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UFC 3-570-02A
01 March 2005
UNIFIED FACILITIES CRITERIA (UFC)
CATHODIC PROTECTION
APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED
UFC 3-570-02A
01 March 2005
UNIFIED FACILITIES CRITERIA (UFC)
CATHODIC PROTECTION
Any copyrighted material included in this UFC is identified at its point of use.
Use of the copyrighted material apart from this UFC must have the permission of the
copyright holder.
U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)
NAVAL FACILITIES ENGINEERING COMMAND
AIR FORCE CIVIL ENGINEER SUPPORT AGENCY
Record of Changes (changes are indicated by \1\ ... /1/)
Change No.
Date
Location
This UFC supersedes TM 5-811-7, dated 22 April 1985. The format of this UFC does not conform to
UFC 1-300-01; however, the format will be adjusted to conform at the next revision. The body of
this UFC is a document of a different number.
1
UFC 3-570-02A
01 March 2005
FOREWORD
\1\
The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and provides
planning, design, construction, sustainment, restoration, and modernization criteria, and applies
to the Military Departments, the Defense Agencies, and the DoD Field Activities in accordance
with USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and
work for other customers where appropriate. All construction outside of the United States is
also governed by Status of forces Agreements (SOFA), Host Nation Funded Construction
Agreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)
Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, the
SOFA, the HNFA, and the BIA, as applicable.
UFC are living documents and will be periodically reviewed, updated, and made available to
users as part of the Services’ responsibility for providing technical criteria for military
construction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval Facilities
Engineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) are
responsible for administration of the UFC system. Defense agencies should contact the
preparing service for document interpretation and improvements. Technical content of UFC is
the responsibility of the cognizant DoD working group. Recommended changes with supporting
rationale should be sent to the respective service proponent office by the following electronic
form: Criteria Change Request (CCR). The form is also accessible from the Internet sites listed
below.
UFC are effective upon issuance and are distributed only in electronic media from the following
source:
x
Whole Building Design Guide web site http://dod.wbdg.org/.
Hard copies of UFC p rinted from electronic media should be checked against
electronic version prior to use to ensure that they are current.
the current
AUTHORIZED BY:
______________________________________
DONALD L. BASHAM, P.E.
Chief, Engineering and Construction
U.S. Army Corps of Engineers
______________________________________
DR. JAMES W WRIGHT, P.E.
Chief Engineer
Naval Facilities Engineering Command
______________________________________
KATHLEEN I. FERGUSON, P.E.
The Deputy Civil Engineer
DCS/Installations & Logistics
Department of the Air Force
______________________________________
Dr. GET W. MOY, P.E.
Director, Installations Requirements and
Management
Office of the Deputy Under Secretary of Defense
(Installations and Environment)
2
TM 5-811-7
REPRODUCTION AUTHORIZATION/RESTRICTIONS
This manual has been prepared by or for the Government and, except to the extent indicated below, is public property and not subject
to copyright.
Copyrighted material included in the manual has been used with the knowledge and permission of the proprietors and is acknowledged
as such at point of use. Anyone wishing to make further use of any copyrighted material, by itself and apart from this text should seek
necessary permission directly from the proprietors.
Reprints or republications of this manual should include a credit substantially as follows: “Department of the Army, USA, Technical
Manual TM 5-811-7, Electrical Protection, Cathodic Protection.”
If the reprint or republication includes copyrighted material, the credit should also state: “Anyone wishing to make further use of
copyrighted material, by itself and apart from this text, should seek necessary permission directly from the proprietors.”
a/b
TM 5-811-7
(a) To allow rectifier aging and film formation, it is considered good practice to use 1.5 as a
multiplying factor:
E = 1.5 x 32.7 = 49.1 V.
(b) The rectifier chosen should produce a d.c. voltage that meets the size requirements of 60-volt,
4-ampere, single-phase.
(8) Locate the rectifier adjacent to tank for the following reasons:
(a) Usually cheaper to install.
(b) Easier to maintain.
(c) Keeps d.c. voltage drop to a minimum.
(9) The d.c. circuit conductors should be installed as follows:
(a) Outside tank — use No.2 AWG high molecular weight polyethylene extruded (HMWPE)
conductor.
(b) Inside tank — use No.8 AWG HMWPE conductor.
(10) The cable should not be stressed or bent.
D-5.
Elevated water tank (ice is expected).
Impressed current cathodic protection is designed for an elevated water tank as shown in figure D-5. The
tank is already built and current requirement tests have been done. Anodes must not be suspended from the
tank roof because heavy ice (up to 2 feet thick) covers the water surface during winter. The anode cables
could not tolerate this weight, so another type of support must be used. Button anodes must be mounted on
the tank's floor and lightweight platinized titanium anodes must be suspended in the riser from the tank
bottom.
D-10
TM 5-811-7
length along the tank bottom equal to 1½ times the spacing of the anode from the bottom.
(b) The anode suspension arrangement for the tank being considered is shown in figure D-9. It can
be seen that stub anodes are required for this design. Ten stub anodes are spaced equally on a circumference
with a radius of 8 feet in a way shown in figure D-8. For smaller diameter tanks, stub anodes may not be
required.
(12) Find the current division between main and stub anodes.
(a) The area of tank bottom protected by stub anodes is found by equation D-10 (fig D-9):
As = • (r22 - r12),
(eq D-10)
2
1
where r = 13 feet (protected segment radius) and r = 2.5 feet (riser radius). Thus,
As = 3.1416 (132 sq ft - 2.52 sq ft)
As = 3.1416 x 162.75
As = 511.3 sq ft.
(b) The maximum current for stub anodes is therefore—
Is = 2.0 x 511.3
Is = 1022.6 milliamperes or 1.02 amperes.
(c) The maximum current for the tank bowl is 15.2 amperes.
(d) The maximum current for main anodes is—
Im = 15.2A - 1.02A
Im = 14.2A.
(13) Find the rectifier voltage rating.
(a) The electrical conductor to the main anode is wire size No.2 AWG, rated at 0.159 ohm per 1000
feet, and has an estimated length of 200 feet. Thus, the resistance of the wire, R, is:
200 ft
R = ——— x 0.159 ohm = 0.032 ohm.
1000 ft
(b) For the voltage drop in the main anode feeder—
E = IR,
where I = 14.2 amperes and R = 0.032 ohm. Thus,
E = 14.2 A x 0.032 ohm
E = 0.45 V.
(c) For the voltage drop through the main anodes—
E = IR,
where I = 14.2 amperes and R = 3.03 ohms. Thus,
E = 14.2 A x 3.03 ohms.
E = 43.0 V.
(d) The total voltage drop in main anode circuit is thus—
ET = 0.45 + 43.0
ET = 43.45 or 45 V.
Use a multiplying factor (safety factor) of 1.5 to get 67.5 volts.
D-19
TM 5-811-7
CHAPTER 1
INTRODUCTION TO CATHODIC PROTECTION
1-1. Purpose.
This manual presents design guidance for cathodic
protection systems.
1-2. References.
a. Government publications.
Department of Transportation
Superintendent of Documents, U.S. Government Printing Office, Washington, DC
20402
Transportation of Natural and Other Gas by
Pipeline: Minimum Federal Safety Standards, Subpart 1 - Requirements Register,
Vol 36, No. 126 (June 30, 1971).
b. Nongovernment publications.
National Association of Corrosion Engineers
(NACE), P.O. Box 218340, Houston, TX
77084
Standard RP-01-69 Control of External
(1972 revision
Corrosion on Underground or Submerged
Metallic Piping Systems
Standard RP-02-72 Direct Calculation of
Economic Appraisals
of Corrosion Control
Measures
one small section of a pipeline may be anodic
because it is in a soil with low resistivity compared
to the rest of the line. Current would leave the
pipeline at that anode site, pass through the soil,
and reenter the pipeline at a cathode site. Current
flows because of a potential difference between the
anode and cathode. That is, the anode potential is
more negative than the cathode potential, and this
difference is the driving force for the corrosion
current. The total system—anode, cathode, electrolyte, and metallic connection between anode and
cathode (the pipeline in fig 1-1)—is termed a
corrosion cell.
1-4. Cathodic protection.
Cathodic protection is a method to reduce corrosion by minimizing the difference in potential
between anode and cathode. This is achieved by
applying a current to the structure to be protected
(such as a pipeline) from some outside source.
When enough current is applied, the whole structure will be at one potential; thus, anode and
cathode sites will not exist. Cathodic protection is
commonly used on many types of structures, such
as pipelines, underground storage tanks, locks, and
ship hulls.
1-3. Corrosion.
1-5. Types of cathodic protection systems.
Corrosion is an electrochemical process in which a
current leaves a structure at the anode site, passes
through an electrolyte, and reenters the structure at
the cathode site as figure 1-1 shows. For example,
There are two main types of cathodic protection
systems: galvanic and impressed current. Figure 1-2
shows these two types. Note that both types have
anodes (from which current flows into the
1-1
TM 5-811-7
electrolyte), a continuous electrolyte from the
anode to the protected structure, and an external
metallic connection (wire). These items are
essential for all cathodic protection systems.
a. Galvanic system. A galvanic cathodic protection system makes use of the corrosive potentials for different metals. Without cathodic protection, one area of the structure exists at a more
negative potential than another, and corrosion
results. If, however, a much less inert object (that
is, with much more negative potential, such as a
magnesium anode) is placed adjacent to the structure to be protected, such as a pipeline, and a
metallic connection (insulated wire) is installed
between the object and the structure, the object will
become the anode and the entire structure will
become the cathode. That is, the new object corrodes sacrificially to protect the structure as shown
in figure 1-2. Thus, the galvanic cathodic protection system is called a sacrificial anode cathodic
protection system because the anode corrodes
sacrificially to protect the structure. Galvanic
anodes are usually made of either magnesium or
1-2
zinc because of these metals’ higher potential
compared to steel structures.
b. Impressed current systems. Impressed current
cathodic protection systems use the same elements
as the galvanic protection system, only the
structure is protected by applying a current to it
from an anode. The anode and the structure are
connected by an insulated wire, as for the galvanic
system. Current flows from the anode through the
electrolyte onto the structure, just as in the galvanic
system. The main difference between galvanic and
impressed current systems is that the galvanic
system relies on the difference in potential between
the anode and structure, whereas the impressed
current system uses an external power source to
drive the current, as figure 1-2b shows. The
external power source is usually a rectifier that
changes input a.c. power to the proper d.c. power
level. The rectifier can be adjusted, so that proper
output can be maintained during the system’s life.
Impressed current cathodic protection system
anodes typically are high-silicon cast iron or
graphite.
TM 5-811-7
CHAPTER 2
CATHODIC PROTECTION DESIGN
2-1. Required information.
Before deciding which type, galvanic or impressed
current, cathodic protection system will be used
and before the system is designed, certain preliminary data must be gathered.
a. Physical dimensions of structure to be protected. One important element in designing a
cathodic protection system is the structure's physical dimensions (for example, length, width, height,
and diameter). These data are used to calculate the
surface area to be protected.
b. Drawing of structure to be protected. The
installation drawings must include sizes, shapes,
material type, and locations of parts of the structure
to be protected.
c. Electrical isolation. If a structure is to be
protected by the cathodic system, it must be electrically connected to the anode, as figure 1-2
shows. Sometimes parts of a structure or system
are electrically isolated from each other by insulators. For example, in a gas pipeline distribution
system, the inlet pipe to each building might
contain an electric insulator to isolate inhouse
piping from the pipeline. Also, an electrical
insulator might be used at a valve along the pipeline
to electrically isolate one section of the system
from another. Since each electrically isolated part
of a structure would need its own cathodic
protection, the locations of these insulators must be
determined.
d. Short circuits. All short circuits must be
eliminated from existing and new cathodic protection systems. A short circuit can occur when one
pipe system contacts another, causing interference
with the cathodic protection system. When
updating existing systems, eliminating short circuits
would be a necessary first step.
e. Corrosion history of structures in the area.
Studying the corrosion history in the area can
prove very helpful when designing a cathodic protection system. The study should reinforce predictions for corrosivity of a given structure and its
environment; in addition, it may reveal abnormal
conditions not otherwise suspected. Facilities personnel can be a good source of information for
corrosion history.
f. Electrolyte resistivity survey. A structure's
corrosion rate is proportional to the electrolyte
resistivity. Without cathodic protection, as electrolyte resistivity decreases, more current is allowed to
flow from the structure into the electrolyte; thus,
the structure corrodes more rapidly. As electrolyte
resistivity increases, the corrosion rate decreases
(table 2-1). Resistivity can be measured either in a
laboratory or at the site with the proper instruments. Appendix A explains the methods and
equipment needed to complete a soil resistivity
survey. The resistivity data will be used to calculate
the sizes of anodes and rectifier required in
designing the cathodic protection system.
Table 2-1. Corrosivity of soils on steel based on soil resistivity
Soil resistivity range (ohm-cm)
0 to 2000
2000 to 10,000
10,000 to 30,000
Above 30,000
Corrosivity
Severe
Moderate to severe
Mild
Not likely
U.S. Air Force.
g. Electrolyte pH survey. Corrosion is also proportional to electrolyte pH (see glossary for definition of pH and other terms). In general, steel's
corrosion rate increases as pH decreases when soil
resistivity remains constant.
h. Structure versus electrolyte potential survey.
For existing structures, the potential between the
structure and the electrolyte will give a direct
indication of the corrosivity. According to NACE
Standard No. RP-01, the potential requirement for
cathodic protection is a negative (cathodic) potential of at least 0.85 volt as measured between the
structure and a saturated copper-copper sulfate
reference electrode in contact with the electrolyte.
A potential which is less negative than -0.85 volt
would probably be corrosive, with corrosivity increasing as the negative value decreases (becomes
more positive).
i. Current requirement. A critical part of design
calculations for cathodic protection systems on
existing structures is the amount of current required per square foot (called current density) to
change the structure’s potential to -0.85 volt. The
current density required to shift the potential indicates the structure's surface condition. A well
coated structure (for example, a pipeline well
coated with coal-tar epoxy) will require a very low
current density (about 0.05 milliampere per square
foot); an uncoated structure would require high
current density (about 10 milliamperes per square
foot). The average current density required for
cathodic protection is 2 milliamperes per square
2-1
TM 5-811-7
foot of bare area. The amount of current required
for complete cathodic protection can be determined
three ways:
Table 2-2. Typical current density requirements for cathodic
protection of uncoated steel
Environment
—An actual test on existing structures using
a temporary cathodic protection setup.
—A theoretical calculation based on coating
efficiency.
—An estimate of current requirements using
tables based on field experience.
(1) The second and third methods above can
be used on both existing and new structures.
Appendix B contains a detailed review of current
requirement testing.
(2) Current requirements can be calculated
based on coating efficiency and current density
(current per square foot) desired. The efficiency of
the coating as supplied will have a direct effect on
the total current requirement, as equation 2-1
shows:
I = (A)(I•)(1.0-CE),
(eq 2-1)
where I is total protective current, A is total structure surface area in square feet, I• is required current density, and CE is coating efficiency. Equation
2-1 may be used when a current requirement test is
not possible, as on new structures, or as a check of
the current requirement test on existing structures.
Coating efficiency is directly affected by the type of
coating used and by quality control during coating
application. The importance of coating efficiency is
evident in the fact that a bare structure may require
100,000 times as much current as would the same
structure if it were well coated.
(3) Current requirements also can be estimated from table 2-2. The table gives an estimate
of current, in milliamperes per square foot, required
for complete cathodic protection. That value,
multiplied by the surface area of the structure to be
protected (in square feet) gives the total estimated
current required. Caution should be used when
estimating, however, as under- or overprotection
may result.
2-2
Neutral soil
Well aerated neutral soil
Wetsoil
Highly acidic soil
Soil supporting active
sulfate-reducing bacteria
Heated soil
Stationary freshwater
Moving freshwater containing
dissolved oxygen
Seawater
Current density (mA/sq ft)
rardb
AFM 88-9a
Ger
0.4
2
1
3
to 1.5
to 3
to 6
to 15
0.4 to 1.5
2 to 3
2.5 to 6
5 to 15
6
3
1
to 42
to 25
to 6
Up to 42
5 to 25
5
5
3
to 15
to 10
5
5
to 15
to 25
a
Data are from Air Force Manual AFM 88-9, Corrosion Control (U.S.
Air Force, August 1962), chap 4, p 203.
b
Data are from J.S. Gerrard, “Practical Applications of Cathodic
Protection," Corrosion, Vol 2 (L.L. Shreir, Ed.), NewnesButterworths, London, 1976, p 11:65. Used with permission.
j. Coating resistance. A coating's resistance
decreases greatly with age and directly affects
structure-to-electrolyte resistance for design calculations. The coating manufacturers supply coating
resistance values.
k. Protective current required. By knowing the
physical dimensions of the structure to be protected, the surface area can be calculated. The product of the surface area multiplied by current density obtained previously in I above gives the total
current required.
l. The need for cathodic protection. For existing structures, the current requirement survey (I
above) will verify the need for a cathodic protection system. For new systems, standard practice is
to assume a current density of at least 2 milliamperes per square foot of bare area will be needed to
protect the structure. (However, local corrosion
history may demand a different current density.) In
addition, cathodic protection is mandatory for
underground gas distribution lines (Department of
Transportation regulations—Title 49, Code of
Federal Regulations, Oct 1979) and for water storage tanks with a 250,000-gallon capacity or
greater. Cathodic protection also is required for
underground piping systems located within 10 feet
TM 5-811-7
of steel reinforced concrete because galvanic corrosion will occur between the steel rebar and the
pipeline.
2-2
Determining type and design of cathodic
protection system.
When all preliminary data have been gathered and
the protective current has been estimated, the
design sequence can begin. The first question to
ask is: which type (galvanic or impressed current)
cathodic protection system is needed? Conditions
at the site sometimes dictate the choice. However,
when this is not clear, the criterion used most
widely is based on current density required and soil
resistivity. If the soil resistivity is low (less than
5000 ohm-centimeters) and the current density
requirement is low (less than 1 milliampere per
square foot), a galvanic system can be used. However, if the soil resistivity and/or current density
requirement exceed the above values, an impressed
current system should be used. Figure 2-1 will be
used in the design sequence. Design sequences for
each type of cathodic protection system are given
in paragraphs a and b below.
2-3
TM 5-811-7
a. Sacrificial anode (galvanic) cathodic protection system design. The following eight steps are
required when designing galvanic cathodic protection systems. Appendix C gives examples of
galvanic cathodic protection designs.
(1) Review soil resistivity. The site of lowest
resistivity will likely be used for anode location to
minimize anode-to-electrolyte resistivity. In addition, if resistivity variations are not significant, the
average resistivity will be used for design
calculations.
(2) Select anode. As indicated in paragraph
1-4, galvanic anodes are usually either magnesium
or zinc. Zinc anodes are used in extremely corrosive soil (resistivity below 2000 ohm- centimeters).
Data from commercially available anodes must be
reviewed. Each anode specification will include
anode weight, anode dimensions, and package
dimensions (anode plus backfill), as table 2-3
shows for magnesium-alloy anodes. In addition, the
anode’s driving potential must be considered (para
a[3] below). The choice of anode from those
available is arbitrary; design calculations will be
made for several available anodes, and the most
economical one will be chosen.
Table 2-3. Weights and dimensions of selected high-potential
magnesium-alloy anodes for use in soil or water
Weight
(lb)
3
5
9
9
12
14
14
17
17
20
24
32
40
48
48
60
Size (in.)
3.75 x 3.75 x 5
3.75 x 3.75 x 7.5
2.75 x 2.75 x 26
3.75 x 3.75 x 13.25
3.75 x 3.75 x 18
2.75 x 2.75 x 41
3.75 x 3.75 x 21
2.75 x 2.75 x 50
3.75 x 3.75 x 26
2.5 x 2.5 x 59.25
4.5 x 4.5 x 23
5.5 x 5.5 x 21
3.75 x 3.75 x 59.25
5.5 x 5.5 x 30
8 x 16
4.5 x 4.5 x 60
Packaged
wt (lb)
12
17
35
27
36
50
42
60
45
70
60
74
105
100
100
—
Packaged
size
(in.)
6 x 10
6 x 12
6 x 31
6 x 17
6 x 23
6 x 46
6.5 x 26
6 x 55
6.5 x 29
5 x 66
7 x 30
8 x 28
6.5 x 66
8 x 38
12 x 25
—
Note: Core material is a galvanized 20-gage perforated steel strip.
Anodes longer than 24 inches have a 9-gage core. The connecting
wire is a 10-foot length of solid No. 12 AWG TW insulated copper
wire, silver-soldered to the core with joints sealed against moisture.
Special wires or other lengths are available.
U.S. Air Force.
(3) Calculate net driving potential for
anodes. The open-circuit potential of standard alloy
2-4
magnesium anodes is approximately -1.55 volts to
a copper-copper sulfate half-cell. The open-circuit
potential of high-manganese magnesium anodes is
approximately -1.75 volts to a copper-copper sulfate half-cell.
(a) The potential of iron in contact with
soil or water usually ranges around -0.55 volt relative to copper-copper sulfate. When cathodic protection is applied using magnesium anodes, the iron
potential assumes some value between -0.55 and 1.0 volt, depending on the degree of protection
provided. In highly corrosive soils or waters, the
natural potential of iron may be as high as -0.82
volt relative to copper-copper sulfate. From this, it
is evident that -0.55 volt should not be used to
calculate the net driving potential available from
magnesium anodes.
(b) A more practical approach is to consider iron polarized to -0.85 volt. On this basis,
standard alloy magnesium anodes have a driving
potential of 0.70 volt (1.55-0.85 0.70) and highpotential magnesium anodes have a driving potential of 0.90 volt (1.75-0.85 0.90). For cathodic
protection design that involves magnesium anodes,
these potentials, 0.70 and 0.90 volt, should be
used, depending on the alloy selected.
(4) Calculate number of anodes needed to
meet groundbed resistance limitations. The total
resistance (RT) of the galvanic circuit is given by
equation 2-2:
RT = Ra + Rw +Rc,
(eq 2-2)
where Ra is the anode-to-electrolyte resistance, Rw
is the anode lead wire resistance, and Rc is the
structure-to-electrolyte resistance. The total resistance also can be found by using equation 2-3:
RT •
•E
,
I
(eq 2-3)
where • E is the anode’s driving potential discussed
in a(3) above and I is the current density required
to achieve cathodic protection (para 2-1). Rc in
equation 2-2 can be calculated by using equation
2.4:
Rc •
R
,
A
(eq 2-4)
where R is the average coating resistance, in ohms
per square feet, at the end of the proposed lifetime
for the system (R is specified by the supplier), and
A is the structure’s surface area in square feet.
Assuming Rw in equation 2-2 is negligible, that
anode-to-electrolyte resistance can then be calculated from equation 2-5:
TM 5-811-7
Ra = RT - Rc,
(eq 2-5)
which gives the maximum allowable groundbed
resistance; this will dictate the minimum number of
anodes required (as number of anodes decreases,
groundbed resistance increases). To calculate the
number of anodes required, equation 2-6 is used:
N •
(0.0052)(• )
8L
[1n
• 1],
(Ra)(L)
d
(eq 2-6)
where N is the number of anodes, is the soil
resistivity in ohms, Ra is the maximum allowable
groundbed resistance in ohms (as computed in eq
2-5), L is the length of the backfill column in feet
(specified by supplier), and d is the diameter of the
backfill column in feet (specified by supplier).
(5) Calculate number of anodes for system's
life expectancy. Each cathodic protection system
will be designed to protect a structure for a given
number of years. To meet this lifetime requirement,
the number of anodes (N) must be calculated using
equation 2-7:
(L) (I)
,
(eq 2-7)
49.3 (W)
where L expected lifetime in years, W is weight (in
pounds) of one anode, and I is the current density
required to protect the structure (in milliamperes).
(6) Select number of anodes to be used. The
greater value of equation 2-6 or 2-7 will be used as
the number of anodes needed for the system.
(7) Select groundbed layout. When the required number of anodes has been calculated, the
area to be protected by each anode is calculated by
equation 2-8:
N •
A •
AT
,
(eq 2-8)
N
where A is area to be protected by one anode, AT
is total surface area to be protected, and N is the
total number of anodes to be used. For galvanic
cathodic protection systems, the anodes should be
spaced equally along the structure to be protected.
(8) Calculate life-cycle cost for proposed design. NACE Standard RP-02 should be used to
calculate the system's life-cycle cost. The design
process should be done for several different anode
choices to find the one with minimal life-cycle cost.
(9) Prepare plans and specifications. When
the design procedure has been done for several
different anodes and the final anode has been
chosen, plans and specifications can be completed.
b. Impressed current cathodic protection system
design. Thirteen steps are required when designing
impressed current cathodic protection systems.
Appendix D gives examples of impressed current
cathodic protection designs.
(1) Review soil resistivity. As with galvanic
systems, this information will contribute to both
design calculations and location of anode groundbed.
(2) Review current requirement test. The required current will be used throughout the design
calculations. The calculated current required to
protect 1 square foot of bare pipe should agree
with the values in table 2-2.
(3) Select anode. As with the galvanic system, the choice of anode is arbitrary at this time;
economy will determine which anode is best. Table
2-4 gives common anode sizes and specifications.
The anodes used most often are made of highsilicon chromium-bearing cast-iron (HSCBCI).
When impressed current-type cathodic protection
systems are used to mitigate corrosion on an
underground steel structure, the auxiliary anodes
often are surrounded by a carbonaceous backfill.
Backfill materials commonly used include coal coke
breeze, calcined petroleum coke breeze, and natural
graphite particles. The backfill serves three basic
functions: (a) it decreases the anode-to-earth
resistance by increasing the anode's effective size,
(b) it extends the system's operational life by
providing additional anode material, and (c) it
provides a uniform environment around the anode,
minimizing deleterious localized attack. The carbonaceous backfill, however, cannot be expected to
increase the groundbed life expectancy unless it is
well compacted around the anodes. In addition to
HSCBCI anodes, the ceramic anode should be considered as a possible alternative for long-term
cathodic protection of water storage tanks and
underground pipes in soils with resistivities less
than 5000 ohm-centimeters. The ceramic anode
consumption rate is 0.0035 ounce per ampere-year
compared to 1 pound per ampere-year for HSCBCI
anodes. Appendix E gives the design and specifications for the ceramic anode.
2-5
TM 5-811-7
Table 2-4.
Weights and dimensions of selected circular highsilicon chromium-bearing cast iron anodes
Anode
weight
(lb)
Anode
dimensions
(in.)
Anode
surface
size (in.)
Package
area
(sq ft)
12
44
60
110
1 x 60
2 x 60
2 x 60
3 x 60
1.4
2.6
2.8
4.0
10 x 84
10 x 84
10 x 84
10 x 84
Reproduced from Harco Corporation, Catalog of Cathodic Protection
Materials, 1971. Used with permission.
(4) Calculate number of anodes needed to
satisfy manufactuere's current density limitations.
Impressed current anodes are supplied with a
recommended maximum current density. Higher
current densities will reduce anode life. To determine the number of anodes needed to meet the
current density limitations, use equation 2-9:
N •
I
(A1 (I1)
,
(eq 2-9)
where N is number of anodes required, I is total
protection current in milliamperes, A1 is anode surface area in square feet per anode, and I 1 is recommended maximum current density output in milliamperes.
(5) Calculate number of anodes needed to
meet design life requirement. Equation 2-10 is used
to find the number of anodes:
(L) (I)
,
(eq 2-10)
(1000) (W)•
where N is number of anodes, L is life in years, and
W is weight of one anode in pounds.
(6) Calculate number of anodes needed to
meet maximum anode groundbed resistance requirements. Equation 2-11 is used to calculate the
number of anodes required:
N •
•K
•P
•
,
(eq 2-11)
NL
S
where Ra is the anodes' resistance, • is soil resistivity in ohm-centimeters, K is the anode shape
factor from table 2-5, N is the number of anodes, L
is length of the anode backfill column in feet, P is
the paralleling factor from table 2-6, and S is the
center-to-center spacing between anode backfill
columns in feet.
Ra •
2-6
Table 2-5.
L/d
5
6
7
8
9
10
12
14
16
28
Shape functions (K) for impressed current cathodic
protection anodes where L is effective anode length
and d is anode/backfill diameter.
K
0.0140
0.0150
0.0158
0.0165
0.0171
0.0177
0.0186
0.0194
0.0201
0.0207
L/d
K
20
25
30
35
40
45
.50
55
60
0.0213
0.0224
0.0234
0.0242
0.0249
0.0255
0.0261
0.0266
0.0270
Reproduced from W.T. Bryan, Designing Impressed Current
Cathodic Protection Systems With Durco Anodes, The Duriron
Company, 1970. Used with permission.
Table 2-6. Anode paralleling factors (F) for various numbers of
anodes (N) installed in parallel
N
2
3
4
5
6
7
8
9
10
12
P
0.00261
0.00289
0.00283
0.00268
0.00252
0.00237
0.00224
0.00212
0.00201
0.00182
N
14
16
18
20
22
24
26
28
30
P
0.00168
0.00155
0.00145
0.00135
0.00128
0.00121
0.00114
0.00109
0.00104
Reproduced from W.T. Bryan, Designing Impressed Current
Cathodic Protection Systems With Durco Anodes, The Duriron
Company, 1970. Used with permission.
(7) Select number of anodes to be used. The
highest number calculated by equation 2-9,2-10, or
2-11 will be the number of anodes used.
(8) Select area for placement of anode bed.
The area with the lowest soil resistivity will be
chosen to minimize anode-to-electrolyte resistance.
(9) Determine total circuit resistance. The
total circuit resistance will be used to calculate the
rectifier size needed.
(a) Calculate
anode
groundbed
resistance. Use equation 2-11.
(b) Calculate groundbed header cable
resistance. The cable is typically supplied with a
specified resistance in ohms per 100 feet. The wire
resistance then is calculated from equation 2-12:
TM 5-811-7
ohms (L)
Rw •
,
(eq 2-12)
100 ft
where L is the structure's length in feet. Economics
are important in choosing a cable, and may indeed
be the controlling factor. To determine the total
annual cable cost, Kelvin's Economic Law can be
used as shown in equation 2-13.
T •
(0.0876)(I 2)(R)(L)(P)
• (0.15)(S)(L),
E
(eq 2-13)
where T is total annual cost in dollars per year, I is
total protection current in amperes, R is cable
resistance in ohms per 1000 feet, L is cable length
in feet, P is cost of electrical energy in farads per
kilowatt-hour, E is the rectifier efficiency expressed
as percent, and S is the cable's initial cost in dollars
per foot.
(c) Calculate structure-to-electrolyte resistance using equation 2-14:
R
,
(eq 2-14)
N
where Rc is the structure-to-electrolyte resistance,
R is the coating resistance in ohms per square feet,
and N is the coated pipe area in square feet.
(d) Calculate total circuit resistance. To
calculate the total resistance, RT, equation 2-15 is
used:
Rc •
RT • Ra • Rw • R c,
(eq 2-15)
where variables are the same as for equations 2-11,
242, and 2-14.
(10) Calculate rectifier voltage. Equation 216 is used to determine voltage output (Vrec) of the
rectifier:
Vrec • (I)(R T)(150%),
(eq 2-16)
where I is total protection current in amperes, RT is
total circuit resistance, and 150 percent is a factor
to allow for aging of the rectifier stacks.
(11) Select a rectifier. A rectifier must be
chosen based on the results of equation 2-16. Many
rectifiers are available commercially; one that satisfies the minimum requirements of (I) and (Vrec) in
equation 2-16 should be chosen. Besides the more
common rectifiers being marketed, a solar cathodic
protection power supply (for d.c. power) may be
considered for remote sites with no electrical
power. Three factors that should be considered
when specifying a solar cathodic protection power
supply are:
—The cost of the solar cathodic protection
power supply in dollars per watt of continuous
power.
—The solar cathodic protection power supply's much higher initial cost compared to selenium
rectifiers operated by a.c. power.
—The additional maintenance required for a
solar cathodic protection power supply, mainly to
keep the solar panels free of dirt deposits. Appendix F discusses rectifier current interference.
(12) Calculate system cost. As with the galvanic cathodic protection system, the choice of
anode for design calculation is arbitrary. When
several anodes have been used in the design calculations, an economic analysis should be done as
recommended in NACE Standard RP-02.
(13) Prepare plans and specifications.
2-7
TM 5-811-7
APPENDIX A
SOIL RESISTIVITY MEASUREMENT
A-1. Introduction.
Since soil resistivity is a major factor affecting the corrosion rate, the design engineer should know how to
measure it. Generally, as soil resistivity decreases, corrosivity increases. In addition, as soil moisture content
increases, resistivity decreases. Soil resistivity typically is measured using one or both of two methods: (1)
testing onsite with the Wenner four-pin method, and/or (2) taking a soil sample to a laboratory for a soil box
resistivity test. It must be stressed that soil resistivity may vary widely within very short distances. Soil
resistivity also changes with depth below the ground surface. Thus, if the soil sample method is used, many
samples must be taken for an accurate map of soil resistivities in the area. The soil box resistivity test also
is much more time-consuming than the four-pin method.
A-2. Wenner four-pin method.
As figure A-I shows, this method is done by placing four pins at equal distances from each other. A current
is then sent through the two outer pins. By measuring the voltage across the two inner pins, the soil resistance
can be calculated using Ohm's Law (V IR). Soil resistivity can be determined using equation A-1:
Resistivity = 191.5 RL ohm-cm,
(eq A-1)
where R is the resistance found above and L is the pin spacing in feet. Buried utilities may produce
interference when using the Wenner method in congested areas. Therefore, it is important to position the pins
perpendicular to the underground pipeline. Placing pins parallel to the pipeline would result in measurements
lower than actual resistivity.
A-1
TM 5-811-7
A-3.
Soil box method.
This resistivity test works on the same principle as the Wenner four-pin test. That is, there are four points
of electrical contact with the soil; the current is driven through the two outer points and the voltage drop is
measured across the two inner points. In the soil box, however, the outer "points" are the metal end plates
of the box, rather than pins. The inner points are pins, just as in the Wenner four-pin method. Figure A-2
gives details of the soil box. Again, for resistance, R=VI from Ohm's Law, but resistivity is now calculated
by equation A-2:
Resistivity • R
WD
,
L
(eq A-2)
where W, D, and L are the soil box dimensions (fig A-2).
A-4.
Temperature effect.
Temperature greatly affects soil resistivity at temperatures below freezing. Thus, soil resistivity should not
be measured on soil at below-freezing temperatures.
A-2
TM 5-811-7
APPENDIX B
CURRENT REQUIREMENT TESTING
B-1.
Required current.
A critical element in designing galvanic and impressed current cathodic protection systems is the current
required for complete cathodic protection. Complete cathodic protection is achieved when the structure
potential is -0.85 volt with respect to a copper-copper sulfate reference electrode.
B-2.
Sample test.
Current requirement tests are done by actually applying a current using a temporary test setup, and adjusting
the current from the power source until suitable protective potentials are obtained. Figure B-1 shows a
temporary test setup. In this setup, batteries can be used as the power supply, in series with heavy-duty
adjustable resistors. The resistors can be adjusted to increase the current until the potential at the location
of interest, such as point A in figure B-1, is at -0.85 volt with respect to a copper-copper sulfate reference
cell. The current supplied is the current required for cathodic protection. The effectiveness of the insulating
joints shown in figure B-1 can also be tested. The potentials at points B and C are measured, first with the
current interruptor switch closed, then with it open. If there is any difference between the two readings at
either point, the joint is not insulating completely.
B-1
TM 5-811-7
APPENDIX C
EXAMPLES OF GALVANIC CATHODIC PROTECTION DESIGN
C-1.
Purpose.
The examples that follow show how to use the design procedure explained in paragraphs 2-1 and 2-2.
C-2.
Aircraft multiple hydrant refueling system.
Galvanic cathodic protection is designed for a standard aircraft hydrant refueling system as shown in figure
C-1. This design is for a system not yet installed.
C-1
TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 5000 ohm-centimeters.
(2) Effective coating resistance at 25 years will be 2500 ohms per square foot, as suggested by the
manufacturer.
(3) Design for 90 percent coating efficiency, based on experience.
(4) Design for 25-year life.
(5) Design for 1 milliampere per square foot of bare pipe after polarization (corrosion history of area
indicates this value is adequate).
(6) Magnesium packaged-type anodes must be used (soil resistivity is greater than 2000 ohmcentimeters).
(7) System is insulated well enough from foreign structures.
(8) All piping is mill-coated with hot-applied coal-tar enamel and wrapped with asbestos felt. Coating
has been tested over the trench for holidays and defects have been corrected. Coating is assumed better than
99.5 percent perfect at installation.
b. Computations (fig C-1).
(1) Find the total outside area of liquid fuel pipes serving the hydrant refueling area (table C-1).
Table C-1. Outside area of liquid fuel pipes
Pipe size
(in.)
Pipe length
(ft)
3 (defueling header)
6 (defueling return)
8 (refueling header)
10 (supply line)
6 (hydrant laterals)
2 x 293 =
586
90
2 293 = 586
90
3 x 960 = 2880
Pipe area (sq ft/ft)
586 x 0.916 = 537
90 x 1.734 = 156
586 x 2.258 = 1323
90 x 2.82 254
2880 x 1.734 = 4994
Total area of POL pipe in square feet
=7264
Reproduced from J.R. Myers and M.A. Aimone, Corrosion Control for Underground Steel Pipelines: A Treatise on Cathodic
Protection, J.R. Myers and Associates, Franklin, OH. Used with permission.
(2) Some experience has shown that steel in this type soil can be cathodically protected with
approximately 1 milliampere per square foot of uncoated surface. Thus, find the required current based on
this value and using equation 2-1:
I = (A)(I’)(1.0 - CE)
= (7264 sq ft)(1.0 mA/sq ft)(1.0 - 0.8)
= 726 mA.
(3) Calculate the number of anodes needed based on maximum groundbed resistance limitations.
(a) Select a 9-pound anode, 3.5 by 3.5 by 13 inches, from table 2.4. Driving potential as provided
by the manufacturer is 0.9 volt.
(b) Calculate total circuit resistance using equation 2-3:
•E
RT •
,
I
RT •
0.9
× 1.23 ohms.
0.726
(c) Calculate structure-to-electrolyte resistance from equation 2-4:
Rc •
C-2
R
N
TM 5-811-7
Rc •
2500 ohms/sq ft
7264 sq ft
Rc • 0.345 ohm.
(d) Find maximum allowable groundbed resistance using equation 2-2:
RT • R a • Rw • Rc
1.23 ohm = Ra + 0.345 ohm (assume Rw is negligible)
0.89 ohm = Ra
(e) Calculate number of anodes from equation 2-6:
(0.0052)(• )
8L
[1n • 1],
N •
(Ra)(L)
d
(0.0052)(500 ohm• cm)
(8)(1.42 ft)
N •
[1n
• 1]
(0.89 ohm)(1.42 ft)
(0.5 ft)
(values for L
and D from
supplier.)
N = 44 anodes.
(4) Calculate number of anodes based on system’s life expectancy and using equation 2-7:
(L)(I)
N •
,
49.3 (W)
(25 yr)(726 mA)
,
49.3 (9 lb/anode)
N = 41 anodes.
(5) Select number of anodes. Since 44 anodes are required to meet maximum allowable groundbed
resistance (e above), that will be the number used.
(6) Select groundbed layout. Determine the area to be covered by each anode using equation 2-8:
AT
A •
N
N •
7264 sq ft
44 anodes
A = 164 sq/ft anode.
(7) Find anode spacing (table C-2).
A •
Table C-2. Requirements for anode spacing
Pipe section
Laterals
Headers
Supply and
return lines
Pipe area
(sq ft)
Number of
anodes
Pipe
length (ft)
Anode
spacing (ft)
4994
1860
30
12
2880
1172
96
98
410
2
180
90
Reproduced from J.R. Myers and MA. Aimone, Corrosion Control for Underground Steel Pipelines: A Treatise on Cathodic
Protection. J.R. Myers and Associates, Franklin, OH. Used with permission.
C-3
TM 5-811-7
(8) Calculate life-cycle cost as recommended in paragraph 2-2. Comparisons with other anode sizes
and types will yield the most economical design.
c.
C-3.
Placement. Locate anodes as shown in figure C-1.
Alternative calculations.
The design examples in paragraphs C-4 and C-5 below use calculations that differ from those used in the text
and in paragraph C-2. Exposure to different methods of calculation should help the design engineer to better
understand the design procedure.
C-4.
Underground steel storage tank.
Galvanic cathodic protection is designed for an underground steel storage tank shown in figure C-2. The tank
is already installed and current requirement tests have been made.
C-4
TM 5-811-7
a.
Design data.
(1) Tank diameter is 12 feet.
(2) Tank length is 40 feet.
(3) Design for 80 percent coating efficiency, based on experience.
(4) Design for 15-year life.
(5) Current requirement is 0.7 ampere.
(6) Packaged 17-pound standard magnesium anodes must be used.
(7) The tank is insulated well enough from foreign structures.
b. Computations.
(1) Find the minimum weight of anodes required for the tank using equation C-I:
W •
YSI
,
E
(eq C-1)
where Y = 15 years, S = 8.8 pounds per ampere-year, I = 0.7 ampere, and E = 0.50 efficiency. Thus,
W •
(15 yr)(8.8 lb/A• yr)(0.7 A)
,
0.50
W= 184.8 lb.
(2) Find the number of magnesium anodes (17 pounds each) required:
N •
c.
C-5.
184.8
• 10.9 (use 12 anodes for symmetry).
17
Placement. Locate anodes as shown in figure C-2.
Gas distribution system.
Galvanic cathodic protection is designed for a gas distribution system in a housing area as shown in figure
C-3.
C-5
TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 4500 ohm-centimeters.
(2) Design for 90 percent coating efficiency, based on experience.
(3) Design for 15-year life.
(4) Design for 2 milliamperes per square foot of bare pipe.
(5) Packaged-type magnesium anodes must be used.
(6) Insulating couplings are used on all service taps. Mains are electrically isolated from all other metal
structures in the area.
(7) All pipe has been precoated at the factory and wrapped with asbestos felt. The coating has been
tested over the trench for holidays and defects have been corrected. The coating is considered to be better
than 99.5 percent perfect when installed.
b. Computations.
(1) Find the total outside area of piping (table C-3).
(2) Find the area of bare pipe to be protected cathodically based on 90 percent coating efficiency:
A = 4288x0.1
A = 429 sq. ft.
(3) Find the maximum protective current required based on 2 milliamperes per square foot of bare
metal:
I = 2 x 429
I = 858 mA or 0.858 A.
Table C-3. Dimensions for finding outside area of pipe
Pipe size
(in.)
3
2
1½
1
Pipe lingth
(ft)
600
1500
1800
2400
3900
Total area of pipe in square feet
Pipe area
(sq ft/lin ft)
0.916
0.622
0.497
0.344
0.278
Pipe area
(sq ft)
550
933
894
826
1084
4288
U.S. Air Force.
(4) Find the weight of anode material required based on maximum current requirement and 15-year
life. Use equation C-1:
YSI
W •
,
E
where Y = 15 years, S = 8.8 pounds per ampere-year, I = 0.858 ampere, and E = 0.50 efficiency. Thus,
(15 yr)(8.8 lb/A• yr)(0.858 A)
,
0.50
W = 227 lb.
W •
Note that the 227-pound value is based on an output current of 0.86 ampere for the cathodic protection
system's full design life, 15 years. Strictly speaking, this is not the true condition, because current output after
new installation is much less due to the high coating efficiency. The average current requirement at first may
be as low as 0.03 milliampere per square foot of pipe.
C-6
TM 5-811-7
(5) Find the current output to ground for a single 17-pound standard packaged magnesium anode using
equation C-2:
i •
Cfy
,
P
(eq C-2)
where C = 120,000, a constant for well coated structures using magnesium, f = 1.00 (table C-4), y = 1.00
(table C-5), P = 4500 ohm-centimeters. Thus,
i •
120,000 × 1.00 × 1.00
4500 ohm• cm
i = 26.7 mA.
Because the structure is well coated, anode spacing will be relatively large.
Table C-4. Galvanic anode size factor (f)
Anode
weight
Standard anodes
3
5
9
17
32
50
50
(packaged)
(packaged)
(packaged)
(packaged)
(packaged)
(packaged-anode dimension 8” dia x 16")a
(packaged-anode dimension 5” x 5” x 31”
Long anodes
(2.75” x 2.75" x 26” backfill 6” x 31”)
(1.5” x 1.5” x 72” backfill 4” x 78”)
(2” x 2” x 72” backfill 5” x 78”)
(2.5” x 2.5”x “60” backfill 5” x 66”)
(3.75” x 3.75” x 60” backfill 6.5” x 66”)
(3” x 3” x 72” backfill 6” x 78”)
Extra-long anodes
(1.6” dia x 10• backfilled to 6” din)
(1.3” x 20• backfilled to 6” din)
(2” dia x 10• backfilled to 8” din)
9
10
18
20
40
42
15
20
25
a
Size
factor
(f)
0.53
0.60
0.71
1.00
1.06
1.09
1.29
1.01
1.71
1.81
1.60
1.72
1.90
2.61
4.28
2.81
In this table, “denotes inches, ‘ denotes feet.
Reprinted from DA. Tefankjian, “Application of Cathodic Protection,” Materials Protection and Performance. Vol 11 (No. 11),
November 1972. Used with permission.
Table C-5. Structure potential factor (y)
Structure-to-electrolyte
potential (volts, relative
to copper-copper sulfate)
-0.70
-0.80
-0.85
-0.90
-1.00
-1.10
-1.20
Magnesium
structure
factor (y)
1.14
1.07
1.00
0.93
0.79
0.64
0.50
Zinc
structure
factor (y)
1.60
1.20
1.00
0.80
0.40
0.00
0.00
Reprinted from l).A. Tefankjian, “Application of Cathodic Protection.•• Materials Protection and Performance. Vol 11 (No. 11),
November 1972. Used with permission.
C-7
TM 5-811-7
(6) Find the number of anodes (n) required from equation C-3:
I
,
i
n •
where I =
(eq C-3)
858 milliamperes and i = 26.7 milliamperes. Thus,
n •
858
26.7
n = 32.1 (use 32 anodes).
(7) Find the anode distribution.
(a) Pipe area protected by one anode:
A = 4288/32
A = l34 sq ft/anode.
(b) Find the anode division (table C-6).
Table C-6. Dimensions for finding anode division
Pipe size
Pipe area
(in.)
(sq ft)
3
550
2
933
1½
895
1
826
¾
1084
Total number of anodes
U.S. Air Force.
C-8
Pipe length
(ft)
600
1500
1800
2400
2900
Number of
anodes
4
7
7
6
8
32
Anode
spacing
(ft)
150
214
257
400
488
TM 5-811-7
APPENDIX D
EXAMPLES OF IMPRESSED CURRENT CATHODIC PROTECTION DESIGN
D-1.
Purpose.
The example in paragraph D-2 below shows how to use the design procedure explained in paragraphs 2-1
and 2-2. Examples in paragraphs D-3 through D-6 are for alternative calculation methods.
D-2.
Steel gas main.
Impressed current cathodic protection is designed for the 6-inch welded gas main shown in figure D-1. This
pipeline is not yet constructed, so measurements cannot be taken.
D-1
TM 5-811-7
a.
Design data.
(1) Average soil resistivity is 2000 ohm-centimeters.
(2) Effective coating resistance at 15 years is estimated at 2500 ohms per square foot.
(3) Pipe has a 6-inch outside diameter.
(4) Pipe length is 6800 feet.
(5) Design for 15-year life.
(6) Design for 2 milliamperes per square foot of bare pipe.
(7) Design for 90 percent coating efficiency based on experience.
(8) The pipeline must be isolated from the pumphouse with an insulating joint on the main line inside
the pumphouse.
(9) HSCBCI anodes must be used with carbonaceous backfill.
(10) The pipe will be coated with hot-applied coal-tar enamel and will be holiday-checked before
installation.
(11) Anode bed must not exceed 2 ohms.
(12) Electric power is available at 120/240 volts a.c. single phase from a nearby overhead distribution
system.
(13) Current requirement test indicates that 2.36 amperes are needed for adequate cathodic
protection.
b. Computations.
(1) Find the gas main’s outside area:
Pipe size - 6 in.
Pipe length - 6800 ft
= 10,681 sq ft.
Pipe area - 6800 x • A = L • d = 6800 • 6
2
12
(2) Check the current requirement using equation 2-1:
I = (A)(I’)(1.0 - CE)
I = 10681 sq ft (2 mA/sq ft)(1.0 - 0.9)
I = 2136 mA,
which agrees with the current requirement test in 13 above.
(3) Select an anode. From table 2-4, choose the 60-pound anode with a 2.8-square-foot surface area
(arbitrary selection).
(4) Calculate the number of anodes needed to meet the anode supplier’s current density limitations; use
equation 2-9:
I
N •
(A1)(I1)
N •
2360 mA
,
(28 sq ft/anode)(1000 mA/sq ft)
(Recommended maximum
current density output for
high-silicon chromiumbearing cast-iron anodes is
1000 mA/sq ft.)
N = 0.84 anode
(5) Calculate the number of anodes required to meet the design life requirements from equation 2-10:
D-2
N •
(L)(I)
(1000)(W)
N •
(15 years)(2360mA)
• 0.59 anode
(1000)(60 lb/anode)
TM 5-811-7
(6) Calculate the number of anodes required to meet maximum anode groundbed resistance
requirements from equation 2-11:
Ra •
(• K)
• P
•
LN
S
• K
N •
•
P
S
2000 ohm• cm (0.016S)
N •
(2000 ohm/cm (0.
7 ft (20 ohm •
20 ft
L (Ra •
N = 2.75 • 3 anodes.
(Values for K and P
from tables 2-6 and
2-7, respectively.)
(7) Select the number of anodes to be used. Since the last calculation resulted in the largest number of
anodes, it will be used. The groundbed resistance, Ra, using three anodes, would equal 1.86 ohms; to insure
compliance with the manufacturer's limitations, four anodes will be used.
(8) Select an area for anode bed placement. The area of lowest resistivity will be used, which is 100
feet from the pipeline.
(9) Determine the total circuit resistance.
(a) Calculate the anode groundbed resistance using equation 2-11:
Ra •
(• K)
• P
•
LN
S
Ra •
2000 ohm• cm (0.0165)
(2000
•
(4 anodes)(7 ft)
(Values for K and P
are from tables 2-6 and
2-7, respectively.)
Ra = 1.46 ohm.
(b) Calculate the groundbed resistance for a 50-foot header cable using equation 2-12. The resistance
specified by the manufacturer is 0.0159 ohm per 100 ft of No.2 AWG cable:
Rw = (ohms/ft)(L)
Rw = (0.0159 ohm/100 ft)(500 ft) = 0.0795 ohm.
(c) Calculate the structure-to-electrolyte resistance from equation 2-14:
Rc •
R
N
Rc •
2500 ohm/sq ft
11,800 sq ft
= 0.212 ohm
D-3
TM 5-811-7
(d) Calculate the total resistance (eq 2-15):
RT = Ra + Rw + Rc
RT = 1.46 ohm + 0.0795 ohm + 0.212 ohm
RT = 1.75 ohms.
(10)
Calculate the rectifier voltage from equation 2-16:
v(rec) = (I)(RT)(150%)
v(rec) = (2.36 A)(1.75 ohms)(150%)
v(rec) = 6.2 V.
c. Select rectifier. Based on the design requirement of 6.2 volts and 2.36 amperes, a rectifier can be
chosen from those marketed. After a rectifier has been chosen, the system's cost can be calculated in
accordance with paragraph 2-2. A comparison with other anode sizes and types will yield the most
economical design.
D-3.
Heating distribution system.
Impressed current cathodic protection is designed for a well coated, buried heating distribution system as
shown in figure D-2. The distribution system has not yet been installed, so measurements cannot be made.
Rectifier size need not be calculated, because it is sized in the field after anode installation.
a.
D-4
Design data.
(1) Average soil resistivity is 1000 ohm-centimeters.
(2) Design for 80 percent coating efficiency based on experience.
(3) Design for 4 milliamperes per square foot of bare metal heating conduits.
(4) Groundbed resistance must not exceed 1.5 ohms.
(5) Graphite anodes must be installed with carbonaceous backfill.
(6) Design for a 15-year life.
TM 5-811-7
(7) Insulating joints must be provided on both steam and condensate lines at the first flange connection
inside all buildings.
(8) All conduit must be metal-bonded together in each manhole.
(9) All conduit will be precoated at the factory and will not have been holiday-checked.
(10) Single-phase electrical power is available at 120/240 volts a.c. from the administration building.
b. Computations.
(1) Find the conduit's total outside area. Because the gage of the metal from which the conduit is made
ranges between 14 and 16, the pipe's outside diameter is considered the same as the inside diameter.
(a) Steam conduit area must be calculated (table D-1).
Table D-1. Dimensions for finding steam conduit area: heat distribution system
Conduit size
(in.)
Conduit length
(ft)
14
1700
12
1125
10
1525
Total area of steam conduit
Conduit area
(sq ft/
(lin ft)
3.67
3.14
2.62
13,768
Conduit
area
(sq ft)
6239
3533
3996
U.S. Air Force
(b) Condensate return conduit area must be calculated (table D-2).
Table D-2. Dimensions for finding condensate return conduit area: heat distribution system
Conduit size
(in.)
8
6
Conduit length
(ft)
1700
2650
Total area of condensate return conduit
Total outside area of all conduit
Conduit area
(sq ft/
(lin ft)
2.09
1.57
Conduit
area
(sq ft)
3553
4161
7713
21481
U.S. Air Force.
(2) Find the area of bare pipe to be cathodically protected based on 80 percent coating efficiency:
A=21,481 x 0.2
A = 4296 sq ft.
(3) Find the maximum protective current required based on 4 milliamperes per square foot of bare
metal:
I = 4296 x 4
I = 17,184 mA or 17.2 A.
(4) Compute the maximum weight of anode material needed for 15 years' life.
(a) Graphite anodes are used.
(b) Average deterioration rate for graphite is 2.0 pounds per ampere-year.
D-5
TM 5-811-7
(c) Find the maximum weight of anode material required (use eq C-1 from appendix C):
W •
YSI
,
E
where Y = 15 years, S = 2.0 pounds per ampere-year, I = 17.2 amperes, and E = 0.50 efficiency. Thus,
W •
(15 yr)(2.0 lb/A• yr)(17.2A)
,
0.50
W = 1032 lb.
c.
Groundbed design.
(1) Anode size is 3-inch by 60-inch (backfilled 10-inch by 84-inch) and weight is 25 pounds per anode
unit.
(2) Find the resistance to earth of a single anode:
Rv •
PK
,
L
(eq D-1)
where P = 1000 ohm-centimeters, L = 7.0 feet (backfilled size), and K = 0.0167, L/d = 8.4 (table 2-6). Thus,
Rv •
(1000 ohm• cm)(0.0167)
,
7.0 feet
Rv = 2.39 ohms
(3) Compute the number of anodes required. The low resistance (2.39 ohms) of a single anode and the
heavy weight of anode material required (1032 pounds) for a 15-year life indicate that the controlling factor
is the amount of anode material, not groundbed resistance. The minimum number of anodes (N) required is
N = 1032/25 = 41.3 or 41 anodes.
These are arranged in a distributed groundbed as shown in figure D-2 based on the following estimates.
(4) Anode distribution:
(a) Conduit area in sections 1 through 6 of figure D-2 are given in table D-3.
Table D-3. Conduit area: heat distribution system
Section
Length (ft)
1
2
3
4
5
6
1700
500
1125
350
400
275
U.S. Air Force.
(b) The area of conduit protected by one anode is —
A = 21,481/41
A = 524 sq ft/anode.
(c) Anodes will be divided as shown in table I)-4.
D-6
Surface area (sq ft)
3553 + 6239 = 9792
785 x 1310 = 2095
1766 x 3533 = 5299
550+ 917=1467
628 + 1048 = 1676
432 + 721=1153
TM 5-811-7
Table D-4. Anode division: heat distribution system
Section
1
2
3
4
5
6
Surface area/
anode protective
area
9792/524 =
2095/524 =
5299/524 =
1467/524 =
1676/524 =
1153/524=
Number of
anodes
19
4
10
3
3
2
U.S. Air Force.
d. Rectifier location. Locate the rectifier in front of the administration building as figure D-2 shows.
The rectifier will be sized after anodes are installed.
D-4.
Black iron hot water storage tank.
Impressed current cathodic protection is designed for the interior of a black iron hot water storage tank
as shown in figure D-3.
a.
Design data.
(1) Tank capacity is 1000 gallons.
(2) Tank dimensions are 46 inches in diameter by 12 feet long.
(3) The tank is mounted horizontally.
(4) Water resistivity is 8600 ohm-centimeters with a pH of 8.7.
(5) The tank's inside surface is bare and water temperature is maintained at 180 degrees Fahrenheit.
D-7
TM 5-811-7
(6) Design for a maximum current density of 5 milliamperes per square foot.
(7) Design for a 5-year life.
(8) Use HSCBCI anodes.
(9) Electrical current is available at 115 volts a.c., single phase.
b. Computations.
(1) Find the tank's interior area using equation D-2:
AT = 2
r2 +
dL,
where r=1.92 feet, d=3.83 feet, and L = 12 feet. Thus,
AT = 2 x 3.1416 x (1.92)2 + 3.1416 x 3.38 x 12
AT = 167.5 sq ft.
(2) Find the maximum protective current required:
I = 167.5 x 5
I = 838 mA or 0.84 A.
(3) Find the minimum weight of anode material needed for a 5-year life (eq C-I from appendix C):
W •
YSI
,
E
where Y = 5 years, S = 1.0 pound per ampere-year, I = 0.84 ampere, and E = 0.50. Thus,
W •
(5 yr)(1.0 lb/A• yr)(0.84 A)
0.50
W = 8.4 lb.
(4) Compute the number of anodes required. An anode 1½ inches in diameter by 9 inches long weighing
4 pounds is chosen as the most suitable size. For proper current distribution, three anodes are required.
(5) Find the resistance of a single anode using equation D-3:
R •
0.012P log (d/D)
L
(eq D-3)
where P =8600 ohm-centimeters, D = 3.83 feet (tank diameter), d = 1½ inches or 0.125 foot (anode
diameter), L = 9 inches or 0.75 foot (anode length). Thus,
R •
0.012 × (8600 ohm• cm) log (3.83 ft/0.125 ft)
0.75 ft
R •
103.2 × log 30.64
0.75
R = 204.5 ohms
This resistance must be corrected by the fringe factor because the anodes are short. The fringe factor is 0.48
from the curve in figure D-4 for an L/d = 9/1.5 = 6:
R = 204.5 x 0.48
R = 98.2 ohms.
D-8
TM 5-811-7
(6) Find the resistance of a three-anode group using an equation based on equation 2-11:
Rn •
1
• . P
Rv •
,
N
S
where Rn = the total anode-to-electrolyte resistance, N = number of anodes, Rv = resistance-to-electrolyte
of a single anode, P = electrolyte resistivity, P = paralleling factor from table 2-7, and S = spacing between
anodes (feet). Thus,
Rn •
1
8600 × 0.00289
98.2 •
3
4
Rn = 38.94 ohms
(7) Find the rectifier rating:
E = IR,
where I = 0.84 ampere and R = 38.94 ohms. Thus,
E = 0.84 x 38.94
E = 32.7 V.
D-9
TM 5-811-7
(a) To allow rectifier aging and film formation, it is considered good practice to use 1.5 as a
multiplying factor:
E = 1.5 x 32.7 = 49.1 V.
(b) The rectifier chosen should produce a d.c. voltage that meets the size requirements of 60-volt,
4-ampere, single-phase.
(8) Locate the rectifier adjacent to tank for the following reasons:
(a) Usually cheaper to install.
(b) Easier to maintain.
(c) Keeps d.c. voltage drop to a minimum.
(9) The d.c. circuit conductors should be installed as follows:
(a) Outside tank — use No.2 AWG high molecular weight polyethylene extruded (HMWPE)
conductor.
(b) Inside tank — use No.8 AWG HMWPE conductor.
(10) The cable should not be stressed or bent.
D-5.
Elevated water tank (ice is expected).
Impressed current cathodic protection is designed for an elevated water tank as shown in figure D-5. The
tank is already built and current requirement tests have been done. Anodes must not be suspended from the
tank roof because heavy ice (up to 2 feet thick) covers the water surface during winter. The anode cables
could not tolerate this weight, so another type of support must be used. Button anodes must be mounted on
the tank's floor and lightweight platinized titanium anodes must be suspended in the riser from the tank
bottom.
D-10
TM 5-811-7
a.
Design data.
(1) Tank height (from ground to bottom of bowl) is 37 feet.
(2) Tank diameter is 24 feet.
(3) High water level in the tank is 34.5 feet.
(4) Overall tank depth is 34.5 feet.
(5) Vertical shell height is 22.5 feet.
(6) Riser pipe diameter is 4 feet.
(7) The tank has a semicircular bottom.
(8) All inner surfaces are uncoated.
(9) Current required for protection — bowl, 7.0 amperes, rise, 1.0 ampere.
(10) Electrical power available is 120/240-volt a.c., single phase.
(11) Tank is subject to freezing.
(12) Design for a 15-year life.
(13) Water resistivity is 4000 ohm-centimeters.
(14) Button-type HSCBCI anodes are used for the tank.
(15) Riser anodes are platinized titanium wire.
b.
Computations.
(1) Find the minimum weight of button anode material required for the tank (eq C-1 from appendix C):
W •
YSI
,
E
where Y = 15 years, S = 1.0 pound per ampere-year, I = 7.0 amperes, and E = 0.50. Thus,
W •
(15 yr)(1.0 lb/A• yr)(7.0 A)
,
0.50
W = 210 lb.
(2) Compute the number of tank anodes needed (button anodes weigh 55 pounds):
N •
210
• 3.82 (use 4 anodes).
55
(3) Find the minimum weight of riser anode material required for the riser (eq C-I from appendix C):
W •
YSI
,
E
where Y = 15 years, S = 1.32 x 10-5 pound per ampere-year, I = 1.0 ampere, and 0 = 0.50. Thus,
(15 yr)(1.32 × 10• 5lb/A• yr)(1.0 A
0.50
-4
W = 3.96 x 10 lb.
W •
(4) Find the number of riser anodes needed. Platinized titanium wire, 0.1-inch in diameter, 3 feet long,
with .001-inch-think platinum over titanium will be used for each anode. The weight of platinum on each
anode is 8.8 x 10-5 pound. Thus,
N •
3.96 × 10• 4
8.8 × 10• 5
• 4.5 (use 5 anodes).
(5) locate anodes as shown in figure D-6.
D-11
TM 5-811-7
(a) Button anodes are mounted on the tank bases at a distance of one-fourth the tank diameter ( ???
feet) from the center. They are mounted on metal angles and plates that are welded to the tank bottom;
polyethylene insulation is required to separate the anode from the metal mounting. Riser anodes are
D-12
TM 5-811-7
suspended in the center of the riser pipe and are spliced to a No.4 AWG cable. The top anode is placed 1 foot
from the tank base. The remaining four anodes are spaced at 4-foot intervals.
(b) Each button anode has its own No.8 AWG 7-strand copper cable (HMWPE) run in conduit to
a resistor box mounted at eye level on a tank leg. The riser anode’s one No.4 AWG 7-strand cable is run in
conduit to the resistor box. If required to get proper current output, a resistor must be installed in the riser
anode circuit at the time of rectifier sizing. The rectifier must be sized after the anodes are installed and must
be mounted at eye level adjacent to the resistor box.
D-6.
Elevated steel water tank.
This impressed current design is for a tank that has not been built; thus, it is not possible to measure current
requirements and other factors. Calculated estimates are used.
a. Design data.
(1) Tank capacity will be 500,000 gallons.
(2) Tank height (from ground to bottom of bowl) will be 115 feet.
(3) Tank diameter will be 56 feet.
(4) The tank’s high water level will be 35 feet.
(5) Overall tank depth will be 39 feet.
(6) Vertical shell height will be 11 feet.
(7) Riser pipe diameter will be 5 feet.
(8) Tank will be ellipsoidal on both top and bottom.
(9) All inner surfaces will be uncoated.
(10) Design for a maximum current density of 2 milliamperes per square foot.
(11) Electric power available will be 120/240-volt a.c., single phase.
(12) String-type HSCBCI anodes will be used.
(13) Design for a 10-year life.
(14) Water resistivity is 4000 ohm-centimeters.
(15) The tank water must not be subjected to freezing.
(16) An assumed deterioration rate is 1.0 pound per ampere-year.
(17) Anode efficiency (assumed) is 50 percent.
b. Computations.
(1) Find the area of wetted surface or tank bowl (figure D-7).
D-13
TM 5-811-7
(a) For the top section (T)—
AT = 2 • rx (approximately),
where r == 28 feet (tank radius), x = 10 feet. Thus,
AT =2 x 3.1416 x 28 ft x 10 ft
A= 1759 sq ft.
(b) For the center section (C)—
Ac =2 • rh,
where r = 28 feet (tank radius) and h = 11 feet. Thus,
` Ac = 2 x 3.1415 x 28 ft x 11 ft
Ac = 1935 sq ft.
(c) For the bottom section (b)—
AB •
2
r a 2 • r 2,
(eq D-5)
(eq D-6)
where r = 28 feet (tank radius) and a 14 feet. Thus,
AB •
2 × 3.1416 × 28 ft × 14 ft 2 • 28 ft 2,
AB = 3894 sq ft.
(d) Therefore, the total wetted area of the tank bowl is—
AT + AC +AB or 7588 sq ft.
(2) Find the riser pipe•s area using equation D-7:
Ar = 2 • rRhR,
where rR = 2.5 feet (riser radius) and hR = 115 feet (riser height). Thus,
AR = 2 x 3.1416 x 2.5 ft x 115 ft
AR = 1806 sq ft
(3) Find the maximum design current for the tank:
IT = 2.0 mA/sq ft x 7588 sq ft
IT = 15,176 mA or 15.2 A.
(4) Find the maximum design current for the riser:
IR = 2.0 mA/sq ft x 1806 sq ft
Ir 3612 mA or 3.62 A.
(5) Find the minimum weight of tank anode material needed (eq C-1 from appendix C):
YSI
,
E
where Y = 10 years, S = 1.0 pound per ampere-year, E = 0.50, and I = 15.2 amperes. Thus,
W •
W •
(10 yr)(1.0 lb/A• yr)(15.2A)
,
0.50
W = 304 lb.
D-14
(eq D-7)
TM 5-811-7
(6) Compute the minimum weight of riser anode material needed (eq C-1):
YSI
,
W •
E
where Y = 10 years, S = 1.0 pound per ampere-year, I = 3.62 amperes, and E = 0.50.
(10 yr)(1.0 lb/A• yr)(3.62A)
,
0.50
W = 72.4 lb.
W •
(7) Find the main anode circle's radius using equation D-8:
r = (DN)/2(• + N)
(eq D-8)
where D = 56 feet and N = 10 (assumed number of anodes). Thus,
56 ft × 10
2(3.1416 • 10)
r = 560/26.28
r •
r = 21.3 ft, use 22 ft.
(8) Determine the spacing for the main anodes. Generally, the distance from the anode to the tank wall
and tank bottom is about equal; this distance should be about one-half the circumference between anodes.
(a) To find circumferential spacing, use equation D-9:
C = (2 • r) / N
(eq D-9)
where r = 22 feet (anode circle radius) and N = 10 (assumed number of anodes). Thus,
2 × 3.1416 × 22 ft
10
C = 13.8 ft, use 14 ft.
C •
(b) The cord spacing is approximately the same as circumferential spacing; 14 feet will be used (fig
D-8).
D-15
TM 5-811-7
(9) Select the main anodes.
(a) The anode unit size chosen is 1c-inch outside diameter, ¾-inch inside diameter, and 9 inches
long. This is a standard sausage-type anode that weighs 1 pound and has a surface area of 0.25 square foot.
(b) The minimum number of anode units per anode string (N), based on a required weight of 304
pounds and 10 anode strings, is computed as follows:
N = 304/(10 x 1)
N = 30.4, use 31 units per string.
(c) Because the inside tank surfaces are uncoated, a maximum structure-to-electrolyte voltage is not
a limiting factor. However, because it is desired to hold the anode current at or below the manufacturer's
recommended discharge rate of 0.025 ampere per anode for this type anode, the minimum number of anodes
will be—
15.2 A
• 60.8 (use 61 anodes per string.
10 × 0.025 A
This number is not practical for the bowl since the distance between the anode hanger and tank bottom is only
28 feet. Table D-5 shows the maximum recommended current discharge per anode for various types of
anodes to insure a 10-year minimum life. Using a type B anode, three anodes per string are required. The
manufacturer does not recommend more than two type B anodes per string assembly because the strings are
fragile. Therefore, the best choice of anode for the main anode strings is type C or CDD. Type CDD is
recommended because the lead wire connection is protected longer by the thicker wall of the enlarged ends.
Two type CDD anodes per string provide a current capacity of 2 amperes 10 strings = 20 amperes. These
anodes are spaced as shown in figure D-9.
Table D-.5. Technical data—commonly used HSCBCI anodes
Anode max
Anode
Weight
discharge
type
Size (in.)
(1b)
(A)
FWa
1c OD x 9
1
0.025
FCb
1½ x 9
4
0.075
G-2
2 OD x 9
5
0.100
G-2½
2½ x 9
9
0.20
Bc,d
1 x 60
12
0.50
C
1½ x 60
25
1.00
CDDc
1½ x 60
26
1.00
Mc
2 x 60
60
2.5
SM
4½ x 60
20
10.0
K-6
6 x 2½
16
0.225
K-12
12 x 3 7/16
53
0.80
B-30
1 x 30
7
0.25
TA-2
2 1/16 x 84
46
6.4
a
For elevated freshwater tank.
b
For distributed system in ground trench.
c
Each end enlarged with cored opening for wire.
d
Not more than two anodes per assembly.
U.S. Air Force.
D-16
Area
(sq ft)
0.2
0.3
0.4
0.5
1.4
2.0
2.0
2.8
5.5
0.5
1.0
0.7
4.0
Max current
density
(A/sq ft)
0.1
0.25
0.25
0.40
0.36
0.50
0.50
0.9
1.8
0.45
0.80
0.36
1.6
TM 5-811-7
(d) Anode current density is computed as follows:
Output •
15.2
2 × 10 × 2
= 0.38 A/sq. ft.
D-17
TM 5-811-7
(10)
Find the main anodes’ resistance (substituting a for d in eq D-3):
R •
0.012P log (D/a)
,
L
where P = 4000 ohm-centimeters, D = 56 feet, L = 2 x 5 feet = 10 feet, and a = 44 x 0.275 = 12.1 feet (0.275
= equivalent diameter factor from curve, fig. D-10). Thus,
(0.102)(4000 ohm• centimeters) log (56 ft/12.1 ft)
10 ft
48 log 4.628
R •
10
R = 3/19 ohms
R •
(a) However, the L/d ratio of two 1½ inch diameter, 60-inch long anodes in tandem is less than 100,
so the fringe factor must be used:
L/d = (2 x 60)/1.5
L/d=80 < 100.
(b) The fringe factor from figure D-4 corresponding to this L/d ratio is 0.95. Thus,
R (adjusted) = 3.19 x 0.95
R = 3.03 ohms.
(11) In designing an elevated water tank, the need for stub anodes must be justified.
(a) The main anode radius has been calculated to be 22 feet. The main anodes are spaced to provide
approximately the same distance from the sides and the bottom of the tank. The main anodes will protect a
D-18
TM 5-811-7
length along the tank bottom equal to 1½ times the spacing of the anode from the bottom.
(b) The anode suspension arrangement for the tank being considered is shown in figure D-9. It can
be seen that stub anodes are required for this design. Ten stub anodes are spaced equally on a circumference
with a radius of 8 feet in a way shown in figure D-8. For smaller diameter tanks, stub anodes may not be
required.
(12) Find the current division between main and stub anodes.
(a) The area of tank bottom protected by stub anodes is found by equation D-10 (fig D-9):
As = • (r22 - r12),
(eq D-10)
2
1
where r = 13 feet (protected segment radius) and r = 2.5 feet (riser radius). Thus,
As = 3.1416 (132 sq ft - 2.52 sq ft)
As = 3.1416 x 162.75
As = 511.3 sq ft.
(b) The maximum current for stub anodes is therefore—
Is = 2.0 x 511.3
Is = 1022.6 milliamperes or 1.02 amperes.
(c) The maximum current for the tank bowl is 15.2 amperes.
(d) The maximum current for main anodes is—
Im = 15.2A - 1.02A
Im = 14.2A.
(13) Find the rectifier voltage rating.
(a) The electrical conductor to the main anode is wire size No.2 AWG, rated at 0.159 ohm per 1000
feet, and has an estimated length of 200 feet. Thus, the resistance of the wire, R, is:
200 ft
R = ——— x 0.159 ohm = 0.032 ohm.
1000 ft
(b) For the voltage drop in the main anode feeder—
E = IR,
where I = 14.2 amperes and R = 0.032 ohm. Thus,
E = 14.2 A x 0.032 ohm
E = 0.45 V.
(c) For the voltage drop through the main anodes—
E = IR,
where I = 14.2 amperes and R = 3.03 ohms. Thus,
E = 14.2 A x 3.03 ohms.
E = 43.0 V.
(d) The total voltage drop in main anode circuit is thus—
ET = 0.45 + 43.0
ET = 43.45 or 45 V.
Use a multiplying factor (safety factor) of 1.5 to get 67.5 volts.
D-19
TM 5-811-7
(14) Select the stub anodes. Because it is desirable to use as small an anode as possible without
exceeding the manufacturers' recommended rate, try using type FC, HSCBCI anode that measures 1½-inches
by 9 inches. Use one anode per string as shown in figure D-9. Anode current density is computed as follows:
Output = 1.02/(10 x 0.03) = 0.34 A/sq ft.
Because this exceeds the recommended maximum anode current density shown in table D-1, the type B
anode is the best choice.
(15) Find the stub anodes' resistance (from eq D-3):
0.012P log (D/a)
,
R •
L
where P = 4000 ohm-centimeter, D = 56 feet, L = 5 feet, and a = 16 0.275 = 4.4 feet (factor from fig D-10)
(0.012)(4000 ohm• centimeters) log (56 ft/4.4 ft)
R •
5 ft
48 log 12.73
R •
5
48 × 1.105
R •
5
R = 10.6 ohms.
L/d = 60/1 = 60 < 100
Using the fringe factor from curve figure D-4, 0.90—
R (adjusted) = 10.6 x 0.90 = 9.54 ohms.
(16) Find the voltage drop in the stub anode circuit.
(a) The electrical conductor to the stub anodes is wire size No.2 AWG, rated at 0.159 ohm/1000
feet, and has an estimated length of 200 feet. Thus,
R = (200 ft/1000 ft) x 0.159 ohm/1000 ft = 0.032 ohm.
(b) To find the voltage drop in the stub anode feeder—
E = IR
where I = 1.02 amperes and R = 0.032 ohm. Thus,
E = 1.02 A x 0.032 ohm
E = 0.033 V.
(c) Find the voltage drop in anode suspension conductors. First, compute the resistance (R) for an
estimated 50-foot-long, No.2 AWG wire rated at 0.159 ohm per 1000 feet:
R = (50/1000) x 0.159 = 0.008 ohm.
E = IR,
where I = 1.02/10 = 0.102 ampere and R = 0.008 ohm. Thus,
E = 1.02 A x 0.008 ohm
E = negligible.
D-20
TM 5-811-7
(d) Find the voltage drop through the stub anodes given that the rectifier output is 80 volts, the
anode current (I) is 1.02 amperes, and the resistance (R) is 9.54 ohms:
E = IR
E = 1.02 A x 9.54 ohms
E = 9.73 V.
(e) Find the total voltage drop in the stub anode circuit.
ET = 0.033 + 9.73
ET = 9.76 V.
(f) Since the stub anode voltage is below the 45 volts calculated for the main tank anode circuit, the
necessary current adjustment can be made through a variable resistor in the stub anode circuit.
(17) Choose a stub anode circuit variable resistor.
(a) The resistor should be able to carry the maximum anode circuit current and have enough
resistance to reduce the anode current by one-half when full rectifier voltage is applied to the anode circuit.
(b) Stub anode circuit data are: rectifier output = 80 volts, anode current = 1.02 amperes, and anode
resistance = 9.54 ohms.
(c) The variable resistor rating is found by–
R = E/I,
where E = 80 volts and I = 1.02/2 or 0.51 ampere. Thus,
R=80/0.51
R = 156.9 ohms
Resistor's ohmic value = 156.9-9.54
= 147.4 ohms.
To find the resistor's wattage rating —
P = I2R
(eq D-11)
P = (1.02)2 x 147.4
P = 153.4 W.
The commercially available resistor that nearest meets the above requirements is a 175-watt, 200-ohm,
1-ampere size.
(18) Find the riser anodes’ resistance. To get the maximum desired current in the riser (3.62 amperes),
the resistance limit is calculated as follows:
R = E/1,
where E = 43.45 volts and I = 3.62 amperes. Thus,
R = 43.5 V/3.62 A
R = 12.0 ohms
(19) Design the riser anode.
(a) Type FW (1-c-inch by 9-inch) string-type anodes cannot be used in the riser because the
maximum anode current discharge of 0.025 ampere per anode would be exceeded. The number of type FW
anodes required would be 145, placed continuously throughout the riser. This number is too high. The best
choice of anode for a flexible riser string is type G-2 (2-inch by 9-inch) high-silicon cast-iron anode.
D-21
TM 5-811-7
(b) The number of units required is found from equation D-3:
R •
0.012P log (D/d)
,
L
or
0.012P lob (D/d)
,
R
where P = 4000 ohm-centimeters, D = 5 feet, d = 2 inches or 0.166 foot, and R = 12 ohms. Thus,
(0.012)(4000 ohm• cm) log (5 ft/0.166 ft)
L •
12 ohms
48 × log 30.1
L •
12
48 × 1.479
L •
12
L •
L = 5.92 ft.
The nubmer of units is thus —
5.92/0.75 = 7.9 or 8 units.
For proper current distribution in the riser pipe, the anode units should not be placed too far apart. It is
generally considered that each anode unit protects a length along the riser pipe equal to 1½ times the spacing
of the anode from the riser pipe wall. Therefore, for a riser height of 115 feet, spacing (center of anode to
tank wall) should be 2.5 feet. The riser length protected by one anode is 1.5 x 2.5 = 3.75 feet, so the number
of units required is 115/3.75=30.7 or 31 units. To satisfy the maximum anode discharge current for a G-2
anode–
3.62 A
• 36.
0.1 A
Therefore, 36 anodes are needed instead of 31 or 8.
(c) To find the anode resistance using 36 anode units, use equation D-3:
0.012P log (D/d)
R •
,
L
where P = 4000 ohm-centimeters, D =5 feet, d = 2 inches or 0.166 foot, and L =36 x 9 inches =324 inches
or 27 feet. Thus,
(0.012)(4000 ohm• cm) log (5 ft/0.166 ft)
27 ft
48 × log 30.1
R •
27
R = 2.63 ohms
R •
The L/d ratio for the riser anode string is 324/2 or 162; thus, no fringe factor correction is used.
(20) Find the voltage drop in the riser anode circuit.
(a) Electrical conductor to riser anodes. For a wire size No.2 AWG, 0.159 ohm per 1000 feet, and
estimated length 200 feet, the resistance (R) is–
R •
D-22
200 ft
0.159 ohm
×
1000 ft
1000 ft
TM 5-811-7
R = 0.032 ohm.
(b) Find the voltage drop in riser anode feeder by–
E = IR,
where I = 3.62 amperes and R = 0.032 ohm. Thus,
E = 3.62 A 0.032 ohm
E = 0.116 V.
(c) Find the voltage drop in the riser anode suspension cables for wire size No.2 AWG, 0.159 ohm
per 1000 feet rating, and estimated length 130 feet;
130 ft
0.159 ohm
R •
×
1000 ft
1000 ft
R = 0.02 ohm.
E = IR
where 1=3.62/2=1.81 amperes average (single current does not flow the full length of the anode string) and
R = 0.02 ohm. Thus,
E = 1.81 A x 0.02 ohm
E = 0.04 V.
(d) Find the voltage drop through riser anodes:
E = IR,
where I = 3.62 amperes and R = 2.63 ohms. Thus,
E = 3.62 A x 2.63 ohms
E = 9.52 V.
(e) Find the total voltage drop in the riser anode circuit:
ET = 0.116V + 0.04V + 9.52V
ET = 9.68 volts.
(21) Select the riser anode circuit variable resistor.
(a) Criteria for the variable resistor are the same as given for the stub anode resistor.
(b) Riser anode circuit data — rectifier output = 80 volts, anode current = 3.62 amperes, anode
resistance = 2.63 + 0.032+0.02 = 2.68 ohms.
(c) Variable resistor rating (resistor should reduce anode current by one-half when full rectifier
voltage is applied)–
r = E/I,
where E = 80 volts and I = 3.62/2 = 1.81 amperes. Thus,
R = 80 V - l.18 A
R = 44.2 ohms.
Resistor ohmic value = 44.2 ohms -2.68 ohms = 41.5 ohms.
Resistor wattage rating = (3.62 A) 2 x 41.5 ohms = 543.8 W.
(d) The commercially available resistor that nearest meets the size requirements is a 750-watt, 50ohm, 3.87-ampere model. This rheostat is 10 inches in diameter and 3 inches deep and is fairly expensive. It
D-23
TM 5-811-7
will not fit into most rectifier cases. In addition, the rheostat consumes large amounts of power. This power
generates heat that can damage components inside the rectifier case unless good ventilation is provided. The
problems found with using a large rheostat can be overcome by using a separate rectifier for the riser anodes.
Although initial cost may be slightly high, power savings will be substantial and heat damage will be avoided.
(22) Size the rectifier for the riser.
(a) Requirements—d.c. output = 3.62 amperes, anode circuit resistance = 2.68 ohms, d.c. voltage
required = IR = 3.62 x 2.68 = 9.70 volts.
(b) Rectifier rating—standard ratings for a rectifier in this size class is 18 volts, 4 amperes.
(23) Find the rectifier d.c. rating for the bowl. Voltage output has been determined to be 80 volts.
Current rating is 15.2 amperes. The commercially available rectifier that nearest meets the above requirements
is 80 volts, 16 amperes.
(24) Determine wire sizes and types. All positive feeder and suspension cables (rectifier to anodes)
must be No.2 AWG HMWPE insulated copper cable. To avoid complication, the negative rectifier cable
(rectifier to structure) must be the same size and type (fig D-11).
(25) Discussion of design.
(a) The design points out the drawbacks of controlling corrosion through cathodic protection
without the aid of a protective coating. For example, when the inside of a tank is coated, the current
requirement is reduced 60 to 80 percent. On large tanks without coating, larger and more costly anodes, wire,
and rectifier units must be used. In addition, the uncoated tank consumes far more power. These costs usually
exceed the cost of a quality coating system over a 10-year period. Corrosion above the water line of a water
storage tank is usually severe because condensation is corrosive. For this reason, protective coatings must
be used above the water line on both large and small water storage tanks to slow corrosion.
D-24
TM 5-811-7
(b) Figures D-11 through D-13 give more guidance in designing cathodic protection systems for
elevated cold water storage tanks.
D-25
TM 5-811-7
(c) HSCBCI was chosen for this design purely as an example. It does not mean this material is better
than other anode types. Other acceptable anode materials include aluminum and platinized titanium or
niobium.
(d) For this design, silicon cells should be specified for the rectifier that protects the bowl, and
selenium cells should be specified for the rectifier that protects the riser. Silicon cells operate more efficiently
at high d.c. output voltages than selenium cells, but require elaborate surge and overload protection. This
protection is not economical in the low power consuming units. A guide for choosing rectifying cells is as
follows: use silicon cells for single-phase rectifiers operated above 72 volts d.c. or for three-phase rectifiers
operated above 90 volts d.c.; use newer, nonaging selenium for single-phase rectifiers operated below 72
volts d.c. or three-phase rectifiers operated below 90 volts d.c.
D-26
TM 5-811-7
APPENDIX E
SPECIFICATIONS FOR CERAMIC ANODE
E-1.
Thickness of ceramic coating.
The coating shall be no less than 0.002 inches thick as shown in figure E-l.
E-1
TM 5-811-7
E-2.
Dissolution rate.
For a current density of 20 amperes per square meter—
- Freshwater, 10 grams per ampere-year.
- Saltwater, 15 grams per ampere-year.
E-3.
Resistivity.
The anode's resistivity shall be 1500 ohm-centimeters.
E-4.
Impact resistance.
A Gardner heavey-duty impact tester (Pacific Scientific, West Silver Spring, MD) shall be used. The anode
will be screwed into a modified holder and a round-nose punch 0.625 inches in diameter will be placed on
the specimen. A 2-pound weight shall be raised to the desired height in a graduated tube and released.
Damage to coatings, such as chipping or cracking, should be observed visually or with low-power (10X)
magnification. The sample should withstand a force of 32 inch-pounds when the weight is dropped; the punch
should be located within an area c inch from the center of the anode without observable damage.
E-2
TM 5-811-7
APPENDIX F
RECTIFIER CURRENT INTERFERENCE*
F-1. This appendix is taken from an unpublished document by T.F. Lewicki of the U.S. Air Force Civil
Engineering Center, Tyndall Air Force Base, FL. It is used in the Air Force Corrosion Reports as an
internal document.
F-2. Cathodic interference.
a.
Method of detecting cathodic interference.
(1) Cathodic interference may be detected by conducting structure-to-soil potential surveys on all
foreign underground structures in the vicinity of the impressed current systems or the structure being
protected. The results are usually plotted as a curve as shown in figure F-1.** To save time, a shortcut
method of detecting cathodic interference may be used. Base maps showing the locations of the proposed
rectifier ground beds, the protected POL system, and all foreign structures should be obtained and studied.
A foreign structure is considered to be a structure which is not part of, or metallically connnected to, the
protected structure. Foreign underground structures that come close to (400 to 1000 feet) the ground bed
and cross or come close to the protected structure at some remote location (greater than 1000 feet from the
ground bed) are prime suspects for cathodic interference. All other suspects should not be ruled out, but the
structures that fit into the category described in the previous sentence will be the most likely structures to
have interference problems. The point at which the foreign structure crosses or comes closest to the protected
structure is the most likely point of cathodic interference or current discharge, commonly referred to as
maximum exposure area. An exception to this would be a case where the foreign structure was well coated.
The holiday in the coating closest to the crossing could be a considerable distance from the crossing.
F-1
TM 5-811-7
(2) To determine if cathodic interference exists at one of these prime suspect areas, the following
should be done:
(a) Refer to figure f-2. Using a high-resistance voltmeter (100,000 ohms per volt or higher) or a
potentiometer-voltmeter circuit, measure the structure-to-earth potential of the foreign structure at the
crossing with all of the proposed rectifiers off. This is called the "natural" or “original” potential. When
making this test, one terminal of the voltmeter should be connected to the foreign structure and the other
terminal connected to a copper-copper sulfate half-cell electrode. The electrode should be placed on the
surface of the earth directly over the structure to be observed.
(b) Turn one of the rectifiers on and record the potential as in a above with the electrode in the same
location. Repeat this procedure with the other rectifiers one at a time to determine which rectifiers are
causing the interference and in what proportion. If the negative structure-to-earth potential is decreased or
swung toward the positive direction when the rectifiers are turned on, current is leaving the structure and
cathodic interference exists.
b. Correction of interference.
(1) When interference has been discovered, permanent corrosion test stations must be installed. During
the preliminary tests for cathodic interference, the minimum size of the wires required for draining the
necessary current should be calculated. Make a temporary bond between the foreign structure wire and the
protected structure wire. The current drained should be measured by placing a milliammeter or am meter in
the circuit. All rectifiers that affect the particular crossing being tested must be turned on for this current drain
test. The following measurements must be recorded: natural structure-to-earth potential of foreign structure;
structure-to-earth potential of foreign structure with all affecting rectifiers ??? structure-to-earth potential
of foreign structure with temporary bond in; current through temporary bond, and potential between the
foreign structure and protected structure with temporary bond in. A meter with a small scale such as the
F-2
TM 5-811-7
M. C. Miller multicombination corrosion engineer's meter that has ammeter scales as large as 10 amperes or
as small as 20 milliamperes or less and a number of small voltmeter scales should be used. The total current
required in a resistance bond to clear the interference current leaving the foreign structure can be calculated
by using the following simple algebraic formula:
• Et
Jd
•
• Er
(eq. F-1)
It
and:
It •
Er × Id
• Et
,
where • Er = Efn - Efr = change in structure-to-earth potential required to clear interference; Et = Efn - Efrd
= change in structure-to-earth potential caused by the temporary bond; Efn = natural structure-to-earth
potential of the foreign structure; Efr = structure-to-earth potential of foreign structure with all affecting
rectifiers on; Efrd structure-to-earth potential of foreign structure with temporary bond in; Id current through
temporary bond; and Tt = total current required through final resistance bond to correct cathodic interference
caused by all of the rectifiers. The minimum size of wire required should also be calculated, but wire smaller
than No.12 AWG should not be used because of poor mechanical strength. The permanent test station should
include two wires thermit welded or brazed to the foreign structure and two wires thermit welded or brazed
to the protected structure. This will allow one wire to be used as a current drain while the other is used to
measure potential.
(2) After the permanent test station has been installed, the resistance bond can be most easily installed
as follows: bare nichrome resistance wire in the range of ½ ohm per foot, or 1 ohm per foot, or 10 ohms per
foot, depending on the total resistance needed, should be used.
(a) Since as many as three rectifiers may be involved in an interference bond in the hydrant refueling
area, a current interrupter in the rectifier circuit for interrupting all the rectifiers simultaneously would not
be practical. All the rectifiers should be turned on and off manually if more than one rectifier is involved. If
only one rectifier causes the interference at a particular site, a current interruptor should be placed in the a.c.
circuit to interrupt the current output.
(b) Place a copper-copper sulfate half-cell electrode on the surface of the earth at the point of
greatest exposure. Measure and record the natural structure to-earth potential of the foreign structure with
all interfering electrical circuits off.
(c) After all interfering sources are turned on, insert a portion of nichrome resistance wire between
the foreign structure wire and the protected structure wire and observe the structure-to-earth potential of the
foreign structure. Increase or decrease the amount of nichrome in the circuit until the structure-to-earth
potential of the foreign (the interfered-with) structure becomes equal to the natural potential measured in b
above. The current flowing through the resistance bond is now the correct amount and the interference has
been cleared. No ammeters or additional test leads can be in series with the resistance bond when this test
is made.
(d) Mark or note the exact contact of the nichrome resistance wire with the test station lead wires.
Add ¼ or ½ inch to each end of these points and cut the nichrome. Crimp an eyelet or fork solderless
terminal at the exact points where the nichrome contacted the test station lead wires. A permanent resistor
has now been fabricated with the exact resistance required for the resistance bond. This resistance may be
installed, removed, and reinstalled for testing purposes without changing the value of the resistance. Install
the fabricated resistor in the test station and check to see that the natural structure-to-earth potential is
achieved. If the potential measured is even a slight amount off, reduce the length of the nichrome or fabricate
a longer piece until the natural structure-to-earth potential is achieved with all rectifiers on and resistance
bond in (Efn will now equal Efrd ). When the proper resistor has been fabricated, the wire should be coiled
on a pencil to make a neat, compact coil that can be installed inside of the test stations without shorting to
the sides of the test station box or the terminals. After the pencil has been removed, the bare nichrome should
not be wrapped with tape to allow proper cooling. Primer and tape may be placed inside the test station box
to prevent a short. The amount of current flowing through the bond will be equal to the value calculated in
F-3
TM 5-811-7
paragraph b above. An ammeter cannot be placed in series with the resistance bond because the resistance
would be increased slightly and the current through the bond would be reduced.
c. Bond installation. The installation of cathodic interference resistance bonds will reduce the cathodic
protection current coming onto the protected POL structure from the electrolyte by an amount equal to the
total current flowing through all of the bonds, This total bond current originates from the rectifiers installed
to protect the protected structure (the POL system in this case). The portion of rectified or d.c. current equal
to the total bond current does not contribute to corrosion mitigation of the POL system. Therefore, the
rectified or d.c. current output may have to be increased as bonds are installed to maintain an adequate POL
structure-to-earth potential of -0.85 volt.
F-4
TM 5-811-7
BIBLIOGRAPHY
Applegate, L. M., Cathodic Protection, McGraw-Hill Book Company, New York, 1960.
Bryan, W. T., Designing Impressed Current Cathodic Protection Systems With Durco Anodes, The Duriron
Company, Dayton, 011, 1970.
Lewicki, T. F., Rectifier Current Interference, U.S. Air Force Civil Engineering Center, Tyndall Air Force
Base, FL, 1974.
Myers, J. R. and Aimone, M. A., Corrosion Control for Underground Steel Pipelines: A Treatise on
Cathodic Protection, James R. Myers and Associates, Franklin, OH, 1980.
Parker, M. E., “Corrosion by Soils," NACE Basic Corrosion Course (A. Brasunus, Ed.), National
Association of Corrosion Engineers, Houston, TX, 1980.
Peabody, A. W., “Principles of Cathodic Protection,” NA CE Basic Corrosion Course (A. Brasunus, Ed.),
National Association of Corrosion Engineers, Houston, TX, 1980.
Segan, E. and Kumar, A., Preliminary Investigation of Ceramic-Coated Anodes for Cathodic Protection,
Technical Report M-333/A133440 U.S. Army Construction Engineering Research Laboratory,
Champaign, IL, 1983.
The Custom VIP Cathodic VIP Cathodic Protection Rectifier. Technical Bulletin, Goodall Electric, Inc., Fort
Collins, CO, January 1982.
TM 5-811-7
GLOSSARY
adsorption - taking up one substance at the surface of another; tendency of all solids to condense on their
surfaces a layer of any gas or solute that contacts such solids.
aeration cell (oxygen cell) - electrolytic cell in which a difference in oxygen concentration at the electrodes
exists, producing corrosion.
amphoterics - materials subject to attack by both acid and alkaline environments; include aluminum, zinc, and
lead (commonly used in construction).
anaerobic - free of air or uncombined oxygen; anaerobic bacteria are those that do not use oxygen in their
life cycle.
anion - a negatively charged ion that migrates toward the anode under influence of a potential gradient.
*anode - electrode at which oxidation of its surface or some component of the solution is occurring
(opposite: cathode).
*bell hole - excavation to expose a buried structure.
cathode - electrode at which reduction of its surface or some component of the solution is occurring
(opposite: anode).
cathodic corrosion - corrosion resulting from a structure's cathodic condition, usually caused by the reaction
of alkaline products of electrolysis with an amphoteric metal.
*cathodic protection - technique to prevent metal surface corrosion by making that surface the cathode
of an electrochemical cell.
cation - positively charged ion of an electrolyte that migrates toward the cathode under the influence of a
potential gradient.
concentration cell - electrolytic cell in which a difference in electrolyte concentration exists between anode
and cathode, producing corrosion.
*continuity bond - metallic connection that provides electrical continuity.
*corrosion - deterioration of a material, usually a metal, because of a reaction with its environment.
* current density - current per unit area.
*electrical isolation - condition of being electrically separated from other metallic structures or the
environment.
*electrosmotic effect - passage of a charged particle through a membrane under the influence of a voltage;
soil can act as the membrane.
*electrode potential - potential of an electrode as measured against a reference electrode. The electrode
potential includes no loss of potential in the solution due to current passing to or from the electrodes (that
is, it represents the reversible work required to move a unit charged from the electrode surface through
the solution to the reference electrode).
electrolyte - chemical substance or mixture, usually liquid, containing ions that migrate in an electric field;
examples are soil and seawater.
electromotive force series (EMF series) - list of elements arranged according to their standard electrode
potentials; the sign is positive for elements with potentials cathodic to hydrogen and negative for those
elements with potentials anodic to hydrogen.
*foreign structure - any structure not intended to be part of the system of interest.
*galvanic anode - metal that, because of its relative position in the galvanic series, provides sacrificial
protection in the galvanic series when coupled in an electrolyte. These anodes are the current source in
one type of cathodic protection.
galvanic cell - corrosion cell in which anode and cathode are dissimilar conductors, producing corrosion
because of their innate difference in potential.
*galvanic series - list of metals and alloys arranged according to their relative potentials in a given
environment.
*holiday - coating discontinuity that exposes the metal surface to the environment.
hydrogen overvoltage - voltage characteristic for each metal-environmental combination above which
hydrogen gas is liberated.
____________________
*Definition is from NACE Standard RP-01.
GLOSS-1
TM 5-811-7
impressed current - direct current supplied by a power source external to the electrode system.
*insulating coating system - all components that comprise the protective coating, the sum of which provides
effective electrical insulation of the coated structure.
*interference bond - metallic connection designed to control electrical interchange between metallic systems.
ion - electrically charged atom or molecule.
*IR drop - voltage across a resistance according to Ohm's Law.
*line current - direct current flowing on a pipeline.
local action - corrosion caused by local cells on a metal surface.
mill scale - heavy oxide layer formed during hot fabrication or heat-treatment of metals. Applied chiefly to
iron and steel.
Molality - concentration of a solution expressed as the number of gram molecules of the dissolved substance
per 1000 grams of solvent.
PH - measure of hydrogen ion activity defined by pH log 10 (1/aH+), where aH+ = hydrogen ion activity =
molal concentration of hydrogen ions multiplied by ion activity coefficient (a = 1 for simplified
calculations).
polarization - deviation from the open circuit potential of an electrode resulting from the passage of current.
*reference electrode - device for which the open circuit potential is constant under similar conditions of
measurement.
* reverse-current switch - device that prevents the reversal of direct current through a metallic conductor.
*stray current - current flowing through paths other than the intended circuit.
*stray current corrosion - corrosion resulting from direct current flow through paths other than the intended
circuit.
*structure-to-electrolyte voltage (also structure-to-soil potential or pipe-to-soil potential) - voltage difference
between a buried metallic structure and the electrolyte, measured with a reference electrode in contact
with the electrolyte.
*structure-to-structure voltage (also structure-to-structure potential) - difference in voltage between metallic
structures in a common electrolyte.
*voltage - electromotive force, or a difference in electrode potentials, expressed in volts.
GLOSS-2
TM 5-811-7
The proponent agency of this publication is the Office of the Chief of Engineers, United
States Army. Users are invited to send comments and suggested improvements on DA Form
2028 (Recommended Changes to Publications and Blank Forms) direct to HQDA (DAENECE-E), WASH DC 20314-1000.
By Order of the Secretary of the Army:
Official:
DONALD J. DELANDRO
Brigadier General, United States Army
The Adjutant General
JOHN A. WICKHAM, JR.
General, United States Army
Chief of Staff
Distribution:
Army: To be distributed in accordance with DA Form 12-34B, requirements for TM 5-800 Series:
Engineering and Design for Real Property Facilities
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