corrosion and cathodic protection © 2007 spectrum external line inspection technologies inc

Corrosion and Cathodic Corrosion and Cathodic ProtectionProtection

© 2007 Spectrum eXternal Line Inspection Technologies © 2007 Spectrum eXternal Line Inspection Technologies Inc.Inc.

CorrosionCorrosion

NACE defines Corrosion:NACE defines Corrosion: Deterioration of a substance (usually a metal) Deterioration of a substance (usually a metal)

or its properties because of a reaction with it’s or its properties because of a reaction with it’s environment.environment.

Pipeline CorrosionPipeline Corrosion

Primarily concerned with the destruction of Primarily concerned with the destruction of the metal by either chemical or the metal by either chemical or electrochemical reaction with a given electrochemical reaction with a given environment.environment.

Electrochemical ReactionsElectrochemical Reactions

Most metals are found as “ores” which are Most metals are found as “ores” which are metallic oxides or salts.metallic oxides or salts.

The ores are converted into the metals by The ores are converted into the metals by refining, which involves adding energy.refining, which involves adding energy.

The more energy added during the refining The more energy added during the refining process, the higher the energy level, or process, the higher the energy level, or potential of the metal.potential of the metal.

The Corrosion CycleThe Corrosion Cycle

The energy stored in the metal during The energy stored in the metal during the refining process is the driving force the refining process is the driving force for corrosion.for corrosion.

The refining-corrosion cycle is The refining-corrosion cycle is somewhat like rolling a ball up a hill and somewhat like rolling a ball up a hill and then watching it roll back down the hill then watching it roll back down the hill the instant you release it.the instant you release it. The job Corrosion Control is to prevent the The job Corrosion Control is to prevent the

ball from rolling back down, or at least to ball from rolling back down, or at least to slow down the rate at which it rolls.slow down the rate at which it rolls.

Electromotive Force Series of Electromotive Force Series of MetalsMetals

MagnesiumMagnesium -2.37 Volts-2.37 Volts AluminumAluminum-1.66 Volts-1.66 Volts ZincZinc -0.76 Volts-0.76 Volts IronIron -0.44 Volts-0.44 Volts LeadLead -0.13 Volts-0.13 Volts CopperCopper +0.34 Volts+0.34 Volts SilverSilver +0.80 Volts+0.80 Volts PlatinumPlatinum +1.20 Volts+1.20 Volts GoldGold +1.60 Volts+1.60 Volts

Most Energy Required for Refining

Least Energy Required for Refining

Greatest Tendency to Corrode

Least Tendency to Corrode

The Electric Circuit of The Electric Circuit of CorrosionCorrosion

The Parts of the Corrosion CellThe Parts of the Corrosion Cell

Anode Anode The metal with the highest Energy level in an The metal with the highest Energy level in an

electrochemical cell. The anode is the electrochemical cell. The anode is the location of the oxidation reaction, which location of the oxidation reaction, which means the anode looses energy (electrons) means the anode looses energy (electrons) and is “corroded” as metal ions are released and is “corroded” as metal ions are released from the metal into the electrolyte by the loss from the metal into the electrolyte by the loss of electrons that are needed to hold the ions.of electrons that are needed to hold the ions.


CathodeCathode The metal with the lowest energy level in an The metal with the lowest energy level in an

electrochemical cell. The cathode is the electrochemical cell. The cathode is the location of the reduction reaction, which location of the reduction reaction, which means the cathode gains energy (electrons) means the cathode gains energy (electrons) and is “plated” with + ions that are present in and is “plated” with + ions that are present in the electrolyte. The cathode metal is un-the electrolyte. The cathode metal is un-reactive.reactive.


Electron BridgeElectron Bridge Any electric conducting pathway between the Any electric conducting pathway between the

anode and cathode.anode and cathode.


ElectrolyteElectrolyte An electrically conductive solution An electrically conductive solution

surrounding the anode and cathode in the surrounding the anode and cathode in the electrochemical cell. The electrolyte is electrochemical cell. The electrolyte is required to complete the electrical circuit.required to complete the electrical circuit.

Rate of CorrosionRate of Corrosion

Electrolyte CompositionElectrolyte Composition pH: Increases if <5, or >12pH: Increases if <5, or >12 Conductivity: Increases with salinity.Conductivity: Increases with salinity. Dissolved Gases: ODissolved Gases: O22, CO, CO22, H, H22S,S,

Potential difference between Anode and Potential difference between Anode and Cathode - EMFCathode - EMF

Temperature - “rule of thumb” every Temperature - “rule of thumb” every 1010ooC doubles the reaction rate.C doubles the reaction rate.

One ampere of current flowing for one year = 20 pounds of iron.

Types of CorrosionTypes of Corrosion

There is a multitude of different types of There is a multitude of different types of corrosion, mostly resulting in different corrosion, mostly resulting in different patterns and amounts of wall loss.patterns and amounts of wall loss. PittingPitting Crevice CorrosionCrevice Corrosion Concentration and/or Differential Aeration Concentration and/or Differential Aeration

CellsCells Scale and sludge - depositsScale and sludge - deposits

Types of CorrosionTypes of Corrosion

There are also several types of There are also several types of environmental cracking that can occur and environmental cracking that can occur and are directly related to corrosionare directly related to corrosion Stress Corrosion Cracking (SCC)Stress Corrosion Cracking (SCC) Sulfide Stress Cracking (SSC)Sulfide Stress Cracking (SSC) Hydrogen related embrittlement and cracking.Hydrogen related embrittlement and cracking.

Cracks are the most dangerous concern to Cracks are the most dangerous concern to pipeline integrity.pipeline integrity.

Controlling CorrosionControlling Corrosion

Eliminate any one of the four parts of the Eliminate any one of the four parts of the corrosion cell and no corrosion will occur.corrosion cell and no corrosion will occur. ElectrolyteElectrolyte

• coatings, internal and external.coatings, internal and external. Anode and CathodeAnode and Cathode

• material selectionmaterial selection• Cathodic Protection.Cathodic Protection.

Cathodic ProtectionCathodic Protection

The goal of cathodic protection system is The goal of cathodic protection system is to polarize the pipe with an abundance of to polarize the pipe with an abundance of e-, which will be available to any e- e-, which will be available to any e- acceptors that would otherwise steal e- acceptors that would otherwise steal e- from the pipe. In order to do this over a from the pipe. In order to do this over a long distance the pipe must have a coating long distance the pipe must have a coating which will prevent the e- from jumping off which will prevent the e- from jumping off at every available chance! at every available chance!


This is why pipe coatings are electric This is why pipe coatings are electric resistors first, physical protection to the resistors first, physical protection to the pipe second. Thus a small pinhole in the pipe second. Thus a small pinhole in the coating will allow e- to escape from the coating will allow e- to escape from the pipe to the soil (electrolyte) and no metal pipe to the soil (electrolyte) and no metal is lost from the pipeline. is lost from the pipeline.


In fact, the charge on the pipeline will In fact, the charge on the pipeline will actually attract the ions that a cathode actually attract the ions that a cathode typically attracts and in the case of a typically attracts and in the case of a pipeline in the ground this often results is pipeline in the ground this often results is the formation of deposits on the outside of the formation of deposits on the outside of the pipe; calcium (C++) forming the pipe; calcium (C++) forming calcareous deposits.calcareous deposits.


Electricians refer to a CP “drain” on a Electricians refer to a CP “drain” on a pipeline system; this is where the Positive pipeline system; this is where the Positive current is drained off of the system. current is drained off of the system. Remember that positive current off of the Remember that positive current off of the pipeline is achieved by putting pipeline is achieved by putting electronselectrons on the pipeline! Rectifiers are always on the pipeline! Rectifiers are always hooked up with the - side to the pipeline hooked up with the - side to the pipeline and the + side to an anode bed.and the + side to an anode bed.


Neg(-)

Pos(+)

Anodes(O xidation)

Cathode(Reduction)

Rectifie r(AC to DC)

Protective Coating

Coating Holidaye- e-

e- e- e-e-

Ca+ Ca+Ca+

E lectron m ovem entC urrent m ovem ent

e-e-

e-e-

e-e-

An+

An+ An+

Tes

t P

ost

Designing Cathodic Protection Designing Cathodic Protection SystemsSystems

Electrolyte Composition - Salt Water, Soil.Electrolyte Composition - Salt Water, Soil. Volume of metal to protect Volume of metal to protect (milliamperes/sq ft)(milliamperes/sq ft).. Quality of coating.Quality of coating. Availability of electricity.Availability of electricity.

Power Requirements = Size and # of Power Requirements = Size and # of AnodesAnodes

Maintaining and Monitoring CPMaintaining and Monitoring CP

Annual adjustive surveys - Pipe to soil Annual adjustive surveys - Pipe to soil potential.potential.

Rectifier inspection and monitoring.Rectifier inspection and monitoring. Interference investigationInterference investigation

Stray AC currentStray AC current competing CP systemscompeting CP systems Telluric currentTelluric current

Close Space Surveys - On/Off PotentialsClose Space Surveys - On/Off Potentials

Effective CPEffective CP

Common to use -850 mV as a minimum Common to use -850 mV as a minimum value to ensure adequate CP.value to ensure adequate CP. Lower or higher values may be required, but Lower or higher values may be required, but

detailed engineering assessments and detailed engineering assessments and surveys are usually required to determine surveys are usually required to determine “spot specific” areas and adjust accordingly.“spot specific” areas and adjust accordingly.

Potentials of -1500 mV and lower can Potentials of -1500 mV and lower can result in excessive Hresult in excessive H22 gas production. gas production.

Coating SurveysCoating Surveys

Coating surveys are conducted to evaluate Coating surveys are conducted to evaluate the coating integrity on buried pipelines. the coating integrity on buried pipelines.

These surveys can be used to determine These surveys can be used to determine potential locations for corrosion, potential locations for corrosion, environmental cracking, or C.P. current environmental cracking, or C.P. current loss. loss.


Because of the cost and time required to Because of the cost and time required to conduct these surveys they are often only conduct these surveys they are often only completed in environmentally sensitive completed in environmentally sensitive areas or in high temperature zones areas or in high temperature zones downstream of plants or boosters where downstream of plants or boosters where coating damage is expected. coating damage is expected.


Utilize the Electrical resistivity of the Utilize the Electrical resistivity of the coating.coating. Apply a current and measure the attenuation Apply a current and measure the attenuation

of the current over a known distance to find of the current over a known distance to find areas of exceptionally high attenuation.areas of exceptionally high attenuation.

Coating SurveysCoating Surveys The most advanced technology is to induce The most advanced technology is to induce

an alternating current on the pipeline, and an alternating current on the pipeline, and then to measure the field strength along the then to measure the field strength along the length of the pipeline from above ground at length of the pipeline from above ground at regular intervals. The strength of the AC regular intervals. The strength of the AC field above ground will attenuate at a field above ground will attenuate at a logarithmic rate that can be calculated, and logarithmic rate that can be calculated, and confirmed. confirmed.


Where non-logarithmic attenuations are Where non-logarithmic attenuations are measured, the location is investigated to measured, the location is investigated to determine if AC leakage to the soil or other determine if AC leakage to the soil or other buried facilities has resulted. If no buried facilities has resulted. If no interference can be determined, then that interference can be determined, then that interval is identified as having faulty or interval is identified as having faulty or compromised coating.compromised coating.

Coating Survey ReportCoating Survey ReportDETAIL #1

COMMENTS: 1) There is a 6" foreign pipeline crossing at 790m.2) There is a small coating anomaly at distance 803m.

CUSTOMER: XYZ PIPELINE CO.

PIPELINE/SECTION: 6" Lateral, 1-10 To 6-13

SURVEY TECHNICIAN: Brian Powlousky DATE: February 20, 1996

C-SCAN & PIN POINT AT 700m - 1000m

0

5

10

15

20

25

30

(m)

706

715

724

733

742

751

760

769

778

787

796

805

814

823

832

841

850

859

868

877

886

895

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931

940

949

958

967

976

985

994

DISTANCE (m)

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B/m

.

0

5

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PIN

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C-SCAN ATTENUATION

PIN POINT

C-SCAN Critical

PIN POINT Critical

HM H U N T E R M c D O N N E L L P I P E L I N E S E R V I C E S I N C .

C - S C A N R E P O R T F O R M A T S A M P L E

corrosion and cathodic protection © 2007 spectrum external line inspection technologies inc

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