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TitleProteccion Catodica AWWA M11
TagsCorrosion Electrochemistry Anode Cathode Nickel
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AWWA M11 89 11 0783350 0003016 T 11




Principies of Corrosion

and Corrosion Control

Corrosion is the deterioration of a substance (usually a metal) or its properties because of a
reaction with its environment.1 Even though the process of corrosion is complex and the
detailed explanations even more so, relatively nontechnical publications on the subject are
available. 2,3

An understanding of the basic principies of corrosion leads to an understanding of the
means and methods of corrosion control. Methóds of corrosion control are discussed in this
chapter and in Chapter 11. Although many of these me~hods apply to all metals, both
chapters deal specifically with corrosion and corrosion control of steel pipe.

10.1 GENERAL THEORY ---------------
All materials exposed to the elements eventually change to the state that is most stable under
prevailing conditions. Most structural metals, having been converted from an ore, tend to
revert to it. This reversion is an electrochemical process-that is, both a chemical reaction
and the flow of a direct electric current occur. Such a combination is termed an
electrochemical cell. Electrochemical cells fall into three general classes:

• galvanic cells, with electrodes of dissimilar metals in a homogeneous electrolyte,
• concentration cells, with electrodes of similar material, but with a nonhomogeneous

• electrolytic cells, which are similar to galvanic cells, but which have, in addition, a

conductor plus an outside source of electrical energy.

Three general types of corrosion are recognized: galvanic, electrolytic, and biochemical.

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Galvanic Corrosion
Galvanic corrosion occurs when two electrodes of dissimilar materials are electrically
connected and exposed in an electrolyte. An example is the common flashlight cell (Figure
10-1). When the cell is connected in a circuit, current flows from the zinc case (the anode)
into the electrolyte, carrying ionized atoros of zinc with it. As soon as the zinc ions are
dissolved in the electrolyte, they lose their ionic charge, passing it on by ionizing atoms of
hydrogen. The ionic charge (the electric current) flows through the electrolyte to the carbon
rod (the cathode). There, the hydrogen ions are reduced to atoros of hydrogen, which
combine to form hydrogen gas. The current flow through the circuit, therefore, is from the
zinc anode to the electrolyte, to the carbon rod cathode, and back to the zinc anode through
the electrical conductor connecting the anode to the cathode. As the current flows, the zinc
is destroyed but the carbon is unharmed. In other words, the anode is destroyed ·but the
cathode is protected.

If the hydrogen gas formed in the galvanic cell collects on the cathode, it will insulate
the cathode from the electrolyte and stop the flow of curren t. As long as the hydrogen film is
maintained, corros ion will be prevented. Removal or destruction of the hydrogen film will
allow corros ion to start again at the original rate. Formation ofthe film is called polarization;
its removal, depolarization. Corrosion cells normally formed in highly corrosive soils or
waters are such that the hydrogen formed on the cathode escapes as a gas and combines with
dissolved oxygen in the electrolyte, thus depolarizing the cathode and allowing corrosion to

In the flashlight battery, the zinc case is attacked and the carbon is not. However, zinc
or any other metal may be attacked when in circuit with one metal, but not attacked when in
circuit with another. A metallisted in Table 10-1 will be attacked if connected in a circuit
with one listed beneath it in the table, if they are placed in a common electrolytic
environment such as water or moist soil.

The order in Table 10-1 is known as the galvanic series; it generally holds true for
neutral electrolytes. Changes in the composition or temperature ofthe electrolyte, however,
may cause certain metals listed to shift positions or actually reverse positions in the table.
For example, zinc is listed above iron in the table, and zinc will corrode when connected to
iron in fresh water at normal temperature. But when the temperature of the water is above

Figure 10-1 Galvanic Ceii-Dissimilar

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Figure 10-2 Galvanic Ceii-Dissimilar

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This is another galvanic cell of dissimilar-electrolyte type. Soil throughout depth of ditch is of uniform kind, but pipe
rests on heavy, moist, undisturbed ground at bottom of ditch while remainder of circumference is in contact with
drier and more aerated soil backfill. Greatest dissimilarity-and most dangerous condition-occurs along narrow
strip at bottom of pipe, which is anode of cell.

Agure 10-tl Corrosion Caused by Differential Aeration of Soil

Electrolytic Corrosion
The transportation industry and other industries use direct current (DC) electricity for
various purposes in their operations. It is common practice with DC circuits to use the
ground as a return path for the curren t. In such cases, the ~h of the current may stray sorne
distance from a straight line between two points i~ a system in order to follow the path of
least resistance. Even where metallic circuits are provided for handling the direct currents,
sorne of the return current may stray from the intended path and return to the generator
either through parallel circuits in the ground or through sorne metallic structure. Because
these currents stray from the desired path, they are commonly referred to as stray earth
currents or stray currents.

The diagrammatic sketch of an electric street-railway system shown in Figure 10-12 is
an example of a system that can create stray DC currents. Many modern subway systems
operate on the same principie. In Figure 10-12, the direct current flows from the generator
into the trolley wire, along this wire to the streetcar, and through the trolley of the car to the
motors driving it. To complete the circuit, the return path of the current is in tended to be
from the motors to the wheels of the car, then through the rails to the generator at the
substation. But beca use of the many mechanical joints along these tracks, all of which offer
resistance to the flow of the electricity:, what usually happens is that a portion of the current,
seeking an easier path to the substation, lea ves the rails, passes into the ¡round, and returns
to the substation through the moist earth. lf, in its journey through the ground, the current
passes near buried metal pipe-which offers an easier path for return than does the ground
around it-the current will flow along the metal walls of the pipe to sorne point near the
substation; there it willleave the pipe to flow through the ground back to the rail, and finally
return to the substation generator.

Areas of the pipe where the current is entering are not corroded. Where the current is
leaving the pipe, however, steel is destroyed at the rate of about 20 lb per ampere-year of
current discharged. To combat electrolysis, an insulated metal conductor must be attached
to the pipe where it will remove and return the current to the source, rather than allowing
the current to escape from the pipe wall. Figure 10-13 diagrammatically shows this method.

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Figure 10-12 Stray-Current Corrosion Caused by Electrified Railway Systems



Figure 10-13 Control of Stray-Current Corrosion

Biochemical Corrosion
Certain soil bacteria create chemicals that may result in corrosion. Bacteria! corrosion, or
anaerobic-bacterial corrosion, is not so much a distinct type of corrosion as it is another
cause of electrochemical corrosion. The bacteria cause changes in the physical and chemical
properties ofthe soil to produce active pseudogalvanic cells. The bacteria! action may be one
of removing the protective hydrogen film. Differential aeration plays a major role in this

The only certain way of determining the presence of anaerobic bacteria, the particular
kind of microorganism responsible for this type of corrosion, is to secure a sample of the soil
in the immediate vicinity of the pipe and develop a bacteria! culture from that sample.
Inspection under a microscope will determine definitely whether harmful bacteria are

Stress and Fatigue Corrosion
Stress corrosion is caused from tensile stresses that slowly build up in a corrosive
atmosphere. With a static loading, tensile stresses are developed at the metal surfaces. At
highly stressed points, accelerated corrosion occurs, causing increased tensile stress and
failure when the metal's safe yield is exceeded.

Corrosion fatigue occurs from cyclic loading. In a corrosive atmosphere, altern~te
loadings cause corrosion fatigue substantially below the metal's failure in noncorrosive

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Figure 10-17 Bonding jumpers Installed on Sleeve-Type Coupling


A. 0-Ring Carnegle Sectlon



B. Rolled Spigot Jolnt

Figure 10-18 Bonding Wire for Bell and Spigot Rubber-Gasketed joint

Current Required



For impressed -current cathodic protection to be effective, sufficient current must flow from
the soil to the pipe to maintain a constant voltage difference at the soil-metal interface,
amounting to 0.25 V or more (approximately 0.80-0.85 V between pipe and copper sulfate
electrode in contact with soil). This mínimum voltage requirement has been determined by
experience, but it may be subject to variations at specific sites.

Design of Cathodic Protection Systems
In many situations, cathodic protection for steel pipelines will not be installed until proven
necessary. However, all joints in steel pipe should be electrically bonded and electrical test
stations provided along the pipeline as necessary.

Corrosion Survey
A corrosion survey, including chemical-physical analyses of the soil, must be performed
along the pipeline right-of-way. Sorne of the measurements taken include soil resistivity,
soil pH, and tests for stray currents.

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l. NACE Basic Corrosion Course. NACE,
Houston, Texas (June 1975).

2. Manual on Underground Corrosion. Co-
lumbia Gas System Service Corp., New
York (1952).

3. HERTZBERG, L.B. Suggested Non-tech-
nical Manual on Corrosion for Water
Works Operators.Jour. AWWA, 48:719
(June 1956).

4. Underground Corrosion, NBS Circ. No.
579, (1957).

5. ELIASSEN, R. & LAMB, J.C. III. Mech-
anism of Interna! Corrosion of Water
Pipe. Jour. AWWA, 45:12:1281 (Dec.

6. LANGELIER, W.F. The Analytical Control
of Anticon;osion Water Treatment.Jour.
AWWA, 28:1500 (Oct. 1936).

7. WEIR, P. The Effect of Interna! Pipe
Lining on Water Quality.Jour. AWWA,
32:1547 (Sept. 1940).

8. LINSEY, R.K., & FRANZINI, J .B. Water-
Resources Engineering. McGraw-Hill Book
Co., New York (1979).

9. PEAiWDY, A. W. Control of Pipeline
Corrosion. Natl. Assn. ofCorrosion Engrs.,
Katy, Texas (1967).

The following references are not cited in
the text.
- BARNARD, R.E. A Method ofDetermining

Wall Thickness of Steel Pipe for Under-
ground Service. Jour. A WWA, 29:791
(June 1937) .

... ,-----~==::::..:::::~

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- Corrosion Control in Water Utilities.
Corrosion Control Committee, Cali-
fornia-Nevada AWWA Sec. (1980).

- DA VIS, C. V. ed. Handbook of App/ied Hy-
draulics. McGraw-Hill Book Co., New
York (1969).

- DENISON, l. A. Electrolytic Measurement
of the Corrosiveness of Soils. NBS Res.
Paper RP 918 (1936).

- LOGAN, K.H. ASTM Symposium on
Corrosion Testing Procedures. Chicago
Meetings (Mar. 1937).

- McCOMB, G.B. Pipeline Protection Using
Coal Tar Enamels. St. Louis, Mo. (June
Scorr, G.N. Adjustment of Soil Cor-
rosion Pit Depth Measurements for Size
and Sample. Prod. Bull. 212. American
Petroleum Institute, New York (1933).

- --- A Preliminary Study of the Rate
ofPitting oflron Pipe in Soils. Prod. Bull.
212. American Petroleum Institute, New
York (1933).

- --- API Coating Tests. Prod. Bull.
214. American Petroleum Institute, New
York (1934).

- Steel Plate Engineering Data-Volume 3.
Amer. Iron & Steel lnst. and Steel Plate
Fabricators Assoc., Inc. (1980).

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