Galvanic anodes system design

Galvanic Anode
Cathodic Protection System Design

by
James B. Bushman, P.E.
Principal Corrosion Engineer
Bushman & Associates, Inc
Medina, Ohio USA

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Page 2 of 10 ©Bushman & Associates, Inc

Galvanic Anode Cathodic Protection System Design
Portions of the following were excerpted from the Appalachian Underground Corrosion Short Course
"Advanced Corrosion Course" text that was edited and revised for applicability to this course
by James B. Bushman, P.E.
Bushman & Associates, Inc. P.O. Box 425, Medina, Ohio,44256
Phone: (330) 769-3694 – Fax: (330) 769-2197

Galvanic anodes are an important and useful to the protected structure. Once
means for cathodic protection of installed, very little maintenance is
underground storage tank systems, pipelines required for the life of the anode. The
and other buried or submerged metallic anode of a galvanic anode system is
structures. The application of cathodic not subject to the same degree of
protection utilizing galvanic anodes is electrical or mechanical malfunction as
nothing more than the intentional creation of that of an impressed current system.
a galvanic electrochemical cell in which two
• Efficient and non-interfering – The
dissimilar metals are electrically connected
relatively low, and normally well
while immersed in a common, electrically
distributed, current output of the
conductive electrolyte. In the "dissimilar
galvanic system can result in a more
metal" cell, the metal higher in the
constant current density at the
electromotive series (or more "active")
protected structure. This minimizes
becomes anodic to the less active metal and
over protection and wasted current.
is consumed during the electrochemical
The low current output reduces the
reaction. The less active metal receives
possibility of interference to a
some degree of cathodic protection at its
minimum. The advantages mentioned
surface due to the current arriving from the
enable the galvanic cathodic protection
anodic metal. The design of a galvanic
system to be utilized efficiently in a
cathodic protection system involves
variety of applications, such as:
consideration of all factors affecting the
proper selection of a suitable anode material a. For well-coated underground
and its physical dimensions, placement, and storage tanks and piping
method of installation.
b. In rural areas and offshore where
ADVANTAGES AND LIMITATIONS power is not available
There are several important advantages to c. For supplemental protection,
using galvanic anodes: such as at pipeline crossings
• No power source is required – Due to d. In isolated corrosive areas ("hot
the fact that the protective current is spots")
generated by the electrochemical e. In highly congested, urban areas
reaction between the metals, no where current distribution and
externally supplied power is required. interference present problems
• Installation and maintenance cost is f. On electrically discontinuous
reduced – Normally, galvanic anodes structures
have the advantage of not requiring g. Well coated pipelines
additional right-of-way purchase since
the anodes are usually installed close


Galvanic Anode System Design edited by James B. Bushman

However, the galvanic anode system is not The efficiency is dependent upon the alloy;
without limitations. The difference in the therefore, it is important that once the proper
potential of the anode and cathode alloy has been selected, the material
(protected structure) that causes the purchased meets the alloy specifications.
protective current to flow is normally quite The next two tables list some typical alloy
small. The small potential difference, or specifications in common usage.
"driving potential," results in very limited
current outputs, especially in high soil The following elements, most commonly
resistivity areas. This fact severely limits the present in magnesium, affect the efficiency
economic use of galvanic systems on: of magnesium anodes used for cathodic
• Large structures protection in soils:
• Poorly-coated structures • Aluminum – Significant effects
AVAILABLE ANODE MATERIALS outside ranges shown
The most commonly used materials for
galvanic anodes on buried structures are • Manganese – Controls to some degree
alloys of magnesium and zinc. the negative impact of iron by
surrounding the iron particles during
When the anode alloy is placed in the
casting solidification
electrolyte for the protection of a structure, a
certain amount of the current is generated
• Nickel – Detrimental to efficiency
due to the self-corrosion of the anode. The
current efficiency is a measure of the actual • Copper – Detrimental to efficiency
current available for cathodic protection of
the primary structure expressed as a • Iron – Detrimental to efficiency, but
percentage of the total current generated. can be controlled to some degree by
Because the anode corrosion rate is directly larger amounts of manganese
proportional to the current output delivered,
the efficiency is an important consideration • Silicon – Detrimental above 0.1
in the selection of anode material. The percent
higher the efficiency is, the more useable
energy per pound of material purchased. • Zinc – Only slightly detrimental in
H-1 Alloy Hi- Hi- higher amounts
Characteristic (AZ-63) Potential Purity
Mag. Alloy Mag. Alloy Zinc • Other (lead, tin, beryllium) – Minor
Solution -1.55 -1.80 -1.10 impurities that do not significantly
potential to
Cu-CuS04 ref.
affect anode efficiency in amounts
cell commonly found, but can be
Faradaic 8.8 8.8 23.5 detrimental above these limits
Consumption
Rate The following two tables provide industry
Current 25-50 50 90+ standard alloy elements for both magnesium
efficiency (%)
and zinc anodes commonly used in cathodic
Actual 250-500 500 360
amps-hrs/lb protection applications. Deviation from
Actual 35-17.5 17.5 26.0 these alloy specifications can result in
lb/amp/year anodes that suffer from pacification, inter-



granular corrosion deterioration and • Reduces self-corrosion of the anode by
excessive consumption rates. promoting a uniform corrosion attack,
thereby improving efficiency
Common alloy specifications - Magnesium
The most commonly used anode backfill
Grade Grade mixture is 75 percent gypsum, 20 percent
Grade
Hi-Pot. “B” “C”
Element
Mg (%)
“A”
Mg Mg Bentonite clay, and 5 percent sodium
Mg (%) sulfate. This mixture is selected because,
(%) (%)
0.010 5.3 - 5.3 - over the wide range of soils likely to be
Al 5.0 - 7.0
max 6.7 6.7 encountered, it has shown the best success in
0.50 - 0.15 0.15 0.15 achieving the desired characteristics. Due to
Mn
1.30 min min min the solubility of backfill components, the
2.5 - 2.O -
Zn 0 2.5 - 3.5
3.5 4.0
backfill tends to "condition" the adjacent
0.10% 0.30% 0.10% soil for several feet.
Si 0
max max max ANODE SELECTION
Cu 0.02 0.02 0.05 0.10
Ni 0.001 0.002 0.003 0.003 After considering the available materials,
Fe 0.03 0.003 0.003 0.003 one must make a suitable selection. The
0.05% criterion for selection is, as one would
each or 0.30% 0.30% 0.30%
Other
0.03% max max max
expect, an analysis of performance versus
max tot. cost. The performance of an anode is
Mg Balance Balance Balance Balance measured by the following criteria:
Common alloy specifications - Zinc • Anode life – Life is a function of three
factors: weight, current output, and
Hi-Purity Zinc
Hi-Amp Zinc
(ASTM B418-67 efficiency. Longer life is achieved
(Mil-A 18001) through heavier weight, lower current
Element for Seawater
Type II)
Use Only
Primarily for output, and high efficiency.
Underground Use
(Percent %)
Percent (%) • Current output – Current output is
Al 0.1 - 0.3 0.005 max governed by electrolyte resistivity,
Cd 0.025 - 0.06 0.003 max anode resistance to electrolyte, and
Fe 0.005 max 0.0014 max
alloy potential. Higher current output
Pb 0.003 max 0.003 max
Zn Remainder Remainder is achieved through lower resistivity,
lower resistance to electrolyte, and
SHAPES, SIZES, AND BACKFILL higher alloy potential.
Galvanic anodes are offered in a wide The costs involved with the installation and
variety of standard shapes and sizes, and operation of galvanic anodes can be
may also be ordered in custom sizes. categorized as follows:
The use of a prepared anode backfill • Material costs--This is based on alloy,
accomplishes the following effects: backfill, and anode size. Generally, the
• Stabilizes anode potential heavier the anode, the lower the cost
per pound of material. More efficient
• Prevents anode polarization, enhancing anode material results in a lower cost
current maintenance per ampere hour of current delivered.
• Lowers anode-to-earth resistance, • Installation costs--The installation cost
increasing current output would not be expected to vary greatly



on a per anode basis regardless of the actual amount of current required. Current
alloy or size of anode selected. testing can be performed utilizing temporary
Therefore, consideration of installation “ground bed” of one or more driven metallic
costs normally involves an rods energized by a test rectifier or storage
investigation of the number of anodes battery.
required. The temporary ground bed is energized and
• Maintenance costs--The cost of its effect upon the structure is measured.
maintenance normally involves only Utilizing proper measurement techniques,
the periodic testing of the cathodic the current output is adjusted until the
protection system, which would not be selected criterion for protection is achieved
substantially affected by the type of with the least amount of current. One or
anode selected. This cost is usually more such temporary ground beds may be
neglected in the selection process. required to analyze sections of the structure,
especially if the physical characteristics of
PRE-DESIGN CONSIDERATIONS
the structure vary significantly. One must
The primary consideration in the design of remember that the resistance to ground of
the galvanic system is the efficient the driven rods is likely to be much higher
distribution of sufficient current to achieve than permanent buried anodes; therefore, the
cathodic protection. Due to the limited range driving voltage required in the test is not
of voltages available the problem of indicative of the actual driving voltage
achieving the desired current becomes one requirement.
of regulating the resistance of the electrical
DESIGN CALCULATIONS
circuit.
The electrical circuit that governs the current
The most important (and least controllable)
output of a galvanic anode is depicted in the
factor affecting the circuit resistance of
next figure.
underground galvanic cathodic protection
systems is soil resistivity. For a small Galvanic Anode Electrical Circuit Components
structure, such as an isolated, very well
coated buried tank, it is often more Grade

economical to overdesign rather than Ia Rw

perform field testing. On the other hand, it is Connecting Wir
e

imperative that testing be conducted for a
poorly coated tank structure. The number of
test points to be considered will vary from
Ea Raf Rcp Rpg Rcg
structure to structure and will depend on the Rcf Ec
Protected
Structure
variation of the resistivity measurements and Anode Backfill containing
the physical characteristics of the structure. 75% Gypsum, 25% Bentonite
and 5% Sodium Sulphate

Areas of predominantly uniform resistivity Galvanic Anode Ingot

will require less frequent measurements than
areas of varying resistivity.
where:
Ea = potential of anode
If the tank structure for which the galvanic
anode design is intended exists, current Ec = potential of cathode
requirement tests should be performed in Ia = anode current
order to more accurately determine the



Raf = anode film resistance is generally considered to be relatively small
when compared to RTAV, the above formula is
Rap = backfill resistance often reduced to the following simplified form:
Rcf = cathode film resistance E −E
Rcg = cathode-to-earth resistance
I = A P

R
A
TAV

Rpg = backfill-to-earth resistance
Rw = resistance of connecting wire This theoretical expression will normally
Rcf is usually negligible in value, compared result in a conservative value of current for
with the other resistive components, whereas anodes in backfill that are installed in the
Raf and Rap are constant for a given anode soil. In addition, it is time-consuming to
in a given backfill. Rcg, the cathode-to- calculate the various resistive factors, and
electrolyte resistance, is heavily dependent often certain. assumptions must be made
on the quality of the structure coating, being that result in an approximate current
nearly negligible for bare structures. calculation. The output of magnesium and
Therefore, Rpg, Rcg, and Rw are the zinc anodes has been fairly well documented
significant and variable components which under varying conditions, and many graphs,
must be considered. charts, and tables have been prepared based
on actual outputs. These references provide
RTAV, the total resistance of a vertically a simplified and reasonably accurate
installed anode in the electrolyte can be determination of anode output under
approximated by H. B. Dwight's equation: conditions normally encountered in the
design of cathodic protection systems for
0.00521   8 L   pipelines, buried tanks, etc. One of the
R = p ln  − 1
L   d  
TAV
widely used references has been prepared by
D. A. Tefankjian. He developed a set of
equations for the output of an anode at a
where: polarized structure potential of -0.85 volts
versus a Cu-CuSO4 reference electrode.
RTAV = Resistance of vertical, rod shaped
anode Correction factors are then applied to adjust
the result for various shapes and structure
p = resistivity of electrolyte, potentials:
L = length of anode rod Imb = 150,000 FY/p
d = diameter of packaged anode Izb = 50,000 FY/p
Once the total anode resistance has been Imc = 120,000 FY/p
calculated, the current output of the anode
can be calculated in accordance with Ohms Izc = 40,000 FY/p
Law:

where:
E −E
I = = amperes
A P
Imb = current output for magnesium anode
R +R +R +R +R
A

on bare structure in milli-amperes
AF AP PG CF W

Since RAF + RAP + RPG is equal to RTAV Izb = current output for zinc anode on bare
calculated above and since RCG + REF + RW structure in milli-amperes



Imc = current output for magnesium anode Note: Anodes are installed vertically.
on coated structure in milli-amperes
Driving voltage correction - Table (y)
Izc = current output for zinc anode on Structure
coated structure in milli-amperes Potential Std. Hi-Pot
Zinc
(vs. Cu- Mag. Mag
P = soil resistivity in ohm-centimeters CuSO4)
F = factor from anode shape table -0.70 1.21 2.14 1.60
Y = factor from driving voltage table -0.80 1.07 1.36 1.20
Anode shape correction - Table (f) -0.85 1.00 1.29 1.00
Anode
-0.90 0.93 1.21 0.80
Weight Packaged -1.00 0.79 1.07 0.40
Alloy Factor
(lbs.) Dimensions (F) -1.10 0.64 0.93 n/a
Mg 3 3" x 3" x 4.5" .53 -1.20 0.50 0.79 n/a
Mg 5 3" x 3" x 7.5" 0.60
Mg 9 3" x 3" x 13.5" 0.71 The equation assumes a minimum resistivity
Mg 9 2.75" x 2.75" x 26" 1.01 of 500 ohm-centimeters and a distance
1.5" x 1.5" x 72" between anode and structure of 10 feet. It
Mg 10 ingot, 4" x 78" 1.71
Package can be seen immediately from the tables that
1.6" dia. x 10' increasing the surface area of the anode
Mg 15 extrusion, 6” x 10’ 2.61 (especially length) or use of a high potential
Backfill alloy has the effect of increasing resultant
Mg 17 4" x 4"x 17" 1.00 current output, assuming other factors are
2" x 2" x 72" ingot,
Mg 18 1.81 equal.
5" x 78" Package
2.5" x 2.5" x 60" For example, compare the current output of
Mg 20 ingot, 5" x 66" 1.60 17-pound standard alloy, high-potential
Package
1.3" dia. x 20'
alloy, and 20-pound (2" dia. x 60")
Mg 20 extrusion, 6” x 20’ 4.28 magnesium anodes. Assume a well coated
Backfill structure, a soil resistivity of 3000 ohm-
2" dia. x 10' centimeters, and an anticipated structure-to-
Mg 25 extrusion, 8” x 10’ 2.81 soil potential of 0.85 volt.
Backfill
Mg 32 5" x 5" x 21" 1.06 Standard 17# H-1 Alloy Magnesium Anode
3.75" x 3.75" x 60"
Mg 40 ingot, 6.5" x 66" 1.72 120,000(1.0)(1.0)
I = = 40mA
Package MC
3000
3" x 3" x 72 ingot,
Mg 42 1.90
6" x 78" Package Standard 17# High Pot. Magnesium Anode
Mg 50 8" dia. x 16" 1.09
Mg 50 5" x 5" x 31" 1.29
120,000(1.0)(1.29)
1.4” x 1.4” x 36” I = = 51.6mA
3000
MC
Zn 18 ingot, 5” x 42” 1.68
Package
2” x 2” x 30” ingot, Long 20# H-1 Alloy Magnesium Anode
Zn 30 1.44
5” x 36” Package
1.4” x 1.4” x 72” 120,000(1.60)(1.0)
Zn 36 ingot, 5” x 78” 1.81 I = = 64mA
3000
MC

Package
2” x 2” x 60” ingot,
Zn 60 1.72 Anodes may be connected in parallel, in
6.5” x 66” Package
order to achieve a higher total current output



at a given location. Unfortunately, the Multiple anode adjusting factors
output of two anodes in parallel which are (Vertically Installed Anodes)
buried less than 30 feet apart (center to Anode Spacing
center spacing) is not quite equal to the sum
(in Feet)
of the current from two separate anodes of
No. of
the same size. Anode
s in 5' 10' 15' 20' 25'
The closer together the anodes are spaced, Bank
the more the current output is restricted
2 1.84 1.92 1.95 1.97 2.03
because the current from one anode tends to
be opposed by the current output from 3 2.46 2.71 2.80 2.85 3.02
adjacent anodes. To determine the 4 3.04 3.46 3.63 3.71 4.01
approximate current output of a multiple 5 3.59 4.19 4.43 4.56 4.98
anode ground bed, multiply the single anode 6 4.13 4.90 5.22 5.41 5.96
current previously calculated by the
7 4.65 5.60 6.00 6.23 6.91
appropriate adjusting factor found in the
8 5.15 6.28 6.77 7.04 7.85
table below.
9 5.67 6.96 7.54 7.88 8.82
The table is calculated for 17-pound
10 6.16 7.64 8.30 8.68 9.75
packaged anodes installed vertically in
parallel. For approximate calculations, it is 11 6.76 8.41 9.14 9.56 10.75
good for any size anodes. 12 7.30 9.12 9.93 10.40 11.71

For a more exact calculation, an adjusting 13 7.83 9.83 10.72 11.23 12.68
factor may be determined from the 14 8.37 10.54 11.51 12.07 13.64
following equation (based upon the E.D. 15 8.91 11.25 12.30 12.91 14.61
Sunde formula for resistance to earth of 16 9.44 11.96 13.09 13.75 15.57
multiple anodes). This equation is provided 17 9.98 12.68 13.89 14.58 16.54
immediately following the table developed
18 10.51 13.39 14.68 15.45 17.50
by Mr. Tefanjian.
19 11.05 14.10 15.47 16.26 18.47
N 20 11.59 14.81 16.26 17.10 19.43
MA =
ADJ
2 L(ln 0.656 N )
1+
  8L  
S ln  − 1 To determine the approximate current output
  d  
of six 17-pound standard alloy anodes
spaced on 10-foot centers in 3000 ohm-
Where:
centimeter soil with a structure potential of
MAADJ = Multiple Anode Adjusting (-)0.85 volts, it was determined earlier that
Factor the current output of a single 17 pound
N = number of anodes in parallel anode under these same conditions = 40
milli-amperes.
L = length of the anode in feet
From the Multiple Anode Adjusting Factor
d = diameter of the anode in feet Table, select 4.90 from the 6 anode row and
S = spacing, center-to-center in feet the 10’ column.



Therefore the output of the six anodes = The utilization factor accounts for a
(40)(4.90) = 196 ma. reduction in output as the surface area of the
anode decreases with time, limiting the
Having arrived at an anode configuration
anode output. This factor is usually assumed
that will produce the required current output
to be 0.85. The equation may then be
is not sufficient in itself. An examination of
reduced to simpler form by substituting the
the estimated life of the anodes must be
constant factors:
undertaken in order to determine whether
the design will provide protection for a
For magnesium:
reasonable period of time. The following
expression may be used to calculate the
estimated life of the anode: 48.5W
L =
I
M

Anode Life = [Faraday
Consumption Rate (Ampere For zinc:
Hours/Pound)/No. of Hours
per Year] x Anode Weight (lbs) 32.5W
L =
x Anode Efficiency x Utilization Z
I
Factor/Anode Current in
Amperes where:

W = Anode metal weight in pounds
I = Current output in milli-amperes
LM = magnesium anode life, years
LZ= zinc anode life, years

The expected life of the cathodic protection
system should be consistent with the design
life, use, and maintenance of the protected
structure.


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