Corrosion Control

by Kellyn Roth and Eric Whitney


"As public health professionals, most people in the water works industy would completely agree that the elimination of lead exposure is a reasonable objective....

For the first time [the Lead and Copper Rule of the Safe Drinking Water Act] gave the water works industry the responsibility of water quality from the tap in the home. This had never been done before....

It became important to recognize that corrosion control is as much an art as it is a science."

- Gerard Higgins, Superviser: Blacksburg, Christiansburg, VPI Water Authority

Push this button to hear Mr. Higgins's thoughts on corrosion control.


Introduction to Corrosion

Regulation of Corrosion

Factors Influencing Corrosion

Testing for Corrosion

Methods of Corrosion Control

References


Introduction to Corrosion

In both water treatment facilities and water distribution systems, corrosion is being recognized as a very serious problem. The photo above, provided by Dr. Deitrich, shows how corrosion can seriously affect pipe conditions. The corrosion and degradation of pipes and facilities used in water treatment operations can cause increased economic costs due to water main replacement. Replacement may be necessary when the increased pipe roughness, caused by corrosion, increases pipe energy costs and reduces distribution system pressures. Leaks are also cause the need for replacement. If pipe materials deteriorate and are carried by the flowing water, these materials may be carried to the consumer and, if toxic, cause serious health problems. External corrosion is due to corrosive soils such as sulfides, and moisture. Internal corrosion is due to corrosive characteristics such as high lead contents and velocity of the drinking water.

Metallic corrosion in both cases is always the result of an electrochemical reaction. This reaction is shown in the figure of a basic annode, cathode, electrolyte, and return current path. Corrosion takes place at the eletrode surface where electrons are generated and travel through the electronic path. This electrode is the anode. The cathode is the electrode to which electron flow.The electrolyte is the conducting solution which is typically water or soil. The return current path is the fourth element that must be present for corrosion to occur (AWWA, 1987). This reaction can be seen below in Figure 1.

corr.GIF

Figure 1. Electrochemical Reaction Causing Corrosion
Prepared by Eric Whitney


 

Regulation of Corrosion

An increased desire and need for corrosion control and regulation is due to the recognition of the economic and health problems it can cause. The Lead and Copper Rule of the Safe Drinking Water Act specifies a lead level of 15 ppm that must be obtained by all water providers. This value is not based on scientific experiments but on simple observations that 15 ppm seems to coincide with corrosion problems.

Corrosion regulation specifies that the water be "non-corrosive." This is determined using the indices discussed under Testing for Corrosion, the Driving Force Index, Langelier Index, and Aggressivity Index. As more methods of measurement and control area being discovered and tested, regulation is predicted to increase and to become more specific.  


Factors Influencing Corrosion

Many variables control the extent of corrosion within a water supply system, several of which are characteristics of the pipe itself. Another factor is the type of metal used in pipe construction. If chemically inactive metals such as the noble metals gold, silver or platinum are used, the rater of corrosion is greatly reduced. Base metals such as magnesium, aluminum, and zinc increase this rate. Steel, lead, and copper pipes fall somewhere in the middle of this range. The amount of exposed area to the water increases this reaction also. If the pipe were painted to provide a protective layer, but pinholes in the paint existed, the corrosion in these pinholes would be greater than if nothing were painted at all (Kerri 1989).The amount of dissolved oxygen in the water controls the corrosion rate.

Some factors influencing corrosion are qualities of the water flowing through and around the system. Salt increases the electric conductivity of the water and would therefore increase corrosion. The presence of calcium carbonate in the water causes a film to form which slows the flow of dissolved oxygen and prevents corrosion. Too much calcium carbonate, however, causes a thick film called scale to form. Scale can increase friction and cause problems with water flow rate. The correct amount of CaCO3 in the water indicates the saturation point; too much is supersaturation (Hammer, 1990). Finally excessive water velocities through the system cause "erosion corrosion."  


Testing for Corrosion

Choose which type of testing:

External

Internal: Physical/Chemical/Laboratory Tests

Internal: Use of Corrosion Indices

External

Several tests can be performed to determine the proper pipe materials for the soil conditions. Each material has its own set of tests that must be examined. These tests are important for all materials; soil resistivity, pH, moisture, and presence of sulfides (AWWA, 1987).

Soil Resistivity

Soil resistivity tests determine the reciprocal of conductivity for a particular soil. Low resistivity indicates a soil will be a good electrolyte. Soil moisture plays a large part in resistivity, and it may be desired to test a soil sample from the designed pipe depth in the laboratory to set soil in a saturated condition. There are several different methods for testing resistivity; all include use of soil probes and a resistance meter. Testing methods vary depending upon preferences of technicians and soil depths.

pH

Soil pH plays a large role in external corrosion. Soils with a pH below 4.0 usually act as an excellent electrolyte, are high in acid and are known to be aggressive. Neutral pH soils (6.5-7.5) indicate sulfate-reducing bacteria may be present under certain conditions. High pH soils (8.5-14.0) are usually high in dissolved salts and exhibit a low resistivity.

Moisture

Water acts as an electrolyte and produces conditions of lower soil resistivity. Soil moisture content varies throughout the year, and general drainage patterns and conditions should be monitored instead.

Sulfides

The presence of sulfides in soils indicates that sulfate-reducing bacteria may be present. A qualitative laboratory test is used to determine the presence of sulfides. This test involves introduction of a solution containing 3% sodium azide in 0.1N iodine into a test tube with a soil sample from the pipe depth. If sulfides are present, they act as a catalyst between iodine and sodium azide, releasing nitrogen. The reaction is:

2NaN3 + I2 -> 2NaI + 3N2

Internal: Physical/Chemical/Laboratory Tests

The most common indicators of corrosion problems are physical ones. They include consumer complaints of dirty, rusty water or an increased number of leaks in the system. Several different methods exist for measuring the corrosion rates in a water distribution system. The simplest method the insertion of small metal scraps in the water main. The metal is removed after a few months and the loss of weight and type of corrosion damage is measured. In this and all tests, standard procedures must be used and true representative samples must be collected. Another simple test is the flow test which measures the change in water flow through a standard black iron pipe under a constant head. After two weeks highly corrosive water will reduce the flow rate by 50%, non-corrosive water will decrease the flow rate by only 10% (Kerri, 1989).

Several chemical tests are also performed in order to quantify the corrosivity of the water. One way takes advantage of the fact that corrosion can only occur in the presence of oxygen. The oxygen concentration can be measured at several points in the system: source inflow, wells, and individual taps. Testing for the presence of toxic metals, such as lead and cadmium, deposited from plumbing is another way to judge the corrosivity of water. The results of testing for metals is varies greatly at different testing locations, making multiple testing a necessity. Another test used is the marble test. The pH, alkalinity and hardness of a water sample is measured, then a small amount of calcium carbonate is mixed into it. If this causes a rise in pH, alkalinity, or calcium then the water was unsaturated, a decrease indicates supersaturation, and no change indicates saturation. Saturation of the water of calcium carbonate occurs in non-corrosive water (Kerri, 1989).

Finally, water can be identified as non-corrosive by testing for the chemical additive used by the water treatment plant to control corrosivity. These chemical additives are discussed in a later section, but include zinc orthophosphate. This compound decreases corrosivity in the pipes, and if its presence is detected from sources far into the distribution system, non-corrosivity is assured. One method of testing for ZnPO3 uses a spectrophotometer as shown in Figure 2 below. Cyanide and cyclohexone are added to the water sample and the presence of zinc causes a blue color to develop. A spectrophotometer is able to detect even very minute amounts of color. Similar procedures are used for other corrosion control compounds.  

Figure 2. Spectrophotomer and Materials used to test for Zinc Orthophosphate
Picture by Kellyn Roth

Internal: Use of Corrosion Indices

Four quantitative indices (Hammer, 1990; Kerri, 1989) are used to determine how close a water is to its equilibrium point and the corrosiveness of the water. The equilibrium point is the point when the water is just saturated with calcium carbonate. At this point the water is stable and will neither dissolve nor deposit calcium carbonate.

1. The first index, and also the simplest, is the Driving Force Index (D.F.I.). The D.F.I. is found using the equation:

D.F.I. = [Ca2+][CO32-] / K'sp*1010

where [Ca2+] is the calcium hardness, [CO32-] is the carbonate alkalinity, and K'sp is the solubility product for calcium carbonate.If this value is less than one, the water is corrosive. If the D.F.I. is greater than one then the water is non-corrosive. A value equal to one indicates stability.

2. The Langelier Index (L.I.) is the most common index used to determine the relative corrosivity of a water. This method reflects the equilibrium pH of a water with respect to calcium and alkalinity. The equation used to find this value is:

L.I. = pH - pHs

where pH is the actual pH of the water, and pHs is the pH at which water of a given calcium content and alkalinity is at the equilibrium point. This method is only considered accurate over the pH range of 6.5 to 9.5. A positive L.I. shows that the water is supersaturated and non-corrosive.

3,4. The final indices used to report corrosivity are the Ryznar Index (R.I) and the Aggressivity Index (A.I.). The Ryznar Index is 2pHs - pH. A value of below 7 denotes supersaturation and non-corrosive. This index can only be used when pHs is 7.00. The A.I. can be found using the formula:

pH + log(Ca2+) + log(Alky).

Several common indices values are shown in Table 1 below.


Methods of Corrosion Control

Choose which type of control:

External

Internal

External

Water utilities employ several different corrosion control systems, including pipe coatings, cathodic protection, use of corrosion resistant materials, and polyethylene casing of ductile iron. These systems must be able to eliminate at least one of the elements required for corrosion to occur: an anode, a cathode, an electrolyte, and a return current.  

Coatings  

Coatings control corrosion by placing a barrier between the pipe and the surrounding environment, eliminating the electrolyte. The effectiveness is dependent upon the integrity of the coating, i.e. how many holes are present, strength of bond with the metal, and ability to insulate the pipe from electrical current. Examples of common coatings are asphalt, concrete, epoxies, polyvinyl chloride (PVC), and polyethylene. It should be noted that none of these coatings are completely unsusceptible to corrosion. Factory coatings may be damaged during any phase from manufacturing to installation, holes in coatings can lead to concentration cells undermining coatings and cause corrosion at site of hole. For these reasons most coatings should be used in conjunction with cathodic protection to reduce the corrosion caused at holes. The use of both methods also reduces the cost of the cathodic protection while increasing the life of the coating.  

Cathodic Protection  

Cathodic protection systems reduce corrosion by establishing the pipe structure as a cathode of a galvanic or electrolytic cell. Direct electrical current from the cell produces a current flowing through the pipe that overcomes any current due to corrosion cells tin which the pipe may be a anode. Current does not flow form the pipe to the cell, preventing corrosion from occurring. Cathodic protection can be used for a variety of pipe materials, including steel, ductile-iron, cast iron, and concrete. Protection of the pipe depends on the four necessary elements of corrosion, an anode, a cathode, a conductive electrolyte, and a return current path. The protected pipe is the cathode, which must be under continuous electricity. The conductive electrolyte is the surrounding soil. The anode and current path are shown in Figure 1. These are dependent upon the system used, galvanic or electrolytic (AWWA, 1987).

 

Advantages

Excellent protection for in place pipes

Design variable depending upon conditions  

Disadvantages

High installation costs

Monitoring and maintenance required

Power source required    

Polyethylene Encasement of Ductile-Iron  

Polyethylene encasement loosely wraps ductile-iron pipes in an 8-mil thick system. This improves the pipes surrounding environment by eliminating direct exposure to soils and reducing pipes surroundings to the area between pipe ad the wrap. Groundwater can enter into space between wrap and pipe, however the initial corrosion reaction depletes the water of its corrosive qualities (AWWA, 1987).  

Advantages

Low initial cost for material and installation

No maintenance

Installed as pipe line is placed

Minor holes have no effect to integrity  

Disadvantages

Difficult to use as system upgrade

Large tears can cause major problems  

Internal

Chemical Additions

Calcium Carbonate: Reduction of corrosivity is often accomplished by treating the water with chemicals to saturate the water with calcium carbonate. Calcium carbonate functions to lower and possibly eliminate the current flowing through the water. Choosing the chemical to achieve calcium carbonate saturation will depend on the water quality characteristics of the water and the cost of the chemicals. Waters with a low hardness and low alkalinity would require quicklime or hydrated lime. Caustic soda or soda ash should be added to waters with high levels of hardness and alkalinity. The chemical dosage is another variable and is a function of temperature, pH, alkalinity, and hardness.

Advantages

Low cost initial cost

Disadvantages

Lime requires expensive equipment for hydration

Scale formation and clogging may be a problem

Lime and caustic soda must be handled with caution

Soda ash may contribute to high blood pressure

Other chemical compounds: Several chemical compounds are also able to form cathodic films that control corrosion in a manner similar to calcium carbonate. Zinc and sodium compounds can be used in several situations. Zinc phosphates are usually proprietary and more expensive, but technical advice and assistance is usually provided by the distributors at no additional charge. Sodium silicate is usually used by individual homeowners and apartment buildings, not water utilities. Sodium polyphosphates are used to control and decrease thick scale formations sometimes caused by calcium carbonate saturation. All chemicals should be fed after filtration to prevent cementing of filer sands (Steel, 1960) The addition of a Zinc compound can be seen in Figure 3 below. The Zinc Orthophosphate is stored in the large container shown.

Advantages

Scale formation is not a problem

Ease of handling and storage

Disadvantages

Must be bought directly from the manufacturer, not a distributer

Phosphate may cause algal blooms if stored in an open reservoir

 
Figure 3. Zinc Storage and Point of Entry
Picture by Kellyn Roth

  Cathodic Protection

Whenever metals, such a water pipes, are buried there is an electrochemical force that damages the material. This process is described above under external controls.


References




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Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1997 Daniel Gallagher
Last Modified: 02/24/1998