CORROSION CONTROL

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CORROSION CONTROL

• MATERIAL SELECTION
• ALTERATION OF ENVIRONMENT
• PROPER DESIGN
• CATHODIC PROTECTION
• ANODIC PROTECTION
• COATINGS WRAPPING

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CATHODIC PROTECTION(is one of the most widely
used methods of corrosion prevention)

• SACRIFICIAL ANODES CATHODIC PROTECTION (SACP)
• IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)

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Cathodic Protection General Application

• Underground piping and tanks for water,
  petroleum, natural gas, sewage, steam, chemical
  and petroleum products.
• Marine structures including docks, ships, piling,
  buoys, log lifts, barges and sewage outfalls.
• Internal protection of tanks and piping.
• Concrete bridge decks, parkades, and piling.
• Hydraulic elevator cylinders.
• Above-ground tank bottoms.

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• In aqueous environment, a steel structure
  corrodes because its interface potential is not
  in the metallic stability domain of Fe.
• Its interface potential is in the stability
  domain of Fe ion (e.g. Fe).

E (volt)
Ecor
Fe
Fe stable
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• Cathodic protection decreases the interface
  potential of a structure down to a potential
  close to its equilibrium potential or in its
  stability domain.

E (V)
Ecor,Fe
Fe2
EFe/Fe
Principle cathodic protection of Fe
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• Immunity, passivation and active corrosion
  domains of Fe

Anodic Protection
Cathodic Protection
Alteration of Environment
Ecor
Ecor
Fe corrodes because its potential is in active
corrosion domain (Fe stability domain)
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Due to the presence of Cl-
In more realistic diagrams
b
a
Cathodic protection of steel/cast iron structures
can be applied in environment containing
aggressive ions (b). It is not recommended when
the pH of environment lt 5
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• Cathodic protection reduces corrosion rate by
  cathodic polarization of corroding metal surface.

E (V)
Ecor,Fe
Anodic pol. curve
Cathodic pol.curve
Fe2
EFe/Fe
Fe stable
Log icor
Log i0
Log i
Principle cathodic protection of Fe
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E (V)
Actual pol. curve
Ecor
Ecor
E1
EFe/Fe
Applied protection current density i1
Log i
Decreasing potential from Ecor to E1 needs
applied protection current density i1
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How does it work ?
Anodic and cathodic sites will be formed on the
unprotected pipe exposed in aqueous environment.
Corrosion current will flow through environment
from anodic to cathodic sites. Discharged current
on anodic site is forced back into the steel pipe
by supplying higher protection current density
than that of the discharged current.
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• Interface potential of steel structures are
  decreased either by electrically connecting to
  sacrificial anodes or by using impressed
  current. Note that the corrosion potential of
  sacrificial anodes have to be significantly lower
  than the corrosion potential of steel structure.
• Both methods have to result in the flow of
  electrons through wire (or other metallic
  connector) toward steel structure direct current
  has to flow from anode through aqueous
  environment (soil, see water etc.) toward steel
  structure.

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Schematic figure of SACP
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Schematic figure of ICCP
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• The current has to flow in a loop and therefore
  the surface of structure that can be protected is
  the submerged surface that received protection
  current from anodes.

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In case of inner pipe surface needs to be
protected by CP, the anodes (usually ICCP anodes)
have to be installed inside the pipe.
Note that the anode spacing for internal
protection of pipeline is limited.
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Consider this
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• Under protection
• Surface area which receives insufficient
  protection current density, still corrodes. This
  insufficient cathodic protection is known as
  under protection.
• Overprotection
• Coating at surface area which receives excess
  protection current have a tendency to disbonding
  and blistering. In extreme case, i.e. when the
  environment contaminated by arsenic, antimony or
  sulphide, ions diffusion of atomic hydrogen into
  the steel may occur and results in hydrogen
  embrittlement. This excess cathodic protection
  current is known as overprotection.
• Criteria for protection and over-protection?

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Protection criteria for steel/cast iron

• Pipe to soil potential -850mV vs. CSE (Cu
  saturated copper sulfate electrode). It has to be
  -950mV vs. CSE when SRB (sulfate reducing
  bacteria) is present.
• - Polarized potential of -850mV criterion
• Cathodic polarization 300mV below Ecor
• Cathodic polarization 100mV during interrupted
• Cathodic polarization to a potential where Tafel
  behavior is achieved
• Net protective current flows from electrolyte
  into the structure surface

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Pipe to soil potential -850mV vs. CSE

• - 850 mV (CSE) with the CP applied
• The full criterion states that adequate
  protection is achieved when a negative (cathodic)
  potential of at least 850mV with the CP applied.
• Probably the most widely use criterion for
  determining an acceptable level of buried and
  submerged steel or cast iron structures. Strictly
  applicable only to steel in neutral environments,
  such as soil and seawater.
• Voltage drops other than those across the
  structure to electrolyte boundary must be
  considered for valid interpretation of this
  voltage measurement.
• The voltage drops resulted from current flow in
  the electrolyte (soil) are generally referred to
  as ohmic or IR voltage drops.

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• IR voltage drop are more prevalent in the
  vicinity of an anode bed or in areas where stray
  current are present and generally increase with
  increasing soil resistivity.
• For bare or very poorly coated structure, IR
  voltage drops can be reduced by placing the
  reference electrode as closed as possible to the
  structure.
• For the majority of coated structures, most of
  the IR drop is cross the coating, and the
  measurement is less affected by reference
  electrode placement.
• The IR voltage drop can also be minimized or
  eliminated by current interrupting all of direct
  current resources of the CP system and measuring
  the instantaneous potential.

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• Polarized potential of -850mV criterion
• This criterion states that adequate protection is
  achieved with a negative polarized potential of
  at least 850mV relative to a saturated
  copper/copper sulfate reference electrode.
• The polarized potential is defined as the
  potential across the structure/electrolyte
  interface that is the sum of the corrosion
  potential and the cathodic polarization.
• The polarization potential is measured directly
  after interruption of all current sources and is
  often referred to as the off- or instant
  off-potential. The difference in potential
  between the native potential and the off or
  polarized potential is the amount of
  polarization that has occurred as a result of the
  application of the CP.

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In this location pipeline is under protection
(not enough protected)
Can be applied only for ICCP systems
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• Cathodic polarization 300mV below Ecor
• This criterion states that adequate protection
  will be achieved when the interface potential
  (pipe to soil) is decreased 300mV (or 400mV
  when SRB is present) below its native potential.
  This criterion is referred to the first criterion
  and the potential of fresh steel structure is
  about -550mV vs. CSE.

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• Cathodic polarization 100mV during interrupted
  
• This criterion states that adequate protection is
  achieved with a minimum of 100 mV of cathodic
  polarization between the structure surface an a
  stable reference electrode contacting the
  electrolyte.
• To determine the magnitude of the shift as a
  result of the formation of polarization, one must
  first determine the native potential of the
  underground structure at test location before
  applying CP. The potential is then re-measured
  after the CP system is energized and the
  structure has had sufficient time to polarize.
  Typical of on potential is continuously monitored
  at one test location, and an off potential is
  made when there is no measurable shift in the
  potential reading for several minutes. The off
  potential is then compared with the native
  potential, if the difference exceeds 100mV, then
  the 100mV criterion has been satisfied at that
  location.

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V vs. SCE
gt100mV
Native potential
IR drops
Protection potential
on
on
off
off
on
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• Cathodic polarization to a potential where Tafel
  behavior is achieved.
• This criterion states that adequate protection is
  achieved with protection potential E1.

Protection potential E1. Note that E1 is gt
than -850 mV. The method can be applied only for
ICCP system
E
Log I
E vs log I curve of a pipeline at certain location
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• Net protective current flows from electrolyte
  into the structure surface
• This criterion qualitatively informs that the
  structure is being cathodically polarized which
  leads to reduce the corrosion rate of structure
  However this criterion cannot be used to evaluate
  the condition of the steel (pipeline).
• It might be used to evaluate the present of stray
  current.

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Each material has its protection criteria
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Influence of interface potential on the
condition of steel of a coated pipeline
In many cases on cathodic protection of buried
pipelines, the potential of the location around
drainage point can reach 1.5V vs. CSE but it is
still accepted.
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Base on technical and economical consideration
cathodic protection is applied together with
coating/wrapping
Cost of protection
Cost for coating alone
Note that there is no coating that is free from
porosity, and coating damage will normally occur
during service.
Cost of cathodic protection alone
Minimum protection cost
0
bare metal
100
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In this case cathodic protection only protect the
exposed area,
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One of the coating properties related to cathodic
protection current requirement is coating
resistance/conductance
Note that the data is for a pipeline having
certain diameter and length
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Sacrificial Anodes Cathodic Protection (Cathodic
Protection with Galvanic Anodes)

• Sacrificial anodes used for steel structured
• Aluminium anode
• Magnesium anode
• Zinc anode

Ecor steel
Average localized protection potential on steel
Ecor sac. anode
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Typical SACP for underground pipeline
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Typical SACP for underground pipeline
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Criteria for sacrificial anodes

• A corrosion potential that is sufficiently
  negative for the specific application in
  general, alloying additions are made to make the
  potential more negative than that of the
  unalloyed basis metal
• A high anode efficiency, which means that
  impurities that result in self corrosion must be
  absent or minimum.
• The ability to remain active and to corrode
  uniformly and NOT to become passive.

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Typical sacrificial anodes for protecting steel
structures
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Anodes selection for soil environment
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Zinc anode(Mol. weight 65.4, ? 7.1 g/cm3)

• Pure Zn is very rare to be used.
• Typical composition 0.3-0.6 Al, 0.003-0.125
  max. Si and 0.025-0.0125 Cd.
• For higher driving voltage, Hg, In, Ca, Li are
  added. However this not frequently used in
  practice.
• It is not recommended to be used gt 500C
• Efficiency 95
• Capacity 780 Ah/kg
• Corrosion potential -1.05 vs. Ag/AgCl

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Aluminium anode(Mol. weight 27, ? 2.7g/cm3)

• Pure Al will be passive due to passive film
  formation, but in chloride containing environment
  Al is susceptible to pitting corrosion.
• Al sacrificial anodes have to contain Hg, In or
  Sn together with Zn to avoid passive film
  formation
• 1. O.35-0.5Zn and 0.035-0.5Hg (eff.95)
• 2. 0.50-5.00Zn and 0.005-0.03In (eff95)
• 3. 4.00-7.00Zn and 0.1Sn (eff. 50-80)
• Capacity 2700-2830 Ah/kg for anodes containing Hg
  or In.
• Efficiency ranges from 90 to 95 .
• Corrosion potential -1.10 to -1.15 vs Ag/AgCl

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Magnesium anode (Mol. weight 24.3, ? 1.7 g/cm3)

• Base on its chemical composition it can divided
  to two types
• 1. 2.7-6.7Al and 0.15-0.20min.Mn
• 2. ?0.03Al and 0.5-1.2min.Mn
• Mn addition decreases corrosion potential of Mg
  anode thereby increasing driving force of the
  anode.
• Anode potential (corrosion potential) Mg 1.50
  to 1.70V vs. Ag/AgCl
• Efficiency is only 50
• Current capacity is 1230 Ah/kg
• It is not recommended to be used in a fuel
  /petroleum tanker.

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• The anode capacity is the current which can be
  produced by a stated weight of anode material,
  and its usually expressed in ampere hours/kg (Ah
  kg-1) Owing to the fact that the efficiency is
  less than 100, the capacity of anode in practice
  is normally less than the theoretical Faradaic
  capacity.

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• Example Anode capacity of aluminium

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HALF CELL POTENTIAL OF SEVERAL STANDARD ELECTRODES
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Formula to calculate total weight and number of
anodes
Total protection current required
Total weight of anodes required
W is the total weight of anodes w is the
weight of an anode chosen
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• Protection current available from single anode
• E1 expected protection potential of structure
• E2 is corrosion potential of a sacrificial anode
  is
• Ran is resistance of single anode
• Resistance of a slender anode (where the ratio of
  length to mean effective radius greater than 10)
  
• In case for protecting submerged marine
  structures
• ? is seawater resistivity, L is the length of
  anode and reff is effective radius of anode.

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Effective radius of the anode is taken as a
radius that is left after 40 of anode is
consumed (utility factor is taken 80)
Minimum number of anodes that has to be provided

For a horizontal anode, the anode resistance may
be presented as
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Water resistivity can be obtained from this
diagram provided that temperature and water
density are known
Given density 1020g/l and temperature 200C,
then resistivity is 25 Ocm
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• Example
• Given Ecor Al anode - 1.05 V vs. Ag/AgCl
• Eprot of steel - 0.80V vs.
  Ag/AgCl
• Anode dimensions 120x 6 x 6cm3
• Water density 1020 g/l
• Water temperature 200C
• Then Water resistivity
• Anode resistance
• Anode output

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• If the ratio of length to mean effective radius
  is small (less than 110) then the resistance of
  anode can be determined as follow
• where L is the length of anode (cm),
• W is the width of anode (cm)
• and T is the thickness of anode (cm)
• Output is higher than that of slender anode,
  consequently the anode consumption is greater and
  the anode life is shorter.

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Formula for the resistance of a horizontal anode
with an cross sectional area of up to 105 cm2,
can be expressed in a graphical form.
This graph is for determining anode resistance in
seawater, different graphs are needed for
different water resistivities.
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• For soil, the resistance of horizontal anode can
  be calculated using the following formula
• where, Rh is the resistance of horizontal
• anode to earth in ohms
• ? is the resistivity of backfill
  material
• (or earth) in ohm.cm
• L is the length of anode in feet
• d is the effective diameter of anode
  in feet
• S is the twice deep of anode in feet
• Resistance for vertical anode can be calculated
  using formula above where ? is the resistivity of
  backfill material (or earth) in ohm.cm

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• Method for choosing the weight and number of
  aluminium and zinc anodes
• For any specific situation, the total anode
  weight, the total current required and the number
  of anodes which will meet these current and
  weight requirements are calculated as follows
• The wetted area of steel to be protected is
  calculated from drawings or direct measurement.
• The total current IT (A) needed is

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• 3. The following formula gives the weight of
  anode material required
• were life is the design life in years
  (1y8760h) and UF is utility factor ( utility
  factor that usually taken is 80).
• 4. The minimum number of anodes required per
  structure is assessed from the following
  formula
• Note the anode selected must satisfy both the
  total weight and total current output
  requirements as follows
• Weight requirement No of anodes x individual
  net weight
• Current requirement No. of anode x individual
  anode current out-put

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• Current densities for protecting steel
• I.e. current density requirements for ships
  hull,
• ballast and tanks

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• Total resistance of each anode to earth consists
  of the resistance of anode to the backfill
  material plus the resistance to the earth of
  backfill column itself.
• The formulas can also be used to calculate the
  resistance of an impressed current anode.
  However one should consider in case of inert
  anodes are used, the effective diameter (or
  effective radius) in the formula has to be
  changed with the initial diameter (or radius) of
  anode because the diameter of anode is reduced
  very slightly.

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Physical shape of anodes for protecting marine
pipes
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Physical shape of the anodes for protecting
marine structures
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Zinc and magnesium anodes used in soil environment
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• Different shapes give different surface area to
  weight relationships, and this results in
  different shapes giving different current outputs
  for the same weight. This means that different
  shape have a different life for the same weight.
  In general the shape is chosen in order to give a
  certain current output for a certain weight and
  thus have a certain life.
• In a number of other instances the anode shape is
  designed to conform with the shape and room
  limitations of the steel structure that it is
  design to protect.

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BACKFILL FOR SACRIFICIAL ANODES

• In a soil environment, zinc or magnesium anodes
  that are used for a cathodic protection system
  have to be surrounded by a backfill.
• Objectives
• To decrease anode resistance.
• To avoid anodes directly contact to soil. Anodes
  directly contact to soil may suffer localized
  corrosion or be passive. I.e. chloride decreases
  the efficiency of magnesium anode while anodes
  that directly contact to phosphate, carbonate or
  bicarbonate in soil can produced film which
  hinders further anode dissolution.
• Backfills tend to attract soil moisture.

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• To increase conductivity of area around the
  anodes
• In high resistivity soils the most common mixture
  of backfill used is
• 75 gypsum
• 20 bentonite
• 5 sodium sulfat
• This backfill has resistivity about 50 ohm.cm

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AppendixTypical anodes used for steel structure

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CP DESIGN FOR A BURRIED PIPELINE USING
SACRIFICIAL ANODES

• Design procedure
• Decide the protection criteria and life time for
  the pipeline. This includes
• - Criterion used for protection
• I.e. pipe to soil potential has to
  be
• - 850mV vs. CSE
• - Cathodic protection system is designed
  for
• T years
• Technical data available related to cathodic
  protection of the pipeline have to be collected.
• Pipe dimension - diameter
• -
  length
• -
  thickness

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• Specific resistance of Coating ? ohm.m2
• Soil resistivity ? ohm.cm
• Backfill resistivity usually 50 ohm.cm
• Required protection current density ip
  mA/m2
• Type of anodes used and its capacity
  (KAh/kg)
• Driving voltage of anode ?E (volt)
• 3. Compute how much the total protection current
  is required
• I0 p D L ip
• 4. Find the total weight of anode required
• where, U is utilization factor usually
  0.8 (80)

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• 5. Find the dimension and weight of anode for
  this
• protection system, w.
• Note that heavier anodes are recommended to
  
• be used in more conductive soil, while
  small
• anodes are provided for high resistivity
  soil.

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• 6. Calculate the number of anodes
• 7. Design the installation method and then
  determine the anode/groundbed spacing.
• For groundbed
• with single anode
• For groundbed
• with multiple anode
• 8. Compute the requirement of protection current
• for the length of pipe of S or SG
• Is p D S ip or
  IsG p D SG ip

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• 9. Calculate the resistance of single
• anode .
• Resistance of anode (Rtotal)
• Ranode to backfill Rbackfll to
  soil
• I.e for a vertical anode
• where Rv is the resistance of vertical anode,
• l is length and r is radius.
• The dimension of backfill is assumed to be
  constant.

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• Effective radius is calculated using the
  following formula
• 10. Determine the current output from a single
  anode/ a
• groundbed
• 
  or
• Rground bed is computed as follows
  

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• where F is the interference factor
• and n is the number of anodes in one
  ground bed.
• 11. If for groundbed with single anode
• Is Ia or
• for groundbed with multiple anodes
• IsG Igrounbed
  design is accepted.
• If for grounbed with single anode
• Is Ia or
• for groundbed with multiple anodes
• IsG Igrounbed
• check the dimension of anode or
  distance between two anodes in a
  groundbed.
• Groundbed with multiple anodes may
  need higher number of anodes.
  

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CASE STUDY 1

• Protection criteria -0.85 V vs. CSE
• Designed life time of CP 20 years
• Data - pipe dimension diameter 14
• 
  thickness 0.5
• length
  10km
• - Coating polyethylene tape,
  overlap 5cm
• - Average resistivity of soil
  2000 ohm.cm
• - Required protection current
  density 0.5 mA/m2
• - Anode used magnesium anodes
• Anode capacity K 1200
  Ah/kg
• ?E 0.7 volt

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• Length of anode 50 cm
• Diameter of anode 12 cm
• Length of backfill 80 cm
• Diameter of backfill 20 cm
• Design the SACP system for the pipeline

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Installation of single anode
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Usually it is recommended to install sacrificial
anode below pipeline because backfill has always
to contain water which is required for
maintaining dissolution of sacrificial anode.
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• Design calculation for multiple galvanic anodes
  installation
• 1. Anode spacing used (Sa) 2 m
• Number of anodes in one ground bed 6
• 2. Rtotal of single anode
• 3. Interference factor

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• 4. Resistance of groundbed
• 
  
• ohm
• 5. Current output from the groundbed
• 
  A
• 6. Groundbed spacing
• 
  m

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• 7. Requirement of protection current
• for the length of pipe of SG
• IsG p D SG ip
  A
• 8. Assessment of design Is Ia ?
• accepted/not - accepted

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Installation of multiple anodes
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How to install test point
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STORAGE TANK

• DATA -diameter 20 m
• -soil resistivity 3000 ohm cm
• -protection current density
• required 15mA/m2
• -Life time of the ICCP system 20
  
• years
• -Anode (14Si-Fe) weight 21 kg
• dimension
  diameter 2
• 
  length 60
• rate of
  consumption 0.5 kg/A y
• utility factor
  0.8 (80)
• -Back fill diameter 8
• length 80

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• Length of wire used is 100 m and its resistivity
  is 0.216 /km.
• Assumption back emf is 2.5 volt
• Design ICCP system for the storage tank. It is
  suggested to use at least 6 anodes.

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Anodes installation for a new storage tank
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Installation design for existing storage tank
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IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)

• WHY SHOULD USE ICCP?
• Steel structure that is to be protected is large
• Protection current requirement is high
• Coating quality varies in a large range
• Resistivity of soil is high
• Source of electric current is available
• Fields for ground bed are available

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• Compared to SACP (sacrificial anode cathodic
  protection)
• ICCP is easy to control
• Number of anodes required is limited
• Lower investment cost for a large structure
• Needs only a few of ground beds
• Has large throwing power

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SACP needs high number of large anodes.
Consequently large structure of rig has to be
constructed.
ICCP needs small number of light anodes.
PROTECTION OF OFFSHORE MARINE OIL DRILLING RIG
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• Disadvantages
• Can cause anodic and cathodic interferences
• Can result in overprotection
• Unsuitable for congested structures
• Needs frequent inspection
• Wrong polarization will damage the structure
• Needs careful design

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Structures that can be protected by ICCP

• PIPELINE (ONSHORE and OFFSHORE)
• STORAGE TANK
• STEEL PILES PIER
• JACKET
• REINFORCING STEEL IN CONCRETE
• PLATFORM, RIG
• SHIP HULL
• Etc.

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Mixed oxide anode is also recommended
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Low tide level
insulator
CONTACT BETWEEN STRUCTURE AND AN ICCP
ANODE/ANODES HAS TO BE AVOIDED
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Anode installation(It is recommended that the
groundbed has to be at lower level than the
pipeline)
Recommended space distance between a pipeline and
a groundbed is 150m
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Deep well installation It can be used for ICCP
of cashing or when field for ground bed is limited
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• Materials used for ICCP anodes
• Rare materials
• Ferrous materials
• Lead materials
• Carbonaceous materials
• Reactive non ferrous metals
• Combination anodes

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Rare materials

• Platinised tantalum
• Platinised niobium
• Platinised titanium
• Platinised silver
• Platinum metals
• Dimensionally stabilized anode (DSA) titanium

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Ferrous materials

• High silicon chromium iron
• High silicon molybdenum iron
• High silicon iron
• Cast iron
• Steel
• Iron
• Stainless steel

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Lead materials

• Lead-antimony-silver
• Lead/platinum bi-electrodes
• Lead dioxide/titanium (lead dioxide on titanium
  substrate)
• Lead dioxide/graphite
• In using lead as an anode the formation and
  maintenance of a hard layer of lead dioxide
  (PbO2) is essential.
• Pure lead fails to passivate and leads to
  chloride formation beneath the lead dioxide thus
  insulating the latter from the lead.
• Pb-6Sb-1Ag produces hard PbO2 (used in commercial)

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• Carbonaceous material
• Graphite (less porous and more reliable than
  carbon)
• Carbon
• Coke breeze
• Reactive non-ferrous materials
• Aluminium
• Zinc

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Combination anodes

• 1. Canned anodes steel casing/carbonaceous
  extender/graphite rod or silicon-iron rod current
  conductors
• 2. Groundbeds carbonaceous extender/graphite rod
  or silicon-iron rod anodes, scrap steel, or
  platinised titanium current conductors
• 3. Co-axial anodes copper-cored platinised
  titanium or platinised niobium

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Recent anode producedis Linear Distributed Anode
(LIDA)
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A tubular anode string from LIDA
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Installation of LIDA anodes in a groundbed
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Canistered anodes
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Backfill for ICCP anodes

• The carbonaceous backfill surrounding an anode is
  essential and serves a number of functions
• 1. To reduce anode resistance. (It has the effect
  of increasing the anode size which results in
  reduction of anode resistance to earth.)
• 2. To extend the service life of anode. (Most of
  the current is transmitted to the backfill from
  the anode by direct contact, so that the greater
  part of material consumption is on the outer
  edges of the backfill column, enabling the anode
  themselves to have an increase life.)
• 3. To provide paths for flowing gas produced
  from anodic reaction

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• Carbonaceous backfill is used for HSI, HSCI,
  graphite, mixed oxide anodes and others.
• Backfill Material
• Coal coke (650-800kg/m3) has met with the
  standard and can directly be used as backfill
• Petroleum coke (700-1100kg/m3) has to be calcined
  before it is used as backfill material
• Natural graphite granules or crushed graphite
  (1100-1300kg/m3) can be used
• In practice, the coke consumption will be of the
  order of 0.25kg/A.Y

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• Typical coal coke specification
• 100 will pass through a 15mm aperture
• 85 will pass through a 10 mm aperture
• 15 will pass through a 5 mm aperture
• Volatile matter 3.25 max.
• Fixed carbon 78
  min.
• Ash 19
  max.
• Sulphur 1.2
  max.
• Resistivity (bulk) 55 Ocm max
• The coke breeze should be thoroughly mixed with
  5-10 by weight of slaked lime, to counter act
  the tendency to lose moisture by electro-osmosis.
  Calcium sulphate is sometime used in very dry
  condition.

118

• Too much fines leads to over-tight compaction and
  gas blocking leading to gas polarization.
• Flake graphite is not recommended as its tends to
  conglomerate and prevent gas emission.
• Resistivity of carbonaceous backfills, Ocm

119
Typical rectifier for cathodic protection
120
Rectifier can be connected to a control circuit
either to produce constant current output or
constant potential output.
121
Economics

• Cathodic protection design involves achieving an
  economic balance between installation costs,
  maintenance costs, initial costs of power units
  and power consumption.
• Because both the cost of the rectifier and the
  cost of electric power consumed are contingent on
  the operating voltage of the system, it is
  desirable to keep the operating voltage as low as
  possible, for this reason a low resistance
  groundbed is desirable when it is economically
  feasible.

122
Typical cost as function of number of anode in
one groundbed