From general experience of life, we probably have some idea about what corrosion is and have experienced the higher levels of corrosion that occur in the presence of moisture and the hostile gas species, such as chlorine, ammonia, sulphur dioxide, hydrogen sulphide and oxides of nitrogen, that are often present in the atmosphere. Whether our experience is of tarnished silver or rotting exhaust pipes, what we have seen is corrosion, defined as "the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment".

The phenomenon of corrosion involves reactions which lead to the creation of ionic species, by either loss or gain of electrons. Take the case of the rusting of iron, where metallic iron is converted into various oxides or hydroxides when exposed to moist air. The equations for this reaction are:

2Fe(solid) + 2H2O(liquid) + O2 (gas) → 2Fe2+ + 4OH- → Fe(OH)2 (solid)  

2Fe(OH)2 (solid) + H2O (liquid) + ½O2 (gas) → 2Fe(OH)3 (solid)


Because the iron loses electrons, chemists refer to this as an oxidation process. In this case, the resulting product is a mix of hydroxides; where copper reacts with acid residues creating chlorides and sulphides, this would also be referred to as an oxidation process.



Factors Affecting Corrosion

Corrosion of bare conductors will happen at a rate that varies substantially, depending on the conditions. The table below indicates some of the factors that affect such corrosion. Note that this is for unprotected materials, and that the addition of an effective surface coating can affect the outcome for the better.

Factors That Affect Corrosion


Nature of the material or alloy
Surface condition/roughness
Conductor configuration
Conductor-conductor spacing


Moisture absorptivity
Nature of any reinforcement


Corrosive elements


Another factor that affects the rate of progress of corrosion is the nature of the "corrosion product". If the material produced by corrosion is insoluble and forms an impervious and tenacious layer, the corrosion reactions becomes self-limiting, as the corrosive medium can no longer diffuse through the corrosion product. A useful example of this is the oxidation of aluminum, which forms a thin protective layer of aluminum oxide, this is a good example of self-passivation.

If, on the other hand, the corrosion product is soluble or porous, corrosion will continue until the material is depleted, and no further reaction can occur. This is seen with the rusting of iron, where the oxide/hydroxide "rust" has a different crystal structure from the iron, and creates only a porous, poorly adherent layer which does not protect against continued attack.



Types of Corrosion

Below is a list of the various types of corrosion.
Uniform corrosion
The reation starts at the surface and proceeds uniformly.
Localized corrosion (pitting corrosion)
The basis metal is eaten away and perforated in places in the manner of holes, the rest of the surface being affected only slightly or not at all.
Wide Pitting corrosion
The corrosion causes localized scarring.
Intergranular corrosion
Imperceptible or barely perceptible from outside, since the corrosion proceeds at the grain boundaries.
Transgranular corrosion
The grain boundary material is retained, since the corrosion proceeds preferentially within the grain.
Galvanic corrosion
Increased corrosion in crevices or cracks or at contact surfaces between two metal articles.
Selective corrosion
Corrosive attack on structural constituents
Exfoliation corrosion
Occurs in deformed articles. Corrosion follows "fiber orientation"
Interfacial corrosion
Frequently observed at water-air interfaces




Corrosion Causes and Treatment

Corrosion of distribution lines, home plumbing and fixtures has been estimated to cost the public hundreds of millions of dollars per year. Lead and cadmium, both toxic metals, occur in tap water almost solely due to corrosion. Three other metals, usually found in high concentrations due to corrosion of piping systems are copper, iron, and zinc. Copper causes blue staining and imparts a metallic bitter taste. Iron corrosion causes reddish or brown water and also imparts a metallic taste. Zinc corrosion does not usually discolor the water, but can also cause a metallic taste. All waters are corrosive to some degree, and a water's corrosive character depends on its physical and chemical constituents. The type of material the water comes in contact with also affects the "corrosivity". For instance, water that may corrode iron pipe may not be as corrosive to copper pipe.

In our area, the most common causes of corrosion of plumbing systems on municipally treated water is from "electrolysis" or electrically induced corrosion. This is often due to improper grounding of electrical systems to water pipes, or by the creation of "galvanic corrosion cells" in home systems due to the dual use of iron and copper piping in the same system. This "electrically-induced" corrosion is aggravated by waters high in total dissolved solids, making the water more conductive.

On private water systems, the most common cause of corrosion is from low pH (less than 7.0) waters. Often these waters are of high quality and are low in buffering calcium minerals, but are high in dissolved carbon-dioxide gas, which can cause the low pH or acidity. Treatment is accomplished by neutralizing the water with the use of an automatic neutralizer. These water filter tanks are filled with a blend of calcium and magnesium carbonates made from naturally occurring minerals, which dissolve into the water, making it less corrosive. Other methods commonly used are pH adjustment by injecting soda ash or a sodium hydroxide solution into the water upstream of a holding or retention tank.



Types of Corrosion

  1. Electro-galvanic
  2. Electrolysis
  3. Bacterial
  4. Chemical


Corrosion is costly, difficult to control, and creates hazards for water  plant operators and customers. It can also contaminate the water.

  1. Electro-galvanic : Iron is not in a pure state in nature. Refining efforts make it relatively pure. When electrical current is passed through it, the outer electron is removed causing the atom to have a positive charge(ionized). Oxygen molecules in water are relatively electrically negative. Therefore, the iron molecules which are slightly positively charged are attracted to the oxygen molecules which are negatively charged with Vander Waals forces and follows the water molecule anywhere it might go. It then becomes, in the presence of free oxygen, FE2O3 which is what we call common rust.
  2. Electrolysis : In electrolysis, acids and bases tend to cause current to flow like in batteries. In the generation of current, the atoms become ionized. Then the ionized iron is attracted to the oxygen molecules which in water is relatively electrically negative. At that point, it does the same as above.
  3. Bacteria : Some bacteria can actually metabolize iron.
  4. Chemical : Chlorine, oxygen, and other compounds can cause direct corrosion without water.



  1. Painting and other surface protection (i.e. plating, etc.) - creates a barrier and keeps water from getting to the metal.
  2. Cathodic protection - Use of a sacrificial electrode to supply electrons to the iron for the reduction of oxygen and slows process of corrosion.
  3. Selection of materials - Corrosion resistant materials like plastics, stainless steels, and brass.



How do you visualize the corrosion process?

                        Fe -----> Current  ------> Fe++


                        Fe++--(O) -->O2--->Fe2O3
                               \          Fe3O4



Things necessary to occur:

H20                         Bacteria
Metal                      Current
O2 or oxidant          Dissimilar Metals

Chemical Treatment

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

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.


Corrosion. Cathodic protection with an external power source.


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. 




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. 


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. 




Causes of 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 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.

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 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 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.
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.

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. Cell

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. 

Corrosion cell.


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.



Anode Reactions

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 section, 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. 




Cathode Reactions

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.

Electrochemical reaction of corrosion.


Corrosion is the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment. There are various types of corrosion that affect different elements. Lead and cadmium, both toxic metals, occur in tap water almost solely due to corrosion. Three other metals, usually found in high concentrations due to corrosion of piping systems are copper, iron, and zinc. Corrosion can often be treated by raising the pH of the water or by adding phosphate in various forms into the water to seal off the corroding piping.

Corrosion is the destructive attack of a metal caused by either a chemical or an electrochemical reaction with the various elements in the environment. There are various types of corrosion that affect different elements. Lead and cadmium, both toxic metals, occur in tap water almost solely due to corrosion. Three other metals, usually found in high concentrations due to corrosion of piping systems are copper, iron, and zinc. Corrosion can often be treated by raising the pH of the water or by adding phosphate in various forms into the water to seal off the corroding piping.