine whether the treated water is corrosive over a span of a few months (in the case of coupons), weeks (for flow tests), or immediately.
Coupons, like the one shown above, are small pieces of the same type of metal used in the distribution system piping. These coupons are inserted into pipes at various locations in the distribution system and are left in place for about three months to give adequate time for corrosion to occur. By weighing the coupon before and after the test period, the amount of metal lost from the coupon due to corrosion can be determined. This is a simple method of corrosion monitoring which is widely used in many distribution systems.
Flow monitoring can also be used to detect corrosion. A new piece of pipe is placed in service and the flow of water through the pipe is measured over time. If the flow becomes lower after a few weeks, then either tubercles or scale have formed on the inside of the pipe, decreasing the area available to carry water.
Dissolved oxygen and toxic heavy metals in the distribution system can be used as indicators of corrosion over a much shorter time frame. There are also a range of tests done at the water treatment plant to determine whether water is stable.
Dissolved oxygen is tested at various points in the distribution system at the same time. If the dissolved oxygen concentration becomes lower further from the treatment plant, then the oxygen is probably being used up by corrosion. However, the operator should be aware of the possibility that D.O. is being used to oxidize organic matter.
Toxic heavy metals, such as copper and lead, are tested at the consumer's tap. High concentrations of these metals in the water indicate corrosion in the distribution system, although in a few cases the metals may have originated in the source water.
Finally, water can be tested directly to determine whether it is stable. Both the Langelier Index and the Marble Test are laboratory tests which can determine the degree of calcium carbonate saturation in the water at the treatment plant. Water which is just saturated with calcium carbonate or which is slightly supersaturated with calcium carbonate is considered stable and safe to release into the distribution system.
Treatment of corrosive water can be either chemical or physical. In this section, we will discuss chemical methods of corrosion control. These chemical are either meant to stabilize the water, to form a protective film on the pipe surface, or to kill problematic bacteria.
Stabilizing the water is often the simplest form of corrosion control. When stabilizing corrosive water, the operator usually adds alkalinity in the form of lime, soda ash, or caustic soda. The goal is to saturate or slightly supersaturate the water with calcium carbonate so that it is stable or slightly scale-forming. When these chemicals are used to stabilize water, they should be fed after filtration to prevent cementing of the filter sand and may be fed before, during, or after chlorination.
Corrosion inhibitors are used to form thin protective films on pipe walls, which will prevent corrosion. The chemicals used for this purpose are more expensive than lime, but also prevent scale which can be a problem when feeding stabilizing chemicals into the water. Sodium silicate is sometimes used by individual customers as an inhibitor but is not widely used by utilities. Glassy phosphates such as sodium hexametaphosphate or tetrasodium pyrophosphate are more widely used, but can increase corrosion rates. Both types of inhibitors require continual application into the water, so dead ends in the distribution system must be flushed at intervals to ensure that fresh water containing the inhibitors reaches these areas as well. A large amount of the inhibitor chemicals ends up forming the film on the pipe walls, but some ends up in the drinking water, though this is not a problem since all inhibitor chemicals are considered safe.
If bacteria are a major component of the corrosion problem, then proper disinfection may be part or all of the answer. Maintaining an adequate chlorine residual in the distribution system will kill the bacteria and prevent corrosion.
Physical protection against corrosion may be very simple or very complex. On the simple end of the spectrum, corrosion can be prevented by breaking the corrosion cell circuit in some manner. Metal pipes can be replaced with nonmetals which are non-conductive and will not corrode. Alternatively, pipes may be lined with portland cement or bituminous or asphaltic compounds to prevent the water from reaching the metal, serving the same purpose.
If galvanic corrosion is a problem, then the two metals can be separated by dielectric couplings. Dielectric couplings are plastic, ceramic, or other non-conductive sections used between the two different types of metal. Since electrons cannot flow through the dielectric coupling, it breaks the circuit and prevents corrosion.
Cathodic protection using a sacrificial anode.
At the more expensive and complicated end of the protection spectrum is cathodic protection, which is the introduction of a different electrical circuit into the pipe. Some cathodic protection systems operate as shown in the picture above, by introducing a sacrificial anode into the pipe. A sacrificial anode is a piece of very active metal (usually zinc or magnesium) which is more galvanically active than any other metal in the system. The sacrificial anode will be the only metal corroded, and even previously active anodes on the pipe wall will become cathodes and will thus be protected. Since the sacrificial anodes slowly corrode away, they must be replaced at intervals, which is the only form of maintenance required on the protection system.
Alternatively, some cathodic protection systems involve the introduction of an external direct current source, known as a rectifier. The rectifier creates a very strong anode since it is constantly producing electrons (an electric current.) This turns the rest of the pipe into a cathode, which prevents any corrosion in the pipe. To complete the circuit, the pipe must be connected back to the rectifier.
Direct current cathodic protection systems have been developed which are fully automatic and will compensate for any changes without operator control. However, they also tend to be very expensive to install.
Stable water is neither scale-forming nor corrosive, both of which characteristics create problems in the distribution system. Scale forms when calcium carbonate precipitates out of hard water. Corrosion occurs when an anode, cathode, conductive connection, and electrolyte create a corrosive cell. In the corrosive cell, the metal of the pipe is oxidized in a series of reactions, producing rust
Corrosion inside a pipe can be caused by electrolysis, oxygen concentration cells, or galvanic corrosion. Many factors can influence the corrosion, including pH, hardness, alkalinity, oxidizing agents, carbon dioxide, dissolved solids, temperature, velocity of flow, bacteria, metal characteristics, and stray electric currents.
Corrosion testing includes monitoring red water complaints and leaks; inspecting old pipes; using coupons; testing flow, dissolved oxygen, and heavy metals; and using the Langelier Index and Marble Test. Chemical treatment involves addition of chemicals to stabilize the water, use of inhibitors to form a protective film on pipes, or addition of disinfectants. Physical protection either breaks the corrosive cell or consists of cathodic protection.
Alabama Department of Environmental Management. 1989. Water Works Operator Manual.
Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.
Ragsdale and Associates. Version III. New Mexico Water Systems Operator Certification Study Guide. NMED Surface Water Quality Bureau: Santa Fe.
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