- Describe corrosion mechanisms
- Explain scale deposition in terms of saturation
- Describe the methods of corrosion control
- Describe methods used to control iron and manganese
Corrosion Control and Water Stabilization
Corrosion is a complex topic involving numerous chemical, electrical, physical, and biological factors. A few concepts describe the major principles of corrosion. Selecting and implementing an effective corrosion-control program requires an understanding of these concepts.
Corrosion is the gradual decomposition or destruction of a material by chemical action, often due to an electrochemical reaction. Corrosion may be caused by:
- Stray current electrolysis
- Galvanic corrosion caused by dissimilar metals
- Differential-concentration cells
Corrosion begins at the surface of a material and moves inward. The severity and type of corrosion depend on the chemical and physical characteristics of the water and the material.
Electrochemical Corrosion: Galvanic Cell
Metallic corrosion in potable water is always the result of an electrochemical reaction. An electrochemical reaction is a chemical reaction where the flow of electric current is an essential part of the reaction. If the electric current is stopped by breaking the circuit, the chemical reaction will stop. Also if the chemical reaction is stopped by removing one the reacting chemicals, the flow of electric current will stop. For corrosion to occur, electric current and chemical reaction must be present.
The deterioration of metal during corrosion is called an electrochemical reaction because a chemical and an electrical process are occurring. The components of the process are:
- Anode-point from which metal is lost and electric current begins. The anode (positive pole) attracts negatively charged particles or ions (anions) in an electrolyte solution. Anions are attracted to the anode under the influence of a difference in electrical potential.
- Cathode-point where the electric current leaves the metal and flows to the anode through the electrolyte. A cathode is the negative pole or electrode of an electrolytic cell or system. It will attract positively charged particles or ions (cations) in an electrolyte solution. Cations are attracted to the cathode under the influence of a difference in electrical potential.
- Electrolyte-conduction solution (water with dissolved salts). A substance that separates into two or more ions when it is dissolved in water.
The anode and cathode must be joined. In an iron pipe, at the anode, a molecule of iron dissolves into the water as a ferrous ion, and two electrons flow to the cathode. At the cathode, the electrons leave the metal at the point of contact with the electrolyte and react with hydrogen in the water to form hydrogen gas. The hydrogen ions are always present in the water from the normal separation of water, and the electrolyte is in contact with the anode and the cathode, completing the circuit.
At the anode, the dissolved iron reacts with oxygen and the water forming a rust film composed initially ferrous hydroxide. Additional water and oxygen then react with the ferrous hydroxide to form the ferric hydroxide, which becomes a second layer over the ferrous hydroxide.
This multilayered rust deposit is known as a tubercle. Tubercles can grow to the point that the carrying capacity of the pipe is significantly reduced. Also during periods of high flow rates, the tubercles may dislodge, resulting in rusty or red-colored water. The formation of a rust coating on the pipe has another important effect on the rate of corrosion. As the rust film forms, it begins to cover and protect the anode, slowing the rate of corrosion. If the rust film is flushed away, the corrosion reaction accelerates.
A more complex form of electrochemical corrosion is caused by the joining of dissimilar metals. This type of corrosion is called galvanic corrosion. Like the corrosion cell, the galvanic corrosion cell has an anode, a cathode, and electrolyte, as well as a connection between the anode and cathode. This galvanic cell, however, has two dissimilar metals at the anode and cathode. The degree to which a particular metal will become anodic (corrode) in a galvanic reaction is related to its tendency to enter into a solution. In this process, the metal reverts to its natural ore state. The relative tendency of various metals to revert to an ore state is demonstrated by their position on an electromotive series or galvanic series in which the most active metals are listed at the top in table 7.1. An electromotive or galvanic series is a list of metals and alloys presented in the order of their tendency to corrode (go into solution). The size and sign of the electrode potential indicate how easily these elements will take on or give up electrons, or corrode.
The higher the level of activity, the greater the tendency for the metal to corrode. Also, the further apart the two metals are on the galvanic series, the greater the galvanic corrosion potential. The more active metal of any two in the galvanic series will always become the anode. When iron and copper water pipes are joined, the iron will corrode if the water contains dissolved oxygen and the copper will be protected. Because of their active positions on the galvanic series, zinc and magnesium make excellent anodes, and they are commonly used as sacrificial anodes in water tanks or for buried pipelines to prevent corrosion. These reactive metals, called base metals, will corrode preferentially to aluminum and iron for example. A metal, such as iron, will react with dilute hydrochloric acid to form hydrogen.
On the cathodic side of the galvanic series, the least active reactive metals are called noble metals. One of the noblest metals is gold. Stainless steel is cathodic to most metals, and it is used in critical chemical industry applications where corrosion potential is great. Stainless steel is used in high-pressure reverse osmosis desalination systems due to its excellent corrosion resistance in the presence of highly conductive seawater.
As the galvanic series demonstrates, when copper and lead solder are in contact, the lead becomes the anode and will corrode in preference to the copper. The relatively high toxicity of lead and its anodic tendencies are why lead has been banned from use in potable water distribution systems by the US Government under the 1986 Safe Drinking Water Act and the Lead and Copper Rule. Other factors, such as water chemistry, biological films, and physical characteristics, like temperature and flow rate, play a role in the severity of the corrosion reaction.
An important feature of the galvanic corrosion is the relative size of the anode and cathode. The level of galvanic electric current increases as the area of the cathode increases. A large cathode will generate a high level of electrical potential. If the current is directed at a small anode, a relatively large amount of metal will dissolve from the available anode area and deep pits will form.
Fe → Fe2+ + 2e-
O2 + 4e- + 2H2O → 4OH
Fe2+ + 2OH- → Fe(OH2)
4Fe(OH)2 + O2 → 2Fe2O3 + 4H2O
Factors influencing Corrosion
Corrosion is complex with many possible variables. The essential elements of a corrosion cell, including the specific case of a galvanic cell, are influenced by increasing or decreasing the rate of the electrochemical reaction. The electrochemical reaction rate is influenced by physical, chemical, and biological factors.
Physical factors that influence corrosion include the type and arrangement of materials used in the system, system pressure, soil moisture, the presence of stray electric currents, temperature, and water flow velocity.
- System construction - The type of materials that make up the anode and cathode influence corrosion; especially, the size of the anode and cathode.
- System pressure - High pressure increases corrosion because gases that can oxidize construction materials increase the maximum concentration of these gases in the water.
- Soil moisture - Contact with moist soil can cause external pipe corrosion because the moisture acts as the electrolyte in the corrosion cell.
- Stray electric current - Grounding of electric circuits to water pipes and can enhance corrosion. Stray current corrosion is more pronounced from direct current grounding than from alternating current grounding.
- Temperature - The rate of chemical reactions usually increases as temperature rise. Because chemical reactions are involved in corrosion, an increased temperature generally has the effect of increasing corrosion. Flow velocity has several significant influences on corrosion. Moderate flow rates are often beneficial while very high or low flow rates increase the rate of corrosion.
- Negative effects: stagnate water flow conditions increases corrosion. However, highly oxygenated water can become more corrosive under higher flow conditions as the movement of water increases the contact of oxygen with the pipe surface. In high velocities, erosion-corrosion can occur, particularly in copper pipes. At rates exceeding 5ft/sec copper tubing will erode. This corrosion is more noticeable at elbows and joints and results in structural damage to the pipe. Circulating hot water systems in buildings are particularly susceptible to erosion-corrosion because of flow velocities and high temperature.
- Beneficial effects: water that has protective properties, such as the tendency to deposit calcium carbonate film, or to which a corrosion inhibitor has been added, will be less corrosive under moderate flow conditions. Film formation requires the deposition of calcium carbonate or the inhibitor (phosphate or silicate) on the surface of the metal. In stagnant water, deposition is limited. Under high flows, it may be scoured off the pipe as it forms or erosion-corrosion may occur faster than the film deposits.
Various chemical factors influence corrosion, such as pH, alkalinity, chlorine residual, levels of dissolved solids, dissolved gases like oxygen and carbon dioxide, and the types and concentrations of various minerals present in water. These factors are:
- Alkalinity - Alkalinity is the capacity of water to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide, and occasionally borate, silicate, and phosphate. Alkalinity is expressed in milligrams per liter of equivalent calcium carbonate. Alkalinity is not the same as pH because water does not have to be strongly basic to have high alkalinity. Alkalinity is also a measure of how much acid must be added to a liquid to lower the pH to 4.5. Alkalinity is a measure of the buffering capacity, or the ability of a particular quality of water to resist a change in pH. The simplest form of corrosion control is to add more alkalinity in the form of lime, soda ash, or caustic soda, or directly add calcium carbonate to the water.
- pH-measures - The amount of hydrogen ions present in the water. The hydrogen ion is extremely active (corrosive) at pH values below 4.0. Chlorine and hydrogen ions are usually present in sufficient concentrations in potable water to have a significant effect on corrosion. Low pH water tends to be corrosive, and high pH water is protective of pipe material. Very high pH water may have a tendency to deposit excessive amounts of scale, however. Certain pH levels offer less protection than would be assumed. At pH values near 8.3, the transition point between carbonate and bicarbonate, the buffering system is weak, and slightly lower pH values may be more protective.
- Dissolved solids - Solids dissolved in water are present as ions that increase the electrical conductivity of the water. Generally, the higher the dissolved solids or salt content of water, the greater the potential for corrosion to occur due to the increased conductivity. However, some solids are involved in scale formation, possibly slowing the rate of corrosion if a protective film is formed. All scale-forming components, such as iron oxide (rust) and calcium carbonate, are present first as dissolved solids in water before they deposit on the surface of pipes and fixtures.
- Hardness - The dissolved form of some of the principal scale-forming components in water is referred to as hardness. Hardness is composed primarily of calcium and magnesium ions. All hardness ions have the common property of forming a scale on the inside of pipes or fixtures under conditions of high enough concentration, and at elevated pH and temperature levels. Planned deposition of calcium carbonate film is one of the most common corrosion-control measures used in water systems. Several methods of measuring the relative level of calcium carbonate saturation in water are used. One of the simpler methods is called the Marble Test, which directly measures whether a water sample will increase in hardness and pH when dosed with an excess of calcium carbonate. The Marble Test and a more extensive determination of the saturation level of calcium carbonate, called the Langelier Index, are used to measure the corrosivity of water and water’s extent of stabilization.
- Chloride and sulfate - Chloride and sulfate ions in water inhibit the formation of protective scales by keeping hardness ions in solution. The relative amount of alkalinity compared to chloric and sulfate affects this tendency. It is recommended that the alkalinity, expressed as calcium carbonate equivalent, be five times higher than the sum of chloride and sulfate ions.
- Phosphate and silicate - These compounds have a tendency to form protective films in water systems when present in high enough concentrations and when in the correct chemical form for the particular conditions of the water. Phosphate and silicate compounds re-added at the water treatment plant as a corrosion-control method.
- Trace metals - Trace metals of significance in corrosion control include copper, iron, lead, and zinc. When these elements are present in high concentrations in water, they are usually indicators of corrosion of the pipes and fittings. Copper and lead usually indicate corrosion of copper pipe and lead solder or service lines. Iron usually results from the corrosion of iron or steel pipe, and zinc results from corrosion of the galvanized pipe. Iron and zinc may be involved in the formation of protective films that limit the rate of corrosion. Zinc is also a common corrosion-control additive, usually in a compound containing zinc and phosphate.
Two types of organisms that can play an important role in corrosion of water distribution systems are iron bacteria and sulfate-reducing bacteria. They can increase the rate of corrosion and the formation of undesirable corrosion byproducts. Iron bacteria use dissolve iron as an energy source, and sulfate-reducing bacteria use sulfate for their energy. Each type of bacteria can grow in dense masses, and they may be relatively tolerant of disinfection by chlorine. They are particularly troublesome in low-flow areas of distribution systems.
Oxygen Concentration Cell
Although galvanic cells are responsible for corrosion problems, by far the most common corrosion cell is the oxygen concentration cell. A dead-end in the distribution system is the end of a water main that is not connected to other parts of the distribution system by means of a connection loop of pipe. An oxygen concentration cell can be started in this dead-end of the waterline. At this location, ferrous ions and oxygen ions react to form solid ferric hydroxide and hydrogen ions. The electrons from the anode reaction flow through the metallic pipe where they react with dissolved oxygen which is replenished by flowing water. The excess hydrogen ions lower the pH and make it more corrosive.
In the dead pipe, an absence of oxygen and low pH make the conditions ideal for the growth of anaerobic bacteria. An anaerobic condition is one in which atmospheric or dissolved oxygen is not present. The action of these bacterial populations on trace organic material and on reducing the sulfate ions to sulfide are responsible for foul odors usually found in the dead ends of water mains.
As ferric hydroxide ages, it forms other minerals such as ferric oxide or iron rust. Eventually, the crust from ferric oxide becomes so thick that negative ions cannot enter the pit where corrosion is occurring, and iron ions cannot escape. The corrosion in the area stops. At this point, the pit is inactive. The reaction of dissolved oxygen with ferrous ions is very slow at low pH values. When the pH is less than 7.0, the reaction is so slow that tubercles do not form and the pits are not self-perpetuating. New pits keep starting in different places, so that corrosion appears to be uniform over the surface of the pipe.
Calcium Carbonate Saturation
Internal corrosion of pipes can be detected through rusty water complaints and by examining the insides of pipes for pitting, tubercles, and other evidence of corrosion. Corrosion from unstabilized water can be predicted by the amount of calcium carbonate saturation. The methods that are used to predict the stabilization of water are the Marble Test, Enslow Column, and the Langelier Index. Water is considered to be stable when it is just saturated with calcium carbonate. In this condition, the water will not dissolve or deposit calcium carbonate. Water treatment plant operators use two approaches to determine the calcium carbonate saturation level of their water: the Calcium Carbonate Precipitation Potential (CCPP) and the Langelier Index.
Marble Test and Enslow Column
To conduct a Marble Test for calcium carbonate saturation, first measure the pH, alkalinity, and hardness of the water sample. Add a pinch of powdered calcium carbonate and then stir the water for at least five minutes. The water should be stirred in a nearly-full stoppered flask to avoid the introduction of carbon dioxide from the air. Also, the water being stirred should be at the same temperature as the water in the distribution system. If the pH, alkalinity, or calcium carbonate increase, the water was undersaturated with respect to calcium carbonate. If they decrease, the water was supersaturated. The CCPP equals the quantitative change in alkalinity (or calcium carbonate) due to the water being exposed to the powdered calcium carbonate.
The Enslow column can be conveniently used to perform the Marble Test. This tube is a column packed with calcium carbonate granules. The pH, alkalinity, and calcium carbonate are measured on a sample stream of water before and after passing through the column. The results are interpreted in the same manner as the Marble Test.
Water is considered stable when it is just saturated with calcium carbonate. In stable water, the calcium carbonate is in equilibrium with the hydrogen ion concentration. If the pH is higher than the equilibrium point (positive Langelier), the water is scale forming and will deposit calcium carbonate. If the pH is lower than the equilibrium point (negative Langelier), the water is considered corrosive.
The Langelier Index (Saturation Index) is the most common index used to indicate how close water is to the equilibrium point, or the corrosiveness of water. This index is based on the equilibrium pH of water with respect to calcium and alkalinity. The Langelier Index can be determined by using the equation:
- Langelier Index = pH-pHs where...
- pH = actual pH of the water
- pHs = pH at which water having the same alkalinity and calcium content is just saturated with calcium carbonate
In this equation, pHs is defined as the pH value at which water of a given calcium content and alkalinity is just saturated with calcium carbonate. For some water of low calcium content and alkalinity, no pH value satisfies this definition; however, for most water, two values of pHs exist. These difficulties can be avoided by defining pHs as the pH where water of given calcium and bicarbonate concentrations is just saturated with calcium carbonate.
A positive Langelier Index (pH is greater than pHs) indicates that the water is supersaturated with calcium carbonate and will tend to form scale. The water is not corrosive. A negative Langelier Index means that the water is corrosive.
The corrosive tendencies of water to particular metals, such as the ones used in distribution systems, are also significantly influenced by the amount of total dissolved solids (TDS). Water containing TDS exceeding 50 mg/L may exhibit corrosive tendencies in spite of a positive Langelier Index. The presence of various ions, such as sulfate and chloride ions in water, may interfere with the formation and maintenance of a uniform protective calcium carbonate layer on metal surfaces. In addition, the presence of these ions will accelerate the corrosion process.
Because of the various water quality indicators involved, the Langelier Index should only be used to determine the corrosive tendencies of water within a pH range of 6.5 to 9.5 provided that a sufficient amount of calcium ions and alkalinity over 40 mg/L as calcium carbonate are present in the water.
Three other indices for calcium carbonate saturation have been used:
- Driving Force Index
- Ryznar Index
- Aggressive Index
Controlling Corrosion and Water Stabilization
To control corrosion, the operator must select the correct chemicals to treat the water, calculate the correct chemical dosage, and determine the proper chemical feeder settings. The water treatment operator must understand cathodic protection to control corrosion and the compounds used for cathodic protection.
Selection of Corrosion Control Chemicals
If the water is corrosive, steps should be taken to reduce the corrosivity of the water. Reducing corrosivity is almost always accomplished by treating the water with chemicals so that the water is saturated or slightly supersaturated with calcium carbonate. Chemicals should be fed after filtration. A slight excess of chemicals could result in a supersaturated solution that could cement together the filer sand grains. Small amounts of turbidity could be introduced with the chemical and might produce misleading results suggesting poor filter performance. The chemical feed can take place before, after, or along with post-chlorination; however, a sample should be taken only after post-chlorination because the chlorine may react with the chemicals used to reduce corrosivity.
The selection of a chemical to achieve calcium carbonate saturation will depend on the water quality characteristics of the water and the cost of the chemicals. The central idea is that all chemical characteristics and pH must be present in the right proportions to achieve calcium carbonate saturation. For water that has low hardness and low alkalinity, quicklime and hydrated lime should be added to increase the calcium content and the pH. For water with sufficient calcium but low alkalinity, soda ash may be used. For water in which the calcium and alkalinity are sufficient but the pH is too low, caustic soda would be indicated, although soda ash can also be used because it raises pH.
Zinc, Silica, and Polyphosphate Compounds
Certain zinc compounds, such a zinc phosphate, are capable of forming effective cathodic films that will control corrosion. These zinc compounds are largely proprietary. The zinc compound treatments are generally more expensive than treatment with lime or caustic, but they have the advantage that scale is less apt to be a problem. Do not use zinc phosphate compounds to control corrosion caused by water that will be stored in an open reservoir. The phosphate may cause algal blooms. Residual chlorine lasts longer in distribution systems using zinc orthophosphate. The reason for this property has been attributed to decreased chorine demand as a result of reducing iron sediments in distribution piping.
Sodium silicate has been used to treat corrosive water. A solution of sodium silicate fed at a rate of approximately 12 mg/L as silica is used for the first month, after which the rate is reduced to 8 mg/L. This method of treatment is used by individual customers, such as apartment houses and large office buildings. It is not commonly used by water utilities.
Sodium polyphosphates, either tetrasodium pyrophosphate or sodium hexametaphosphate, have been used for corrosion control. Solutions of these compounds may form protective films but, because they react with calcium, they reduce the effective calcium concentration and thereby, increase corrosion rates. The major use of these chemicals in water treatment is to control scale formation in water that is supersaturated with calcium carbonate.
The deterioration of asbestos-cement pipe may be prevented by maintaining calcium carbonate saturation. Evidence suggests that the zinc treatment is also effective for this purpose as are treatments using traces of iron, manganese, or silica in water. Any deterioration of this type of pipe will cause an increase in pH and calcium content of water as it passes through the pipe. Tests for pH and calcium content should be performed after the pipe has been in service for two months or longer because all asbestos pipe contains at least traces of free lime, which will result in an initial increase in water pH when the pipe is placed in service.
The Lead and Copper Rule describers the health concerns and regulations for water affected by lead and copper. The water treatment plant operator must understand the monitoring and treatment requirements for these contaminates and must know the public education and reporting requirements.
The health concerns concerning exposure to lead are described by the EPA:
Lead is a common, natural, and often useful metal found throughout the environment in lead-based paint, air, soil, household dust, food certain types of pottery, porcelain, and pewter, and water. Lead can pose a significant risk to health if too much of it enters the body. Lead builds up in the body over many years and can cause damage to the brain, red blood cells, and kidneys. The greatest risk is to young children and pregnant women where amounts of lead that will not harm adults can slow down normal mental and physical development of growing bodies. In addition, a child at play often comes into contact with sources of lead contamination, like dirt and dust, which rarely affect an adult.
Lead in drinking water, although rarely the sole cause of lead poisoning, can significantly increase a person’s total lead exposure, particularly the exposure of infants who drink baby formulas and concentrated juices that are mixed with water. The EPA estimates that drinking water can make up 20 percent or more of a person’s total exposure to lead.
Lead is unusual among drinking water contaminants in that it seldom occurs naturally in water supplies like rivers and lakes. Lead enters drinking water primarily as a result of the corrosion, or wearing away, of materials containing lead in the water distribution system and household plumbing. These materials include lead-based solder used to join copper pipe, brass, and chrome-plated brass faucets, and in some cases, pipes made of lead that connect the house to the water main.
When water stands in lead pipes or in plumbing systems containing lead for several hours or more, the lead may dissolve into the drinking water. The water that is first drawn from the tap in the morning, or later in the afternoon after returning from work or school, can contain fairly high levels of lead.
The health effects of copper include stomach and intestinal distress. Prolonged doses result in liver damage. Excess intake of copper or the inability to metabolize copper is called Wilson disease.
Iron and Manganese
- Iron and manganese are frequently found together in natural waters and produce similar adverse environmental effects and color problems. Excessive amounts of iron and manganese are usually found in groundwater and in surface water contaminated by industrial waste discharges.
- Before 1962, these elements were covered by a single recommended limit.
- In 1962, the US Public Health Service recommended separate limits for iron and manganese to reflect more accurately the levels at which adverse effects occur for each.
- Each is highly objectionable in large amounts in water supplies for domestic and industrial use.
- Each element imparts color to laundered goods and plumbing fixtures.
- Taste thresholds in drinking water are considerably higher than the levels that produce staining effects.
- Each element is part of the daily nutritional requirements; however, these requirements are not met by the consumption of drinking water.
The SMCL for iron is 0.3 mg/L.
- At levels greater than 0.05 mg/L some color may develop, staining of fixtures may occur, and precipitates may form.
- The magnitude of the staining effect is directly proportional to the concentration.
- Depending on the sensitivity of taste perception, a bitter, astringent taste can be detected from 0.1 mg/L to 1.0 mg/L.
- Precipitates that are formed create not only color problems but also lead to bacterial growth of slimes and of the iron loving bacteria, Crenothrix, in wells and distribution piping.
The daily requirement is 1 to 2 mg; however, intake of larger quantities is required as a result of poor absorption.
- The limited amount of iron permitted in water (because of objectionable taste or staining effects) constitutes only a small fraction of the amount normally consumed and does not have toxicologic (poisonous) significance.
The SMCL for manganese is 0.05 mg/L.
- A concentration of more than 0.02 mg/L may cause a buildup of coatings in distribution piping.
- If these coatings slough off, they can cause brown blotches in laundry items and black precipitates.
- Manganese imparts a taste to water above 0.15 mg/L.
- The application of chlorine, even at low levels, increases the likelihood of precipitation of manganese at low levels.
- Unless the precipitate is removed, precipitates reaching pipelines will promote bacterial growth.
- Toxic effects are reported as a result of inhalation of manganese dust or fumes.
- Liver cirrhosis has arisen in controlled feeding of rats.
- Neurological effects have been suggested; however, these effects have not been scientifically confirmed.
- Daily intake of manganese from a normal diet is about 10 mg.
- Manganese is essential for proper nutrition.
- Diets deficient in manganese will interfere with growth, blood and bone formation, and reproduction.
Iron and Manganese can be controlled through aeration where iron is oxidized to ferric oxide and removed through filtration. This method works for low levels of iron in the water. Iron and manganese can be oxidized using chlorine or chlorine dioxide. Ozone also will oxidize iron. Iron and manganese can further be oxidized using potassium permanganate. The operator should use caution in using potassium permanganate because excessive doses will cause water to become pink.
Excessive amounts of iron and manganese can also be removed using ion exchange and green sand. In ion exchange, iron and manganese ions are exchanged for sodium ions.
- Describe corrosion mechanisms.
- Explain scale deposition in terms of saturation.
- Describe the methods of corrosion control.
- Describe methods used to control iron and manganese.
- _____ is the gradual decomposition or destruction of a material by chemical action, often due to an electrochemical reaction.
- Anode, cathode, and electrolyte are components that make up _______.
- In corrosion, at the______, the dissolved iron reacts with oxygen and the water forming a rust film composed initially of ferrous hydroxide. Additional water and oxygen then react with the ferrous hydroxide to form the ferric hydroxide, which becomes a second layer over the ferrous hydroxide.
- Junction of dissimilar metals
- The ______ in corrosion is the point where the electric current leaves the metal.
- Junction of dissimilar metals
- The ________is the conduction solution (water with dissolved salts). It is a substance that separates into two or more ions when it is dissolved in water.
- Junction of dissimilar metals
- A multilayered rust deposit is known as a _______. They can grow to the point that the carrying capacity of the pipe is significantly reduced.
- _______ is the capacity of water to neutralize acids. This capacity is caused by the water’s content of carbonate, bicarbonate, hydroxide, and occasionally borate, silicate, and phosphate.
- None of these apply
- The _____ is the most common of the indices used to indicate how close water is to the equilibrium point, or the corrosiveness of water. This index is based on the equilibrium pH of water with respect to calcium and alkalinity.
- Driving Force Index
- Ryznar Index
- Aggressive Index
- Langelier Index
- The corrosive tendencies of water to particular metals, such as the ones used in distribution systems, are also significantly influenced by the amount of ________. Water containing 50 mg/L may exhibit corrosive tendencies in spite of a positive Langelier Index.
- Total dissolved solids
- If the water is corrosive, steps should be taken to reduce the corrosivity of the water. Reducing corrosivity is almost always accomplished by treating the water with chemicals so that the water is saturated or slightly supersaturated with ____.
- A base
- Calcium carbonate
- Sodium hydroxide
- Calcium hydroxide
- The SMCL for iron is ____.
- 0.05 mg/L
- 0.3 mg/L
- 1.3 mg/L
- 15 ppb
- The SMCL for manganese is _____.
- 0.05 mg/L
- 0.3 mg/L
- 1.3 mg/L
- 15 ppb
- The MCL for copper is ______.
- 0.05 mg/L
- 0.3 mg/L
- 1.3 mg/L
- 15 ppb
- The action level for lead is ______.
- 0.05 mg/L
- 0.3 mg/L
- 1.3 mg/L
- 15 ppb