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Cathodic Protection Design

Chemical Engineering (University of Engineering and Technology Lahore)

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CATHODIC PROTECTION DESIGN

from Swain Classnotes (1996)

STRUCTURE
Metal, Design Life, Dimensions, Coatings, Other

LOCATION
Environmental Conditions, Other Structures

C.P. CRITERIA
Potential for Cathodic Protection

C.P. CURRENT DEMAND

Initial, Mean, Final

C.P. TYPE
Impressed Current, Sacrificial Anode

ANODE AND HARDWARE SELECTION

Current Output, Design Life, Placement

COST AND IMPLEMENTATION

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Table of Contents
1.0 Introduction .................................................................................................................................. 3
2.0 Structure ...................................................................................................................................... 3
3.0 Location ....................................................................................................................................... 3
4.0 CP Criteria ................................................................................................................................... 4
4.1 Potential Values ...................................................................................................................... 4
4.2 300 mV Shift ............................................................................................................................ 5
4.3 100 mV Shift ............................................................................................................................ 5
4.4 E-log-I Curve ........................................................................................................................... 6
4.5 Anodic Current Discharge Points ............................................................................................ 6
5.0 Cathodic Protection Current Demand ......................................................................................... 6
5.1 Recommended Practice RP B401, Det Norske Veritas .......................................................... 6
5.1.1 Uncoated Steel ............................................................................................................... 7
5.1.2 Coated Steel ................................................................................................................... 7
5.1.3 Pipeline Coatings............................................................................................................ 9
5.1.4 Concrete ......................................................................................................................... 9
5.2 Current Requirements for Pipelines in Soils of Different Types............................................ 10
5.3 Current Requirements for Ship Protection ............................................................................ 11
6.0 Cp Type ..................................................................................................................................... 12
7.0 Anode Selection......................................................................................................................... 13
7.1 Anode Resistance to Ground ................................................................................................ 16
7.2 Anode Ground Beds.............................................................................................................. 18
7.3 Anode Current Output ........................................................................................................... 18
7.4 Anode Size, Weight, Number, Distribution and Design Life ................................................. 18
8.0 Cost and Implementation........................................................................................................... 21
8.1 Oil Platform Example............................................................................................................. 21
8.1.1 Structural Details .......................................................................................................... 21
8.1.2. Current Demand ........................................................................................................... 21
8.1.3. Sacrificial Anode Design for Uncoated Structure ......................................................... 22
8.1.4. Sacrificial Anode Design Coated Structure .................................................................. 26
8.1.5 Impressed Current Anode Design ................................................................................ 27
8.2 Ship Hull Protection............................................................................................................... 29

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1.0 INTRODUCTION
Metallic structures in contact with water, soil, concrete, and moist air are subject to
corrosion. Cathodic protection (CP) is one of the few methods that successfully
mitigates corrosion. It can be applied in any situation where the environment
surrounding the metal acts as a conductor for electric current. It has been successfully
applied to offshore structures, ships, boats, propellers, moorings, pipelines, storage
tanks, piers, jetties, bridges, aquaria, instrumentation etc.

This handout is designed as an introduction to CP design. As such, it does not cover all
aspects of the subject. Therefore, the student should realize the limitations of his/her
knowledge and consult other literature or experts in the field when necessary.

2.0 STRUCTURE
CP design begins with a thorough understanding of the structure to be protected. This
includes the following information:
• Metal type(s)
• Operating conditions
• Dimensions and surface area
• Coatings
• Data from previous structures and CP systems
• Design life

3.0 LOCATION
The environmental conditions are determined by the location of the proposed
installation. Factors such as climate, electrolyte conductivity and chemistry, physical
loading, and biological activity, all impact CP requirements. These factors are generally
allowed for in the CP current demand and polarization potential criteria.

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4.0 CP CRITERIA
Potential measurements are the most commonly used criteria to ascertain the level of
CP afforded to metals and alloys. CP potential values vary according to the metal and
the environment. Corrosion is likely to occur at potentials which are more positive than
the protected value. Damage may also occur if the metal is overprotected (i.e. the
potential too negative). The most common error associated with potential
measurements is a result of IR drop. This is the displacement of measured metal
potential due to current flow through the electrolyte. High electrolyte resistivity and high
current densities can cause significant differences between the measured and actual
metal potential.

4.1 Potential Values

The measurement of potential with respect to a standard reference electrode is
probably the most common method of evaluating the degree of cathodic protection
afforded to a structure. Typical cathodic protection potentials for commonly used metals
ref. Ag/AgCl reference electrode (seawater) are provided in Table 4.1. A more detailed
summary of protection potentials for steel in seawater is provided in Table 4.2.

Table 4.1 Approximate freely corroding and protected potentials of metals in

seawater (may vary according to velocity and conditions).
Metal or Alloy Freely Corroding Protected Potential
Potential (V) ref. Ag/AgCl
(V) ref. Ag/AgCl
316,304 Stainless (passive) -0.10 -0.75
Copper Alloys -0.35 -0.70
316,304 Stainless (active) -0.50 -0.75
Steel -0.60 -0.80
Aluminum Alloys -0.75 -1.00
Zinc/Aluminum Anodes -1.05
Magnesium Anodes -1.50

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Table 4.2 Potential values for corrosion and protection of steel in seawater
V ref. Ag/AgCl Condition V ref. Zn
Heavy Corrosion
-0.60 Freely Corroding Steel +0.50
-0.70 Some Protection +0.40
-0.80 Cathodic Protection +0.30
-0.90 Some Over +0.20
-1.00 Protection +0.10
-1.10 0.00
-1.20 Over Protection -0.10
-1.30 May Cause -0.20
-1.40 Paint Blistering and Flaking -0.30
-1.50 -0.40

4.2 300 mV Shift

The NACE Standard, RP-02-85 states that a minimum negative (cathodic) voltage shift
of 300mV, produced by the application of protective current should provide CP to iron
and steels. The voltage shift is measured between structure surface and a stable
reference electrode contacting the electrolyte. This criteria does not apply to structures
in contact with dissimilar metals.

4.3 100 mV Shift

The NACE Standard, RP-02-85 states that a minimum negative (cathodic) voltage shift
of 100mV measured between the structure surface and a stable reference electrode
contacting the electrolyte should provide CP to iron and steel. This polarization voltage
shift is determined by interrupting the protective current and measuring the instant off
and polarization decay. The instant off value is obtained immediately following the
interruption of the CP current. The voltage shift is equivalent to the IR drop created by
the CP current and electrolyte resistance. The polarization decay is measured as the
change in potential over a period of time from the instant off value.

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4.4 E-log-I Curve

The NACE Standard, RP-02-85 states that a structure-to-electrolyte voltage at least as
negative (cathodic) as that originally established at the beginning of the Tafel segment
of the E-log-I-curve should provide CP to iron and steel. This structure-to-electrolyte
voltage shall be measured between the structure surface and a stable reference
electrode contacting the electrolyte at the same location where voltage measurements
were taken to obtain the E-log-I curve.

4.5 Anodic Current Discharge Points

The NACE Standard, RP-02-85 states that a net protective current from the electrolyte
into the structure surface as measured by an earth current technique applied to
predetermined current discharge (anodic) points of the structure should provide CP to
iron and steel.

5.0 CATHODIC PROTECTION CURRENT DEMAND

The cathodic protection current demand is the amount of electricity required to polarize
the structure to a level that meets the criteria described in Section 4. This may be
obtained from a trial polarization of the structure at the installation site, a trial
polarization of a metal test coupon at the installation site, or from conservative
estimates obtained from historical information obtained from previous structures
operating under the prescribed conditions.

For planning and design purposes, it is often possible to rely on conservative estimates
provided by recommended practice. There are several sources for this information.
The most current one is Recommended Practice RP B401, Cathodic Protection Design,
Det Norske Veritas Industri Norge AS, 1993.

5.1 Recommended Practice RP B401, Det Norske Veritas

The CP current densities are calculated for different environmental conditions and
conditions of the steel (i.e. uncoated, coated, concrete reinforcing steel, pipeline).

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5.1.1 Uncoated Steel

Three design current densities are given: initial, final, and average.
Initial This is the current density required to effect polarization of the initially
exposed bare steel surface. It assumes some atmospheric rusting and/or
millscale. The initial current density is higher because of lack of
calcareous scales (cathodic chalks). A proper initial current density
enables rapid formation of protective calcareous scales.
Final This is the current density required to protect the metal surface with
established marine growth and calcareous layers. It takes into account
the current density required to repolarize the structure in the event of
removal of these layers by storms, cleaning operations etc.
Average This is the anticipated current density required once the cathodic
protection system has reached its steady state. The average or
maintenance current density is used to calculate the minimum mass of
anode material required to protect the structure throughout the design life.

Table 5.1 Initial, final, and average current densities for various climatic
conditions and depths (climatic conditions are based on yearly range
of average surface water temperatures).
Design Current Densities (A/m2)
Tropical >20oC Sub-Tropical 12o-20oC Temperate 7o-12oC Arctic <7oC
Depth Initial Final Average Initial Final Average Initial Final Average Initial Final Average
(m)

0 - 30 0.150 0.090 0.070 0.170 0.110 0.080 0.200 0.130 0.100 0.250 0.170 0.120
>30 0.130 0.080 0.060 0.150 0.090 0.070 0.180 0.110 0.080 0.220 0.130 0.100

5.1.2 Coated Steel

The use of coatings on steel dramatically reduces the current demand on the cathodic
protection system. This can save on the cost and structural weight associated with
sacrificial anode systems. The CP current demand of a coated offshore jacket may be
estimated by multiplying the bare steel current demand by a coating breakdown factor
(fc). The coating breakdown factor does not allow for mechanical damage to paint

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coatings. These areas are treated as bare metal surface. For CP design purposes the
average and final coating breakdown factors for a design life of tr years are as follows:
tr
f c (average) = k1 + k 2
2
f c ( final ) = k1 + k 2 t r
When the design life of the CP system exceeds that of the coating system then fc
(average) is calculated as follows:

f (average) = 1 −
(1 − k ) 1
2

c
2k 2 t r

If the calculated value exceeds 1, then fc = 1 shall be applied to the design.

Table 5.2 Constants (k1 and k2) for calculation of paint coating breakdown
factors.
Category Description k1 k2 k2
0-30m >30m

I One layer of primer coat, about 50 μm nominal 0.10 0.10 0.05

DFT.

II One layer of primer coat, plus minimum one layer 0.05 0.03 0.02
of intermediate top coat, 150 - 250 μm nominal
DFT.
III One layer of primer coat, plus minimum two 0.02 0.015 0.012
layers of intermediate/top coats, 300 μm nominal
DFT.
IV One layer of primer coat, plus minimum three 0.02 0.012 0.012
layers of intermediate/top coats, 450 μm nominal
DFT.

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5.1.3 Pipeline Coatings

The coating breakdown factors as shown in table 5.2 apply equally to both buried and
non-buried pipelines. It is assumed that coatings and field joint systems have been
chosen to be compatible with the maximum design temperature of the pipeline.

For pipelines with the following coating systems, another coating breakdown factor is
calculated.
⇒ asphalt + concrete weight coating
⇒ fusion bonded epoxy + adhesive + polyethylene or polypropylene
⇒ polychloroprene rubber
⇒ equivalent coating systems based on an inner layer dedicated to corrosion
protection and one or more outer layers for mechanical protection.
This is as follows:
f c (average) = 0.05 + 0.002(t r − 30)

f c ( final ) = 0.07 + 0.004(t r − 20)

5.1.4 Concrete
It is now recognized that cathodic protection of concrete reinforcing steel is necessary to
ensure the long term integrity of the structure. Also, any CP system designed to protect
metallic appendages and components must be designed to allow for current drain from
CP to the reinforcement. The cathodic current density is determined by transport of
oxygen to the steel by capillary action of pore water driven by evaporation in the
atmospheric zone and internal dry compartments. The current densities are, therefore,
dependent on depth and climatic conditions.

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Table 5.4 Design current densities for concrete reinforcing steel.

(NOTE: design currents refer to the area of the reinforcing steel)

Design Current Densities (A/m2)

Depth (m) Tropical >20oC Sub-Tropical Temperate 7o- Arctic <7oC
12o-20oC 12oC
5 to -10 0.0030 0.0025 0.0015 0.0010
<-10 0.0020 0.0015 0.0010 0.0008

5.2 Current Requirements for Pipelines in Soils of Different Types

The current demands for steel pipelines are determined by the soil type (conductivity,
pH, moisture, temperature) and the condition of the steel (coating type). An example of
typical CP current demand for a pipeline with different coating conditions is presented
as follows.

Table 5.3 Range of current required to protect 10 miles of 36" diameter pipe in
soil with average resistivity of 1000 ohm-centimeters. Current
required is that needed to cause a 0.3 Volt drop across the effective
resistance between pipeline and remote earth. [from A.W.Peabody,
Control of Pipeline Corrosion,NACE, 1967]
Effective Coating Resistance in Ohms Current Required, Amps
for One Average Square Foot
Bare Pipe (minimum 1 mA/ft2) 500
10,000 14.91
25,000 5.964
50,000 2.982
100,000 1.491
500,000 0.2982
1,000,000 0.1491
5,000,000 0.0298
Perfect Coating 0.000058

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5.3 Current Requirements for Ship Protection

Information with regard to current density requirements for ship hull protection is limited.
One source of information is the Technical and Research Report R-21, Fundamentals
of Cathodic Protection for Marine Service, The Society of Naval Architects and Marine
Engineers, January 1976. It must be remembered that this was compiled before the
development of modern day bottom coatings. It may, therefore, be better to use the
DNV practice for coated steel and to include an allowance for damaged surfaces.

Table 5.5 Protective current densities for ships. [from Technical and Research
Report R-21, Fundamentals of Cathodic Protection for Marine
Service, The Society of Naval Architects and Marine Engineers,
January 1976]
Specific Area Current Density, mA/m2
External Hull 22-54
Rudders (Coated and for velocities not exceeding 5 knots. 490
Current demand maybe 3 or more times greater underway)
Propellers (For velocities not exceeding 5 knots. Current 150 -170
demand maybe 3 or more times greater underway)
Coated Tanks 11
Segregated Ballast 150
Washed Cargo / Clean Ballast 130
Dirty Ballast Tanks 86

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6.0 CP TYPE
The CP type determines how the cathodic current is supplied to the structure. CP can
be applied by either an impressed current system or by a sacrificial anode system.
Impressed current CP systems use an external DC current source and a variety of
anode materials to supply the cathodic current. Sacrificial anode CP systems generate
the cathodic current from the corrosion of metals less noble than the metal to be
protected.

The choice between impressed and sacrificial cathodic protection depends many factors
and may be just personal preference. There are, however, situations where one or the
other provides the correct choice. The advantages and disadvantages of each type of
CP system are described in Table 6.1.

Table 6.1 Advantages and disadvantages of impressed current and sacrificial

anode CP systems.
Impressed Current Sacrificial Anodes
Advantages
Variable control of current and potential Self contained
Can be automated Can be self adjusting
Light weight and fewer anodes Polarity of connections always correct
Varied anode geometry Needs no supervision
Long life with inert anodes Simple to install
Disadvantages
Complex installation and maintenance Expensive method of generating electricity
Requires external power source No variable control
Anodes require dielectric shields Anodes add weight
Anodes may be damaged Anodes have finite life
Probability of stray current corrosion Small lead resistance reduces current

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7.0 ANODE SELECTION

Anodes, for both impressed current and sacrificial anodes, are selected according to
their size and chemical composition. This determines the current output and design life.
Specifications for impressed current anodes are provided in Table 7.1 and for sacrificial
anodes in Tables 7.2 and 7.3.

Table 7.1 Impressed current anodes.

Anode Material Recommended Maximum Consumption Comments
Current density Voltage, V Rate, g/A-yr
2
A/m
Scrap Steel Varies - 200 - 9,000 Difficult life
prediction
Graphite 10 - 30 - 450 Very brittle
Silicon-Chromium- 10 - 100 - 90 - 250 Very brittle
Cast Iron
Lead-Silver 250 - 500 - 30 - 90 Heavy, Poor
mechanical
properties
Lead-Platinum 100 - 2 - 60
Magnetite 10 - 500 - 40 Very Brittle
Platinized 250 - 700 9 0.01 5 μm thick Pt film
Titanium provides 10 year
life
Platinized 500 - 1000 100 0.01 5 μm thick Pt film
Tantalum provides 10 year
life
Platinized 500 - 1000 100 0.01 5 μm thick Pt film
Columbium provides 10 year
life
Lithium-Ferrite 15 - 2000 9.7 1-2 Lightweight and
Ceramic tough

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Table 7.2 Sacrificial anode types and use.

Anode Preferred Use Approx. Potential
Volts ref. Ag/AgCl
Magnesium, High Soils with resistance > 2000 Ω-cm -1.75
Potential
Magnesium, Standard Soils with resistance < 2000 Ω-cm, and in aqueous -1.50
environments with controllers if necessary
Zinc, Hi-Amp Seawater, brackish water, saline mud. Temps < -1.05
o
60 C
Zinc, Hi-Purity Underground, fresh water, and saline environments -1.05
o
> 60 C
Galvalum I Submerged seawater, max. temp 25 oC -1.05
Galvalum II Saline mud -1.04
Galvalum III Seawater, brackish water, saline mud -1.10
Reynode -1.05
Al-Sn-In Alloy -1.05

Table 7.3 Sacrificial anode properties.

Property Anode Material Type
Magnesium Zinc Galvalum 1 Galvalum II Galvalum
III
3
Density, kg/m 1940 7130 2700 2700 2700
Electrochem Equiv, g/coulomb 0.126E-3 0.339E-3 0.093E-3 0.093E-3 0.093E-3
Theoretical Ah/Kg 2,205 819 2,987 2,987 2,987
Current Efficiency % 0.55 0.95 0.95 0.57 0.85
Actual Ah/Kg 1,212 780 2,830 1,698 2,535
Actual Kg / Amp / Year 7.95 11.25 3.10 5.16 3.46
Potential V, ref. Ag/AgCl -1.75 -1.05 -1.05 -1.04 -1.10

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Table 7.4 Sacrificial Anode Composition.

Percent of Total Weight
Mg Zn Al Cd Cu Fe Hg In Mn Ni Pb Si
Magnesium, rem 0.01 0.02 0.03 0.50 - 0.001
High Potential 1.30
Magnesium, rem 2.5 - 5.3 - 0.05 0.003 0.15 0.003 0.30
Standard 3.5 6.7
Zinc, Hi-Amp rem 0.1 - 0.025 - 0.005 0.006
0.4 0.060
Zinc, Hi-Purity rem 0.003 0.0014 0.003

Galvalum I 0.35 - rem 0.035 - 0.14 -

0.48 0.048 0.21
Galvalum II 3.5 - rem 0.035 -
5.0 0.048
Galvalum III 2.8 - rem 0.01 - 0.08 -
3.5 0.02 0.12

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7.1 Anode Resistance to Ground

The current output from an anode is determined by its shape, electrolyte resistance, and
driving potential. The shape and electrolyte resistance determine the anode resistance
to ground which is calculated from standard anode resistance formulae. The most
commonly used formulae are presented in Table 7.4 and seawater conductance values
in Table 7.5.

Table 7.4 Anode resistance to ground formulae.

Anode Type Resistance Formula
Long Slender stand-off L ≥ 4r ρ ⎛ ⎛ 4L ⎞ ⎞
R= ⎜ ln⎜ ⎟ − 1⎟
2πL ⎜⎝ ⎝ r ⎠ ⎟⎠
(Modified Dwight)

Long Slender stand-off L < 4r ⎡ ⎧ 2⎤

ρ ⎢ ⎪ 2 L ⎛⎜ ⎛ r ⎞
2
⎞⎫⎪ r ⎛ r ⎞ ⎥
R= ln ⎨ 1+ 1+ ⎜ ⎟ ⎟ + − 1+ ⎜ ⎟
2πL ⎢ ⎪ r ⎜ ⎝ 2L ⎠ ⎟⎬ 2 L ⎝ 2L ⎠ ⎥
⎣⎢ ⎩⎝ ⎠⎪⎭ ⎦⎥
Long flush mounted ρ
R= (Lloyds)
L ≥ 4 x width and thickness 2S
Short flush-mounted, bracelet 0.315 ρ
R= (McCoy)
and other flush mounted shapes A

where: R is anode resistance, ohms

ρ is electrolyte resistivity, ohm-cm
L is anode length, cm
a+b
S is the mean of the anode sides =
2

anode cross - sectional area

r is equivalent radius, cm, =
π
2
A is the exposed surface area of anode, cm

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Table 7.5 Specific Conductance of Seawater

Note: Resistivity, ρ, is the reciprocal of conductance. Tabled values are expressed in in
(Ω-1-cm-1).

Chlorinity, ppt Temperature, oC

0 5 10 15 20 30
1 0.001839 0.002134 0.002439 0.002763 0.003091 0.003431
2 0.003556 0.004125 0.004714 0.005338 0.005971 0.006628
3 0.005187 0.006016 0.006872 0.007778 0.008702 0.009658
4 0.006758 0.007845 0.008958 0.010133 0.011337 0.012583
5 0.008327 0.009653 0.011019 0.012459 0.013939 0.015471
6 0.009878 0.011444 0.013063 0.014758 0.016512 0.018324
7 0.011404 0.013203 0.015069 0.017015 0.019035 0.021121
8 0.012905 0.014934 0.017042 0.019235 0.021514 0.023868
9 0.014388 0.016641 0.018986 0.021423 0.023957 0.026573
10 0.015852 0.018329 0.020906 0.023584 0.026367 0.029242
11 0.017304 0.020000 0.022804 0.025722 0.028749 0.031879
12 0.018741 0.021655 0.024684 0.027841 0.031109 0.034489
13 0.020167 0.023297 0.026548 0.029940 0.033447 0.037075
14 0.021585 0.024929 0.028397 0.032024 0.035765 0.039638
15 0.022993 0.026548 0.030231 0.034090 0.038065 0.042180
16 0.024393 0.028156 0.032050 0.036138 0.040345 0.044701
17 0.025783 0.029753 0.033855 0.038168 0.042606 0.047201
18 0.027162 0.031336 0.035644 0.040176 0.044844 0.049677
19 0.028530 0.032903 0.037415 0.042158 0.047058 0.052127
20 0.029885 0.034454 0.039167 0.044114 0.049248 0.054551
21 0.031227 0.035989 0.040900 0.046044 0.051414 0.056949
22 0.032556 0.037508 0.042614 0.047948 0.053556 0.059321

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7.2 Anode Ground Beds

Anode ground beds are used to increase the anode current output in soils. They
typically comprise an excavation which is filled with low conductance carbonaceous
material into which the anode(s) are placed. The total resistance of the system then
becomes the resistance of the anode to the carbonaceous backfill plus the resistance to
earth of the backfill itself. The anode resistance is reduced by the low resistance of the
backfill (typically 50 ohm-cm for coke breeze), and the resistance of the backfill to earth
is reduced by the large surface area of the backfill in contact with the soil. Standard
anode to ground resistance formulae are used to obtain the resistance values.

Because of the variables involved in ground bed sites, experience is invaluable in

attaining competence in their design. They are designed with regard to the current
demand of the structure to be protected, to the soil resistance, and to other structures
and stray current effects.

7.3 Anode Current Output

Anode current output is calculated using Ohm’s Law:
V
I=
R
This is where V is the driving potential, and R is the anode resistance. The driving
potential is determined by the anode type. The driving potential for sacrificial CP
systems is determined by the environment (Table 7.2), but for impressed current
systems it is determined by the rectifier and controller voltage output (Table 7.1). The
anode resistance to ground is found from the anode resistance formulas (Table 7.4).

7.4 Anode Size, Weight, Number, Distribution and Design Life

The CP system must be designed to provide the required current to every part of the
structure for the required design life. This requires determining anode size, weight,
number, and distribution.

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The calculations for impressed current CP systems are relatively simple. In this case, it
is only necessary to match the number of anodes of known current output to the total
current demand of the structure, and to be sure that the anode distribution insures an
even and well balanced current distribution.

The calculations for sacrificial CP systems are a little more complex. Not only must the
number of anodes satisfy the current demand of the structure, but they must also have
sufficient mass to provide electricity for the design life of the structure.

Anode size and shape are determined by the following factors:

• Requirements for minimum and maximum current output
• Requirement for mounting and attachment
• Requirement for streamlining
• Requirement for weight of anode material (sacrificial)
• Commercial availability

The minimum and maximum current outputs are calculated as described in sections 7.1
and 7.2. The types of mounting methods range from welding steel cores, cast into the
anode materials, directly to the structure, to mounting complex dielectric shields with
screw in platinized titanium impressed current anodes. Where streamlining is required,
recesses may be built into the structure to house both impressed and sacrificial anode
types.

Sacrificial CP design requires that the weight of anode material is sufficient to supply
current for the design life of the structure. This is calculated by the following formula:

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(8760 )YC h

W =
yr

ZU

where :
W = weight of anode material
Y = design life (yrs)
C = current demand (Amps)
Z = anode capacity
U = utilization factor (0.9 for aluminum and zinc)

Finally, due to practical considerations, anode selection may ultimately be determined

by commercial availability. It is often too expensive to customize anode size and
geometry for one job. Therefore, except for large and specialized requirements, CP
design centers around standard, commercially available anode types.

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8.0 COST AND IMPLEMENTATION

There are many permutations possible in CP design, however to be successful it must

satisfy economic constraints and be easy to install and operate. Examples of CP
designs are presented for an oil platform and ship.

8.1 Oil Platform Example

8.1.1 Structural Details
Water Depth 110m
No. of Legs 4
No. Horizontal Frames 5
No. of Nodes Below Surface 75
Total Submerged Surface Area 63,000 m2
Total Pile Surface Area in Mud 6,000 m2
Allowance for Risers, Conductors, Wells 220 Amps
Design Life 35 years

8.1.2. Current Demand

Total = (submerged S. A. )(CP current density) + (pile S. A. )(CP current density) + allowance

Submerged Steel (A) Piles (A) Allowance Total (A)

(A)
North Sea
Initial 11,340 150 220 11,710
Mean 5,670 120 220 6,010
Final 7,560 90 220 7,870
Gulf of Mexico
Initial 6,930 150 220 7,300
Mean 3,780 120 220 4,120
Final 5,040 90 220 5,350

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The following design current densities were used for calculating the current demand
shown in the previous table:

Submerged Steel Piles

(mA/m2) (mA/m2)
North Sea
Initial 180 25
Mean 90 20
Final 120 15
Gulf of Mexico
Initial 110 25
Mean 60 20
Final 80 15

Example calculation (for North Sea, Initial)

CP current for submerged steel = (submerged S. A. )(CP current density) = (63,000m 2 )(0.180 mA2 )
CP current for submerged steel = 11340
, A

CP current for piles = (pile S. A. )(CP current density) = (63,000m 2 )(0.025 mA2 )
CP current for piles = 150A

Total CP current = 11,340A + 150A + 220A

Total CP current = 11710
, A

8.1.3. Sacrificial Anode Design for Uncoated Structure

The following table is the weight required for 35 year CP design life (using mean
current density)
Location Zinc, kg Aluminum, kg
NORTH SEA 2,624,880 723,462
GULF OF MEXICO 1,799,419 495,953

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Example calculation for the weight of zinc required, North Sea:

W=
(8760 )YC
h
yr

ZU

W=
( )
8760 yrh (35 yr )(6,010 A)
(780 )(0.9) Ah
kg

W = 2,624,880kg

Number of Anodes Required to Provide Initial Current Demand to Polarize the

Structure

Assume the following anode dimensions: 2,500 mm long

250 mm width
207 mm thick

NORTH SEA GULF OF MEXICO

Seawater Resistivity, ohm-cm 30 20
Anode Resistance, Ohms 0.0641 0.0427
Anode Current Output, Amps 3.900 5.855
Number of Anodes for Initial Current 3003 1247
Demand
Mass of Zinc Anodes, kg 2,768,766 1,149,734
Mass of Aluminum Anodes, kg 1,048,047 435,203

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The previous table was created using the following methodology:

Find Mass of One Anode, Zinc

m = ρ zinc ∀
m = (7,130 mkg3 )(2.5m)(0.25m)(0.207m)
m = 922kg
Find Mass of One Anode, Aluminum
m = ρ Al ∀
m = (2,700 mkg3 )(2.5m)(0.25m)(0.207 m)
m = 349kg

Find the Resistance of the Anode Using Table 7.4 (example for North Sea)
ρ ⎛ ⎛ 4L ⎞ ⎞
R= ⎜ ln⎜ ⎟ − 1⎟
2πL ⎜⎝ ⎝ r ⎠ ⎟⎠
⎛ ⎛ ⎞ ⎞
⎜ ⎜ ⎟ ⎟
30Ωcm ⎜ ⎜ 4(250cm) ⎟ − 1⎟
R= ⎜ ln
2π (250cm) ⎜ ⎟ ⎟
⎜⎜ ⎜ (25cm)(20.7cm) ⎟ ⎟⎟
⎝ ⎝ π ⎠ ⎠
R = 0.0641Ω

Calculate single anode current output (example for North Sea):

V
I=
R
(−0.800V − (−1.05V ))
I=
0.0641Ω
I = 3.90 A

Find the # of Anodes Necessary to Produce the Initial Current (example for North Sea):
Initial current demand
# of anodes =
current output per anode
11,710A
# of anodes = A
= 3002.5 anodes = 3003 anodes
3.90 anode

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Find the Mass of the Anodes (example for zinc anodes, North Sea):

Total Mass of Zinc Anodes = (# of anodes)(mass per anode)

kg
Total Mass of Zinc Anodes = (3003 anodes)(922 anode ) = 2,768,766 kg

Let us then compare the mass of anodes required for the design life and the mass
required for the initial polarization of the structure:

NORTH SEA GULF OF MEXICO

Mass of Zinc to protect the 2,624,880 1,799,419
structure for 35 years, kg
Mass of Zinc required to 2,768,766 1,149,734
provide the initial current, kg
Mass of Aluminum to protect 723,462 495,953
the structure for 35 years, kg
Mass of Aluminum required to 1,048,047 435,203
provide the initial current, kg
# of Zinc Anodes Needed 3003 1952
# of Aluminum Anodes 3003 1421
Needed

Anode Distribution and Spacing

For sacrificial anodes this maybe based on the current demand of the structure and the
maximum current output of the anode.

NORTH SEA
Maximum Current Output 3.900 Amps
Maximum Current Demand 180 mA/m2
Maximum Area Protected 3.900 A/(0.180A/m2) = 21.7m2

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GULF OF MEXICO
Maximum Current Output 5.855 Amps
Maximum Current Demand 110 mA/m2
Maximum Area Protected 5.855 A/(0.110 A/m2) = 53.2m2

If the structural member is relatively large, say 3 m diameter, then a single anode
placed in the center of a 22 or 53 m2 area will not be too far from the extremities of the
cathode it is protecting. For smaller members, allowances have to be made for
attenuation, and anode sizes must be selected to ensure that the anode protects half
way to the next anode.

8.1.4 Sacrificial Anode Design Coated Structure

The effect of coating a structure on CP design can be seen by applying the DNV criteria
to Example 1. It can be seen that significant savings in anode material can be achieved
if a Category 2, 3 or 4 coating is used. The category 1 coatings are only helpful for
short design life. This interpretation of the interaction between coatings and CP is still
open to debate with many experts in the Oil Industry questioning the low performance
criteria assigned to Category I coatings.

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2
Structure Area, m : 63,000
Allowance, Amps 220
Piles, Amps: 120
Zinc Anodes, Ah/kg 780 Cost, $/tonne, Feb 1995 1,025
Aluminum Anodes, Ah/kg 2,830 Cost, $/tonne, Feb 1995 2,000

2 2 2
CD mA/m tr, years Category k1 k2 f (ave) f (final) CP (ave) mA/m CP (final) mA/m
North Sea
90 35 1 0.1000 0.1000 1.8500 3.6000 90 90
90 35 1 0.1000 0.0500 0.9750 1.8500 88 90
90 35 2 0.0500 0.0300 0.5750 1.1000 52 90
90 35 2 0.0500 0.0200 0.4000 0.7500 36 68
90 35 3 0.0200 0.0150 0.2825 0.5450 25 49
90 35 3 0.0200 0.0120 0.2300 0.4400 21 40
90 35 4 0.0120 0.0120 0.2220 0.4320 20 39
90 35 4 0.0120 0.0120 0.2220 0.4320 20 39

Category I II III IV
Total Current Demand, Amps 6,010 3,600 1,942 1,599
Wt Aluminum, kg 723,465 433,387 233,745 192,451
Cost Aluminum 1,446,931 866,774 467,490 384,903
Number of Anodes 3,003 3,000 2,500 2,500
Weight/Anode, kg 482 289 187 154
Cost, $ / Anode 1,156 693 449 370
Cost Installation $ / Anode 450 400 400 400
Total Cost CP, $ 4,823,983 3,280,258 2,121,975 1,923,767

Gulf of Mexico
60 35 1 0.1000 0.1000 1.8500 3.6000 60 60
60 35 1 0.1000 0.0500 0.9750 1.8500 59 60
60 35 2 0.0500 0.0300 0.5750 1.1000 35 60
60 35 2 0.0500 0.0200 0.4000 0.7500 24 45
60 35 3 0.0200 0.0150 0.2825 0.5450 17 33
60 35 3 0.0200 0.0120 0.2300 0.4400 14 26
60 35 4 0.0120 0.0120 0.2220 0.4320 13 26
60 35 4 0.0120 0.0120 0.2220 0.4320 13 26

Category I II III IV
Total Current Demand 4,120 2,514 1,408 1,179
Wt Aluminum 233,535 142,473 79,802 66,839

8.1.5 Impressed Current Anode Design

Assuming the use of platinized titanium anodes with the following dimensions.
length = 1000 mm
diameter = 25 mm
Maximum driving potential = 9V

Then the surface area of the anode is:

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πdL = 0.078m2

NORTH SEA GULF OF MEXICO

Anode Resistance, Ohms 0.2277 0.1518
Maximum Anode Current Output, 39.5 59.3
Amps
Maximum current density on anode 506 760
surface, A/m2
Number of anodes required 296 123

The distribution of the anodes is critical.

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8.2 Ship Hull Protection

Ship Hulls require protection from both corrosion and the development of biofouling
accumulations. The former is achieved by both coating and cathodic protection
systems and the latter by antifouling coatings. In addition to the basic costs of the
coating and cathodic protection systems, allowances must be made for dry dock costs,
loss of revenue, and increased fuel consumption and lost performance due to the
increase in skin friction drag caused by poor hull maintenance and biofouling.
SHIP HULL COATING
GRIT BLAST
NACE #1, with 1 - 2 mil anchor profile
BARRIER COAT
Hard Boiled Mastic, two component epoxy amine
Two coats, 3 mils D.F.T.
Coverage per one mil dry - 564 sq.ft./gal.
Cost, $23.00/gal.
Cost for 6 mils D.F.T. $0.24/sq.ft.

CATHODIC PROTECTION DESIGN

SHIP ORIANA
TYPE PASSENGER
LENGTH, m 245
BREADTH, m 30
DRAFT, m 9.75
BLOCK COEFFICIENT, Cb 0.6
WETTED SURFACE AREA (1.7xLxD)+(CbxLxB) m2 8,471
CURRENT DEMAND FOR HULL, (@22mA/m2) Amps 186
CURRENT DEMAND FOR RUDDERS, Amps 15
CURRENT DEMAND FOR PROPELLERS, Amps 35
TOTAL CURRENT DEMAND, AMPS 236

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Sacrificial Anode Calculations

Current Output per Anode
Assume anode dimensions: 500mm x 115mm x 65mm
V
The current output, I=
R
R= the anode resistance calculated using Lloyds' Formula for plate anodes
Lloyds' Formula
ρ
R=
2S
20Ω − cm
R=
2(30.75cm)
R = 0.325Ω

Current output per anode,

V
I=
R
0.25V
I= = 0.769 A
0.325Ω

Number of anodes required to supply current,

Total current required
# of anodes needed =
Current output per anode
236 A
# of anodes needed = A
= 307 anodes
0.384 anode

The total current capacity for one anode is:

Volume of Anodes * Density * Capacity of Galvalum 1
= 3.738x10-3 m3 * 2,700 kg/m3 * 2,830 Ah/kg = 28,558 Ah

The design life of the system will be

= 28,558 A hrs * 307 anodes * (0.9) = 33,435 hrs or 3.8 years
236 Amps

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