Key Words and Concepts
Alkalinity – The capacity of water to neutralize acids. This is caused by carbonate, bicarbonate,
hydroxide, borate, silicate and phosphate.
Anode – The positive pole or electrode of an electrolytic system, such as a battery. The anode attracts
negatively charged particles or ions. (anions)
Cathode – The negative pole or electrode of an electrolytic cell or system. The cathode attracts
positively charged particles or ions. (cations)
Cathodic Protection – An electrical system for the prevention of rust, corrosion and pitting of metal
surfaces which are in contact with water or soil.
Corrosion – The gradual decomposition or destruction of a material by chemical action, often due to an
electrochemical reaction.
Galvanic Corrosion – A form of localized corrosion caused by the electrical connection of two dissimilar
metals.
Hardness – A characteristic of water caused mainly by the salts of calcium and magnesium.
Langelier Index – An index reflecting the equilibrium pH of a water with respect to calcium and
alkalinity.
Oxidation – Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from
a compound or element.
Reduction – The addition of hydrogen, removal of oxygen or the addition of electrons to an element or
compound.
Sacrificial Anode – An easily corroded material, deliberately installed in a pipe or tank. The intent is to
sacrifice this anode to corrosion, while the water supply facilities remain relatively corrosion free.
Stray Current Corrosion – A corrosion activity resulting from stray electrical current originating from
some source outside the plumbing system, such as electric rail systems.
Water System Corrosion
Corrosion Chemistry
Concentration cell corrosion in iron system piping is the result of an electrochemical reaction
between impurities in the pipe
One impurity acts as a cathode and the other an anode in relation to each other
Electrical system created in which electrons are transferred from the anode to the cathode
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Corrosion Factors
Corrosion is the result of chemical reactions
Anything impacting the chemical reactions near the anode or cathode will change corrosion rate
Factors include:
o Dissolved oxygen concentration
o Total dissolved solids concentration
o pH
o Alkalinity
o Temperature
o Flow velocity
o Type of metal
o Electrical current
o Bacteria population and concentration
Dissolved Oxygen
Accelerates the rate of corrosion
The more dissolved oxygen, the faster corrosion occurs
Total Dissolved Solids
Impacts corrosion rate because there must be electrical contact between anode and cathode
Pure water is a poor conductor, but water with high levels of TDS conducts electricity very well
pH and alkalinity
Both have the same effect on the rate of corrosion.
As pH and alkalinity go up, the corrosion rate goes down.
Higher Water Temperatures
Increases rate at which chemical reactions occur which increases the corrosion rate
Flow Velocity
Can either speed up or slow down the corrosion rate of the interior of a pipe
If water is corrosive and has a high DO concentration, higher flow velocity will increase the
corrosion rate
If the water is above the point of calcium saturation or if chemicals have been added to inhibit
corrosion, the corrosion rate will go down as the depositing compounds will be distributed more
quickly and coat the pipe (assumes flow is < scouring velocity)
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Type of Metal
Plays a big role in the rate of corrosion
Metals that give up electrons easily will corrode faster than those metal that don’t
When two different metals are in electrical contact with each other, one metal will act as an
anode and the other as a cathode resulting in galvanic corrosion
If electrical current is passed through metal system piping, the corrosion rate will increase
o Called stray current corrosion and is often the result of electrical railway systems near
system piping or household electrical systems that have been improperly grounded to
the plumbing system
Bacteria
Can accelerate the rate of corrosion because they produce carbon dioxide (CO2)
CO2 reacting with water forms carbonic acid which will reduce the water’s pH and create a
corrosive condition
Excessive bacterial growth results in slime coating on inside wall of pipe which traps the CO2
produced by the bacteria causing a localized environment that is very corrosive
The two bacteria groups that cause most of the corrosion problems within water systems are:
o Iron bacteria
o Sulfate reducing bacteria
Corrosion Types
Two broad categories:
o Localized corrosion
o Uniform corrosion
Localized pitting corrosion is the result of either galvanic or concentration cell corrosion
o Causes a more rapid failure of system integrity than uniform corrosion.
Uniform corrosion occurs evenly over the entire surface of the pipe, tank, or other components
o Usually the result of aggressive water having a low pH and alkalinity that attacks
unprotected metal surfaces
System Scaling
Overview
Scaling can be good or bad
Mild scale on the interior of piping systems can provide a protective layer that inhibits corrosion
Uncontrolled scale formation can eventually lead to piping that is almost completely blocked by
mineral deposits and must be replaced
Scale Formation Chemistry
Scale is formed when cations calcium and magnesium (Ca++ and Mg++) combine with other
dissolved minerals and precipitate out of solution and coat the pipe wall
Typically in the form of calcium carbonate (CaCO3) but can be magnesium carbonate (MgCO3)
and calcium sulfate (CaSO4)
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Scale Formation Factors
Factors affecting the amount of CaCO3 water can hold in solution are:
o pH
o Alkalinity
o Total dissolved solids
o Temperature
Corrosion and Scaling Control
Overview
There are multiple ways for water systems to control corrosion and scaling
The decision will depend on the chemical characteristics of source water and effects of the
selected method on other treatment processes.
There are three basic methods used to control corrosion and scaling:
o Adjusting pH and alkalinity
o Formation of a calcium carbonate coating
o Use of corrosion inhibitors and sequestering agents
Systems that have multiple water sources with different chemical characteristics may have a
complex problem that requires an outside expert to help to determine the best course of action
Adjusting pH and Alkalinity
Soft water with a pH of less than 7 will be corrosive to household plumbing and could result in
increased lead and copper levels if lead and copper are present in the plumbing system.
Generally, a moderate increase in the pH and alkalinity of treated water will reduce the rate of
corrosion
There are several different chemicals that can be used to increase the pH and alkalinity of
treated water:
o Lime
o Soda ash
o Sodium bicarbonate
o Caustic Soda
Calcium Carbonate Coating
To protect against lining failures and even to protect unlined metal surfaces inside the
distribution system, water utilities create the proper conditions to cause a coating of CaCO3 to
be deposited on the interior surface of the system
The water pH is adjusted to just above the saturation point for calcium carbonate which causes
it to precipitate out of solution and form a scale on the pipe and tank surfaces
A protective coating is created that will inhibit corrosion.
Lime is often used because it increases the calcium hardness along with alkalinity which is
necessary for the coating reaction
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Inhibiting and Sequestering
Used when low levels of calcium hardness and alkalinity and it is not an option to create a
calcium carbonate coating on the interior of system piping and tanks
Polyphosphates and silicates have been used with good success to create protective layers on
the interior of their system components.
Some polyphosphate compounds can be used as sequestering agents which prevent scale
formation.
o Sequestering agent “ties up” the Ca and Mg so they cannot react and form a scale
o Sequestering compounds stay in the water and will be ingested by the drinking public
Sequestering agent must be suitable for drinking water and meet the NSF 60 standard
Polyphosphates are also used to prevent iron from precipitating and turning the water rust color
Corrosion Control Chemicals
Increase pH and Alkalinity
Chemical Name Chemical Formula
Unslaked lime (quicklime) CaO
Slaked lime (hydrated lime) Ca(OH)2
Sodium bicarbonate NaHCO3
Sodium carbonate (soda ash) Na2CO3
Sodium hydroxide (caustic soda) NaOH
Decrease pH and Alkalinity
Chemical Name Chemical Formula
Carbon dioxide CO2
Sulfuric Acid H2SO4
Form Protective Coating
Chemical Name Chemical Formula
Sodium silicate Na2O(SiO2)n
Sodium hexametaphosphate (NaPO3)n .
Na2O
Sodium zinc phosphate (MPO3)n .
M2O
Zinc orthophosphate Zn3(PO4)2
Sequestering Agents
Chemical Name Chemical Formula
Sodium hexametaphosphate (NaPO3)n .
Na2O
Tetrasodium pyrophosphate Na4P2O7 .
10 H2O
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Chemical Feed Equipment
Lime
Slaked lime requires volumetric or gravimetric dry feeder for adding lime to solution chamber
where a slurry is formed by adding water to the solution chamber with the slaked lime
Unslaked lime uses dry feeder which adds lime to a slaker to create a slurry
The lime slurry is then added to the water requiring pH and alkalinity adjustment.
The distance between the slurry mix point and the injection point should be minimal because
the lime slurry will cake out on any surface and must be cleaned or cleared on a regular basis.
Soda Ash / Sodium Bicarbonate
Gravimetric or volumetric feeders can be used to feed soda ash and sodium bicarbonate
Soda ash does not dissolve into solution quickly, so larger dissolving chambers and good mixing
equipment are required to ensure adequate mixing
Sodium bicarbonate feed systems should be constructed of caustic resistant materials
o PVC or stainless steel are two materials that are well suited for sodium bicarbonate feed
systems due to their resistance to corrosion while contacting caustic solutions
Sodium Hydroxide
Liquid that can be fed by metering pumps made for handling caustic solutions
Causes rapid deterioration of valves and fittings containing copper, brass, bronze or aluminum
All feed system piping should be made of caustic resistant material such as PVC
Sulfuric Acid
Should be fed with a metering pump that is corrosion resistant
Some feed systems are made of heat welded HDPE piping because of its resistance to sulfuric
acid at the acid strength used at water treatment plants
Sodium Silicate
Sodium silicate can be fed with a positive displacement metering pump straight from the
delivery container or from a day tank
No special system requirements
Phosphate Compounds
Liquids typically added straight from the shipping container or from a day tank
Metering pumps should be designed for corrosive material and the piping system with all of its
valves and fittings should be made of PVC or stainless steel
Limestone Contactors
Easy way to reduce lead and copper corrosion in systems
Water flows through container packed with crushed limestone dissolving some calcium
carbonate which raises the pH and alkalinity which reduces lead and copper corrosion
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Corrosion Control Processes
Process Selection
Sources of data for process selection:
o Inspection of replaced system piping for corrosion
o Records of water leaks
o Records of reduced flow rates
o Records of customers complaining about colored water
o Lab results of system water quality compared to raw water lab results
Langelier Saturation Index (LSI)
Can be used to predict whether water will tend to dissolve or precipitate calcium carbonate.
o If it dissolves calcium carbonate, it is considered corrosive.
o If it precipitates calcium carbonate, it will cause scaling.
LSI looks at saturation pH of a given water sample and compares it to actual pH of the water
LSI = pHactual – pHsaturation
The saturation pH formula takes into account:
o Calcium ion concentration
o Alkalinity
o pH
o Temperature
o Total dissolved solids
LSI = 0 → stable
LSI = + → deposit
LSI = ‐ → dissolve
Operator Suggestions
Get approval from the primary agency before initiating any new treatment process
If a corrosion inhibitor is used, investigate to ensure it is compatible with other chemicals in
current use
Notify all industrial and medical customers as well as the WWTP of the proposed change and
verify it will not cause problems for them
Consider the pros and cons of handling and feeding the various chemicals that may be used for
the corrosion control program:
o Storage space required
o Chemical shelf life
o Ease of handling chemical
o Problems associated with the chemical
o Special feed equipment required
o Chemical hazards and special handling requirements
o Cost of chemical per unit of water produced
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Adjusting pH and Alkalinity
The stability of water can be changed by adjusting the pH and alkalinity levels using:
Lime
Caustic soda
Soda ash
Sodium bicarbonate
Carbon dioxide
Sulfuric acid
Protective Coatings
Lime
Can be added to the treated water to cause CaCO3 to precipitate out of the water and form a
scale on the pipe walls in the distribution system
Thin coating will protect the pipes from corrosion
Process must be monitored carefully because excessive scale will increase system head loss and
can clog home plumbing systems
Protective coating should uniformly cover the pipe interior for best results
A good coating can be produced and maintained if the distribution system water quality
consistently meets the following requirements:
o Calcium and alkalinity are maintained ≥ 40 mg/L as CaCO3
o Maintain a 4‐10 mg/L as CaCO3 oversaturation condition
o pH range of 6.8 to 7.3
Polyphosphates
Can be used to create a protective coating on pipe interiors for systems that lack naturally
present CaCO3 in their source water.
Two compounds used are:
o Sodium zinc phosphate
o Zinc orthophosphate
Dosed in the range of 0.5 ‐ 3 mg/L (initial dose higher to establish protective coating)
Operational Control
Regular monitoring of the corrosion control program is essential
Monitoring involves water quality testing at the treatment plant and in the distribution system,
pipe inspections, and coupon testing
o Daily water quality testing should include:
Calcium ion concentration
Alkalinity
pH
Temperature
Total dissolved solids
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Pipe inspection is another way to see how the corrosion control program is working
o Can occur on portions of pipe removed from the system for repair or on “spool pieces”
installed for the specific purpose of corrosion control inspections.
A spool piece is a short section of pipe easily removed for inspection
Coupons testing includes inserting pieces of metal into the system for a minimum of 120 days
o Coupons are weighed prior to beginning the test and end of test
o Loss of weight indicates corrosion is occurring
o Weight gain indicates scaling
Regulations
Lead and Copper Rule (LCR)
Enacted in 1991 to reduce public exposure to high lead and copper concentrations in water
All community water systems and nontransient noncommunity water systems are required to
monitor for lead and copper levels at the customer’s taps
If excessive levels of lead or copper found, they must institute a corrosion control program
o Program is intended to reduce the amount of lead and copper that is leaching into the
customer’s household plumbing systems because of aggressive water
o Program may include adjusting the pH and alkalinity of the system to create the
conditions that will not allow the water to dissolve or “accept” additional ions such as
lead and copper
o Program may also include the use of corrosion inhibiting chemicals to prevent the lead
and copper in household plumbing systems from leaching into the water
