Cathodic Cathodic Protection of Protection of

PipelinePipeline

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ContentsContents CHAPTER ONECHAPTER ONE : : principle of corrosion principle of corrosion CHAPTER TwoCHAPTER Two : : Forms of corrosionForms of corrosion CHAPTER THREECHAPTER THREE :: Environment EffectsEnvironment Effects CHAPTER FOURECHAPTER FOURE : : corrosion protectioncorrosion protection CHAPTER FIVECHAPTER FIVE : : preparation of pipelinepreparation of pipeline CHAPTER SIXCHAPTER SIX : : Corrosion of pipelineCorrosion of pipeline CHAPTER SEVENCHAPTER SEVEN : : Cathodic protection of Cathodic protection of pipeline pipeline CHAPTER EIGHTCHAPTER EIGHT : : Case StudyCase Study

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Ch.1 principles of Ch.1 principles of corrosioncorrosion IntroductionIntroduction

Corrosion is the destructive attack of a material by reaction with its environment. Corrosion is the destructive attack of a material by reaction with its environment. The serious consequences of the corrosion process have become a problem of The serious consequences of the corrosion process have become a problem of worldwide significance. In addition to our everyday encounters with this form of worldwide significance. In addition to our everyday encounters with this form of degradation, corrosion causes plant shutdowns, waste of valuable resources, loss degradation, corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and or contamination of product, reduction in efficiency, costly maintenance, and expensive over design; it also jeopardizes safety and inhibits technological expensive over design; it also jeopardizes safety and inhibits technological progress.progress.

1.1 corrosion definition 1.1 corrosion definition Corrosion is the deterioration of materials by chemical interaction with Corrosion is the deterioration of materials by chemical interaction with

their environment. The term corrosion is sometimes also applied to the their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to degradation of plastics, concrete and wood, but generally refers to metals.metals.

OrOr Destruction of a metal by chemical or electrochemical reaction with Destruction of a metal by chemical or electrochemical reaction with its' environment.its' environment.

OrOr A process in which a metal is destroyed by a chemical reaction A process in which a metal is destroyed by a chemical reaction

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The The cathode cathode is that portion of the metal surface where reduction takes place and does not is that portion of the metal surface where reduction takes place and does not dissolve.dissolve.

The The anode anode is that portion of the metal surface that is corroded. It is the point at which metal is that portion of the metal surface that is corroded. It is the point at which metal dissolves, or goes into solution. When metal dissolves, the metal atom loses electrons and isdissolves, or goes into solution. When metal dissolves, the metal atom loses electrons and is

oxidisedoxidised

a. a. Corrosion occurs by an electrochemical process.Corrosion occurs by an electrochemical process. The phenomenon is similar to that which takes place when a carbon-zinc “dry” cell The phenomenon is similar to that which takes place when a carbon-zinc “dry” cell

generates a direct current. Basically, an anode (negative electrode), a cathode generates a direct current. Basically, an anode (negative electrode), a cathode (positive electrode), an electrolyte (environment), and a circuit connecting the anode (positive electrode), an electrolyte (environment), and a circuit connecting the anode and the cathode are required for corrosion to occur (see Figure 1-1). and the cathode are required for corrosion to occur (see Figure 1-1).

Figure (1-1), dry cellFigure (1-1), dry cell

1.2 corrosion principles1.2 corrosion principles

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the predominant cathodic reaction isthe predominant cathodic reaction is O2 + H2O + 4e- → 4(OH)O2 + H2O + 4e- → 4(OH) eq (1-2 ) eq (1-2 ) The cathodic reaction that usually occurs in deaerated acids isThe cathodic reaction that usually occurs in deaerated acids is 2H+ + 2e- → H2 2H+ + 2e- → H2 eq (1-3)eq (1-3) In aerated acids, the cathodic reaction could beIn aerated acids, the cathodic reaction could be O2 + 4H+ + 4e- → 2H2OO2 + 4H+ + 4e- → 2H2O eq (1- 4) eq (1- 4) b. The number of electrons lost at the anode must equal the number of electrons gained at b. The number of electrons lost at the anode must equal the number of electrons gained at

the cathodethe cathode.. For example, if iron (Fe) was exposed to an aerated, corrosive water, the anodic reaction For example, if iron (Fe) was exposed to an aerated, corrosive water, the anodic reaction

would bewould be 2Fe → 2Fe ++ + 4e- (anodic) eq (1-5)2Fe → 2Fe ++ + 4e- (anodic) eq (1-5)

O2 + 2H2O + 4e- → 4(OH- ) (cathodic)O2 + 2H2O + 4e- → 4(OH- ) (cathodic) eq (1-6)eq (1-6) These can be summed to give the overall oxidation reduction reactionThese can be summed to give the overall oxidation reduction reaction 2Fe + O2 + 2H2O → 2Fe ++ +4(OH- )2Fe + O2 + 2H2O → 2Fe ++ +4(OH- ) eq (1-7) eq (1-7)

c. After dissolution c. After dissolution ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++ ); these will combine with ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++ ); these will combine with

hydroxide ions (OH- ) formed at the cathode to give a corrosion product called rust hydroxide ions (OH- ) formed at the cathode to give a corrosion product called rust (FeOOH or Fe2O3 x H2O).(FeOOH or Fe2O3 x H2O).

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1.3 classification of corrosion1.3 classification of corrosion * General / Uniform Corrosion * General / Uniform Corrosion ::       Atmospheric Galvanic Stray-currentStray-current General biologicalGeneral biological High-temperature

* Localized Corrosion* Localized Corrosion Filiform Filiform Crevice Pitting Localized microbiological

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Ch:2 Forms of Ch:2 Forms of corrosioncorrosion

2.1 Uniform corrosion or general corrosion2.1 Uniform corrosion or general corrosion as sometimes called, is defined as a type of corrosion attack (deterioration) that is more or less as sometimes called, is defined as a type of corrosion attack (deterioration) that is more or less

uniformly distributed over the entire exposed surface of a metal (see illustration below).uniformly distributed over the entire exposed surface of a metal (see illustration below).

Fig (2-1) , uniform corrosion

2.1.1 Mechanisms2.1.1 MechanismsThe anodic reaction in the corrosion process is always the oxidation reaction:

M = M+ + e-  eq (2-1)In acidic environments, i.e., pH < 7,  the cathodic process is mainly the reduction of hydrogen ions:

2H+ + 2e = H2       eq (2-2)

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With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2-2) take With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2-2) take place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The cathodes and anodes are located randomly and alternating with time. The end result is a more or less cathodes and anodes are located randomly and alternating with time. The end result is a more or less uniform loss of dimension.uniform loss of dimension.

Fig (2-2) Real uniform corrosion

2.1.2 Prevention or Remedial Action2.1.2 Prevention or Remedial Action• Uniform corrosion or general corrosion can be prevented through a number of methods: • Use thicker materials for corrosion allowance • Use paints or metallic coatings such as plating, galvanizing or anodizing • Use Corrosion inhibitors or modifying the environment • Cathodic protection (SA/ICCP) and Anodic Protection • selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy) • utilize coatings to act as a barrier between metal and environment. • modify the environment or add chemical inhibitors to reduce corrosion rate

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2.2 Galvanic 2.2 Galvanic Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the

presence presence of an electrolyte (i.e. conductive solution).of an electrolyte (i.e. conductive solution).

Fig (2-3) Galvanic corrosion 2.2.1 Mechanism2.2.1 MechanismDifferent metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active  metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode.

Fig (2-4) Real example of Galvanic corrosion See the details @ www.metallurgy.eg.vg 99

2.2.3 Prevention or Remedial Action2.2.3 Prevention or Remedial Action • selection of alloys which are similar in electrochemical behavior and/or alloy content. selection of alloys which are similar in electrochemical behavior and/or alloy content. • area ratio of more actively corroding material (anode) should be large relative to the area ratio of more actively corroding material (anode) should be large relative to the

more inert material(cathode). more inert material(cathode). • use coatings to limit cathode area. use coatings to limit cathode area. • insulate dissimilar metals. insulate dissimilar metals. • use of effective inhibitor. use of effective inhibitor. • Select metals/alloys as close together as possible in the galvanic series. Select metals/alloys as close together as possible in the galvanic series.

2.3 crevice2.3 crevice Crevice corrosion is a localized form of corrosion usually associated with a stagnant Crevice corrosion is a localized form of corrosion usually associated with a stagnant

solution on the micro-environmental level This form of attack is generally associated solution on the micro-environmental level This form of attack is generally associated with the presence of small volumes of stagnant solution in occluded interstices, with the presence of small volumes of stagnant solution in occluded interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of

sand, sand,

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Fig (2-5) Crevice corrosion 2.3.1 MECHANISM2.3.1 MECHANISM. Autocatalytic process are three stage : 2.3.1.1 Stage one of a crevice formation ( Induction )

Fig (2-6) Stage one of a crevice formation

2.3.1.22.3.1.2 Stage two of a crevice formation (Restricted Convection)Stage two of a crevice formation (Restricted Convection)

Fig (2-7) Stage two of a crevice formation (Restricted Convection)

2.3.1.3Stage three of a crevice formation(Obstruction and Electromigration)

Fig (2-8) Stage three of a crevice formation(Obstruction and Electromigration) 1212

2.3.2 Prevention2.3.2 Prevention Crevice corrosionCrevice corrosion can be designed out of the system can be designed out of the system Use welded butt joints instead of riveted or bolted joints in new equipment Use welded butt joints instead of riveted or bolted joints in new equipment Eliminate crevices in existing lap joints by continuous welding or soldering Eliminate crevices in existing lap joints by continuous welding or soldering Use solid, non-absorbent gaskets such as Teflon. Use solid, non-absorbent gaskets such as Teflon. Use higher alloys for increased resistance to crevice corrosion Use higher alloys for increased resistance to crevice corrosion design installations to enable complete draining (no corners or stagnant zones)design installations to enable complete draining (no corners or stagnant zones)

2.4 Pitting2.4 Pitting Pitting: Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion

Fig(2-9) Morphology of pitting 1313

2.4.2 Mechanisms2.4.2 Mechanisms For a homogeneous environment, pitting IS caused by the MATERIAL that may contain For a homogeneous environment, pitting IS caused by the MATERIAL that may contain

inclusions (MnS to pit initiation )inclusions (MnS to pit initiation )

Fig (2-10) Real pitting corrosion 2.4.3 Prevention or Remedial Action2.4.3 Prevention or Remedial Action * Pitting corrosion can be prevented through: • Proper selection of materials with known resistance to the service environment • Control pH, chloride concentration and temperature • Cathodic protection and/or Anodic Protection • increase velocity of media and/or remove deposits of solids from exposed metal

surface

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2.5 Stress-corrosion cracking (SCC)2.5 Stress-corrosion cracking (SCC) Stress-corrosion crackingStress-corrosion cracking (SCC) is a cracking process that requires the simultaneous action of (SCC) is a cracking process that requires the simultaneous action of

a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate aa

alloy without applied or residual stress.alloy without applied or residual stress.

Fig(2-11) stress corrosion cracking

2.5.1 Mechanisms2.5.1 MechanismsStress corrosion cracking results from the conjoint action of three components: (1) a susceptible material; (2) a specific chemical species (environment) and(3) tensile stress.

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2.5.2 Prevention2.5.2 Prevention Stress corrosion crackingStress corrosion cracking can be prevented through : can be prevented through : Control of stress level (residual or load) and hardness. Control of stress level (residual or load) and hardness. Avoid the chemical species that causes SCC. Avoid the chemical species that causes SCC. Use of materials known not to crack in the specified environment. Use of materials known not to crack in the specified environment. Control temperature and or potential Control temperature and or potential

2 .6 intergranular corrosion2 .6 intergranular corrosion Intergranular corrosion is sometimes also called "Intergranular corrosion is sometimes also called "intercrystalline corrosionintercrystalline corrosion" or "" or "interdendritic interdendritic corrosioncorrosion".".

Fig (2-12) intergranular corrosion

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2.6.1 Mechanisms2.6.1 Mechanisms This type of attack results from local differences in composition, such as coring commonly This type of attack results from local differences in composition, such as coring commonly

encountered in alloy castings. Grain boundary precipitation, notably chromium carbides in encountered in alloy castings. Grain boundary precipitation, notably chromium carbides in stainless steels, is a well recognized and accepted mechanism of intergranular corrosion. stainless steels, is a well recognized and accepted mechanism of intergranular corrosion. The precipitation of chromium carbidesThe precipitation of chromium carbides consumed the alloying element - chromium from a consumed the alloying element - chromium from a narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack The chromium depleted zone becomes the preferential path for corrosion attack or crack propagation if under tensile stresspropagation if under tensile stress. .

Fig (2-13) intergranular corrosion

2.6.2 Prevention2.6.2 Prevention•Intergranular corrosion can be prevented through: •Use low carbon (e.g. 304L, 316L) grade of stainless steels •Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). Titanium and niobium are strong carbide- formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion. •Use post-weld heat treatment.

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2.7 selective leaching2.7 selective leaching *Dealloying*Dealloyingis the selective corrosion of one or more components of a solid solution alloy. It is also called is the selective corrosion of one or more components of a solid solution alloy. It is also called partingparting, ,

selective leaching selective leaching oror selective attack selective attack. Common dealloying examples are decarburization, . Common dealloying examples are decarburization, decobaltification , denickelification, dezincification, and graphitic corrosion or graphitizationdecobaltification , denickelification, dezincification, and graphitic corrosion or graphitization

*Decarburization*Decarburization is the selective loss of carbon from the surface layer of a carbon-containing alloy is the selective loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.due to reaction with one or more chemical substances in a medium that contacts the surface.

**DezincificationDezincification is the selective leaching of zinc from zinc-containing alloys. Most commonly found is the selective leaching of zinc from zinc-containing alloys. Most commonly found in copper-zinc alloys containing less than 85% copper after extended service in water containing in copper-zinc alloys containing less than 85% copper after extended service in water containing dissolved oxygen.dissolved oxygen.

Fig (2-14) forms of dezincification

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2.7.1 Mechanisms2.7.1 Mechanisms Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the

same electrolytesame electrolyte. Modern alloys contain a number of different alloying elements that exhibit . Modern alloys contain a number of different alloying elements that exhibit different corrosion potentials. The potential difference is the driving force for the preferential different corrosion potentials. The potential difference is the driving force for the preferential attack on the more "active" element in the alloy.attack on the more "active" element in the alloy.

Fig (2-15) dezincification

2.7.2 Prevention2.7.2 Prevention Dealloying, selective leaching and graphitic corrosion can be prevented through the following methods: •Select metals/alloys that are more resistant to dealloying. For example, inhibited brass is more resistant to dezincification that alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron. •Control the environment to minimize the selective leaching

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2 .8 Erosion Corrosion2 .8 Erosion Corrosion Erosion corrosion is the corrosion of a metal which is caused or accelerated by the Erosion corrosion is the corrosion of a metal which is caused or accelerated by the

relative motion of the environment and the metal surface.as shown in Fig(2-16)relative motion of the environment and the metal surface.as shown in Fig(2-16)

Fig(2-16) Erosion corrosion 2.8.1 Mechanism2.8.1 MechanismThere are several mechanisms described by the conjoint action of flow and corrosion that result in flow-influenced corrosion: * Mass transport-control: Mass transport-controlled corrosion implies that the rate of corrosion is dependent on the convective mass transfer processes at the metal/fluid interface.

Fig (2-17) Real of erosion corrosion 2020

*Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal *Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes. It is often accepted that a critical fluid either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given materialvelocity must be exceeded for a given material

2.8.2 Prevention or Remedial Action2.8.2 Prevention or Remedial Action selection of alloys with greater corrosion resistance and/or higher strength. selection of alloys with greater corrosion resistance and/or higher strength. re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the

environment. environment. reduction in the corrosive severity of the environment. reduction in the corrosive severity of the environment. use of corrosion resistant and/or abrasion resistant coatings. use of corrosion resistant and/or abrasion resistant coatings. cathodic protection.cathodic protection.

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Ch:3 Environment Ch:3 Environment Effects Effects 3.1 3.1 Atmospheric CorrosionAtmospheric Corrosion

Atmospheric corrosion can be defined as the corrosion of materials exposed to Atmospheric corrosion can be defined as the corrosion of materials exposed to air and its pollutants, rather than immersed in a liquid. Atmospheric corrosion air and its pollutants, rather than immersed in a liquid. Atmospheric corrosion can further be classified into dry, damp, and wet categories.can further be classified into dry, damp, and wet categories.

3.1.1 Types of atmospheres and environments3.1.1 Types of atmospheres and environments Rural.Rural. This type of atmosphere is generally the least corrosive and normally This type of atmosphere is generally the least corrosive and normally

does not contain chemical pollutants, but does contain organic and inorganic does not contain chemical pollutants, but does contain organic and inorganic particulates. The principal corrodents are moisture, oxygen, and carbon dioxide. particulates. The principal corrodents are moisture, oxygen, and carbon dioxide. Arid and tropical types are special extreme cases in the rural category.Arid and tropical types are special extreme cases in the rural category.

UrbanUrban. This type of atmosphere is similar to the rural type in that there is little . This type of atmosphere is similar to the rural type in that there is little industrial activity. Additional contaminants are of the SOx and NOx variety, from industrial activity. Additional contaminants are of the SOx and NOx variety, from motor vehicle and domestic fuel emissions.motor vehicle and domestic fuel emissions.

IndustrialIndustrial. These atmospheres are associated with heavy industrial . These atmospheres are associated with heavy industrial processing facilities and can contain concentrations of sulfur dioxide, chlorides, processing facilities and can contain concentrations of sulfur dioxide, chlorides, phosphates, and nitrates.phosphates, and nitrates.

MarineMarine. Fine windswept chloride particles that get deposited on surfaces . Fine windswept chloride particles that get deposited on surfaces characterize this type of atmosphere. Marine atmospheres are usually highly characterize this type of atmosphere. Marine atmospheres are usually highly corrosive, and the corrosivity tends to be significantly dependent on wind corrosive, and the corrosivity tends to be significantly dependent on wind direction, wind speed, and distance from the coast. It should be noted that an direction, wind speed, and distance from the coast. It should be noted that an equivalently corrosive environment is created by the use of deicing salts on the equivalently corrosive environment is created by the use of deicing salts on the roads of many cold regions of the planet. roads of many cold regions of the planet.

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3.2 Corrosion By Water3.2 Corrosion By Water Nearly all corrosion problems which occur in oilfield production operations are due to Nearly all corrosion problems which occur in oilfield production operations are due to

the presence of water.the presence of water.3.2.1 Effect OF Electrolyte Composition3.2.1 Effect OF Electrolyte Composition There are two aspects to the effects of electrolyte composition on the corrosion There are two aspects to the effects of electrolyte composition on the corrosion

circuit. The first is the conductivity of the electrolyte and the effect of electrolyte on the circuit. The first is the conductivity of the electrolyte and the effect of electrolyte on the base corrosion potential of the system.base corrosion potential of the system.

3.2.2 PHYSICAL VARIABLES 3.2.2 PHYSICAL VARIABLES The variables of temperature, pressure, and velocity need to be accounted The variables of temperature, pressure, and velocity need to be accounted

for when designing and implementing a corrosion control program. Correct for when designing and implementing a corrosion control program. Correct application inhibitors and cathodic protection as corrosion control methods application inhibitors and cathodic protection as corrosion control methods are very dependent on these variables. Temperature and pressure are are very dependent on these variables. Temperature and pressure are interrelated, and the corrosivity of a system is further influenced by velocityinterrelated, and the corrosivity of a system is further influenced by velocity

3.3 Soil in the Corrosion Process3.3 Soil in the Corrosion Process Soil has been defined in many ways, often depending upon the particular Soil has been defined in many ways, often depending upon the particular

interests of the person proposing the definition. In discussion of the soil as interests of the person proposing the definition. In discussion of the soil as an environmental factor in corrosion, no strict definitions or limitations will be an environmental factor in corrosion, no strict definitions or limitations will be applied; rather, the complex interaction of all earthen materials will come applied; rather, the complex interaction of all earthen materials will come within the scope of the discussion.within the scope of the discussion.

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* The Corrosion Process in Soil* The Corrosion Process in Soil Although the soil as a corrosive environment is probably of greater complexity than any other Although the soil as a corrosive environment is probably of greater complexity than any other

environment, it is possible to make some generalizations regarding soil types and corrosion.environment, it is possible to make some generalizations regarding soil types and corrosion.

Fig (3-4) corrosion by soil

3.3.3 Types of Soil Moisture1. Free ground water. At some depth below the surface, water is constantly

present. This distance to the water table may vary from a few metres to hundreds of metres, depending upon the geological formations present. Only a small amount of the metal used in underground service is present in the ground water zone. Such structures as well casings and under-river pipelines are surrounded by ground water. The corrosion conditions in such a situation are essentially those of an aqueous environment.

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2.2. Gravitational waterGravitational water. Water entering soil at the surface from rainfall or . Water entering soil at the surface from rainfall or some other source moves downward. This gravitational water will flow at some other source moves downward. This gravitational water will flow at a rate governed largely by the physical structure regulating the pore a rate governed largely by the physical structure regulating the pore space at various zones in the soil profile. An impervious layer of clay, a space at various zones in the soil profile. An impervious layer of clay, a ‘puddled’ soil, or other layers of material resistant to water passage may ‘puddled’ soil, or other layers of material resistant to water passage may act as an effective barrier to the gravitational water and cause zones of act as an effective barrier to the gravitational water and cause zones of water accumulation and saturation. This is often the situation in highland water accumulation and saturation. This is often the situation in highland swamp and bog formation. Usually gravitational water percolates rapidly swamp and bog formation. Usually gravitational water percolates rapidly to the level of the permanent ground waterto the level of the permanent ground water..

3. Capillary water. Most soils contain considerable amounts of water held in the capillary spaces of the silt and clay particles. The actual amount present depends upon the soil type and weather conditions. Capillary moisture represents the important reservoir of water in soil which supplies the needs of plants and animals living in or on the soil. Only a portion of capillary water is available to plants. ‘Moisture-holding capacity’ of a soil is a term applied to the ability of a soil to hold water present in the form of capillary water. It is obvious that the moisture-holding capacity of a clay is much greater than that of a sandy type soil. Likewise, the degree of corrosion occurring in soil will be related to its moisture-holding capacity, although the complexities of the relationships do not allow any quantitative or predictive applications of the present state of knowledge

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Ch 4 : corrosion Ch 4 : corrosion protectionprotection

4.1 FACTORS THAT CONTROL THE CORROSION RATE4.1 FACTORS THAT CONTROL THE CORROSION RATE Certain factors can tend to accelerate the action of a corrosion cell . Certain factors can tend to accelerate the action of a corrosion cell .

These include : These include : (a)(a) Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This Establishment of well-defined locations on the surface for the anodic and cathodic reactions. This

concentrates the damage on small areas where it may have more serious effects, this being concentrates the damage on small areas where it may have more serious effects, this being described as “local cell action”. Such effects can occur when metals of differing electrochemical described as “local cell action”. Such effects can occur when metals of differing electrochemical properties are placed in contact, giving a “galvanic couple”.properties are placed in contact, giving a “galvanic couple”.

(b)(b) Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent Stimulation of the anodic or cathodic reaction. Aggressive ions such as chloride tend to prevent the formation of protective oxide films on the metal surface and thus increase corrosion. Sodium the formation of protective oxide films on the metal surface and thus increase corrosion. Sodium chloride is encountered in marine conditions and is spread on roads in winter for de-icing. chloride is encountered in marine conditions and is spread on roads in winter for de-icing.

The rate at which attack is of prime importance is usually expressed in one of two The rate at which attack is of prime importance is usually expressed in one of two ways:ways:

(1)(1) Weight loss per unit area per unit time, usually mdd (milligrams per square Weight loss per unit area per unit time, usually mdd (milligrams per square decimeter per day)decimeter per day)

(2)(2) A rate of penetration, i.e. the thickness of metal lost. This may be expressed in A rate of penetration, i.e. the thickness of metal lost. This may be expressed in American units, mpy (mils per year, a mil being a thousandth of an inch) or in metric American units, mpy (mils per year, a mil being a thousandth of an inch) or in metric units, mmpy (millimetres per year). units, mmpy (millimetres per year).

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Table (4-1) Corrosion protection techniques

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4.2 Material selection4.2 Material selection4.2.1 Alloy steels4.2.1 Alloy steels The corrosion resistance of steels can be markedly improved by adding other metals The corrosion resistance of steels can be markedly improved by adding other metals

to produce alloys. The most resistant of the common steel alloys is stainless steel.to produce alloys. The most resistant of the common steel alloys is stainless steel.4.2.2 4.2.2 Stainless steelsStainless steels These steels owe their corrosion resistance to the formation of a passive surface These steels owe their corrosion resistance to the formation of a passive surface

oxide film, basically oxide film, basically Cr2O3Cr2O3

4.3 Corrosion Prevention4.3 Corrosion Prevention By retarding either the anodic or cathodic reactions the rate of By retarding either the anodic or cathodic reactions the rate of

corrosion can be reduced. This can be achieved in several ways :corrosion can be reduced. This can be achieved in several ways : 4.3.1 Conditioning the Metal4.3.1 Conditioning the Metal This can be sub-divided into two main groups:This can be sub-divided into two main groups:(a) (a) Coating the metalCoating the metal, in order to interpose a corrosion resistant , in order to interpose a corrosion resistant

coating between metal and environment. The coating may consist coating between metal and environment. The coating may consist of:of:

(i) another metal , e.g. zinc or tin coatings on steel,(i) another metal , e.g. zinc or tin coatings on steel,(ii) a protective coating derived from the metal itself, e.g. aluminium (ii) a protective coating derived from the metal itself, e.g. aluminium

oxide on “anodised” aluminium,oxide on “anodised” aluminium,(iii) organic coatings, such as resins, plastics, paints, enamel, oils and (iii) organic coatings, such as resins, plastics, paints, enamel, oils and

greases.greases.

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*Coating type*Coating type : : 1. Internal Lining1. Internal Lining

• Description:Internal coating using a two component liquid epoxy based paint.

• Features:This coating system has excellent anti-friction properties and good resistance to chemicals.

Fig (4-1) internal coat

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2. Fusion Bonded Epoxy (FBE) Powder Coating2. Fusion Bonded Epoxy (FBE) Powder Coating

Fig (4-2) Fusion Bonded Epoxy (FBE) Powder Coating

•Description:Stand alone coating system.

•Features:This coating system has adequate mechanical properties and effective anti-corrosion properties with resistance to high temperature operating service up to 120°C depending on raw materials used

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3. Dual Fusion Bonded Epoxy (D-FBE ) coating3. Dual Fusion Bonded Epoxy (D-FBE ) coating

Fig (4-3) Dual Fusion Bonded Epoxy (D-FBE ) coating

• Description:2-layer coating system composed of FBE primer (first layer), FBE topcoat (top layer).•Features:This coating system has good mechanical properties and effective anti-corrosion properties and resistance to high temperature operating service up to 110°C or 150°C depending on raw materials used

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4.4. Concrete Weight Coating (CWC)Concrete Weight Coating (CWC)

Fig (4-7) Concrete Weight Coating (CWC) Fig (4-7) Concrete Weight Coating (CWC) Description:

Weight coating system composed of cement, water, aggregates, heavy or light depending on the required density, and reinforcement.

Features:Concrete weight coating is used to provide pipe stability on the sea bed as well as superior mechanical protection. It can be manufactured in a range of densities to suit the project specification.

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(b)(b) Alloying the metalAlloying the metal to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed

with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium oxide chromium oxide Cr2O3Cr2O3 . .

4.3.2 Conditioning the Corrosive Environment 4.3.2 Conditioning the Corrosive Environment (a) (a) Removal of OxygenRemoval of Oxygen By the removal of oxygen from water systems in the pH range 6.5 - 8.5 one of the components By the removal of oxygen from water systems in the pH range 6.5 - 8.5 one of the components

required for corrosion would be absent. required for corrosion would be absent. (b) (b) Corrosion InhibitorsCorrosion Inhibitors A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous

environment, reduces the rate of metal wastage.environment, reduces the rate of metal wastage.

(i) (i) anodic inhibitorsanodic inhibitors Anodic inhibitors are thus classified as “dangerous inhibitors”. Other examples of anodic Anodic inhibitors are thus classified as “dangerous inhibitors”. Other examples of anodic

inhibitors include orthophosphate, nitrite, ferricyanide and silicates.inhibitors include orthophosphate, nitrite, ferricyanide and silicates.

(ii) cathodic inhibitors(ii) cathodic inhibitors Cathodic inhibitors are classed as safe because they do not cause localised corrosion.Cathodic inhibitors are classed as safe because they do not cause localised corrosion.

((iii) adsorption type corrosion inhibitorsiii) adsorption type corrosion inhibitors The main functional groups capable of forming chemisorbed bonds with metal surfaces are The main functional groups capable of forming chemisorbed bonds with metal surfaces are

amino (amino (NH2NH2), carboxyl (), carboxyl (COOHCOOH) )

(iv) mixed inhibitors(iv) mixed inhibitors

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4.3.3 Electrochemical Control4.3.3 Electrochemical Control the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the the rate of corrosion reactions may be controlled by passing anodic or cathodic currents into the

metal metal

Anodic protectionAnodic protection Fontana and Greene’ state that ‘anodic protection can be classed as one of the most Fontana and Greene’ state that ‘anodic protection can be classed as one of the most

significant advances in the entire history of corrosion science’, but point out that its significant advances in the entire history of corrosion science’, but point out that its adoption in corrosion engineering practice is likely to be slow. Anodic protection may be adoption in corrosion engineering practice is likely to be slow. Anodic protection may be described as a method of reducing the corrosion rate of immersed metals and alloys by described as a method of reducing the corrosion rate of immersed metals and alloys by controlled anodic polarisation, which induces passivity. Therefore, it can be applied only controlled anodic polarisation, which induces passivity. Therefore, it can be applied only to those metals and alloys that show passivity when in contact with an appropriate to those metals and alloys that show passivity when in contact with an appropriate electrolyte. This decrease in corrosion increases the life of components/plant as well as electrolyte. This decrease in corrosion increases the life of components/plant as well as reducing the contamination of the liquid, so is particularly beneficial in the manufacture, reducing the contamination of the liquid, so is particularly beneficial in the manufacture, storage and transport of chemicals such as acids. Edeleanu first demonstrated the storage and transport of chemicals such as acids. Edeleanu first demonstrated the feasibility of anodic protection and also tested it on small-scale stainless-steel boilers feasibility of anodic protection and also tested it on small-scale stainless-steel boilers used for sulphuric acid solutionsused for sulphuric acid solutions . .

Finally the anodic protection isFinally the anodic protection is : : • • suitable for active-passive alloys (e.g. stainless steel, nickel alloys, titanium)suitable for active-passive alloys (e.g. stainless steel, nickel alloys, titanium) • • requires a broad potential range for passivityrequires a broad potential range for passivity • • need sizable/expensive electrical equipmentneed sizable/expensive electrical equipment

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• risky if potential “slips” into the active/pitting region• used often for very aggressive solutions when other methods fail, e.g. for protection of tanks storing of strong acids (e.g. sulphuric, phosphoric, nitric)

Fig(4-11) Anodic protection

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Ch :5 preparation of Ch :5 preparation of pipelinepipeline

5.1 Pipeline Construction5.1 Pipeline Construction 5.1.1 Stringing5.1.1 Stringing At steel rolling mills where the pipe is fabricated, pipeline representatives will carefully inspect At steel rolling mills where the pipe is fabricated, pipeline representatives will carefully inspect

new pipe to assure that it meets industry and federal government safety standards. For corrosion new pipe to assure that it meets industry and federal government safety standards. For corrosion control, the outside surface will be treated with a protective coatingcontrol, the outside surface will be treated with a protective coating

Fig(5-1) Stringing Process

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5.1.2 Trenching5.1.2 Trenching The trenching crew will use a wheel trencher or backhoe to dig the pipe trench. The U.S. The trenching crew will use a wheel trencher or backhoe to dig the pipe trench. The U.S.

Department of Transportation (DOT) requires the top of the pipe to be buried a minimum of 30 Department of Transportation (DOT) requires the top of the pipe to be buried a minimum of 30 inches below the ground surface in rural areas, so the depth of the trench will be at least five to inches below the ground surface in rural areas, so the depth of the trench will be at least five to six feet deep for pipe 30 to 36 inches in diametersix feet deep for pipe 30 to 36 inches in diameter

Fig (5-2) Trenching Process

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5.1.3 Pipe Bending5.1.3 Pipe Bending The pipe bending crew will use a bending machine to make slight bends in the pipe to account for The pipe bending crew will use a bending machine to make slight bends in the pipe to account for

changes in the pipeline route and to conform to the topographychanges in the pipeline route and to conform to the topography The bending machine uses a series of clamps and hydraulic pressure to make a very smooth, The bending machine uses a series of clamps and hydraulic pressure to make a very smooth,

controlled bend in the pipe. All bending is performed in strict accordance with federally prescribed controlled bend in the pipe. All bending is performed in strict accordance with federally prescribed standards to ensure integrity of the bend. standards to ensure integrity of the bend.

Fig (5-3) pipe bending Process

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5.1.4 Welding5.1.4 Welding The pipe gang and a welding crew will be responsible for welding, the process that The pipe gang and a welding crew will be responsible for welding, the process that

joins the various sections of pipe together into one continuous length. The pipe gang joins the various sections of pipe together into one continuous length. The pipe gang uses special pipeline equipment called side booms to pick up each joint of pipe, align uses special pipeline equipment called side booms to pick up each joint of pipe, align it with the previous joint and make the first part (pass) of the weld. The pipe gang it with the previous joint and make the first part (pass) of the weld. The pipe gang then moves down the line to the next section repeating the process. The welding then moves down the line to the next section repeating the process. The welding crew follows the pipe gang to complete each weld.crew follows the pipe gang to complete each weld.

A second quality-assurance test ensures the quality of the ongoing welding operation. A second quality-assurance test ensures the quality of the ongoing welding operation. To do this, qualified technicians take X-rays of the pipe welds to ensure the To do this, qualified technicians take X-rays of the pipe welds to ensure the completed welds meet federally prescribed quality standards. The X-ray technician completed welds meet federally prescribed quality standards. The X-ray technician processes the film in a small, portable darkroom at the site. If the technician detects processes the film in a small, portable darkroom at the site. If the technician detects any flaws, the weld is repaired or cut out, and a new weld is made. Another form of any flaws, the weld is repaired or cut out, and a new weld is made. Another form of weld quality inspection employs ultrasonic technology.weld quality inspection employs ultrasonic technology.

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5.1.5 Coating5.1.5 Coating Line pipe is externally coated to inhibit corrosion by preventing moisture from coming into direct Line pipe is externally coated to inhibit corrosion by preventing moisture from coming into direct

contact with the steel. contact with the steel.  Normally, this is done at the mill where the pipe is manufactured or at another coating facility Normally, this is done at the mill where the pipe is manufactured or at another coating facility

location before it is delivered to the construction site. location before it is delivered to the construction site. 

Fig (5-4) Coating Process

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5.1.6 Lowering In5.1.6 Lowering In Lowering the welded pipe into the trench demands close coordination and skilled operators. Lowering the welded pipe into the trench demands close coordination and skilled operators.

Using a series of side-booms, which are tracked construction equipment with a boom on the side, Using a series of side-booms, which are tracked construction equipment with a boom on the side, operators simultaneously lift the pipe and carefully lower the welded sections into the trench. operators simultaneously lift the pipe and carefully lower the welded sections into the trench. Non-metallic slings protect the pipe and coating as it is lifted and moved into position.Non-metallic slings protect the pipe and coating as it is lifted and moved into position.

Fig (5-5) Lowering In Process

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5.1.7 Backfilling5.1.7 Backfilling Once the pipe has been placed in the trench, the trench can be backfilled. This is accomplished Once the pipe has been placed in the trench, the trench can be backfilled. This is accomplished

with either a backhoe or padding machine depending on the soil makeup. As with previous with either a backhoe or padding machine depending on the soil makeup. As with previous construction crews, the backfilling crew takes care to protect the pipe and coating as the soil is construction crews, the backfilling crew takes care to protect the pipe and coating as the soil is returned to the trench. As the operations begin, the soil is returned to the trench in reverse order, returned to the trench. As the operations begin, the soil is returned to the trench in reverse order, with the subsoil put back first, followed by the topsoil with the subsoil put back first, followed by the topsoil

Fig (5-6) Backfilling Process

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5.1.8 Hydrostatic Test  5.1.8 Hydrostatic Test  Before the pipeline is put into natural gas service, the entire length of the pipeline is pressure Before the pipeline is put into natural gas service, the entire length of the pipeline is pressure

tested using water. The hydrostatic test is the final construction quality assurance test. tested using water. The hydrostatic test is the final construction quality assurance test. Requirements for this test are also prescribed in DOT’s federal regulations. Depending on the Requirements for this test are also prescribed in DOT’s federal regulations. Depending on the varying elevation of the terrain along the pipeline and the location of available water sources, the varying elevation of the terrain along the pipeline and the location of available water sources, the pipeline may be divided into sections to facilitate the testpipeline may be divided into sections to facilitate the test

  5.1.9 Restoration5.1.9 Restoration The final step in the construction process is restoring the land as closely as possible to its original The final step in the construction process is restoring the land as closely as possible to its original

conditioncondition

Fig (5-7) restoration Process 4343

Ch :6 Corrosion of Ch :6 Corrosion of pipelinepipeline

6.1 6.1 TYPES OF CORROSIONTYPES OF CORROSION6.1.1 6.1.1 General CorrosionGeneral Corrosion

This type of corrosion is chemical orThis type of corrosion is chemical or electrochemical in nature. However, there are no discrete electrochemical in nature. However, there are no discrete anode or cathode areas. This form of corrosion is uniform over the surface of the metal exposed anode or cathode areas. This form of corrosion is uniform over the surface of the metal exposed to the environment. The metal gradually becomes thinner and eventually fails.to the environment. The metal gradually becomes thinner and eventually fails.

6.1.2 Concentration Cell Corrosion.6.1.2 Concentration Cell Corrosion. This type of corrosion is caused by an electrochemical corrosion cell. The potential This type of corrosion is caused by an electrochemical corrosion cell. The potential

difference (electromotive force) is caused by a difference in concentration of some difference (electromotive force) is caused by a difference in concentration of some component in the electrolytecomponent in the electrolyte

Liquids tend to be more uniform, but can vary in the concentration of some components such asLiquids tend to be more uniform, but can vary in the concentration of some components such as

oxygen varies by depth and flow ratesoxygen varies by depth and flow rates

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Fig (6-1). Concentration Cell Caused by Different Environments

6.1.2.1 Dissimilar Environment.Pipelines tend to pass through many different types of soils. The metal exhibits different electrical potentials in different soils The electrical potential in those soils determines which areas become anodic and which areas become cathodic.

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6.1.2.2 Oxygen Concentration. Pipelines or tanks that are exposed to an electrolyte with a low oxygen concentration are Pipelines or tanks that are exposed to an electrolyte with a low oxygen concentration are

generally anodic to the same material exposed to an electrolyte with a high oxygen contengenerally anodic to the same material exposed to an electrolyte with a high oxygen conten

Fig (6-2) Concentration Cell Caused by Different Concentrations of Oxygen

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6.1.2.3 Moist/Dry Electrolyte. Pipelines or tanks that are exposed to areas of low and high water content in the electrolyte also Pipelines or tanks that are exposed to areas of low and high water content in the electrolyte also

exhibit different potentials in these different areas. Generally, the area with more water content exhibit different potentials in these different areas. Generally, the area with more water content becomes the anode in this electrochemical corrosion cell. becomes the anode in this electrochemical corrosion cell.

Fig(6-3) Concentration Cell Caused by Different Concentrations of Water

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6.1.2.4 Non-Homogeneous Soil. Pipelines or tanks that are exposed to an electrolyte that is not homogeneous exhibit different Pipelines or tanks that are exposed to an electrolyte that is not homogeneous exhibit different

electrical potentials in the different components of the soil. This can occur in any soil that is a electrical potentials in the different components of the soil. This can occur in any soil that is a mixture of materials from microscopic to substantially sized components. The area(s) with the mixture of materials from microscopic to substantially sized components. The area(s) with the higher potential becomes the anode in this electrochemical corrosion cell higher potential becomes the anode in this electrochemical corrosion cell

Fig(6-4) Concentration Cell Caused by Non-Homogeneous Soil

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6.1.2.5 Concrete / Soil Interface. Pipelines or tanks that are in contact with cement and exposed to another electrolyte exhibit Pipelines or tanks that are in contact with cement and exposed to another electrolyte exhibit

different potentials in each area. The area not in contact with cement becomes the anode in this different potentials in each area. The area not in contact with cement becomes the anode in this electrochemical corrosion cell. electrochemical corrosion cell.

Fig(6-5) Concentration Cell Caused by Concrete and Soil Electrolytes

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6.1.3 Galvanic Corrosion.6.1.3 Galvanic Corrosion.

This type of corrosion is caused by an electrochemical corrosion cell developed by a potential This type of corrosion is caused by an electrochemical corrosion cell developed by a potential difference in the metal that makes one part of the cell an anode, and the other part of the cell the difference in the metal that makes one part of the cell an anode, and the other part of the cell the cathodecathode

Different metals have different potentials in the same electrolyte. This potential Different metals have different potentials in the same electrolyte. This potential difference is the driving force, or the voltage, of the cell. As with any electrochemical difference is the driving force, or the voltage, of the cell. As with any electrochemical corrosion cell, if the electrolyte is continuous from the anode to the cathode and there corrosion cell, if the electrolyte is continuous from the anode to the cathode and there is a metallic path present for the electron, the circuit is completed and current will flow is a metallic path present for the electron, the circuit is completed and current will flow and electrochemical corrosion will occur. and electrochemical corrosion will occur.

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6.1.3.1 Dissimilar Metals.6.1.3.1 Dissimilar Metals. The most obvious form of this type of corrosion is when two different kinds of metal are in the The most obvious form of this type of corrosion is when two different kinds of metal are in the

electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different metals are connected, the metal with the most negative potential is the anode; the less negative metals are connected, the metal with the most negative potential is the anode; the less negative metal is the cathodemetal is the cathode

Fig(6-6) Galvanic Corrosion Cell Caused by Different Metals

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6.1.3.2 Old-to-New Syndrome.6.1.3.2 Old-to-New Syndrome. This type of corrosion can also be rather severe. Steel is unique among metals because of the This type of corrosion can also be rather severe. Steel is unique among metals because of the

high energy put into the process of producing the steel . New steel is more active, than corroded high energy put into the process of producing the steel . New steel is more active, than corroded steel.steel.

Fig(6-7) Galvanic Corrosion Cell Caused by Old and New Steel

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6.1.3.3 Marred or Scratched Surface.6.1.3.3 Marred or Scratched Surface. A marred or scratched surface becomes anodic to the surrounding metallic surface. This is A marred or scratched surface becomes anodic to the surrounding metallic surface. This is

similar to the old-to-new syndrome, where new steel is anodic to the old steel. This similar to the old-to-new syndrome, where new steel is anodic to the old steel. This electrochemical corrosion cell is set up by the difference in the electrical potential of the electrochemical corrosion cell is set up by the difference in the electrical potential of the scratched surface compared to the remaining surface of the structure.scratched surface compared to the remaining surface of the structure.

Fig(6-8) Galvanic Corrosion Cell Caused by Marred and Scratched Surfaces

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6.1.3.4 Simultaneous Sources of Corrosion.6.1.3.4 Simultaneous Sources of Corrosion. Each of these previously discussed types of electrochemical corrosion cells may cause Each of these previously discussed types of electrochemical corrosion cells may cause

significant corrosion, but in many cases there are a combination of many different types of significant corrosion, but in many cases there are a combination of many different types of corrosion simultaneously at work to make corrosive situations even worse on the metal surface. corrosion simultaneously at work to make corrosive situations even worse on the metal surface.

Fig(6-9) Combination of Many Different Corrosion Cells at Work

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6.1.4 Stray Current Corrosion.6.1.4 Stray Current Corrosion. This type of electrochemical corrosion cell is caused by an electromotive force from an external This type of electrochemical corrosion cell is caused by an electromotive force from an external

source affecting the structure by developing a potential gradient in the electrolyte or by inducing a source affecting the structure by developing a potential gradient in the electrolyte or by inducing a current in the metal, which forces part of the structure to become an anode and another part a current in the metal, which forces part of the structure to become an anode and another part a cathodecathode

Fig (6-10) Stray Current Corrosion Cell Caused by External Anode and Cathode

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6.1.4.1 DC Transit Systems.6.1.4.1 DC Transit Systems. Electrified railroads, subway systems, street railway systems, mining systems, and trolleys Electrified railroads, subway systems, street railway systems, mining systems, and trolleys

that operate on DC are major sources of stray current corrosion. These systems may operate that operate on DC are major sources of stray current corrosion. These systems may operate load currents of thousands of amperes at a common operating potential of 600 voltsload currents of thousands of amperes at a common operating potential of 600 volts

Figure 2-13. Stray Current Corrosion Cell Caused by a DC Transit System

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6.1.4.2 High Voltage Direct Current (HVDC) Electric Transmission6.1.4.2 High Voltage Direct Current (HVDC) Electric Transmission

Lines. Power distribution systems are another source of stray currents. Most power systems are Lines. Power distribution systems are another source of stray currents. Most power systems are AC, although sometimes DC systems with grounded neutral may be used. These transmission AC, although sometimes DC systems with grounded neutral may be used. These transmission lines, under fault conditions, may use the earth as the return path for the DC current. lines, under fault conditions, may use the earth as the return path for the DC current.

Fig(6-11) Stray Current Corrosion Cell Caused by an HVDC Transmission System

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Ch7 Cathodic Ch7 Cathodic protection of pipelineprotection of pipeline

**DEFINITIONSDEFINITIONS Cathodic ProtectionCathodic Protection : : Reduction of corrosion rate by shifting the corrosion Reduction of corrosion rate by shifting the corrosion

potential of the potential of the electrode toward a less oxidizing potential by applying an external electrode toward a less oxidizing potential by applying an external

electromotive force.electromotive force. Groundbed :Groundbed : One or more anodes installed below the earth's surface for the One or more anodes installed below the earth's surface for the

purpose of purpose of supplying cathodic protection. supplying cathodic protection. Rectifier :Rectifier : A device which converts alternating current to direct current. A device which converts alternating current to direct current.

7.1 7.1 CCATHODIC PROTECTIONATHODIC PROTECTION Cathodic protection is the most widely applied electrochemical corrosion Cathodic protection is the most widely applied electrochemical corrosion

control technique. This is accomplished by applying a direct current to the control technique. This is accomplished by applying a direct current to the structure which causes the structure potential to change from the corrosion structure which causes the structure potential to change from the corrosion potential (Ecorr) to a protective potential in the immunity region. potential (Ecorr) to a protective potential in the immunity region.

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7.2 Applications :7.2 Applications : 1. Petroleum & Petrochemical:1. Petroleum & Petrochemical: underground piping and storage tanks, above ground storage tank underground piping and storage tanks, above ground storage tank

bottoms, internal surfaces of water storage tanks, heat exchangers bottoms, internal surfaces of water storage tanks, heat exchangers and storage well casingsand storage well casings

2. Marine:2. Marine: ships, barges, buoys, steel or reinforced concrete dock structures, ships, barges, buoys, steel or reinforced concrete dock structures,

offshore pipelines, offshore drilling and production platforms offshore pipelines, offshore drilling and production platforms

3. Reinforced concrete structures:3. Reinforced concrete structures: bridges, parking garages and foundationsbridges, parking garages and foundations

44. Electrical Power Industry:. Electrical Power Industry: cooling water pipelines & intakes, grounding systems, tower cooling water pipelines & intakes, grounding systems, tower

footings, penstocks, condensers footings, penstocks, condensers

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7.3 Type of cathodic protection7.3 Type of cathodic protection 7.3.1 GALVANIC CATHODIC PROTECTION SYSTEM7.3.1 GALVANIC CATHODIC PROTECTION SYSTEM Galvanic anodes are most efficiently used on electrically isolated coated structures. Galvanic anodes are most efficiently used on electrically isolated coated structures.

The current output of a galvanic anode installation is typically much less than that The current output of a galvanic anode installation is typically much less than that which is obtained from an impressed current cathodic protection system. which is obtained from an impressed current cathodic protection system.

Fig(7-1) Sacrificial Anode CP System in Seawater 6060

*Anodes Materials*Anodes Materials

Zinc anodesZinc anodes are also available in many shapes and sizes. They are appropriate in are also available in many shapes and sizes. They are appropriate in soils with very low resistivities (750 ohm-cm to 1500 ohm-cm). Favorable soils with very low resistivities (750 ohm-cm to 1500 ohm-cm). Favorable environments are sea water and salt marshes. Short chunky shapes are suitable for environments are sea water and salt marshes. Short chunky shapes are suitable for low resistivity areas, but long slender shapes should be employed in higher resistivity low resistivity areas, but long slender shapes should be employed in higher resistivity areas.areas.

Aluminum anodesAluminum anodes are not commonly used in earth burial applications. Some are not commonly used in earth burial applications. Some proprietary aluminum alloy anodes work well in a sea water environmentproprietary aluminum alloy anodes work well in a sea water environment

Magnesium anodesMagnesium anodes are available in a variety of shapes and sizes, bare or are available in a variety of shapes and sizes, bare or

prepackaged with the most popular being the 17 lb. prepackaged anode. As a prepackaged with the most popular being the 17 lb. prepackaged anode. As a general guideline, one may assume magnesium anodes to be acceptable where soil general guideline, one may assume magnesium anodes to be acceptable where soil resistivities are between 1,000 ohm-cm and 5,000 ohm-cm. Short chunky shapes are resistivities are between 1,000 ohm-cm and 5,000 ohm-cm. Short chunky shapes are suitable for low resistivity areas, but long slender shapes should be employed in suitable for low resistivity areas, but long slender shapes should be employed in Higher resistivity areasHigher resistivity areas

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7.3.1.2 7.3.1.2 Advantages Advantages :: Self-powered so no external power source is required. Self-powered so no external power source is required. Easy field installation. Easy field installation. Low maintenance requirement. Low maintenance requirement. Less likely to cause stray current interference problems on other structures. Less likely to cause stray current interference problems on other structures. When the current requirement is small, a galvanic system is more economical than an When the current requirement is small, a galvanic system is more economical than an

impressed current system.impressed current system.

7.3.1.37.3.1.3 Disadvantages Disadvantages :: Low driving voltage. Low driving voltage. Limited to use in low resistivity soils. Limited to use in low resistivity soils. Low maintenance requirement. Low maintenance requirement. Not an economical source of large amounts of CP current. Not an economical source of large amounts of CP current. Very Little capacity to control stray current effects on the protected structureVery Little capacity to control stray current effects on the protected structure

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7.3.2 IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM7.3.2 IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM An impressed current system is used to protect large bare and coated structures and structures in high resistivity An impressed current system is used to protect large bare and coated structures and structures in high resistivity

electrolytes. Design of an impressed current system must consider the potential for causing coating damage and electrolytes. Design of an impressed current system must consider the potential for causing coating damage and the possibility of creating stray currents, which adversely affect other structures See fig (7-2) .the possibility of creating stray currents, which adversely affect other structures See fig (7-2) .

Fig(7-2) Impressed Current Cathodic Protection System

videovideo

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**An impressed current system consists of the following components:An impressed current system consists of the following components: Rectifier (current supply) Rectifier (current supply) Counter electrode Counter electrode Reference electrodeReference electrode

7.3.2.1 7.3.2.1 AdvantagesAdvantages Flexibility Flexibility Applicable to a variety of applications Applicable to a variety of applications Current output may be controlled Current output may be controlled Not constrained by low driving voltage Not constrained by low driving voltage Effective in high resistivity soilEffective in high resistivity soil

7.3.2.1 7.3.2.1 DisadvantagesDisadvantages Increased maintenance Increased maintenance Higher operating costs Higher operating costs May cause interference on other structures May cause interference on other structures

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Table (7-1) Comparison of CP System Characteristic

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7.4 CATHODIC PROTECTION - THEORY7.4 CATHODIC PROTECTION - THEORY Carbon steel and stainless steel (depending on the temperature) exposed to seawater will suffer Carbon steel and stainless steel (depending on the temperature) exposed to seawater will suffer

from corrosion. The following reactions will occur on the surface from corrosion. The following reactions will occur on the surface Anodic reaction: Anodic reaction: Fe → Fe2+ + 2e- eq(7-1) Fe → Fe2+ + 2e- eq(7-1) Cathodic reactionsCathodic reactions: O2 + 2H2O + 4e- → 4OH- eq(7-2): O2 + 2H2O + 4e- → 4OH- eq(7-2) 2H+ + 2e- → H2(g) eq(7-3)2H+ + 2e- → H2(g) eq(7-3)

These reactions can be shown schematically in a over voltage diagram (E - logi) according to Figure

Fig(8-3) Over voltage diagram (E-log I) for steel in seawater 6666

The protection current can be supplied in two different ways, as schematically shown in Figure (7-The protection current can be supplied in two different ways, as schematically shown in Figure (7-5):5):

Impressed current from an external power source Impressed current from an external power source Sacrificial anodesSacrificial anodes

Fig(7-5) A schematic picture of the cathodic protection principle witha) sacrificial anodes b) impressed current.

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Anode ShapeAnode Shape

Fig(7-6) Anode Shape

a) Stand off,b) Flush mounted,c) Bracelet (Jotun Cathodic Protection – today Skarpenord Corrosion)

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Ch.8 Case StudyCh.8 Case Study SUMED MissionSUMED Mission contribution to world economy growth, development and prosperity through transporting crude contribution to world economy growth, development and prosperity through transporting crude

oil efficiently and at competitive cost in addition to offering relevant services that complement and oil efficiently and at competitive cost in addition to offering relevant services that complement and augment the main activity.augment the main activity.

* * The System consists ofThe System consists of : : Off-shore facilities at Ain Sukhna.Off-shore facilities at Ain Sukhna. On -shore facilities at Ain Sukhna.On -shore facilities at Ain Sukhna. Main pumping stations at Ain Sukhna.Main pumping stations at Ain Sukhna. 2 pipelines 42” O.D., 319.349 km long.2 pipelines 42” O.D., 319.349 km long. Nile crossing relief station.Nile crossing relief station. Intermediate future pumping stations piping at Sidi Kerir arrangement.Intermediate future pumping stations piping at Sidi Kerir arrangement. Off-shore facilities at Sidi Kerir.Off-shore facilities at Sidi Kerir. On -shore facilities at Sidi KerirOn -shore facilities at Sidi Kerir

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**River crossingRiver crossing The crossing of water courses encountered along the pipeline route has been The crossing of water courses encountered along the pipeline route has been

achieved according to the design drawing of each individual crossing.achieved according to the design drawing of each individual crossing. For minor crossing standard drawings and procedures have been adapted such For minor crossing standard drawings and procedures have been adapted such

as shown on drawing 3000-GC-D-66605, from which results a minimum cover as shown on drawing 3000-GC-D-66605, from which results a minimum cover of 2m an minimum distance between pipelines of 10 m.of 2m an minimum distance between pipelines of 10 m.

The crossing pipes have been over-weighted by a continuous concrete coating The crossing pipes have been over-weighted by a continuous concrete coating of a thickness variable from 11 to 15 cm.of a thickness variable from 11 to 15 cm.

If rocky soil is encountered minimum coverage of 1 m has been adopted.If rocky soil is encountered minimum coverage of 1 m has been adopted. The line pipe coating is of the reinforced type for a length equal to the over-The line pipe coating is of the reinforced type for a length equal to the over-

weighted section plus 5 m at both ends.weighted section plus 5 m at both ends.

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• Prepared byPrepared by : :

Khaled Mohamed yousif Emad Mohamed Mahmoud Taha Abd El - razq Ramadan

Suez Canal UniversityFaculty of Petroleum & Mining Engineering Metallurgy & Materials Engineering Dept.

Supervised by

Prof . Dr. Mohamed abd El Fattah El Zeky

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