UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Electrification Administration

**BULLETIN 1751F-670**

**SUBJECT: [Outside Plant Corrosion Considerations]{.underline}**

**TO:** All Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Seven years from effective date

**OFFICE OF PRIMARY INTEREST:** Outside Plant Branch, Telecommunications
Standards Division

**PREVIOUS INSTRUCTIONS:** This bulletin replaces REA Telecommunications
Engineering & Construction Manual (TE&CM) 670, Corrosion Considerations
in Telecommunications in Outside Plant, Issue 2, dated January 1982.

**FILING INSTRUCTIONS:** Discard REA Telecommunications Engineering &
Construction Manual (TE&CM) 670, Corrosion Considerations in
Telecommunications in Outside Plant, Issue 2, dated January 1982, and
replace it with this bulletin. File with 7 CFR 1751 and on REANET.

**PURPOSE:** This bulletin provides REA borrowers, consulting engineers,
contractors and other interested parties with information concerning the
various aspects of corrosion which should be considered in the
preparation of a system design.

Blaine D. Stockton, Jr. 09/02/93

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Administrator Date

**TABLE OF CONTENTS**

1\. General . . . . . . . . . . . . . . . . . . . . . . . . . 6

2\. Corrosion Theory . . . . . . . . . . . . . . . . . . . . 6

3\. Types of Corrosion . . . . . . . . . . . . . . . . . . . 8

4\. Corrosion Behavior of Metals Used in Outside Plant

Construction . . . . . . . . . . . . . . . . . . . . . . 15

5\. Corrosion Prevention Measures . . . . . . . . . . . . . 19

6\. Environmental Conditions Affecting the Selection of

Cable or Wire Shields . . . . . . . . . . . . . . . . . 23

7\. Soil Measurements . . . . . . . . . . . . . . . . . . . 27

**TABLES & FIGURES**

Table 1 Galvanic Series of Metals in Sea Water . . . . . 10

Table 2 Soil Corrosivity Versus Resistivity . . . . . . . 27

Table 3 Typical Soil Resistivity Values . . . . . . . . . 28

Figure 1 Example of Galvanic Corrosion . . . . . . . . . . 30

Figure 2 Example of Concentration Cell Corrosion . . . . . 31

Figure 3 Example of Stray Current Corrosion . . . . . . . 32

Figure 4 Anchor Rod Corrosion - Galvanic Couple . . . . . 33

Figure 5 Anchor Rod Corrosion - Differential Aeration Cell 34

Figure 6 Coordinated Stray Current Protection . . . . . . 35

Figure 7 Estimated Average Earth Resistivity

Northeast Region . . . . . . . . . . . . . . . . 36

Figure 8 Estimated Average Earth Resistivity

Southeast Region . . . . . . . . . . . . . . . . 37

Figure 9 Estimated Average Earth Resistivity

North Central Region . . . . . . . . . . . . . . 38

Figure 10 Estimated Average Earth Resistivity

Western Region . . . . . . . . . . . . . . . . . 39

Figure 11 Estimated Average Earth Resistivity

Southwest Region . . . . . . . . . . . . . . . . 40

Figure 12 Soil Resistivity Measurement Procedure . . . . . 41

INDEX:

Outside Plant

Corrosion

Telecommunications

**ABBREVIATIONS**

ASTM American Society For Testing And Materials

AWG American Wire Gauge

B Distance between electrodes

cm Centimeter

cm/m Centimeter per meter

dc Direct current

EMF Electromotive Force

m Meter

NOx Nitrogen oxides

ohm·cm Ohm·centimeter

OH- Hydroxyl ion

P Average earth resistivity

pH Potential of hydrogen

R Resistance

REA Rural Electrification Administration

SOx Sulfur oxides

TE&CM Telecommunications Engineering and Construction Manual

Ð Greek Letter, Pi, = 3.1416

**DEFINITIONS**

**[Active]{.underline}:** A state in which a metal tends to corrode
(opposite of passive). In a corrosion cell the active or corroding metal
is the metal which is more negative in potential.

**[Amphoteric Metal]{.underline}:** A metal susceptible to corrosion in
both acidic or alkaline environments.

**[Anode]{.underline}:** The electrode of an electrolytic cell at which
oxidation or corrosion occurs (opposite of cathode). (Electrons flow
away from the anode in the external circuit. It is usually the electrode
where corrosion occurs and metal ions enter solution.)

**[Cathode]{.underline}:** The electrode of an electrolytic cell at
which reduction (and practically no corrosion) occurs (opposite of
anode). (Electrons flow toward the cathode in the external circuit.)

**[Cathodic Polarization]{.underline}:** Polarization of the cathode; a
reduction from the initial potential resulting from current flow effects
at or near the cathode surface. Potential becomes more active (negative)
because of cathodic polarization.

**[Cathodic Protection]{.underline}:** Reduction or elimination of
corrosion by making the metal a cathode by means of an impressed direct
current or attachment to a sacrificial anode.

**[Concentration Cell]{.underline}:** A cell involving an electrolyte
and two identical electrodes, with the potential resulting from
differences in the chemistry of the environments adjacent to the two
electrodes.

**[Differential Aeration Cell]{.underline}:** A concentration cell
caused by differences in oxygen concentration along the surface of a
metal in an electrolyte.

**[Electrolyte]{.underline}:** A substance that dissociates into ions in
solution or when fused, thereby becoming electrically conducting.

**[Electromotive Force Series]{.underline}:** A list of elements
arranged according to their standard electrode potentials within an
electrolyte, with [noble]{.underline} metals such as gold being positive
and [active]{.underline} metals such as zinc being negative.

**[Galvanic Cell]{.underline}:** A cell consisting of two dissimilar
metals in contact with each other and with a common electrolyte
(sometimes refers to two similar metals in contact with each other but
with dissimilar electrolytes; differences can be small and more
specifically defined as a concentration cell).

**[Galvanic Corrosion]{.underline}:** Corrosion associated with the
current resulting from the electrical coupling of dissimilar electrodes
in an electrolyte.

**[Galvanic Series]{.underline}:** A list of metals and alloys arranged
according to their relative corrosion potentials in a given environment.

**[Holiday]{.underline}:** A discontinuity (hole or gap) in a protective
coating.

**[Local Cell]{.underline}:** An electrochemical cell created on a metal
surface because of a difference in potential between adjacent areas on
that surface.

**[Monel]{.underline}:** A corrosion-resistant alloy of nickel, copper,
iron, and manganese.

**[Noble]{.underline}:** The positive (increasingly oxidizing) direction
of electrode potential.

**[Oxidation]{.underline}:** Loss of electrons by a constituent of a
chemical reaction. (Also refers to the corrosion of a metal that is
exposed to an oxidizing gas at elevated temperatures.)

**[pH]{.underline}:** A measure of the acidity or alkalinity of a
solution, numerically equal to 7 for neutral solutions, increasing with
alkalinity and decreasing with increasing acidity.

**[Passive]{.underline}:** The state of the metal surface characterized
by low corrosion rates in a potential region that is strongly oxidizing
for the metal.

**[Pitting]{.underline}:** Corrosion of a metal surface, confined to a
point or small area, that takes the form of cavities.

**[Polarization]{.underline}:** The change from the open-circuit
electrode potential as the result of the passage of current.

**[Reduction]{.underline}:** The gain of electrons by a constituent of a
chemical reaction.

**[Stray Current Corrosion]{.underline}:** The corrosion caused by
electric current from a source external to the intended electrical
circuit, for example, extraneous current in the earth.

**[Stress Corrosion Cracking]{.underline}:** A cracking process that
requires the simultaneous action of a corrodent and sustained tensile
stress. (This excludes corrosion-reduced sections which fail by fast
fracture. It also excludes intercrystalline or transcrystalline
corrosion which can disintegrate an alloy without either applied or
residual stress.)

**1. GENERAL**

**1.1** Corrosion of metals is one of the many problems that has to be
considered by engineers when designing aerial, direct buried, or
underground telecommunications systems. If not controlled, corrosion can
result in costly replacement of facilities. The information and
recommendations in this bulletin are advisory.

**1.2** The Rural Electrification Administration (REA)
Tele-communications Engineering and Construction Manual (TE&CM) 218,
[Plant Annual Cost Data for System Design Purposes]{.underline} (Planned
conversion to REA Bulletin 1751B-230), provides cost data showing the
depreciation rates and maintenance expenses to be applied to the various
plant items in different areas throughout the United States. However, in
some locations, corrosive conditions may exist which can greatly
increase maintenance expense and cause the early replacement of some
plant items. This can result in a severe economic burden on the
operating telecommunications company.

**1.3** Corrosive atmospheric conditions may exist over an entire
service area of a telecommunications system, or may be confined to
relatively small areas within the total service area. Rural environments
which are usually free from industrial contaminants, represent most of
the total land area of the United States. Exposed metals are expected to
provide a relatively long service life when installed in rural areas.
For example, galvanized steel has long been used in the
telecommunications and electric power utilities and to learn of
conductors and strand still in use after 40 years of service is not
uncommon. The life expectancy of galvanized steel will vary with the
amounts of contaminants present in the atmosphere. In severely corrosive
environments, Class A galvanized coatings on steel conductors and strand
may last for as little as 3 years.

**1.4** The processes of corrosion are not always simple and their
effects are not easily predicted. In many cases the effects of one type
of corrosion will mask the effects of another type of corrosion so that
neither is recognized. The following sections discuss the theory of
corrosion and the application of this theory to practical situations in
telecommunications systems.

**1.5** A comprehensive set of definitions of terms relating to
corrosion can be found in American Society For Testing And Materials
(ASTM) G 15, [Standard Terminology Relating to Corrosion and Corrosion
Testing]{.underline}.

**2. CORROSION THEORY**

**2.1** Corrosion may follow any one of several general patterns.
Basically stated, corrosion results from the instability of a
construction material with the environment in which the material is
placed. All materials, both organic and inorganic, can react with their
environments and may eventually lose their usefulness for a given
application. Some nonmetals tend to decompose, usually as the result of
chemical interaction with some component of their environment.

**2.2** Corrosion is the result of electrochemical, chemical, or
biological reaction between a metal and its surroundings. Many of the
destructive effects of corrosion are due to electrochemical reaction.
The basic requirements for corrosion to occur in a given corrosion cell
are as follows:

a\. An anode and cathode must be present to form a cell;

b\. An electrically conducting medium (electrolyte)

contacting both the anode and cathode must be present;

c\. A metallic connection between the anode and cathode

must be present; and

d\. A direct current (dc) potential between the anode and

cathode causing current to flow must be present.

**2.2.1** The anode is that metallic part of a corrosion cell from which
metal ions separate from the metal and flow into the surrounding
electrolyte. The metal is consumed (corroded) at the anode by this
action. The cathode is that metallic part of a corrosion cell to which
positively charged ions in the electrolyte flow and react with the
negatively charged electrons in the metal. The cathode metal in an
individual corrosion cell does not normally corrode, except when
amphoteric metals are involved. Amphoteric metals are metals that
corrode in either acidic or alkaline environments. If such metals are
made excessively cathodic, the chemical reaction at the cathode can make
the environment extremely alkaline and can cause corrosion to occur.
Aluminum and lead are examples of amphoteric metals.

**2.2.2** The anodes and cathodes can be adjacent (local cells) or far
apart. The current can be self-generated or it can be impressed on the
system from an outside source. The conducting medium can be water, wet
soil or any solution which conducts electricity.

**2.3** The anode corrodes. Metal at the anode chemically reacts with
the electrolyte causing minute pieces of metal to dissolve and flow into
the electrolyte. In the process the minute pieces of metal lose
negatively charged electrons to the anode metal and the minute pieces of
metal enter into the electrolyte as positively charged ions. The loss of
electrons by the metal ion at the anode is referred to as
[oxidation]{.underline}. The excess electrons left by the metal ions at
the anode flow through the metal or through an external metallic path to
the cathode. At the cathode-electrolyte interface electrons leave the
metal and chemically react with positively charged ions in the
electrolyte. The electron gain by the positively charged ions in the
electrolyte at the cathode is referred to as [reduction]{.underline}.

**2.3.1** Since the generally accepted concept for the flow of electric
current is opposite to the direction of electron flow, current flow in a
corrosion cell is from anode to cathode in the electrolyte and from
cathode to anode in the metal or metallic path.

**2.3.2** The quantity of metal lost by the corrosion process is
directly proportional to the amount of direct current which flows
through the corrosion cell and early in the 19th century was
quantitatively shown by Michael Faraday to be in accordance with the
following equation:

W = k \* I \* t

Where:

W = Weight Loss in grams

k = Electrochemical Equivalent in grams/coulombs

I = Current in amperes

t = Time in seconds

It can be seen that for a given amount of current over a given period of
time, the electrochemical equivalent (k) is the variable which
determines the actual weight loss of the metal or material. Each metal
or material has its own electrochemical equivalent which is a natural
characteristic of that metal or material. For example, one ampere of
current flowing for one year will corrode about 9 kilograms of steel.

**2.4** When the positive metal ions leave the metal anode (corrosion
sites), the remaining electrons flow to cathodic sites where they are
discharged during the cathodic reactions. The principal cathodic
reactions include hydrogen ion reduction to form hydrogen gas, oxygen
reduction to form hydroxyl ions (OH-), and reduction of water to
hydroxide ions. Cathodic polarization occurs when the reaction products
cover the cathodic sites. The rate of corrosion in soils is often
controlled by the rate at which the reactants are supplied to, or the
reaction products are removed from the cathodic sites (cathodic
depolarization). Similarly, polarization may occur at the anode due to
concentration of metal ions although this phenomena is of lesser
importance in the soil. In the presence of moisture, the metal ions
react with hydroxide ions to form hydroxides which in turn react with
oxygen to form oxides thus producing both cathodic and anodic
depolarization.

**3.** **[TYPES OF CORROSION]{.underline}**

**3.1** **[Galvanic Corrosion]{.underline}**

**3.1.1** The corrosion resulting from current generated from
differences in potential of metals in an electrolyte is referred to as
galvanic corrosion.

**3.1.2** Dissimilar metals, coupled electrically and placed in a
conductive solution, satisfy the requirements for the basic corrosion
cell. Two electrically coupled metals, in the presence of an
electrolyte, constitute a special corrosion cell, called a galvanic
cell, wherein one of the metals becomes cathodic and the other becomes
anodic and corrodes. If the potential difference between the metals is
small, corrosion usually will be slow. Conversely, if the potential
difference between the metals is large, corrosion will usually be rapid.

**3.1.3** During the galvanic corrosion process, an electric current
flows as a result of the difference in the electrode potential of the
two metals coupled electrically. The magnitude of the current determines
the degree of corrosion of the more vulnerable material over and above
the material\'s natural rate of deterioration alone in the same
electrolyte. The quantity of galvanic corrosion is defined as the
corrosion resulting from the electrochemical reaction. In the past
[galvanic]{.underline} has been erroneously defined as the total
corrosion including normal decomposition.

**3.1.4** An example of [Galvanic Corrosion]{.underline} is shown in
Figure 1. The galvanized steel 3-bolt clamp and the galvanized steel
anchor rod act as anodes and the copper-steel guy strand acts as the
cathode. The corrosion of the anchor rod and the 3-bolt clamp will be
increased over the normal rate of corrosion by galvanic effect, unless
the hardware is provided with some coating to prevent air and moisture
from penetrating to the dissimilar metal connection.

**3.1.5** From the galvanic corrosion behavior of metals and alloys,
metals can be arranged in a series which will indicate their general
tendency to form galvanic cells. Such a series can be used to predict
the probable direction and the extent of galvanic effects. Table 1 is
such a galvanic series and includes most metals used in outside plant
environments. The table is qualitative and is intended only to indicate
trends. The arrangement of the metals in the series is based on actual
corrosion testing experience within sea water. Since more observations
of galvanic behavior have been made in sea water than in any other
single environment, the galvanic series listed in Table 1 should be used
as a first approximation of the probable direction and extent of the
galvanic effects in other environments in the absence of data more
directly applicable to such environments. The only way of predicting the
direction and extent of the galvanic effects on a material in a specific
environment is to perform potential measurements in the specific
environment. The galvanic series listed in Table 1 should not be
confused with the Electromotive Force (EMF) Series. In the EMF series
the metals are arranged according to the potentials developed when
immersed in a solution of their own ions of prescribed composition as to
strength and chemical nature. The EMF series is of theoretical value and
cannot be relied upon to predict the potential differences between
metals in outside plant operating conditions.

TABLE 1

Galvanic Series of Metals in Sea Water

ANODIC END (Most Active)

Magnesium\
Magnesium Alloys

Zinc

Aluminum

Cadmium

Steel or Iron\
Cast Iron

18:8 Stainless Steel (active)\
18:8:3 Stainless Steel (active)

Lead-Tin Solders\
Lead\
Tin

Nickel (active)\
Inconel (active)\
Hastelloy (active)

Brass\
Copper\
Bronze\
Copper:Nickel Alloys\
Monel

Silver Solder

Nickel (passive)\
Inconel (passive)

18:8 Stainless Steel (passive)\
18:8:3 Stainless Steel (passive)\
Hastelloy C (passive)

Silver

Graphite\
Gold\
Platinum

CATHODIC END (Least Active)

**3.1.6** Some of the metals in Table 1 are grouped together to indicate
that the grouped members have little tendency to produce galvanic
corrosion on each other and, therefore, are relatively safe to use in
contact with one another. However, the coupling of metals widely
separated in the series can result in galvanic corrosion of the metal
higher in the list, i.e., the one most anodic. Usually the farther apart
the two metals are in the series the greater the corrosive tendency. The
relative position of a metal within a group sometimes changes with the
environment, but rarely does a metal change from group to group. Note
that stainless steel alloys 18:8 and 18:8:3 appear in two places in
Table 1. They frequently change positions as indicated in the table
depending upon the corrosiveness of the sea water. These two alloys
demonstrate relatively good resistance to corrosion in their passive,
more noble, condition.

**3.1.7** The surface areas of the metals forming a galvanic cell are a
major consideration in the rate of corrosion. A small piece of aluminum
coupled with a large piece of copper will corrode much faster than a
piece of aluminum equal in area to the copper. If circumstances dictate
that dissimilar metals be in contact with one another, the area of the
active (anodic) metal should be large with respect to the area of the
passive (cathodic) metal.

**3.2** **[Concentration Cell Corrosion]{.underline}**

**3.2.1** Another source of corrosion is ion concentration variance
within an electrolyte surrounding a single piece of metal. Usually this
form of corrosion originates in or around crevices. Early detection and
prevention depend largely upon knowledge of the corrosion mechanism.

**3.2.2** In general, the lower the metal ion concentration, the greater
the tendency of the metal to dissolve, i.e., the lower the metal ion
concentration, the higher is its solution potential. When a single metal
surface is exposed to a corrosive environment which is not uniform in
its metallic salt concentration from point to point, corrosion results
in areas of lower concentration. Differences in oxygen concentration at
a metal surface within a solution also develop different potentials over
the points of contact with metallic surfaces. In both cases, the zone of
greatest attack will be in the area of the more dilute ion or oxygen
concentration. This explains the rapid metal destruction associated with
lap joints, crevices, or deposits that interfere with the flow of
electrolyte over the entire surface of the metal.

**3.2.3** As mentioned in Paragraph 3.2.2, differences in oxygen
concentration on a metal surface will cause voltage differences to arise
and thus accelerate the rate of corrosive attack. These oxygen
concentration cells are typically associated with crevices or deposits
that hinder diffusion of oxygen into the solution under the deposit. The
lower concentration of oxygen creates an anode fostering corrosion which
can eventually weaken the structure. With some alloys the effect of the
oxygen concentration cell is exacerbated by electrolytic cells formed
between the relatively small areas of activated alloy (anode) within
crevices and large areas of passive alloy (cathode) around crevices.
These cells are similar to, but generally more destructive than, normal
oxygen concentration cells because corrosion activity is concentrated in
small areas. This type of corrosion is most likely to be found in
aluminum and stainless steel, although any metal which has been treated
to form a protective coating may also be affected because of the
possibility for holidays or aberrations in the coatings where corrosion
may concentrate.

**3.2.4** The concentration cell corrosion depicted in Figure 2 shows
one cause for failure of corrosion-resistant steel (stainless steel)
cable suspension strand in industrial and marine atmospheres. Dirt and
soot from an industrial area or salt deposits in a marine atmosphere can
settle upon the surface, enter into the interstices of the strand and
retain moisture. A differential aeration cell is set up and the steel
under the deposit becomes active (anodic) to the passive steel
(cathodic) on the outer surface of the strand. Pitting of the stainless
steel on the inside of the strand under the deposits will result, which
may lead to stress corrosion cracking. Galvanized steel strand on the
other hand will behave quite differently under similar conditions. The
zinc coating on the galvanized steel strand will protect the steel until
the zinc coating is completely consumed. The outside surfaces of the
strand can show extensive corrosion while the inner surfaces of the
strand are relatively unaffected.

**3.2.5** Some metals form tight protective oxide films that normally
resist corrosion. If the film is damaged, the film will repair itself if
sufficient oxygen is present to oxidize the base metal. In the absence
of sufficient oxygen to keep the oxide film in repair, the metal under
the damaged film becomes active relative to the film protected surfaces
and, as in the case of the oxygen cell, corrosion pitting will occur
under the deposit. Similarly, concentration cell corrosion may occur on
metal surfaces which are covered with nonmetallic materials. If
manufacturing voids, cracks, scratches, or other accidental openings in
a protective coating occur, moisture can enter under the coating and
cause a corrosion site. Severe pitting may result from local corrosion
with the net damage more severe than if the entire metal area was
exposed to the elements.

**3.2.6** Dissimilar surface conditions on a metallic surface such as
scratches or other abrasions and nonhomogeneous soil backfills can cause
galvanic corrosion. For example, wrench abrasions on a metal surface
will be anodic to the area around it and begin to corrode if there is an
electrolyte. Also dissimilar soils in a backfill will induce galvanic
corrosion. Where rocks and clay in a sand matrix are in contact with an
exposed metal surface, pitting of the metal will occur from galvanic
action.

**3.3** **[Bacteriological Corrosion]{.underline}**

**3.3.1** Certain types of bacteria can establish conditions on the
surface of underground metallic structures which will result in more
rapid corrosion by existing cells. The most significant type of bacteria
which can cause corrosion is the [anaerobic]{.underline} type which
means that the bacteria thrive in the absence of oxygen. The anaerobic
bacterium which is the most common problem-causing corrosion bacterium
is known as [Desulfovibrio Desulfuricans]{.underline}. This form of
bacteria reduces any sulfates present at the underground metal surface
to produce hydrogen sulfide and consumes hydrogen in the process. At the
metallic surface, hydrogen is consumed at the cathodic surfaces of the
existing corrosion cells. This intensifies the action of the corrosion
cell by depolarization. From the above it is apparent that the bacteria
do not themselves attack the metal, but do result in the intensification
of existing corrosion cells.

**3.3.2** Anaerobic bacteria are likely to be found in heavy, dense,
water logged soils where oxygen content is at a minimum. They can exist,
however, under less favorable conditions or under a material coating
which locally restricts the oxygen content.

**3.3.3** Bacteria require organic material as a food supply. This food
supply is normally available to some degree in the earth. However, if a
major food supply is combined with a restricted oxygen content, the rate
of corrosion can be intensified. An example of this action is a piece of
wood lying against an underground metallic structure.

**3.4** **[Stray Current Corrosion]{.underline}**

**3.4.1** Stray current corrosion is due to direct currents of external
origin leaving a metal. Since corrosion is electrochemical in nature,
metals ordinarily will corrode whenever current leaves the metal to pass
into the electrolyte, regardless of the current source. Stray current
corrosion can be caused by [dynamic]{.underline} stray current and
[static]{.underline} stray current.

**3.4.1.1** Dynamic stray currents are currents which change with time.
Sources of dynamic stray current can cause changes in the areas where
current enters and leaves the structure and can cause reversal of
current flow on a structure in some instances. This all results in
changes in the location of the anodic and cathodic areas.

**3.4.1.2** Static stray currents are currents which are from a steady
state external dc voltage source which results in fixed anodic and
cathodic areas on the affected structure with a relatively constant
current flow on the structure.

**3.4.1.3** Static stray currents tend to be more corrosive than dynamic
stray currents per ampere of current leaving the underground structure
and entering the environment. This is because the variable
characteristics of dynamic stray currents, particularly where current
reversals occur, cause them to take on some of the characteristics of
alternating current which is relatively an insignificant source of
corrosion. Therefore, the alternating current characteristic of dynamic
stray current tends to reduce its corrosive impact to some degree.
However, dynamic stray currents can cause serious corrosion problems
where large stray currents are encountered.

**3.4.2** Stray current corrosion can be either man-made or natural.
Some of the man-made sources of stray direct current are as follows:

a\. Variable speed drives in transit systems;

b\. Load-carrying devices;

c\. Mining operations;

d\. Welding operations; and

e\. High voltage direct current electric power

transmission systems.

The natural source of stray current is referred as
[telluric]{.underline} or earth current of magnetic origin. These are
direct currents in the earth\'s crust which result from the variations
in the earth\'s magnetic field which in turn result from variations in
solar activity. The telluric currents are dynamic in nature because the
anodic and cathodic areas on the structure are constantly changing as a
result of reversals in the direction of current flow on the structure.
The telluric currents usually affect long structures such as pipelines
and tend to cause more corrosion problems in some parts of the world
other than the United States.

**3.4.3** Stray current corrosion was noted years ago in cities where
direct current was used widely in industrial applications. In the rural
areas lead sheathed telecommunications cables buried by
telecommunications companies have been corroded by cathodic protection
applied to cross-country pipelines to prevent corrosion of the
pipelines.

**3.4.4** In pipeline cathodic protection, a negative potential is
impressed on the pipeline which causes electrical earth currents to flow
to the pipeline, protecting it against corrosion. Since the conductivity
of most metals exceeds the conductivity of average soils, buried
metallic pipes or cables act as low resistance paths and tend to collect
stray earth currents which may be present in the surrounding earth
electrolyte. At the point where the stray currents enter these auxiliary
conductors the earth becomes anodic and the pipe or cable becomes
cathodic. But where the currents leave the auxiliary conductor (a pipe
or cable) to enter the cathodically protected pipeline (See Figure 3),
the metallic auxiliary conductor becomes anodic, and the resulting
localized corrosion can be quite severe.

**3.4.5** A comparison of stray current corrosion to galvanic corrosion
reveals several differences. The damage caused where the direct current
from an external source leaves a grounded metal is independent of oxygen
concentration of the surrounding environment. The flow of stray currents
usually develops sufficient voltage to force hydrogen discharge so
corrosion is not limited to the oxygen reduction reaction. Likewise,
stray current corrosion is not affected by deposits of corrosion
products or protective coatings unless the coatings are perfect
insulators. Defects in the coatings can actually result in more severe
localized corrosion since the current is concentrated at these sites.

**4. CORROSION BEHAVIOR OF METALS USED IN OUTSIDE PLANT**

**CONSTRUCTION**

**4.1** **[Atmospheric Environments]{.underline}**

**4.1.1** Corrosive atmospheric environments have caused REA to become
increasingly aware of the need for special attention in the choice of
materials and construction practices. There are areas of the United
States where plant maintenance and replacement costs are excessively
high because of the corrosive nature of the atmospheres. The atmospheres
are grouped into three general classifications which are industrial,
marine, and rural.

**4.1.2** Industrial atmospheres are among the most corrosive. The
atmosphere around industrial plants may contain sulfur oxides and carbon
dioxides in relatively high concentrations. Sulfur oxide and carbon
dioxide both form acid films on exposed metals and, since most
pollutants present in the atmosphere attract moisture, atmospheres high
in these contaminants usually are very corrosive. Long range transport
of industrial pollutants is responsible for acid deposition over large
areas of the United States.

**4.1.3** Marine atmospheres are corrosive because of the sea air and
because of the industry usually located in those areas. The depth of
these coastal areas vary from approximately three kilometers to as much
as thirty kilometers from the ocean. In some areas, such as along the
Gulf Coast of Louisiana and Texas, typical marine corrosive atmospheres
exist as far inland as 160 kilometers due to a combination of salt
atmosphere and industrial fumes. Sea air contains chlorides and may
contain traces of sulfur, carbon dioxide, and other components that,
over a period of time, can become concentrated on metal surfaces.
Furthermore, in areas immediately adjacent to the shore, the salt spray
from the ocean thrown up by strong breezes contributes to the buildup of
sea salt deposits on metals which keeps them wet a good portion of the
time.

**4.1.4** Years ago, exposed metals were expected to obtain their
longest possible service life in rural atmospheres which represented
approximately 80 percent of the United States. However, industrial
pollutants which consist of sulfur oxides (SOx) and nitrogen oxides
(NOx) are subject to chemical transformation in the atmosphere to form
acids. These pollutants, which can be carried hundreds and sometimes
thousands of kilometers cause acid deposition over wide areas of the
United States. Consequently rural atmospheres can become corrosive.

**4.1.5** Galvanized steel is widely used in the REA program for both
hardware items and cable messenger strand. In industrial atmospheres the
corrosion products formed on galvanized steel are zinc sulfate and zinc
carbonate with zinc sulfate predominating. The zinc sulfate is soluble
and is washed off by rain and blown off by wind. The zinc carbonate is
removed chiefly by the erosion action of wind driven dirt and dust. This
loss of corrosion products results in an accelerated deterioration of
zinc in industrial atmospheres. The chlorides present in both industrial
and marine atmospheres will attack the zinc coating on galvanized steel
to form a zinc chloride. This zinc chloride compound will attract and
retain moisture and then will react with other elements in the air to
form zinc oxychloride and zinc carbonate. The resulting coating is
porous and powdery and can be washed off quite easily. Because of the
moisture-laden winds along the coast, corrosion products can build up
faster than the rain can wash them off. Since the material remains wet
most of the time, the corrosion will progress until the zinc is
completely changed to compounds leaving base steel unprotected. The
chemical composition of the base steel will determine the integrity of
the oxide coating formed after the zinc has been sacrificed. Carbon
steel will develop an oxide coating which will be porous and flaky. The
oxide is not soluble but may be readily dislodged. Thus the carbon steel
will continue to corrode. The oxide coating on the low alloy steel will
be less porous and more adherent than the oxide on the carbon steel. The
rate of diffusion of corrosive elements to the base metal will be low;
consequently, the low alloy steel will be more corrosion resistant.

**4.1.6** Copper and copper alloys, as well as copper-covered steel, are
used to a great extent in the REA telecommunications program for
conductors, shielding, and a variety of hardware items. Copper is the
most sensitive to the sulfur oxides present in industrial atmospheres.
New copper exposed to an industrial atmosphere will form a copper oxide
coating which is not porous but is quite brittle. The sulfur dioxide and
moisture in the air will combine with the copper oxide to convert part
of it to basic copper sulfate. Copper corrosion products provide the
familiar green [patina]{.underline} appearance on copper. The recognized
low rate of copper corrosion in the atmosphere is due to the protective
patina. As long as the patina is not disturbed, the corrosion reaction
will continue, but at a very slow rate. However, if the cuprous oxide
underlayer is disturbed, fresh copper is exposed anew to the accelerated
corrosion process.

**4.1.7** The somewhat protective copper coating on a copper-covered
steel conductor can be dislodged by vibration or abrasion under tension.
Copper and copper-covered steel will probably corrode at a faster rate
in a marine atmosphere than in an industrial environment because of the
presence of chlorides and traces of sulfur. The surfaces of the metals
are likely to remain wet a large share of the time because of salt
spray. The copper will react with oxygen to produce a layer of basic
copper oxide which is converted to basic copper sulfate and basic copper
chloride in the salt air. The same copper oxide would be present as in
the industrial environment but its combination with the chloride would
cause the corrosion reaction to be stronger. The coating would probably
be more porous which would hold more moisture and would allow more air
to diffuse through to the base metal to continue the corrosion of the
copper. Because there is a relatively thin layer of copper over the base
steel in the copper-covered steel conductors, the tendency for the steel
to be exposed to the atmosphere is very real. Once the steel is exposed
to the atmosphere, a galvanic corrosion cell is formed with the steel
anodic to the copper. Since the exposed area of the steel (anode) is
small compared to the area of the copper (cathode) on the surface of the
wire, the galvanic cell that results is active and the corrosion of the
steel will be rapid.

**4.1.8** Certain grades of stainless steel have been used for cable
suspension and guy strands in industrial and marine areas to provide
additional service life over that offered from Class C galvanized steel
strand. Corrosion resistant steel forms a protective oxide coating to
corrosive elements in the industrial and marine atmospheres by
preventing moisture penetration to the base metal. However, when dirt
and soot from industrial atmospheres and salt deposits from marine
atmospheres settle upon the surface and into the interstices of the
strand, an oxygen concentration cell is created. The corrosion resulting
from the concentration cell and the subsequent stress corrosion cracking
of the strand is quite serious. Cable suspension strand made of
stainless steel can appear perfect but fail because of corrosion pitting
inside the strand that is not apparent from visual inspection. Some
outside plant engineers favor the use of Class C galvanized steel strand
over the corrosion resistant steel because of the oxygen concentration,
stress corrosion phenomenon. Although the Class C galvanized steel
strand has a shortened life span in a corrosive atmosphere, replacement
can be planned whereas the failure of the corrosion resistant steel
strand can not be predicted with certainty. The serious effects of
premature or unexpected failure of a suspension strand carrying
telecommunications cable should be a significant factor in selecting
strand.

**4.1.9** In addition to the three basic types of atmospheres found
within the United States, there may exist commercial operations that
could change the characteristics of the atmosphere in the environment
which could result in accelerated corrosion of telecommunications
outside plant products. Such operations exist in areas of high air
pollution and include drilling or mining installations, chemical or
fertilizer plants, burning slag dumps, sour gas field, etc. Also in hot
humid areas, vegetation such as Spanish Moss can absorb moisture and in
contact with a metallic structure can create a galvanic corrosion cell
which could lead to serious corrosion. All of the above factors must be
considered when evaluating the possibility of atmospheric corrosion of
outside plant facilities.

**4.2** **[Underground Environments]{.underline}**

**4.2.1** Underground equipment in the REA telecommunications program
consists of anchors, anchor rods, ground rods, buried plant terminal
housings, and buried cables and wires. A majority of currently installed
telecommunications cable and wire plant is buried directly in the soil.
The various shielding and armoring materials, when exposed to the
environment of different soils, are subjected to all of the corrosion
mechanisms. These corrosion mechanisms include the effects of dissimilar
metals (galvanic couples), soil conditions, differential aeration, stray
currents, and bacteria. From past experience the corrosion of anchor
rods has proven to be the most important aspect of outside plant
construction. The corrosion of galvanized steel anchor rods may be
caused by dissimilar metal affects (galvanic couples), soil conditions
(differential salt concentration or differential aeration) or by stray
currents.

**4.2.2** The general practice has been to use galvanized or
copper-covered steel ground rods for grounding of metallic facilities or
structures. Also a requirement on joint use construction, where the
power system is of the multigrounded neutral type, is to bond the guy
strand to the neutral either directly or through the cable messenger
(suspension strand). The zinc and iron in a galvanized steel anchor rod,
when buried in conducting earth, tend to develop a dc voltage that is
more negative (anodic) than that of buried copper (cathodic). When the
buried steel and copper are connected together by the power system
neutral, as shown in Figure 4, currents will begin to flow through the
completed circuit from the steel through the conducting earth to the
copper on the ground rod and the copper pole-butt grounds. Corrosion
associated with such current flow is an example of galvanic corrosion.

**4.2.2.1** Polarizing effects occur at the steel and copper surfaces in
most soils and interfere with the flow of current so that it soon
decreases to the point where no serious damage results. In the more
aggressive soils, however, the current may continue to flow so that the
steel is corroded at an excessive rate. Excessive underground corrosion
is generally noticed on anchor rods first, and in many cases anchor rod
corrosion is believed to be the entire problem, but other buried
structures may be affected. Any buried structure connected to the power
system neutral may be corroded as a result of galvanic action. Another
example of corrosion due to galvanic action is the connection of
telephone system copper covered ground rods to steel anchors and anchors
rods.

**4.2.2.2** Ungalvanized anchors are considered an important contributor
to corrosion of steel anchor rods and may be the major contributor where
the corrosion is not associated with currents in guys due to copper
grounds. The mill scale on hot-rolled steel and oxide films on other
steel is cathodic to galvanized and bright surfaces in much the same way
as copper causes corrosion. Galvanizing of anchors requires prior
removal of all oxide films, and the galvanizing offers additional
corrosion protection to the anchor rod as well as to the anchor.

**4.2.3** Differential aeration corrosion of an anchor rod can occur,
regardless of the material used, even though it is electrically isolated
from the power system\'s multigrounded neutral. A differential aeration
corrosion cell (See Figure 5) may begin on the surface of the anchor rod
where the lower portion of the rod, in contact with a soil low in
oxygen, becomes anodic to the upper portion of the rod which is in
contact with a soil having higher oxygen contents. Most corrosion
failures of anchor rods occur within 15 centimeters of the anchor, and
other structures also likely to have the greatest damage deep in the
ground where moisture is present and oxygen excluded.

**4.2.4** Stray currents from cathodic protection applied to
cross-country pipelines have caused REA borrowers a number of corrosion
problems. Stray currents from these cathodically protected pipeline
systems can cause rapid corrosion of anchor rods, ground rods, metal
sheathed cables, and other metallic structures placed underground. The
first signs of stray current corrosion damage is likely to be failure of
one or more anchor rods in low or wet locations. In the majority of
cases this anchor rod failure can be noted from slack guys. This initial
corrosion signal should initiate an investigation into the problem
before further damage occurs.

**5. CORROSION PREVENTION MEASURES**

**5.1** **[Atmospheric Environment]{.underline}**

**5.1.1** The atmospheric environment that exists in the area of
proposed construction should be determined by the engineer during the
early stages in the design of the telecommunications system. Corrosive
atmospheric conditions may exist over an entire service area of a
telecommunications system or may be confined to relatively small areas
within the total service area. In heavily populated industrial areas
that are spread over several square kilometers, the entire project area
may be corrosive and require special consideration. On the other hand,
in an area that is generally classified rural, a commercial operation
could affect the service life of just a few kilometers of plant. One of
the most valuable sources of information for the engineer is the
historical performance of the various materials that are being replaced
in a particular area. Obviously, if the material in the plant being
replaced has been in service for 20 to 25 years there is no reason for
serious concern.. When it is not possible to determine the service life
of the existing plant because of a lack of records or personnel who
remember when a certain section of line was built, the local electric
utility may be able to furnish the necessary information. Many of the
materials and the problems in the use of these materials are common to
both the telecommunications and the electric utilities. Also a survey of
the commercial and industrial activities in the service area should
pinpoint those areas where special corrosion preventive measures should
be taken.

**5.1.2** Once the type of atmosphere in the proposed area of
construction has been identified, a determination of materials best
suited for use in that particular environment should be made. For best
results consider areas immediately adjacent to the east coast and to the
northwest coast of the United States to be marine, whereas the areas
within 30 kilometers of the Gulf Coast of Louisiana and Texas and the
coast of California south of San Francisco would be classified as
marine-industrial. In the Gulf Coast areas of Louisiana and Texas, the
belt from 30 kilometers to 160 kilometers from the coast would be
classified as industrial because of the numerous oil wells, chemical
processing, and oil and gas refinery operations.

**5.1.3** Galvanized steel has been widely used for many years in the
telecommunications industry for hardware items. Generally speaking,
hardware items such as 3-bolt clamps, cable suspension clamps, machine
bolts, thimble-eye bolts, etc., are available only in galvanized steel.
However, in the past several years aluminum covered steel and aluminum
alloy hardware items have been developed for use in outside plant
construction. In severely corrosive industrial areas , where galvanized
steel would have a shortened service life, the use of aluminum covered
steel or aluminum alloy hardware is recommended where available. If the
use of aluminum covered steel or aluminum alloy hardware is not used in
these situations, galvanized steel hardware made from low alloy steel
should be used in preference to carbon steel.

**5.1.4** One of the most important considerations in material selection
for a given environment is to choose metals that will give good
performance when fastened together. Metals that are widely separated in
the galvanic series (Table 1) should not be placed in contact with one
another. If the use of dissimilar metals cannot be avoided, then
remember that the least damage occurs when the ratio of exposed area of
noble (protected) to the exposed area of less noble (corroding) metal
surfaces is kept as small as possible. Also, stranded wire used for
bonding purposes should be tinned copper to minimize corrosion from
joining dissimilar metals. When grounding wire is required to contact
the soil, the bare copper should be insulated. Whenever possible a
connection of dissimilar metals should be coated. When properly applied,
the coating materials will prevent moisture and air from contacting the
dissimilar metal connection and thus prevent galvanic corrosion at the
jointing interface.

**5.1.5** Galvanized steel has been used for many years in the
telecommunications industry for cable messenger strand and for guy
strand. The material is not adequate for applications in the more
corrosive industrial and marine atmospheres. Experience to date
indicates that aluminum-covered steel messenger strand and guy strand
should be used in areas high in sulfur content, such severely corrosive
industrial atmospheres and near sulfur mining activities.

**5.1.6** Polyethylene jacketed and insulated facilities are expected to
experience good performance characteristics in all types of atmospheres.
Theses products should be installed in areas that are highly corrosive.

**5.2** **[Underground Environments]{.underline}**

**5.2.1** Corrosion of underground telecommunications systems is the
most difficult to prevent because of the many factors that singly, or in
combination, affect the course of the electro-chemical reaction. The
major problem in corrosion prevention is to decide the corrective
measures that will give satisfactory results at the least cost. Care
must be taken to insure that the corrective measures taken to cure the
corrosion does not reduce the intended performance of the
telecommunications system. The important need is to adequately recognize
the processes that are happening or can happen and the alternatives
available, so that future corrosion problems can be minimized. There
should be no need to reengineer for corrosion each time that a system is
designed, constructed, or modified.

**5.2.2** The guidelines set forth in the following paragraphs are
intended to minimize underground corrosion of buried metal that is
connected to or a part of the electrical grounding system and to reduce
the difficulty and the cost of applying cathodic protection where needed
for further reducing the amount of underground corrosion.

**5.2.2.1** Galvanic corrosion, caused by connecting dissimilar metals
underground, has been the most prevalent and most serious type of
corrosion experienced by the telecommunications and electric utility
industries. In order to eliminate or to minimize galvanic corrosion of
galvanized anchor rods, plumbing and well casings, and other metals
connected to the grounding system, REA suggests that grounding of
electrical protection apparatus on telecommunications systems follow the
guidelines contained in this bulletin.

**5.2.2.2** Where a driven ground rod is required for electrical
protection (grounding) of the subscriber\'s premise in buried plant
construction, a galvanized steel ground rod should be used.

**5.2.2.3** On pole line construction, where a driven ground rod is
required for electrical protection purposes and is to be interconnected
with the guy and anchor assemblies, a galvanized steel ground rod should
be used. In the event the ground rods are not interconnected with the
guy and anchor assemblies, a copper-covered steel ground rod may be
used.

**5.2.2.4** Where joint use construction is contemplated with a
multigrounded neutral power system inquiries should be made of the
electric company as to its history of underground corrosion. A
corrective program may have already been established. If so, the
telecommunications system should follow a similar program. If there has
been no experience of underground corrosion on the electric system,
standard telecommunications construction practices can probably be
followed with satisfactory results.

**5.2.2.5** The use of galvanized anchors, in addition to the galvanized
anchor rods, will further reduce the rate of corrosion. A substantial
amount of protection against corrosion can be achieved at very little
increase in cost. This is especially important in areas of low earth
resistivity.

**5.2.2.6** Stray current corrosion, resulting from cathodic protection,
can be reduced in underground telecommunications plant by either
electrically insulating the telecommunications facilities from the stray
currents or installing a mitigation bond between the telecommunications
facilities and the foreign structure being cathodically protected.
Usually the anchor rods and anchors can more easily and more
economically be protected by insulating them from the dc source; however
a mitigation bond can easily be provided between the dc source and the
multigrounded neutral. If stray current corrosion is being experienced
on buried metallic sheathed cables, the best method for reducing its
effects probably will be to install a mitigation bond.

**5.2.2.7** When new construction is being contemplated in the vicinity
of a buried pipe system, municipal water system, or a gas utility
system, a determination should be made as to the corrosion protection
measures being employed between the telecommunications facilities and
the foreign structure being cathodically protected. Every effort should
be made to outline a program with these companies that will assure the
telecommunications facilities of the maximum practical corrosion
protection from the effects of these cathodically protected foreign
structures. Figure 6 shows an example of a coordinated effort to arrest
the corrosive effects on the buried cable of the stray currents
generated by the cathodic protection applied to the pipeline. A
mitigation bond was placed between the buried metal shielded cable and
the cathodically protected pipeline to provide a low resistance path
back to the rectifier station for the stray currents.

**6. ENVIRONMENTAL CONDITIONS AFFECTING THE SELECTION OF**

**CABLE OR WIRE SHIELDS**

**6.1** Many factors, such as construction personnel, equipment, rock,
etc., can damage the cable or wire sheath during construction.
Manufacturing defects, lightning, and rodents can also contribute to
sheath damage. A greater incidence of sheath damage is experienced in
gopher areas and exposure of the metallic shield is more prevalent.
Filled cable and wire designs and good construction practices have
substantially reduced sheath damage, but are not likely to eliminate it
completely. The proper selection of shielded cable or wire for use in
problem areas will provide maximum corrosion protection for buried cable
or wire.

**6.2** The evaluation of the corrosivity of a soil is complex. Some of
the soil properties that affect corrosion are soil resistivity, stray
currents, pH, drainage or aeration, moisture content, and soluble salts
present. Studies sponsored by REA have shown that [NO SINGLE PROPERTY OF
A SOIL]{.underline} is an accurate indicator of its corrosivity to a
particular metal.

**6.3** Shields incorporating copper, copper alloys, and copper/alloy
steel and copper/stainless steel combinations comprise a significant
portion of the cable or wire system. Copper exhibits slow, uniform
corrosion in most soil conditions. However, in highly acidic or in
sulfur-containing soils, corrosion of copper may accelerate. Because
copper is cathodic to most metals in the outside plant (i.e., carbon
steel or cast iron), it will receive cathodic protection from these
metals within reasonable proximity, thus extending its life. However, in
newer installations the water and gas systems are often nonmetallic or
electrically insulated, which eliminates the cast iron or steel as a
sacrificial anode. When galvanically coupled with other copper
structures such as power system neutrals, these shields are not
susceptible to accelerated corrosion. In many environments copper
exhibits satisfactory corrosion resistance; however in an aggressive
environment copper may corrode.

**6.3.1** When certain chemical constituents are present in the soil
such as sulfides, bicarbonates, and ammonium compounds, corrosion of
copper can occur. In soils contaminated by fertilizers, cinders, and
decaying organic matter, the rate of corrosion will be greater than that
observed in less aggressive environments.

**6.3.2** Marine and industrial environments, which may transport
pollutants great distances, frequently contain sulfur compounds such as
hydrogen sulfide which may permeate the jacket and cause corrosion of
copper. The soils containing sulfate ions and anaerobic, sulfide
generating bacteria have in some isolated instances also caused treeing
of the cable or wire jacket.

**6.3.3** Copper clad stainless steel or copper clad alloy steel
laminates have demonstrated satisfactory corrosion resistance in tests
and in service in a wide range of environments. The outer layer performs
comparably to solid copper and to copper alloys in soils where copper
has satisfactory corrosion resistance. In more aggressive soils i.e.,
low pH, high sulfide levels, where copper corrosion is accelerated, test
and fields results have shown that the stainless steel or alloy steel
remains passive and acts as a corrosion barrier protecting the inner
copper layer from corrosion. Thus the shield integrity may be prolonged.

**6.3.4** The primary mechanisms which can contribute to the corrosive
attack of copper are as follows:

a\. Direct attack in acidic, sulfide, and ammonia

containing soils;

b\. Electrolytic corrosion from external sources of

direct current; and

c\. Corrosion under the influence of alternating current.

**6.4** The use of plastic coated aluminum since the early 1960\'s
indicates its use as one approach in controlling corrosion for shields
in buried cables used in the outside plant. Plastic coated aluminum has
demonstrated satisfactory corrosion resistance in fields tests,
laboratory tests, and service in a wide range of environments for over
25 years.

Note: For uniformity, the words coated and coating are

used in this bulletin to describe metallic components

covered with plastic materials whether by solvent release

coating, film laminating or extrusion coating. The intent

of the reference is to address the performance of the

coated materials, not to describe the process of

preparation.

**6.4.1** The plastic coating restricts the area of aluminum subjected
to corrosion. The polymer coating forms a chemical bond with the metal
which effectively counteracts the mechanical forces generated during the
corrosion process that attempt to detach the coating. Also, the chemical
bond prevents the migration of moisture and chemicals along the
interface of the copolymer coating and aluminum thus retarding
corrosion.

**6.4.2** The forces which damage the outer jacket also will likely
damage the plastic coating. In this case, localized corrosion of the
aluminum may occur wherever the coatings are damaged due to the same
factors which corrode bare aluminum. However, coatings which are
chemically bonded to a metal substrate will generally restrict the
corrosion to the vicinity of the damaged region. In outside plant areas
where the jacket is more susceptible to mechanical damage, the coated
aluminum should be bonded to the jacket since bonded-sheath improves the
puncture resistance and toughness of the cable and thus reduces the
possibility of cable damage.

**6.4.3** Since galvanized steel is recommended for use as ground rods,
galvanic corrosion of the plastic coated aluminum shield is greatly
reduced. However, where the plastic coated aluminum shield is coupled to
a copper shield, careful consideration needs to be given to the area of
cathode (copper), anode (coated aluminum), and distance of the
electrolyte between the metals.

**6.4.4** If the plastic coating on the aluminum is damaged, corrosion
may be initiated. Factors which are likely to corrode the bare aluminum
are as follows:.

a\. Direct attack in soils;

b\. Galvanic corrosion;

c\. Dynamic and static stray currents from external

sources; and

d\. Differential aeration.

Aluminum is an amphoteric metal and it corrodes more rapidly in highly
acidic and highly alkaline media, i.e., at pH levels below 5.0 and
greater than 8.5. The rate of corrosion also depends on the specific
ions present. Chlorides and sulfates are considered aggressive towards
aluminum. Also when aluminum is coupled to copper and other passive
metals, corrosion of aluminum can be severe due to galvanic effects.
Although these mechanisms are pertinent to exposed bare aluminum at
defects in plastic coated aluminum, the rate at which corrosion occurs
for plastic coated aluminum is mitigated by the presence of the plastic
coating, as described in the previous paragraphs.

**6.4.5** Marine and industrial environments, which may transport
pollutants great distances, frequently contain sulfur compounds such as
hydrogen sulfide which may permeate through the jacket and cause
corrosion.

**6.4.6** The primary mechanisms which can contribute to the corrosive
attack of bare aluminum are:

a\. Direct attack in highly acidic and highly

alkaline soils;

b\. Chlorides and sulfates;

c\. Electrolytic corrosion from external sources of

direct current;

d\. Corrosion under the influence of alternating current;

e\. Galvanic effects when coupled to copper;

f\. Galvanic effects when coupled to other passive metals;

and

g\. Differential aeration.

**6.5** The corrosion of bare steel due to the various factors thus far
described for aluminum and copper in the text is well recognized. Bare
steel is more susceptible to corrosion in acidic rather than neutral or
alkaline media. The plastic coatings on both aluminum and steel reduce
the corrosion from the environment. The use of coated steel provides the
first line of defense from corrosive elements if the jacket is damaged
and also protection to the coated aluminum for it to perform vital
shielding functions. If the plastic coating on the steel is damaged,
corrosion may be initiated with subsequent dissolution of the steel.
Further corrosion of the exposed steel may proceed according to the
primary mechanisms referenced in Paragraph 6.4.6.

**6.6** Certain locations in the country may have severe corrosion
conditions which are more harmful to one type of cable or wire shield or
cable armor than another. In such areas, the engineer should determine
the cable or wire material which is best suited for these particular
soil conditions. To determine a potentially corrosive environment, the
engineer should measure the earth resistivity, identify the pH level of
the soil, and gather historical data from company files and from other
utilities in the exchange area. Soil resistivity is an indicator of
corrosivity as shown in Table 2; as well as, is historical data.
However, the latter should be viewed in light of changing soil
conditions caused by acid rain, long-range transport of industrial
pollutants in the atmosphere, deicing practices, fertilizers, etc. If
there has been no prior construction in the vicinity of a new site, the
soil resistivity and pH at the new site should be determined and
compared to measurements taken at similar sites with some long-term in
place construction. For example, if soil measurements along a new cable
or wire route indicate a neutral pH with low soil resistivity and a
similar site with long lived coated aluminum cable plant is nearby with
a good performance history, coated aluminum shielded cable could
probably be used at the new site with no problems.

TABLE 2

Soil Corrosivity Versus Resistivity

Soil

Resistivity Corrosivity

[(Ohm·cm)]{.underline} [(description)]{.underline}

Below 500 Very Corrosive

500 to 1000 Corrosive

1000 to 2000 Moderately Corrosive\
2000 to 10,000 Mildly Corrosive

Above 10,000 Progressively Less Corrosive

**6.7** Soil properties can be obtained from the local Soil Conservation
office or samples can sent to an independent laboratory for property
measurements. Soil resistivity is the easiest measurement and should be
made on site following the recommended techniques listed in Section 7.
Figures 7 through 11 show the general soil resistivity throughout the
United States but are not meant as a substitute for local measurements.

**7. SOIL MEASUREMENTS**

**7.1** **[Soil Resistivity]{.underline}**

**7.1.1** Soil resistivity can be measured in a number of ways. The most
accurate method is to obtain some soil at the cable or wire burial depth
and measure the resistivity in the laboratory under controlled
conditions which include saturating the soil with distilled water. A
field measurement can be made at the soil burial depth using the four
terminal method shown in Figure 12. Four small test electrodes (stubs of
number 8 American Wire Gauge (AWG) wire will suffice) are placed to the
same depth and equal distances apart in a straight line. Four separate
lead wires connect the electrodes to the four terminals on the soil
resistivity tester as shown in Figure 12. Hence, the name of the test:
[The Four Terminal Method]{.underline}.

**7.1.2** The theory behind the Four Terminal Method showed that if
electrode depth [A]{.underline} is kept small compared to the distance
between electrodes [B]{.underline}, the following equation applies:

P = 2ÐBR

Where:

P = Average soil resistivity to depth B in ohm·cm\
Ð = 3.1416\
B = Distance between electrodes\
R = Instrument reading in ohms

**7.1.3** Since copper cable or wire is usually buried at a depth of 0.6
to 1 meter (m), the average soil resistivity should be measured to at
least 1.2 m. For buried fiber optic cable, the average soil resistivity
should be measured to at 1.5 m because this cable is usually buried at a
depth of 1 to 1.2 m. Where a cable is to be buried at a depth greater
than 1.2 m, the average soil resistivity should be measured at the 1.2 m
depth plus 25 percent. The electrode depth should not exceed 10 percent
of the electrode spacing dimension. A 7.6 centimeter (cm) electrode
depth should be adequate. Black vinyl tape may be placed 7.6 cm from the
ends of the electrodes as a depth gauge. Assuming an instrument reading
of 30 ohms is obtained at 1.2 m electrode spacing, the soil resistivity
would be calculated as follows:

P = 2ÐBR

= 2 \* (3.1416) \* (1.2 m x 100 cm/m) \* (30)

= 754 \* (30 ohms)

= 22,620 ohm·cm

Therefore, if the 1.2 m electrode spacing is maintained the soil
resistivity is 754 times the instrument reading in ohms. If an electrode
spacing of 0.86 m is maintained a multiplier of 500 will result yielding
resistivity at 0.86 m which is a realistic cable or wire placement
depth. Typical soil resistivity values to be expected are given in Table
3.

TABLE 3

Typical Soil Resistivity Values

Soil

Resistivity

[Soil Types]{.underline} [(Ohm·cm)]{.underline}

Sand Saturated with Sea Water 100 - 200

Marsh 200 - 400

Clay 200 - 15,000

Sand and Gravel 5,000 - 100,000

Clay Mixed with Sand Gravel 1,000 - 135,000

Shale 1,000 - 50,000

Rock 50,000 - 1,000,000

Limestone 500 - 400,000

Note: A description of the Four Terminal Method is

described in ASTM G 57, latest issue, [Standard Method for]{.underline}

[Field Measurement of Soil Resistivity Using the Wenner]{.underline}

[Four Electrode Method]{.underline}.

**7.2** **[Soil pH]{.underline}**

**7.2.1** Soil pH can be measured with a number of commercially
available battery-powered meters. These high-impedance meters measure
the potential difference between a sensing electrode whose potential
changes with the hydrogen ion concentration and a reference electrode
(usually calomel). Commercially available combination electrodes
consisting of a saturated calomel reference electrode and an electrode
combined as one single electrode are available. The easiest and most
practical method to measure soil pH is the use of a commercial meter in
conjunction with a combination electrode.

**7.2.2** The pH meter and electrodes are standardized by immersion in
standard solutions of known pH and adjustment of the
[standardized]{.underline} dial in accordance with the manufacturers\'
recommendations or ASTM G 51, latest issue.

**7.2.3** The contact area of the glass electrode or combination
electrode should be pressed against undisturbed soil at the location of
interest. With the electrodes in place, set the meter to read pH, allow
sufficient time for equilibrium to be established, then take the meter
reading. Again allow the meter to equilibrate, then reread the meter.
The values obtained should agree within 0.2 units to be acceptable.

**7.2.4** It is best to bore and make the pH measurement at the depth of
interest. A subsurface probe for accomplishing this is described in ASTM
G 51, latest issue, [Standard Test Method for pH]{.underline} [of Soil
for Use in Corrosion Testing]{.underline}. Soil samples can be brought
to the surface with a boring tool or post-hole digger and the
measurement made in the field on the soil thus obtained. The least
desirable method is to make the measurement on soil returned to the
laboratory, however, if this is done the measurement should be made
within 24 hours from the time the sample was obtained. Under no
circumstances should a surface pH measurement be construed as the pH of
the soil at the cable depth.

FIGURE 1 EXAMPLE OF GALVANIC CORROSION

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FIGURE 2 EXAMPLE OF CONCENTRATIONS CELL CORROSION

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FIGURE 3 EXAMPLE OF STRAY CURRENT CORROSION

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FIGURE 4 ANCHOR ROD CORROSION - GALVANIC COUPLE

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FIGURE 5 ANCHOR ROD CORROSION - DIFFERENTIAL AERATION CELL

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FIGURE 6 COORDINATED STRAY CURRENT PROTECTION

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FIGURE 7 ESTIMATED AVERAGE EARTH RESISTIVITY

NORTHEAST REGION

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FIGURE 8 ESTIMATED AVERAGE EARTH RESISTIVITY

SOUTHEAST REGION

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FIGURE 9 ESTIMATED AVERAGE EARTH RESISTIVITY

NORTH CENTRAL REGION

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FIGURE 10 ESTIMATED AVERAGE EARTH RESISTIVITY

WESTERN REGION

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FIGURE 11 ESTIMATED AVERAGE EARTH RESISTIVITY

SOUTHWESTERN REGION

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FIGURE 12 SOIL RESISTIVITY MEASREMENT PROCEDURE

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