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Pipe Lines

CATHODIC PROTECTIONS

CATHODIC PROTECTIONS

Systems Requiring Cathodic Protection (CP)Systems Requiring Cathodic Protection (CP)

• Natural gas distribution systemNatural gas distribution system

• Fuel distribution systemFuel distribution system

• Potable water distribution systemPotable water distribution system

• Compressed air distribution systemCompressed air distribution system

• Underground fuel storage tanks and ground level tank bottomsUnderground fuel storage tanks and ground level tank bottoms

• Offshore platforms, floating and sub-sea structuresOffshore platforms, floating and sub-sea structures

• Jetties and harbor structureJetties and harbor structure

Cathodic Protections

Types of Cathodic Protection SystemTypes of Cathodic Protection System

• Galvanic system (Sacrificial anode system)Galvanic system (Sacrificial anode system) - Galvanic system (Sacrificial anode system)- Galvanic system (Sacrificial anode system)

- Sacrificial anodes- Sacrificial anodes - A conductor connecting the anode to the structure or direct welding to the structure- A conductor connecting the anode to the structure or direct welding to the structure - Secure and minimum resistance connections between conductor and structure - Secure and minimum resistance connections between conductor and structure - Secure and minimum resistance connections between conductor and anode- Secure and minimum resistance connections between conductor and anode

• Impressed-current systemImpressed-current system - Secure and minimum resistance connections between power source and structure.- Secure and minimum resistance connections between power source and structure. - Secure and minimum resistance connections between power source and anode.- Secure and minimum resistance connections between power source and anode.

Cathodic Protections

Corrosion ProcessCorrosion Process•The corrosion of metals is an electrochemical processThe corrosion of metals is an electrochemical process•CorrosionCorrosion is the degradation of a metal by its chemical combination with a nonmetal such as is the degradation of a metal by its chemical combination with a nonmetal such as oxygen, sulfur and the like.oxygen, sulfur and the like.

Cathodic Protections

Corrosion ProcessCorrosion Process

Most metals corrode on contact withMost metals corrode on contact with

•Water (and moisture in the air),Water (and moisture in the air), •Acids,Acids,

• Bases,Bases,

• Salts,Salts,

• Oils,Oils,

• Aggressive metal polishes, and Other solid and liquid chemicalsAggressive metal polishes, and Other solid and liquid chemicals

Cathodic Protections

Corrosion ProcessCorrosion Process

Metals also corrode when exposed to gaseous materials like Metals also corrode when exposed to gaseous materials like

• Acid vapors, Acid vapors,

• Formaldehyde gas, Formaldehyde gas,

• Ammonia gas, Ammonia gas,

• Sulfur containing gasesSulfur containing gases

Cathodic Protections

Corrosion CellCorrosion Cell

Four parts constitute what is called the ‘Four parts constitute what is called the ‘corrosion cell’corrosion cell’..

• Anode,Anode,

• Cathode,Cathode,

• Electrolyte, Electrolyte,

• Metallic pathMetallic path

Cathodic Protections

Corrosion CellCorrosion CellAnodeAnode• Location where corrosion occursLocation where corrosion occurs

• Positive electrode in an electrolytic cellPositive electrode in an electrolytic cell

• Point in a corrosion cell where electricity is passed by chemical means from the surface of Point in a corrosion cell where electricity is passed by chemical means from the surface of the metal to the electrolytethe metal to the electrolyte

• Chemical reaction is an oxidation reactionChemical reaction is an oxidation reaction

• Metal loosing an electron and combining with another element, usually oxygenMetal loosing an electron and combining with another element, usually oxygen

% Design Pressure

Cathodic Protections

Corrosion CellCorrosion CellCathodeCathode

• Location where protection occursLocation where protection occurs

• Negative electrode in an electrolytic cellNegative electrode in an electrolytic cell

• Point in a corrosion cell where electricity is passed by chemical means from the Point in a corrosion cell where electricity is passed by chemical means from the electrolyte to the surface of the metalelectrolyte to the surface of the metal

• Chemical reaction is a reduction reactionChemical reaction is a reduction reaction

• The metal passing electrons to the electrolyteThe metal passing electrons to the electrolyte

Cathodic Protections

Corrosion CellCorrosion CellElectrolyteElectrolyte

• Third part of the corrosion cell is the electrolyteThird part of the corrosion cell is the electrolyte

• Location where ions flowLocation where ions flow

• Electrolyte is any material in contact with both the anode and the cathode which will Electrolyte is any material in contact with both the anode and the cathode which will allow ions to migrateallow ions to migrate

• The part of a corrosion cell which allows oxidation and reduction reactions to occurThe part of a corrosion cell which allows oxidation and reduction reactions to occur

• Can be any soil or liquid adjacent to and in contact with the anode and cathodeCan be any soil or liquid adjacent to and in contact with the anode and cathode

Cathodic Protections

Corrosion CellCorrosion CellMetallic PathMetallic Path

•Completes the circuit and allows the electrons to flowCompletes the circuit and allows the electrons to flow

•Can be any conductor which allows electrons to flow from the anode to the cathodeCan be any conductor which allows electrons to flow from the anode to the cathode

•Electron flow must be present for electrochemical corrosion to occurElectron flow must be present for electrochemical corrosion to occur

• In the case of a tank or pipeline, this can be the tank or pipe itself, or it can be a In the case of a tank or pipeline, this can be the tank or pipe itself, or it can be a metallic bond to different metallic structuremetallic bond to different metallic structure

Cathodic Protections

Corrosion CellCorrosion Cell

• The electrode which is more electrically active, or more negative in voltage, undergoes the The electrode which is more electrically active, or more negative in voltage, undergoes the corrosion, so by definition is the corrosion, so by definition is the anodeanode

• The electrode which is more noble (less negative in potential), passes electrons to the The electrode which is more noble (less negative in potential), passes electrons to the electrolyte (reduction reactions) and by definition is the electrolyte (reduction reactions) and by definition is the cathodecathode and does not undergo and does not undergo corrosion (oxidation reactions). corrosion (oxidation reactions).

Potential and its MeasurementPotential and its Measurement

Cathodic Protections

CorrosionCorrosion

FACTORS AFFECTING THE RATE OF CORROSIONFACTORS AFFECTING THE RATE OF CORROSION

Cathodic Protections

1.1. Electrical effectsElectrical effects2.2. Chemical effectsChemical effects3.3. Relative size of the anodic and cathodic areaRelative size of the anodic and cathodic area

• Electrical effectsElectrical effects

• Potential DifferencePotential Difference

• Resistivity of the ElectrolyteResistivity of the Electrolyte

• Contact ResistanceContact Resistance

• Coating of the StructureCoating of the Structure

• Polarization of the StructurePolarization of the Structure

• Amount of Current FlowAmount of Current Flow

Cathodic Protections

1.1. Chemical effectsChemical effects

• TemperatureTemperature

• Ion concentrationIon concentration

• Electron concentrationElectron concentration

• pH of the electrolytepH of the electrolyte

• Coating of the structureCoating of the structure

• Polarization of the structurePolarization of the structure

Effects of CorrosionEffects of Corrosion

Cathodic Protections

• Loss of commodityLoss of commodity

• Property damage including fireProperty damage including fire

• Expensive repairsExpensive repairs

• Contamination of water suppliesContamination of water supplies

• Loss of livestockLoss of livestock

• Deterioration of public relationsDeterioration of public relations

Cathodic Protections

Means Used to Prevent or Control CorrosionMeans Used to Prevent or Control Corrosion

• Coating, paintingCoating, painting

•Application of oil or tar, pitch, gypsum, white lead , red leadApplication of oil or tar, pitch, gypsum, white lead , red lead

• GalvanizingGalvanizing

• Sacrificial alloys of magnesium and aluminumSacrificial alloys of magnesium and aluminum

•Cathodic protectionCathodic protection

Cathodic Protection (CP)Cathodic Protection (CP)

Cathodic Protections

The principle of cathodic protection is in connecting The principle of cathodic protection is in connecting

an external anode to the metal to be protected an external anode to the metal to be protected

and the passing of an electrical dc current and the passing of an electrical dc current so that all areas of the metal surface become cathodic so that all areas of the metal surface become cathodic and therefore do not corrodeand therefore do not corrode

• The external anode may be a The external anode may be a galvanic anodegalvanic anode, , • where the current is a result of the potential difference where the current is a result of the potential difference between the two metalsbetween the two metals

• Or it may be an impressed current anode, Or it may be an impressed current anode, • where the current iswhere the current is impressed impressed from from • an external dc power sourcean external dc power source

Cathodic Protections

Cathodic Protection (CP)Cathodic Protection (CP)

• CP is applied simply by maintaining a dc circuit and its effectiveness may be monitored CP is applied simply by maintaining a dc circuit and its effectiveness may be monitored continuouslycontinuously

• Commonly applied to a coated structure to provide corrosion control to areas where the Commonly applied to a coated structure to provide corrosion control to areas where the coating may be damagedcoating may be damaged

• May be applied to existing structures to prolong their lifeMay be applied to existing structures to prolong their life

• Its main use is to protect steel structures buried in soil or immersed in water Its main use is to protect steel structures buried in soil or immersed in water

• Cannot be used to prevent atmospheric corrosion on metalsCannot be used to prevent atmospheric corrosion on metals

• Two basic ways:Two basic ways:

1.1. By Sacrificial Anode (or Galvanic Anode) Cathodic ProtectionBy Sacrificial Anode (or Galvanic Anode) Cathodic Protection

2.2. By Impressed Current Cathodic ProtectionBy Impressed Current Cathodic Protection

Cathodic Protections

Cathodic Protection (CP)Cathodic Protection (CP)

• Sacrificial anode or Galvanic cathodic protectionSacrificial anode or Galvanic cathodic protection - by using the galvanic series to select a more active - by using the galvanic series to select a more active metal, metal, install that metal in the electrolyte and provide a metallic install that metal in the electrolyte and provide a metallic path.path.

• Impressed Current Cathodic ProtectionImpressed Current Cathodic Protection - applying a source of DC current which forces the - applying a source of DC current which forces the current to current to flow from an installed anode (s) to the structure, flow from an installed anode (s) to the structure, causing the causing the structure to be a cathodestructure to be a cathode

• The current required for cathodic protection depends upon the metal being protected The current required for cathodic protection depends upon the metal being protected and the environmentand the environment

• The potentials required to determine adequate protection (criteria) are establishedThe potentials required to determine adequate protection (criteria) are established

• To achieve these protective potentials, current must flow from the anode to the structure To achieve these protective potentials, current must flow from the anode to the structure being protectedbeing protected

• The amount of current required to protect a given structure is proportional to the area of The amount of current required to protect a given structure is proportional to the area of the structure which is exposed to the electrolytethe structure which is exposed to the electrolyte

• For coated structures, the amount of current required is much lower than for bare For coated structures, the amount of current required is much lower than for bare structures, as only those areas where the coating has been damaged or has structures, as only those areas where the coating has been damaged or has deteriorated require or will receive currentdeteriorated require or will receive current

• Current requirements for coated structures are best determined by actual testing after the Current requirements for coated structures are best determined by actual testing after the structure is installedstructure is installed

Cathodic Protections

Cathodic Protection (CP)Cathodic Protection (CP)

Sacrificial Anode Cathodic ProtectionSacrificial Anode Cathodic Protection A galvanic cathodic protection system consists ofA galvanic cathodic protection system consists of sacrificial anodes , wiring or sacrificial anodes , wiring or

conductor connecting the anode to the structure and inspection station conductor connecting the anode to the structure and inspection station installed near installed near the surface of the groundthe surface of the ground

The current required for cathodic protection is supplied by the corrosion of an active The current required for cathodic protection is supplied by the corrosion of an active metal.metal.

Galvanic systems have limited life spansThe current required for cathodic protection Galvanic systems have limited life spansThe current required for cathodic protection is supplied by the corrosion of an active metalis supplied by the corrosion of an active metal

Anode efficiency , which accounts for the anode’s self corrosion rate and the Anode efficiency , which accounts for the anode’s self corrosion rate and the corrosion rate for the amount of cathodic protection current.corrosion rate for the amount of cathodic protection current.

The simplest systems consist of an anode fabricated from an active metal such as The simplest systems consist of an anode fabricated from an active metal such as zinc which is directly connected to the structure in an area where it will be exposed zinc which is directly connected to the structure in an area where it will be exposed to the same environment as the structure being protectedto the same environment as the structure being protected

Cathodic Protections

Cathodic Protections

Sacrificial Anode Cathodic ProtectionSacrificial Anode Cathodic Protection

Cathodic Protections

Application of Galvanic Cathodic ProtectionApplication of Galvanic Cathodic Protection Limited by the small potential difference (normally less than 1 volt DC) which Limited by the small potential difference (normally less than 1 volt DC) which

can be obtainedcan be obtained Economical use on Economical use on small or well-coated structures in low resistivity electrolytessmall or well-coated structures in low resistivity electrolytes The electrolyte resistivity determines the amount of current which the limited The electrolyte resistivity determines the amount of current which the limited

voltage will supply voltage will supply The amount of metal exposed to the electrolyte determines the amount of The amount of metal exposed to the electrolyte determines the amount of

current requiredcurrent required Commonly used as galvanic anodesCommonly used as galvanic anodes

- Magnesium- Magnesium

- Zinc- Zinc

- Aluminum alloy- Aluminum alloy

Application of Galvanic Cathodic ProtectionApplication of Galvanic Cathodic Protection

Cathodic Protections

Typical examples of Sacrificial AnodesTypical examples of Sacrificial Anodes