Lesson 8:

Corrosion Control

 


Corrosion




Objective

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?

 

 

Reading Assignment

Along with the online lesson, read Chapter 8: Corrosion Control, in your textbook Operation of Water Treatment Plants Volume I .

 

 

Lecture

Introduction

Corrosion occurs because metals tend to oxidize when they come in contact with water, resulting in the formation of stable solids. Corrosion in water distribution systems can impact consumer's health, water treatment costs, and the aesthetics of finished water. Corrosive water corrodes structures, lines, and plumbing fixtures. It is stabilized by adjusting its pH and alkalinity by adding lime, sodium hydroxide (lye), sodium carbonate, or sodium bicarbonate to bring the pH and alkalinity above the corroding level.

Corrosion is the dissolving of metals like iron, lead, copper, cadmium, zinc, tin, and antimony from the structures, pipes, plumbing fixtures, and solders. It is natural to return the processed metals into their natural dissolved state. It is a complex process because a large number of environmental variables are involved. A generally accepted explanation is that it is an electrochemical reaction that is formed of three parts: anode, cathode, and an electrolyte solution. The anode, the positive electrode, is the site where the reducing agent, a metal (iron, lead, or copper), is getting dissolved by losing electrons. The cathode, the negative electrode, is where an oxidizing agent (oxygen or hydrogen) accepts the electrons. The electrolyte solution, the conducting medium, is the water with dissolved electrolytes. Completion of reaction needs all these three parts. It is like an electric circuit.

Natural water with dissolved substances has all three required parts for corrosion to occur; thus, almost any metal in contact with water will corrode. The rate of corrosion will depend on the nature of metal and water characteristics. Other metals, such as lead and copper, are corroded the same way as iron and are mostly converted into their oxides.

The corrosion process is an oxidation/reduction reaction that returns refined or processed metal to their more stable ore state. With respect to the corrosion potential of your drinking water, the primary concerns include the potential presence of toxic metals, such as lead and copper; deterioration and damage to the household plumbing, and aesthetic problems such as stained laundry, bitter taste, and greenish-blue stains around basins and drains.

The primary health concern is the potential for the presence of elevated levels of lead and copper in the water. The primary source of the lead includes the use of lead pipes, lead lined tanks, and use of 50/50 lead/tin solder. Corrosion will occur anywhere a galvanic cell or field can be or has established. To establish the field all that is needed is two dissimilar metals that are connected directly or indirectly by an electrolyte, such as water. This is the same chemical reaction that occurs within a battery.

Nearly all metals will corrode to some degree. The rate and extent of the corrosion depend on the degree of dissimilarity of the metals and the physical and chemical characteristics of the media, metal, and environment. In water that is soft, corrosion occurs because of the lack of dissolved cations, such as calcium or magnesium in the water. In scale forming water, a precipitate or coating of calcium or magnesium carbonate forms on the inside of the piping. This coating can inhibit the corrosion of the pipe becase it acts as a barrier, but it can also cause the pipe to clog. Water with high levels of sodium, chloride, or other ions will increase the conductivity of the water and promote corrosion. Corrosion can also be accelerated by:

  1. low pH (acidic water) and high pH (alkaline water) - it is possible that a chemical scale may form that would help to protect against corrosion, but if a bacteria becomes established the scale, such as SRB (sulfur reducing bacteria), you may experience a problem related to Microbiologically Induced Corrosion or MIC;
  2. high flow rate within the piping can cause physical corrosion;
  3. high water temperature can increase biological rate of growth and chemical corrosion;
  4. oxygen and dissolved carbon dioxide or other gases can induce corrosion;
  5. high dissolved solids, such as salts and sulfates, can induce chemical or bio-chemical corrosion;
  6. if the mass ratio (CMSR) of chloride to sulfate is > 0.2, but < 0.5 there is an elevated concern, but if the CMSR is > 0.5 and the alkalinity of the water is less than 50 mg calcium carbonate/L the concern should be significant;
  7. corrosion related bacteria, high standard plate counts, and electrochemical corrosion can result in pinhole leaks and isolated corrosion and aesthetic water quality problems, and
  8. presence of suspended solids, such as sand, sediment, corrosion by-products, and rust can aid in physical corrosion and damage and facilitate chemical and biochemical corrosion.

 

If it is necessary to flush or run your cold water in the morning for a few minutes before you drink because the water has a bitter taste, your water is probably corrosive. If you see blue-green stains in your basins or some staining along the joints of your copper piping, your water is probably corrosive. As corrosive water stands or seals in pipes or tanks, it leaches metals from the piping, tanks, well casing, or other metal surfaces that water is in contact. If you see pink standing on the waters edge, this may not be corrosion, but pink bacteria. Pink bacteria is an airborne bacteria.

 

 

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. 

 

 

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. 

These are some of the important classes of corrosion based on its physical, chemical, and biological nature:

 

Physical Corrosion

Physical corrosion is the erosion of a pipe surface due to a high velocity (of over 5 ft/sec), particulate matter, or dispersing gas bubbles.

 

 

Galvanic/Bimetallic Corrosion

Galvanic/bimetallic corrosion occurs when two dissimilar metals are connected together in the water lines, such as lead and copper, lead solder on copper line, and a galvanized iron fitting connected to a copper pipe. In the electromotive series, elements are arranged according to their oxidizing and reducing strength. Cesium, the most active reducing agent (metal), is at the top. Mercury, the least active, is at the lowest level of the metals. Position of a metal in the series determines whether it will corrode. A metal at a higher level in the electromotive series becomes an anode and will corrode, while the one below it becomes the cathode. This principle is applied in the cathodic protection method of the corrosion control. In case of lead solder on a copper pipe, lead acts as an anode and copper as a cathode, causing the corrosion of lead. Similarly, lead will dissolve when lead and copper lines are connected together.

Galvanic series.

 

 

Stray Current Corrosion

Stray current corrosion occurs due to grounding of appliances through the pipes. Corrosion occurs where the stray current leaves the pipe. It is the cause of mysterious red water complaints in some homes. Grounding of appliances should never be through the water lines.

 

 

Localized or Pitting Corrosion

Localized or pitting corrosion starts when an area of an otherwise coated metal surface is exposed due to faulty coating, damage, or stress. This area becomes anodic and forms a pit, and the surrounding area serves as a cathode. As the corrosion proceeds, the pit grows and has blackish-gray ferrous hydroxide inside and an insoluble rusty, ferric hydroxide layer on the outside. This outside growth is known as a tubercle. High velocity, flushing against the normal flow direction in the lines, or water hammer can rupture these tubercles and cause the red water complaints. Pitting can seriously damage the water lines.

 

 

Bacterial Corrosion

Bacterial corrosion is caused by bacteria, such as iron bacteria, nitrogen bacteria, and sulfur bacteria, which become active under low dissolved oxygen and low disinfectant conditions. These conditions normally prevail at the dead ends of pipes, causing corrosion, slime formation, high heterotrophic plate counts, and sometimes even the presence of coliform bacteria. Water quality deteriorates and then the water, generally, has a bad smell. When velocity in the water mains increases, some of this smelly water reaches homes and causes taste and odor complaints. Generally, water utilities have a regular unidirectional flushing plan to resolve this problem.

 

 

Characteristics Influencing Corrosion

Introduction

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 later in this lesson.

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.

Corrosive Water
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. 

Baylis Curve

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 earlier, 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

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.
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. 

 

 

Other Factors

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. 

 

 

 

Impacts of Corrosion

Corrosion can cause higher costs for a water system due to problems with:

  • decreased pumping capacity, caused by narrowed pipe diameters resulting from corrosion deposits;
  • decreased water production, caused by corrosion holes in the system, which reduce water pressure and increase the amount of finished water required to deliver a gallon of water to the point of consumption;
  • water damage to the system, caused by corrosion-related leaks;
  • high replacement frequency of water heaters, radiators, valves, pipes, and meters because of corrosion damage; and
  • customer complaints of water color, staining, and taste problems.

 

Besides the aesthetic concerns, the corrosion process can result in the presence of toxic metals in your drinking water. These metals include chromium, copper, lead, and zinc. The following are the recommended maximum contaminant levels for regulated public water supplies for the aforementioned metals:

  • chromium: 0.05 ppm
  • copper: 1 ppm
  • lead: 0.05 ppm
  • zinc: 5 ppm

 

If a public water supply is corrosive, the state requires that the water be treated to make the water non-corrosive.

 

 

 

Diagnosing Corrosion

The following events and measurements can indicate potential corrosion problems in a water system:

Consumer complaints: Many times a consumer complaint about the taste or odor of water is the first indication of a corrosion problem. Investigators need to examine the construction materials used in the water distribution system and in the plumbing of the complainants' areas.

Corrosion indices: Corrosion caused by a lack of calcium carbonate deposition in the system can be estimated using indices derived from common water quality measures. The Langelier Saturation Index (LSI) is the most commonly used measure and is equal to the water pH minus the saturation pH (LSI = pH water - pH saturation). The saturation pH refers to the pH at the water's calcium carbonate saturation point (i.e. the point where calcium carbonate is neither deposited nor dissolved). The saturation pH is dependent upon several factors, such as the water's calcium ion concentration, alkalinity, temperature, pH, and presence of other dissolved solids, such as chlorides and sulfates. A negative LSI value indicates potential corrosion problems.

Sampling and chemical analysis: The potential for corrosion can also be assessed by conducting a chemical sampling program. Water with a low pH (less than 6.0) tends to be more corrosive. Higher water temperature and total dissolved solids also can indicate corrosive.

Pipe examination: The presence of protective pipe scale (coating) and the condition of pipes' inner surfaces can be assessed by simple observation. Chemical examinations can determine the composition of pipe scale, such as the proportion of calcium carbonate, which shields pipes from dissolved oxygen and thus reduces corrosion.

 

 

Effects of System Design

In many cases, corrosion can be reduced by properly selecting distribution and plumbing system materials and by having a good engineering design. For example, water distribution systems designed to operate with lower flow rates will have reduced turbulence and, therefore, decreased erosion of protective layers. In addition, some piping materials are more resistant to corrosion in a specific environment than others. Finally, compatible piping materials should be used throughout the system to avoid electrolytic corrosion.

Other measures that help minimize system corrosion include:

  • using only lead-free pipes, fittings, and components;
  • selecting an appropriate system shape and geometry to avoid dead ends and stagnant areas;
  • avoiding sharp turns and elbows in the distribution and plumbing systems;
  • providing adequate drainage (flushing) of the system;
  • selecting the appropriate metal thickness of piping, based on system flow and design parameters;
  • avoiding the use of site welding without replacing the pipe lining;
  • reducing mechanical stresses, such as flexing of pipes and "water hammer" (hydraulic pressure surges);
  • avoiding uneven heat distribution in the system by providing adequate coating and insulation of pipes;
  • providing easy access for inspection, maintenance, and replacement of system parts; and
  • eliminating the grounding of electrical circuits to the system, which increases the potential for corrosion.

 

 

Reducing System Corrosion

Corrosion in a system can be reduced by changing the water's characteristics, such as adjusting pH and alkalinity; softening the water with lime; and changing the level of dissolved oxygen (although this is not a common method of control). Any corrosion adjustment program should include monitoring. This allows for dosage modification, as water characteristics change over time.

 

 

pH Adjustment

Operators can promote the formation of a protective calcium carbonate coating (scale) on the metal surface of plumbing by adjusting pH, alkalinity, and calcium levels. Calcium carbonate scaling occurs when water is oversaturated with calcium carbonate. Below the saturation point, calcium carbonate will redissolve; at the saturation point, calcium carbonate is neither precipitated nor dissolved. The saturation point of any particular water source depends on the concentration of calcium ions, alkalinity, temperature, and pH, and the presence of other dissolved materials, such as phosphates, sulfates, and some trace metals. It is important to note that pH levels well suited for corrosion control may not be optimal for other water treatment processes, such as coagulation and disinfection. To avoid this conflict, the pH level should be adjusted for corrosion control immediately prior to water distribution, and after the other water treatment requirements have been satisfied.

 

 

Lime Softening

Lime softening (which, when soda ash is required in addition to lime, is sometimes known as lime-soda softening) affects lead's solubility by changing the water's pH and carbonate levels. Hydroxide ions are then present, and they decrease metal solubility by promoting the formation of solid basic carbonates that "passivate" or protect the surface of the pipe. Using lime softening to adjust pH and alkalinity is an effective method for controlling lead corrosion. However, optimum water quality for corrosion control may not coincide with optimum reduction of water hardness. Therefore, to achieve sound, comprehensive water treatment, an operator must balance water treatment, an operator must balance water hardness, carbonate levels, pH and alkalinity, as well as the potential for corrosion.

 

 

Dissolved Oxygen Levels

The presence of excessive dissolved oxygen increases water's corrosive activity. The optimal level of dissolved oxygen for corrosion control is 0.5 to 2.0 parts per million. However, removing oxygen from water is not practical because of the expense. Therefore, the most reasonable strategy to minimize the presence of oxygen is to:

  • exclude the aeration process in the treatment of groundwater,
  • increase lime softening,
  • extend the detention periods for treated water in reservoirs, and
  • use the correct size water pumps in the treatment plant to minimize the introduction of air during pumping.

 

 

Commercial Pipe Coatings and Linings

The nearly universal method of reducing pipe corrosion involves lining the pipe walls with a protective coating. 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, but this method is much more expensive. Mechanically applied coatings and linings differ for pipes and water storage tanks. The most common types of pipe linings include coal-tar enamels, epoxy paints, cement mortar, and polyethylene. Water storage tanks are most commonly lined to protect the inner tank walls from corrosion. The most common types of water storage tank coatings and linings include coal-tar paints and enamels, vinyls, and epoxy.

 

 

Testing

Corrosion Indicators

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.

 

 

Long-Term Testing

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. 

Coupon

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. 

 

 

Short-Term Testing

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. 

 

 

Langelier Saturation Index

The Langelier Saturation Index is a means of evaluating water quality data to determine if the water has a tendency to form a chemical scale. In order to use this index, the following laboratory analysis is needed: pH, conductivity, total dissolved solids, alkalinity, and total hardness. The Saturation Index is typically either negative or positive and rarely 0. A Saturation Index of zero indicates that the water is "balanced" and is less likely not to cause scale formation. A negative Saturation Index suggests that the water is undersaturated with respect to carbonate equilibrium and the water may be more likely to have a greater corrosive potential.

A corrosive water can react with the household plumbing and metal fixtures resulting in the deterioration of the pipes and increased metal content of the water. This reaction could result in aesthetic problems, such as bitter water and stains around basins/sinks, and in many cases elevated levels of toxic metals. A positive Saturation Index suggests that water may be scale forming. The scale, typically a carbon residue, could clog or reduce the flow in pipes, cause buildup on hot water heaters, impart an alkali taste to the water, reduce the efficiency of the water heaters, and cause other aesthetic problems.

The Langelier Index is an approximate indicator of the degree of saturation of calcium carbonate in water. It is calculated using the pH, alkalinity, calcium concentration, total dissolved solids, and water temperature of a water sample collected at the tap. If the:

  • Langelier Index is negative, then the water is under saturated with calcium carbonate and will tend to be corrosive in the distribution system

  • Langelier Index is positive, then the water is over saturated with calcium carbonate and will tend to deposit calcium carbonate forming scales in the distribution system

  • If Langelier Index is close to zero, then the water is just saturated with calcium carbonate and will neither be strongly corrosive or scale forming.

 

The Langelier Index is one of several tools used by water operators for stabilizing water to control both internal corrosion and the deposition of scale. Water supply operators can optimize their water supply systems and identify leakage potentials with the Langelier Index. Experience has shown that a Langelier Index in the range of -1 to +1 has a relatively low corrosion impact on metallic components of the distribution system. Langelier Index values outside this range may result in laundry stains or leaks.

A perfect score on the Langelier Saturation Index (LSI) is zero (0.00). Zero is perfectly balanced water; saturated with the perfect amount of calcium carbonate, and has a stable pH. Being the universal solvent, if water is out of balance, it will naturally try to find its own balance and equilibrium, because it wants to be at 0.00 LSI. For instance, if there is not enough calcium, water will dissolve and extract it from the most readily available source. Usually in pools, that means the cement in the plaster or pebble finish.

The LSI is basically a way to determine if water is corrosive (negative LSI) or scale-forming (positive LSI). LSI between -0.30 and +0.30 is the widely accepted range, while 0.00 is perfect equilibrium. Water wants to be in equilibrium, and will find a way to get there. Under-saturation is corrosive, and over-saturation is scale-forming. Water can only hold so much calcium in solution. Water will stop at nothing to find equilibrium...so when it's hungry for calcium, it will aggressively look for it. When the water does not have a readily available source of calcium, corrosion and degradation can occur anywhere in the equipment. Another important thing to remember: water cannot over-saturate itself. It will take only what it can hold, and nothing more.

 

Analogy

When you add sugar to your drink and stir it, it will dissolve. Add more sugar, and it will dissolve too. But at some point, when you add too much sugar, what happens to that sugar? It just swirls around at the bottom of the glass, unable to dissolve. That's because you have exceeded the drink's saturation limit; it can no longer hold any more sugar. If you insist upon dissolving more sugar, there are a couple of things you can do. First, you can make it a much larger drink, which changes the volume, and reduces the saturation. You can also increase the temperature, say, to a boil. If you have ever made desserts, you know that boiling water can hold a LOT more sugar than cold water, because its saturation properties have changed. Now, replace the word "sugar" with "calcium". The LSI tells us how saturated the water is with calcium. The properties can change with six factors, not just water temperature. And it's worth noting that unlike sugar, cold water can hold more calcium in solution, and that's why cold water is more aggressive than warm.

 

 

 

Treatment

Almost any metal in contact with water will corrode to some extent, depending on the environmental conditions. In water treatment, the corrosion process can be slowed but not stopped completely unless none of the metal part is exposed to water. A full protection needs a protective barrier between water and the metal. This barrier is provided either by the manufacturer of the pipes as a coating - such as cement lining with the tar coating, paint, plastic, or rubber - or provided by the treated water as a coating of calcium carbonate, a phosphate, or a silicate.


Chemical Treatment

Treatment of corrosive water can be either chemical or physical.  In this section, we will discuss chemical methods of corrosion control.  These chemicals 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. 

 

Corrosion Inhibitors

Treating water provides protective coatings to help prevent corrosion. These include:

  • Calcium carbonate coating is the most commonly used due to the presence of natural hardness in the water. CaCO3 coating size and rate of deposition depend, mainly, upon the pH, alkalinity, and the calcium carbonate content of the water.
  • Phosphate coating is provided by the three types of phosphates: orthophosphates, polyphosphates, and zinc phosphates. They form a protection layer on the cathodic site by reacting with corrosion products and metal.
    • Orthophosphates are simple phosphate compounds, such as phosphoric acid, sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate.
    • Polyphosphates are long-chain phosphates formed by reacting phosphoric acid with sodium or potassium compounds. A common example of a polyphosphate is sodium hexametaphosphate,. In the presence of calcium and iron ions at a low pH, they form a protective coating on the cathodic site. At high pH and low dose, they dissolve iron and calcium by a sequestering mechanism, thus preventing excessive scale formation. For corrosion control, they require high velocity of water and a high dose. Polyphosphates remove corrosion products from the anode, form positively charged colloidal particles, and deposit them on the cathode area. Increasing polarization of the cathode reduces corrosion.
    • Zinc phosphates contain zinc in various concentrations (10 to 30 percent) with ortho- or polyphosphates. The protective film is formed of zinc compounds. The higher zinc concentration acts faster by rapid film formation. The higher the pH, the lower the zinc content is required for an effective control.
  • Silicate coating has been used by some water utilities in the eastern U.S. and Canada. Sodium silicate is the most commonly used chemical. It is available as a dry or liquid product. Silicates are used for waters with pH 7-9. For proper corrosion control, a 4 to 30 mg/L dose is required. Silicates form a protective thin layer of their compounds over the anode, the corroded metal part; therefore, unlike phosphates, they are anodic inhibitors. This film, unlike calcium carbonate, does not become very thick. If the application is stopped, the film breaks down and protection stops. Silicates can be combined with zinc or with phosphates for better corrosion control by protecting the anode and the cathode. Silicates are more expensive than phosphates; therefore, their use is not very common.

 

 

 

Physical Protection

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. 

Sacrificial anode.
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. 

 

 

Cathodic Protection

Cathodic protection is an electrical method for preventing corrosion of metallic structures. However, this expensive corrosion control method is not practical or effective for protecting entire water systems. It is used primarily to protect water storage tanks. A limitation of cathodic protection is that it is almost impossible for cathodic protection to reach down into holes, crevices, or internal corners.

Metallic corrosion occurs when contact between a metal and an electrically conductive solution produces a flow of electrons (or current) from the metal to the solution. The electrons given up by the metal cause the metal to corrode rather than remain in its pure metallic form. Cathodic protection stops this current by overpowering it with a stronger, external power source. The electrons provided by the external power source prevent the metal from losing electrons, forcing it to be a "cathode", which will then resist corrosion, as opposed to an "anode", which will not.

There are two basic methods of applying cathodic protection. One method uses inert electrodes, such as high-silicon cast iron or graphite, which are powered by an external source of direct current. The current impressed on the inert electrodes forces them to act as anodes, thus minimizing the possibility that the metal surface being protected will likewise become an anode and corrode. The second method uses a sacrificial anode. Magnesium or zinc anodes produce a galvanic action with iron, so that the anodes are sacrificed (or suffer corrosion), while the iron structure they are connected to is protected.

 

 

 

Review

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. 

 

 

 

Assignments

Please answer the following questions and email to your instructor:

  1. List the factors that can accelerate corrosion.
  2. List and describe the different types of corrosion.
  3. Describe how corrosion can cause higher costs for water systems.
  4. Describe the events and measurements that can indicate potential corrosion problems in a water system.
  5. List and describe the corrosion inhibitors that can be used to help prevent corrosion.

 

 

Quiz

Answer the questions in the Lesson 8 quiz. You may take the quiz online and submit your grade directly into the database for grading purposes.