Cathodic Protection ME 472-061 Corrosion Engineering I ME, KFUPM - - PowerPoint PPT Presentation

Cathodic Protection

ME 472-061 Corrosion Engineering I ME, KFUPM

• Dr. Zuhair M. Gasem

• Dr. Z. Gasem

ME 472-061 KFUPM

2

References:

ASM Handbook, vol 13, pp. 466-477 Corrosion for Science and Engineering, K.R.

Trethewey and J. Chamberlain, chapter 16

Handbook of Corrosion Engineering, P.R.

Roberge

Cathodic Protection in ARAMCO’s Engineering

Encyclopedia

• Dr. Z. Gasem

ME 472-061 KFUPM

3

Fe2+ Fe2+

H H2

2

H H+

+

H H+

+

H H+

+

H H+

+

H H+

+

H H+

+

H H H H

e e

Metal Cathode Anode Acid Solution

Electron Flow

Anodic and Cathodic Reactions of Iron in Acids

Corrosion Cell on a Metal Surface

• Dr. Z. Gasem

ME 472-061 KFUPM

4

Basic Physics of CP of Iron in Acids

Electrons from an external source are forced to flow into

the structure to be protected resulting in:

• Increased cathodic reaction (2H+ + 2e- → H2)
• Decreased anodic reaction and hence reduced corrosion rate

Electrons from external source

e e e e e e e e e e e e e e e Electrolyte

H+ H+ H+ H+ H+ H+ H+

e e

H+ Fe2+ H H H H H H H H H2 H2 H2 H2 H+ H+ H+ H+ H+ H+

• Dr. Z. Gasem

ME 472-061 KFUPM

5

Polarization Principle of CP of Iron in Acids

Before CP:

ianode = icathode

= icorr

E = Ecorr

After CP:

icorr = ia icathode = ic iapp = ic – ia E = ECP ECP

• Dr. Z. Gasem

ME 472-061 KFUPM

6

Polarization Principle of CP of Iron in Water

The cathodic reaction for

corrosion of steel and iron in aerated-water is usually (O2+ 2H2O+ 2e→4OH-) under concentration polarization.

Before CP:

• ianode = icathode = iL
• E = Ecorr

After CP:

• icorr = ia
• icathode = ic = iL
• iapp = ic – ia
• E = ECP

ECP

• Dr. Z. Gasem

ME 472-061 KFUPM

7

Cathodic Protection

Summary of cathodic protection:

• CP makes the structure’s potential more negative which promotes cathodic

reactions and slows anodic reaction

• Increases icathode
• Decreases ianode
• Need to supply iapp = icathode - ianode

Where CP is used?

• CP is often applied to coated structures, with the coat providing the

primary form of corrosion protection and the CP system acts as a supporting protection.

The main applications of CP include:

• Buried pipeline
• Acids storage tanks
• Offshore steel structures such as platforms and oil rigs
• Ships
• Concrete structures exposed to seawater such as bridges

• Dr. Z. Gasem

ME 472-061 KFUPM

8

CP of Buried Pipelines

Before CP is applied:

Anodes and cathodes are on the same surface of the

pipe

The soil is the electrolyte Ionic current flow b/w the anode and the cathode in

the external surfaces

Electrons flow in the metal from anode to cathode

• Dr. Z. Gasem

ME 472-061 KFUPM

9

CP of Buried Pipelines

After CP is applied:

• The structure to be protected becomes the cathode
• The anode is an external electrode:
• Amore active metal (sacrificial anode)
• An inert anode with impressed DC current (Impressed

current)

• The soil is the electrolyte
• Ionic current flow b/w the anode and the cathode in the

external surfaces

• Electrons flow between the anode and cathode through an

insulated copper wire.

cathode Sacrificial anode e- +ve ions current in electrolyte

• Dr. Z. Gasem

ME 472-061 KFUPM

10

CP of Buried Pipelines

Sources of current

• Sacrificial anode system
• Impressed current system (note the - polarity from the rectifier)

• Dr. Z. Gasem

ME 472-061 KFUPM

11

Electrochemical reaction in sacrificial Anode CP System

Cathodic reactions on the

steel structure:

• In aerated wet soil
• O2+ 2H2O+ 4e- ⇒ 4OH-
• In aerated wet acidic soil
• O2+ 4H+ + 4e- ⇒ 2H2O
• In neutral seawater
• O2+ 2H2O+ 4e- ⇒ 4OH-
• In de-aerated soil or water
• 2H2O+ 2e- ⇒ 2OH- + H2

Anode reactions

• At active anode in

sacrificial anode CP system (Mg, Al, Zn)

• M → M+ n + ne-

• Dr. Z. Gasem

ME 472-061 KFUPM

12

CP of pipelines

Note that cathodic protection

current will only protect external surfaces on buried structures, because the anode-electrolyte- cathode is at external surfaces.

Above ground, structures cannot

be protected by cathodic protection because the current discharged from the current source can not travel through the atmosphere (no electrolyte).

CP is not usually used to protect

internal surfaces of pipelines because of difficulty in placing anodes.

internal surfaces of pipelines can

be protected by: inhibitors, coatings, or by using a corrosion resistant alloy.

• Dr. Z. Gasem

ME 472-061 KFUPM

13

Protection Criteria

How much current is needed to protect

the pipeline?

Little current will lead to ineffective protection High current will lead to disbonding of

coatings and hydrogen embrittlement (more power consumption and higher cost)

Experience show that we should keep the

pipeline potential less than a protection potential.

• Dr. Z. Gasem

ME 472-061 KFUPM

14

Protection Criteria

In less corrosive soil, E< -0.850 mV wrt

Cu/CuSO4 reference electrode

this reference electrode is used because it is

less sensitive to temperature variation (0.318

• s. SHE)

In Saudi’s Aramco, the protection potential

for cross-country pipeline is -1.1 V vs Cu/CuSO4 (due to highly corrosive soil)

More –ve potential means more current

required and more operation cost.

• Dr. Z. Gasem

ME 472-061 KFUPM

15

Reference Electrodes

Common reference electrodes used in CP

Cu/CuSO4 in soil

CuSO4 + 2e- ↔ Cu+ SO4

2-

E vs. SHE 0.318 V

AgCl in seawater

AgCl + e- ↔ Ag + Cl- E vs. SHE 0.222 V

• Dr. Z. Gasem

ME 472-061 KFUPM

16

Potential Protection

Why < -0.850 mV vs.

Cu/CuSO4?

From Pourbaix diagram,

Fe is stable below -0.6 V vs SHE

Cu/CuSO4 is more + ve

than SHE by 0.318 V

Hence, Fe is stable and

corrosion is minimum if potential is (-0.6-0.318=

• 0.918 V vs Cu/CuSO4)

• Dr. Z. Gasem

ME 472-061 KFUPM

17

Potential and Corrosion of Buried Steel

Severe overprotection (disbonding of coatings, hydrogen blistering, HE)

• 1.1 to -1.4

Overprotection

• 0.9 to -1.1

Cathodic protection

• 0.8 to -0.9

Slow corrosion

• 0.7 to -0.8

Corrosion

• 0.6 to -0.7

Intense corrosion

• 0.5 to -0.6

Corrosion condition

Potential (V vs. Cu/CuSO4)

• Dr. Z. Gasem

ME 472-061 KFUPM

18

NACE Standards for CP

• Dr. Z. Gasem

ME 472-061 KFUPM

19

Saudi Aramco’s Potential Requirements

Structure Minimum Required Potentials

Buried Cross-Country Pipelines

• 1.10 volts versus CuSO4 electrode.

Buried Plant Piping, Tank

• 1.00 volt versus CuSO4 electrode.

Bottom Externals,

• 850 mV versus CuSO4 electrode.

Isolated Buried Casings Water Tank Interiors

• 0.90 volts vs. AgCl electrode

Marine structures

• 0.90 volt or more negative versus AgCl electrode

• Dr. Z. Gasem

ME 472-061 KFUPM

20

Soil Corrosivity

Soil is composed mainly of mineral particles (mainly SiO2). Soil is composed of a mixture of:

• Fine sand (0.02-0.2 mm)
• Coarse sand (0.2-2 mm)
• Slit (0.002-0.02 mm)
• Clay (< 0.002 mm)

The soil particles are covered with thin surface film of moisture with

dissolved salts and gases.

The total volume of soil consists of solid particles and pores filled

with moisture and air.

Soils with a high proportion of sand have very limited storage

capacity for water whereas clays are excellent in retaining water

Air in the pores contains 10-20 times as much CO2 as atmospheric

air.

Soils with high moisture content, high electrical conductivity, high

acidity, and high dissolved salts will be most corrosive.

• Dr. Z. Gasem

ME 472-061 KFUPM

21

Soil Corrosivity

Variables affecting soil corrosivity:

Water is the electrolyte for electrochemical corrosion

reactions

Oxygen: the oxygen concentration decreases with

increasing depth of soil

pH: soils usually have a pH range of 5-8

• Soil acidity is produced by decomposition of acidic plants,

industrial wastes, and acid rain

• Alkaline soils tend to have high sodium, potassium,

magnesium and calcium contents which form calcareous deposits on buried structures with protective properties against corrosion.

Chloride level: harmful for metals sulfate level: harmful for concreter

• Dr. Z. Gasem

ME 472-061 KFUPM

22

Soil Corrosivity

For CP design against

corrosion, the most important property of a soil in determining its corrosivity is its electrical conductivity.

The table shows soil corrosion

severity ratings.

Soil corrosion causes corrosion

in underground petroleum storage tanks, pipelines, and water distribution systems.

Soil resistivity is measured by

Wenner 4-pin method

Very corrosive Corrosive Moderately corrosive Mildly corrosive Essentially non- corrosive

Corrosivit y Rating

<1,000 1,000 to 5,000 5,000 to 10,000 10,000 to 20,000 Dry sand Clay with saline water (sabkha) >20,000

Soil resistivity (ohm cm)

• Dr. Z. Gasem

ME 472-061 KFUPM

23

Soil Corrosivity

Electrolyte

Resistivity (ohm-cm)

Seawater (Gulf)

16

Raw water 200-2000 Drinking water 2000-5000

10,000 2,000 1,000 500 Progressively Less Corrosive Mildly Corrosive Moderately Corrosive Corrosive Very Corrosive

Ohm-cm

Seawater Resistivity

• Dr. Z. Gasem

ME 472-061 KFUPM

24

Current density required to reach Ecp for steel in moving and standing seawater and in soil

5-11 11-32 45-75 160- 270 Stagnant seawater 1.1-0.54 5.4-11 11-16 43-54 soil 11-16 32-54 75-105 325- 375 Moving seawater

Applied CP current Initial CP current Applied CP current Initial CP current

Coated steel (mA/m2) Bare Steel (mA/m2) Environment

ASM Handbook Vol#13 p.476

• Dr. Z. Gasem

ME 472-061 KFUPM

25

Given a coated offshore structure with a

surface area of 5,000 m2, 2/3 is immersed in seawater, calculate the amount of initial current and applied current necessary to cathodically protect the structure.

The initial and applied current density

requirement for coated seawater structures is 35 and 10 mA/m2.

iinitial = 5,000* 2/3* 35= 116,666 mA = 117 A iapplied = 5,000* 2/3* 10 = 33,333 mA = 34 A

Current Calculation for Design

• Dr. Z. Gasem

ME 472-061 KFUPM

26

Given a 150 m section of 0.75 m diameter steel pipe coated with fusion bonded epoxy (FBE), calculate the total amount of current required. Assume that 10% of the coating was damaged during installation. Assume that the required current density for FBE coated buried pipeline is 0.1 mA/m2 while for uncoated steel is 1 mA/m2 .

surface area = πDL= 354 m2 iinitial = bare area* 1 mA/m2 + coated area* 0.1 mA/m2

• = 354* 0.1* 1 + 354* 0.9* 0.1 = 67.3 mA

Note that if the whole pipe is not coated, then the

current requirement would be

• = 354* 1= 354 mA (5 times more than above)

Example

• Dr. Z. Gasem

ME 472-061 KFUPM

27

Coatings in CP Systems

• Bare structures require more current than

Bare structures require more current than coated structures coated structures

Economical applications of CP for buried

pipelines applied only for coated pipelines.

Always assume 5-10% of coated area as bare

due to damage during pipe installation.

• Dr. Z. Gasem

ME 472-061 KFUPM

28

Sacrificial Anode CP Systems

a more active metal than steels

can act as a sacrificial anode.

The galvanic series indicate that

Mg, Zn, and Al are more active than steels.

A number of anodes are

electrically connected to the steel structure to be protected to provide the needed current.

The amount of current output is

increased by increasing the number of anodes.

Usually applied in:

• Low current requirement

application

• Soil resistivity < 10,000 (Ω.cm)

• Dr. Z. Gasem

ME 472-061 KFUPM

29

Sacrificial Anode CP Systems

Advantages of sacrificial CP:

No external power source needed Ease of installation, low maintenance, low cost Provides uniform distribution of current

Disadvantages

Limited current and power output High resistivity environments or large structures

require a large number of anodes

Periodic replacement of anodes

• Dr. Z. Gasem

ME 472-061 KFUPM

30

Volts Metal (referenced to Cu/CuSo4) Commercially pure magnesium

• 1.75

Magnesium alloy (6% Al, 3%, Zn, 0.15% Mn)

• 1.6

Zinc

• 1.1

Aluminum alloy (5% Zn)

• 1.1

Commercially pure aluminum

• 0.8

Mild steel (clean and shiny)

• 0.5 to -0.8

Mild steel (rusted)

• 0.2 to -0.5

Cast Iron

• 0.5

Lead

• 0.5

Mild steel in concrete

• 0.2

Copper, brass, bronze

• 0.2

High silicon cast iron

• 0.2

Mill scale on steel

• 0.2

Carbon, graphite, coke +0.3

Galvanic Series in soil and seawater

• Dr. Z. Gasem

ME 472-061 KFUPM

31

Sacrificial anodes

In soil, special backfills

are used with sacrificial anodes to improve anode efficiency.

Anodes are packaged in

porous bags prefilled with backfill materials such as

• clay. Clay:
• absorbs moisture from the

soil and reduce anode resistance of anode/electrolyte

• distribute the anodic

reaction all over the anode

• Increase the life of the

anode

• Dr. Z. Gasem

ME 472-061 KFUPM

32

Sacrificial anodes

Anodes are packaged in

bags filled with backfill material

Commercial anodes (

• 60 inch in length
• 4 Kg

Anodes for buried structures (pipes,

tanks):

• Pure Mg
• Mg alloy (Mg+ 6Al+ 3Zn+ 0.2Mn)
• Pure Zn

For marine applications

• Al alloy containing 5% Zn is

used

• Zn alloy

• Dr. Z. Gasem

ME 472-061 KFUPM

33

Design of Sacrificial Anode CP

Select Protection Criterion (depends on the environment)

• For buried steel (NACE standard gives protection potential = -850 mV
• vs. Cu/CuSO4 )

Measure resistivity of environment (slide# 34):

• Soil resistivity ranges from (500-20,000 Ω* cm)
• Seawater ranges from ( 10-50 Ω* cm)

Estimate cathodic current requirement which depends on the

environment and the surface area to be protected using either:

• Current requirement table (see slide# 24)
• Current requirement test (see slide# 35)

Select a suitable sacrificial anode and calculate the theoretical

capacity and the driving voltage (slide# 37 and 38)

Estimate the number of anodes needed based on groundbed

resistance (slide# 41 and 42)

Estimate anode life and replacement period

• Dr. Z. Gasem

ME 472-061 KFUPM

34

Wenner 4-pin Method to Measure Soil Resistivity

This method is done by placing four pins at equal distances from each other. A current is passed through the two outer pins using a power supply. the voltage across the two inner pins is measured using a voltmeter. the resistance can be calculated using Ohm's law (Resist = ∆V/I). Soil resistivity = 191.2* ∆V/I* d (d in feet) ohm-cm, where R is the soil resistance and d is the pin spacing in feet.

Power supply voltmeter

• Dr. Z. Gasem

ME 472-061 KFUPM

35

Current Requirement Test

The current may be increased

gradually until the voltmeters at positions A and B reaches -0.85 V with respect to a copper sulfate reference electrode placed directly above the pipe. Current requirement test:

• A small DC power system is used

(10 A)

• A temporary anode ground bed is

installed

• Potential loggers are installed at

selected test locations to monitor potentials

• A current is applied and the

potential is measured

• The current that brings the

potential of the whole pipe below the protective potential is used the required current for protection

• Dr. Z. Gasem

ME 472-061 KFUPM

36

Sacrificial Anode

Anodes must have:

High driving potential to generate sufficient current Stable operating potentials over a range of current

• utputs (Eanode does not vary a lot with i)

High capacity to deliver current per unit mass Does not passivate Theoretical capacity: the total charge in coulombs

produced by the corrosion (dissolution) of a unit mass of the anode material [units in (A* hr)/Kg].

High Efficiency (efficiency = actual

capacity/theoretical capacity* 100)

• Dr. Z. Gasem

ME 472-061 KFUPM

37

Calculating the theoretical capacity

MW of Mg is 24.3 g/mol, density= 1.74 g/cm3 Mg→Mg+ 2 + 2e- (one mole of Mg produces 2

moles of electrons)

Take 1 Kg of Mg as a basis:

1000g * mole/24.3g= 41.2 mole of Mg # of e- mole= 2* 41.2= 82.4 moles of e- 82.4 moles of e- * 96500 Coulomb/(mole e)=

795,1600 Coulomb/(Kg of Mg)

795,1600 Coulomb/Kg * 1 hr/3600s= 2,200 (A.hr)/Kg

• Dr. Z. Gasem

ME 472-061 KFUPM

38

Calculating driving potential

ED = Eanode – Ep + Epolar

• ED = driving potential
• Eanode = anode potential
• Ep = protection potential
• Epolar = change in potential
• f anode due to current

flow (polarization); usually taken as 0.1 V

ED

• Dr. Z. Gasem

ME 472-061 KFUPM

39

Driving Potential

Example: calculate the driving potential for Mg

in soil assuming:

• Eanode = -1.75 vs Cu/CuSO4
• Ep (buried pipeline cross country) = -1.0 V
• Epolar = 0.1 V
• ED = -1.75 – (-1.0) +0.1 = -0.65 V

Example: calculate the driving potential for Al

alloy (5%Zn) in soil assume Ep = -0.85 V and Epola=0 and Eanode=-1.1V

• ED = -1.1- (-0.85) = -0.25 V

• Dr. Z. Gasem

ME 472-061 KFUPM

40

Sacrificial Anodes

• 1.1
• 1.1
• 1.75 V (vs

Cu/CuSO4) potential > 90% > 90% 50-60% Efficiency 2640 780 1232 Actual capacity 2980 810 2200 (A* hr)/Kg Theoretical capacity Al Zn Mg

• Dr. Z. Gasem

ME 472-061 KFUPM

41

Anode bed (groundbed) resistance

In both systems, the flow of current

is analogous to a simple resistive circuit.

The highest resistance to current flow

is due to the anode/electrolyte resistance (Rab)

RS (structure/electrolyte) resistance. RLW (lead wire) resistance. Rab = Resistance of

anode/electrolyte; depends on the anode shape and the resistivity of the environment.

Rtotal = Ra+ RLW+ RS (RS and RLW) can be neglected Rtotal ≈ Rab

R Battery Resistor E

Electric Circuit I Rtotal

• Dr. Z. Gasem

ME 472-061 KFUPM

42

Anode bed (ground bed) resistance

Dwight’s equation for single

vertical anodes:

Ra = ρ/(2πLa)* (ln(8La/Da) -1)

(for slender anodes mounted at least 0.3 m away from the steel structure)

• La= length of anode (cm)
• ρ = soil resistivity (Ω.cm)
• Da = anode diameter (cm)

Anode current output

i = ED/Ra

C A

Rab

• Dr. Z. Gasem

ME 472-061 KFUPM

43

Anode bed (ground bed) resistance

• Example: Calculate the maximum current output for

zinc anodes (L = 150 cm and D = 15 cm) in sacrificial anode CP system. Assume very corrosive soil (ρ= 1000

Ω* cm)

1.

Calculate the driving potential assuming Epola = 0.1V, EZn = -1.1 vs Cu/CuSO4, Ep (buried pipeline) = -0.85 V

• ED = -1.1 – (-0.85)-0.1 = -0.15 V

2.

Calculate the ground bed resistance

• Ra = ρ/(2πLa)* (ln(8La/Da) -1)
• Ra = 1000/(2π* 150)* (ln(8* 150/15)-1) = 3.6 Ω

3.

Calculate the maximum current output from each anode i = ED/Ra = 0.15/3.6 = 0.042 A = 42 mA

• Dr. Z. Gasem

ME 472-061 KFUPM

44

Anodes distribution

Example: suppose you need 1

A to protect a steel structure by using Zn anodes as in the previous example.

Each anode provides 0.042 A. Then, you need to have

1/(0.042) ≈ 24 anodes to give sufficient protection.

Distance of anode to cathode:

• Too far: high resistance in the

soil leads to voltage drop

• Too short: current distribution

is not uniform

• Needs experience (usually less

than a meter)

• Dr. Z. Gasem

ME 472-061 KFUPM

45

Design Example

Example: design a CP system for a section of coated steel buried pipe assuming that the current required to shift the pipeline potential to the EP was approximated by a current requirement test to be 500 mA. Zn anodes (L= 150 cm, D= 15 cm) are available. (Density of Zn = 7.14 g/cm3). Assume Zn efficiency is 90% and ρ= 1000 Ω* cm. From the previous example, each Zn anode produces 42 mA. Then, # anodes = 500/42 = 12 anodes. Total mass of anodes = 12* vol* density = 12* 3.14* D2* L/4* 7.14 g/cm3= 2270 Kg Total charge available = efficiency* theoretical capacity* mass= 0.9* 810* (A.hr/Kg)* 2270 Kg= 1,593,540 (A* hr) Replacement period = 1,593,540 (A* hr)/0.5A= 189540 hr = 364 years

• Dr. Z. Gasem

ME 472-061 KFUPM

46

Sacrificial Anode CP System

Install 12 anodes evenly

distributed along the

• pipeline. Keep each anode

30-100 cm away from the

• pipe. The system design

life is indefinite.

Monitoring Sacrificial CP

• Measure the potential of

the pipe and make sure it is -850mV

• Monitor the current flow

from each anode at the junction box

• Dr. Z. Gasem

ME 472-061 KFUPM

47

Applications: buried tanks, pipelines, internal protection of heat exchangers and vessels, ship hulls, marine structures

Aluminum alloy anode AA-036348

Anodes

• Dr. Z. Gasem

ME 472-061 KFUPM

48

Impressed Current CP (ICCP) System ICCP is used if:

high current is required high resistance electrolyte

• Dr. Z. Gasem

ME 472-061 KFUPM

49

ICCP

Components of ICCP

system

A transformer: to

reduce the voltage from high to low voltage

A rectifier to convert

AC to DC

A current distributor

(junction box)

Anodes with backfills

• Dr. Z. Gasem

ME 472-061 KFUPM

50

Reactions

The anode potential is set at

high + ve potential and the following oxidations reactions become possible:

• 2H2O→ O2 + 4H++ 4e- (in

water or in wet soil)

• 2Cl-→Cl2 + 2e- (in salt or

brackish water)

• C+ 2H2O→ CO2 + 4H++ 4e-

(in graphite anodes)

Reactions at the cathode:

• In aerated wet soil
• O2+ 2H2O+ 4e- ⇒ 4OH-
• In aerated acidic solution
• O2+ 4H+ + 4e- ⇒ 2H2O
• In neutral seawater
• O2+ 2H2O+ 4e- ⇒ 4OH-
• In de-aerated soil or water
• 2H2O+ 2e- ⇒ 2OH- + H2

anode Cathode

• Dr. Z. Gasem

ME 472-061 KFUPM

51

Advantages and Disadvantages of Impressed Current Systems

Advantages

• Higher Current and power outputs
• Adjustable protection levels (controlled current)
• Large areas of protection
• Low number of anodes
• Can be used to protect poorly coated structure

Disadvantages

• Complex equipment and installation costs
• Higher maintenance costs
• Possible interference problems with foreign

structures

• Risk of incorrect polarity connections

• Dr. Z. Gasem

ME 472-061 KFUPM

52

Components of ICCP

An external power source

AC transmission lines A solar power system

Anodes are not necessarily more active

than the structure to be protected

Two types of anodes

inert or non-consumable anodes: platinized

anodes (a few micrometers thick coating of platinum on Ti or Niobium), graphite,

consumable anodes (scrap steel, high-Si Cr cast

iron)

• Dr. Z. Gasem

ME 472-061 KFUPM

53

Anode to coke resistance Coke-to-earth Resistance Soil Coke breeze

Anodes for ICCP

Anodes are used with

carbonaceous backfill called coke-breeze to:

increases the effective

size of the anode

lowers the anode-to-

ground resistance.

extends the life of the

anode.

Anode

• Dr. Z. Gasem

ME 472-061 KFUPM

54

Anodes in ICCP

applications Consumption rate (Kg/(A* yr) Material

Seawater, concrete, 8x10-6 Platinized niobium (inert) Marine, soil 1-0.25 High Si-cast iron Marine, soil 7-9 Scrap steel Soil, Potable water 0.1-1 Graphite (inert) = = 8x10-6 Platinized Ti (inert)

• Dr. Z. Gasem

ME 472-061 KFUPM

55

Applications

Ships (with coatings) Offshore platforms Buried pipelines (pref

method)

Oil well casing Concrete Structures

(offshore bridges)

• Dr. Z. Gasem

ME 472-061 KFUPM

56

Buried pipelines

• Dr. Z. Gasem

ME 472-061 KFUPM

57 Producing Zone Remote Surface Anode Bed Junction Box Rectifier Perforations

Well Casing

• Dr. Z. Gasem

ME 472-061 KFUPM

58

ICCP Design

1.

Evaluation of electrolyte resistivity

2.

Estimating the current requirement

• Current requirement test
• Current requirements theoretical estimation

3.

Selecting anode material and current distribution

• Uniform current distribution
• Avoid interference (stray current)

4.

Determine the anode bed ground resistance

5.

Determine number of required anodes

6.

Select the power source capacity

• Dr. Z. Gasem

ME 472-061 KFUPM

59

• 1. Electrolyte Resistivity Survey

Measure soil resistivity along the pipeline The data from a soil resistivity survey along a 6

km section of pipeline is shown below. The lowest effective soil resistivity points are the most favorable anode bed locations.

• Dr. Z. Gasem

ME 472-061 KFUPM

60

• 2. Interruption Test for Bare Pipeline in Impressed

Current System

In impressed current system for bare structure, the

applied current is high and IR drop can not be neglected.

Thus, protection Criteria for bare pipeline must check

I* RΩ effect.

The protection criterion for bare steel pipeline uses

interruption test where a negative (cathodic) change in potential of > 300 mV must take place immediately after CP current is applied.

potential time

CP Power off CP power on 300 mV

• Dr. Z. Gasem

ME 472-061 KFUPM

61

• 3. Current distribution

Variation of electrolyte

resistivity b/w anode and cathode (largest current flows along least resistant path)

Defects in coatings:

current concentrates at defects

• Dr. Z. Gasem

ME 472-061 KFUPM

62

Stray Currents

• Stray Currents: currents

flowing in the electrolyte from external sources other than the applied CP.

• Sources of stray

currents:

• Subway system
• Interference with

another CP system

• Welding equipment
• Electrical power

transmission lines

• Dr. Z. Gasem

ME 472-061 KFUPM

63

• 4. Number of Required

Anodes

N = number of impressed current anodes Y = design life in years I = total current required in amperes C = anode consumption rate in kg/A-yr W = weight of a single anode in kg

N = Y*C*I/W

• Dr. Z. Gasem

ME 472-061 KFUPM

64

• 5. Resistance calculation

Dwight’s equation for single vertical

anode:

Ra = ρ/(2πLa)* (ln(8La/Da) -1)

A group of vertical or horizontal anodes

(buried 6 ft below ground):

• Vertical anodes ( Rv = ρ* F/537)
• Each anode is 8-12 in in diameter and 10 ft in length
• Horizontal anodes (RH = ρ* F/483)
• Each anode is 10 ft in length and 6 ft below surface
• F is called adjusting factor (F= 1 for single anode)

• Dr. Z. Gasem

ME 472-061 KFUPM

65

F : Adjusting Factor

• Dr. Z. Gasem

ME 472-061 KFUPM

66

• 4. Power Source Selection

The size of the power source is

determined by:

the amount of current required to protect the

structure (I)

the voltage required to force the current

through the anode ground bed resistance (R)

E= I* R

• Dr. Z. Gasem

ME 472-061 KFUPM

67

Example: Design for ICCP

Design an impressed current system to protect a buried pipeline coated with fusion bonded epoxy (FBE) using the following information:

• Horizontal anodes 6 ft below ground with 20

ft spacing

• Anode material: High silicon-cast iron (

C= 0.5 Kg/(A* yr))

• Anode dimensions with backfill: 25 cm dia. x

300cm, weight= 50 Kg

• Pipeline length: 500 m and 115 mm in

diameter

• the anode to soil resistance is 0.24
• hm
• Neglect cable resisitivity.
• Soil resistivity: 2,000 ohm-cm
• Required current density is 0.2 mA/m2 for

FBE.

• Design life of 20 years

• Dr. Z. Gasem

ME 472-061 KFUPM

68

Example: Design for ICCP

Current required = surface area * current density

• = π d L * i= 3.14* 0.115* 500 * 0.2 = 36.1 A

# anodes= Y* I* C/W = 20* 36.1* 0.5/50 = 7.2 anodes .

Then use 8 anodes.

To calculate the resistance:

• RH = ρ* F/483 (for 8 horizontal anodes F= 0.184)
• = 2000* 0.184/483 = 0.76Ω

Total R= 0.76+ 0.24= 1 Ω E = I* R= 1* 36.1 = 36.1 V Hence, use a DC power with a minimum current

supply of 40 A and a minimum voltage of 40 V (1600 Watt rating).