217801872 Cathodic Protection

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TECHNICAL DATA

GENERAL

Corrosion Definitions:

Anode - An electrode at which oxidation of the surface or some component of the solution isoccurring. Practically, this is the electrode at which corrosion occurs.

Ampere - The practical unit of electrical current equal to that produced by one volt applied across aresistance of one ohm.

Ampere-hour - The unit quantity of electricity equal to that produced by one ampere of current in aone hour period.

Cathode - An electrode at which reduction is occuring. Practically, this is the electrode at which

protection occurs in a cathodic protection system.

Corrosion - The deterioration of material, usually metal, from a reaction with its environment.

Current Density - The applied electrical current per unit area.

Electrolyte - The common environment with which both a cathode and anode are in contact.Practically, this is the soil or water to which a metal structure is exposed.

Galvanic Anode - A metal that provides cathodic protection when connected to other metals, as aresult of its relative position in the Galvanic eries.

Galvanic Series - A listing of metals in order of reactivity when exposed to an electrolyte.

Impressed Current Cathodic Protection - A cathodic protection system derived from the applicatioof external electrical energy from sources such as common electrical power, thermoelectricgenerators, and solar panels.

Ohm - The practical unit of electrical resistance equal to the resistance of a circuit in which a potentidifference of one volt produces a current of one ampere.

Sacrificial Cathodic Protection - A cathodic protection system derived from the internal electrical

energy developed by coupling to more reactive metals such as aluminum, magnesium, and !inc.

Volt - The practical unit of electrical potential difference required to produce a current flow of oneampere across a resistance of one ohm.

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Introduction to Corrosion

Introduction"orrosion is usually defined as the deterioration of a metal or its properties caused by a reaction withits environment. #ost metals occur naturally in the form of oxides and are usually chemically stable.$hen exposed to oxygen and other oxidi!ing agents, the refined metal will try to revert to its naturaloxide state. %n the case of iron, the oxides will be in the form of ferrous or ferric oxide, commonly&nown as rust.

#etallic corrosion generally involves the loss of metal at a particular location on an exposed surface."orrosion occurs in various forms ranging from a generali!ed attac& over the entire surface to asevere concentrated attac&. %n most cases, it is impossible or economically impractical to completelyarrest the corrosion process' however, it is usually possible to control the process to acceptablelevels.

The (.. government funded a detailed study of the annual cost of corrosion in )*+. The total costof metallic corrosion to the (.. economy was estimated at of the G/P 01+2 billion dollars3. 4fthat, approximately 52 was defined as avoidable. Ad6ustment based on today7s current economyputs total current costs in the 1522 billion range with over 1)22 billion of that avoidable.

Corrosion Process

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#etallic corrosion is caused by the flow of direct current from one part of the metal surface to anotheThis flow of direct current causes the loss of metal at the point where current discharges into theenvironment 0oxidation or anodic reaction3. Protection occurs at the point where current returns to thmetal surface 0reduction or cathodic reaction. The rate of corrosion is proportional to the magnitude othe corrosion current. 4ne ampere of direct current removes approximately twenty pounds of steel inone year. $here corrosion occurs and to what extent depends upon the environment to which themetal is exposed.

8our conditions must be met for corrosion to occur. 9limination of any of the four conditions will haltthe corrosion reaction.

• Anode - the oxidation reaction occurs here. "urrent discharge into the environment and metaloss are associated with this reaction.

• Cathode - the reduction reaction occurs here. "urrent acceptance and metal protection areassociated with this reaction.

• Electrolyte - the environment to which both the cathode and the anode are exposed. Theelectrolyte must have the capacity to conduct electrical current through the flow of ions.

• Metallic path - the anode and the cathode must be connected via a metallic connection thatconducts electrical current flow through the flow of electrons.

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Causes of Corrosion

"orrosion is a natural process. The primary driving force of corrosion is based upon thetransformation of iron from its natural state to steel. The refining of iron ore into steel requires theaddition of energy. teel is essentially an unstable state of iron and corrosion is the process of ironreturning to its natural state. The energy used in the refining process is the driving force of corrosion

"orrosion cells are established on underground pipelines for a variety of causes. A primary cause of

corrosion is due to an effect &nown as galvanic corrosion. All metals have different natural electricalpotentials. $here two metals with different potentials are connected to each other in a commonenvironment, current will flow causing corrosion to occur. The coupling of steel to a different metal,such as copper, will cause a corrosion circuit to be established. :irect coupling of copper to steel wilcause the steel to corrode much faster than normal. Another form of this is the coupling of rusty pipeto new, clean steel. The natural difference in potential causes the new steel to corrode more rapidlythan the old steel. 4ther causes of pipeline corrosion cells include the effect of different soil types,oxygen availability, stray current interference and microbiological growth.

Two other unique causes 0and sometimes related3 are stress and hydrogen.

• Stress Corrosion occurs when even a very small pit forms in a metal under stress. The

concentrated stress either deepens and extends the pit, or crac&s the protective film whichtends to form. (nder continued exposure to the corrosive medium and stress, the crac&extends by alternate corrosion and stress failure.

• ydro!en Em"rittlement and hydrogen attac& results when atomic hydrogen penetrates intothe grain boundaries of steel producing microcrac&s, blistering and loss of ductility. The atomihydrogen combines into molecules and results in blistering and laminations.

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#orms of Corrosion

"orrosion exhibits itself in a number of ways. A brief description of some of these is provided below.

• General Corrosion is the most common form of corrosion. %t exhibits itself in an overall attacof the metal surface with no apparent concentrations. An example is the effect of atmospheric

corrosion.

• Pittin! Corrosion results in a locali!ed, concentrated attac& and has the appearance of holesor craters.

• Crevice Corrosion occurs in shielded areas where stagnant corrosive electrolyteaccumulates. This type of corrosion occurs under bolt heads, gas&et surfaces, and overlappinmetal connections.

• Erosion-Corrosion is a combination of electrochemical and mechanical damage that occursin environments of high fluid velocities or mechanical movement between two metals.

• Selective $eachin! results in one constituent of an alloy being selectively removed, leaving aporous replica of the original alloy. An example is the de!incification of brass or bron!e and thgraphiti!ation of cast iron where iron is removed selectively, leaving a replica composed ofcarbon or graphite.

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Control of Corrosion

The five general methods used in the control of corrosion are coating, cathodic protection, materialselection, environmental modification, and design practices. "ontrol of underground corrosion isprimarily achieved by two methods; coating and cathodic protection. An effective external coating caprovide corrosion protection to over ** of the exposed pipe surface. The protective coating isusually applied to the pipe or tan& before burial. The coating serves to electrically insulate the metalfrom the soil. %f the metal could be completely isolated, then the establishment of corrosion cellswould be prevented and no corrosion current would flow. y forcing the metal surface to accept current from the environment, theunderground metal becomes a cathode and protection occurs. The external source can use outside

A" power through a rectifier and groundbed or by attaching sacrificial metals such as magnesium oraluminum to the structure to be protected. %t is used extensively in preventing corrosion tounderground and submerged steel structures' such as pipelines, production well casings, and tan&s.

9ffective application of cathodic protection can provide complete protection to any exposed areas fothe life of the structure. The combination of an external coating and cathodic protection provides the

most economical and effective choice for protection of underground and submerged pipelines. 8orbare or ineffectively coated existing pipeline systems, cathodic protection often becomes the onlypractical alternative for corrosion protection.

"athodic protection is a mandated requirement of federal and state regulations governingunderground transmission pipelines, gas distribution systems, and underground fuel tan&s. Theserequirements include installation, monitoring, and maintenance of cathodic protection systems.

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Introduction to Cathodic Protection

Introduction"athodic protection is an electrochemical technique for preventing corrosion of a metal exposed to aelectrolyte. The process involves application of :" electrical current to the metal surface from anexternal source. The external source can be either a commercial power source or through connectioto sacrificial metals such as magnesium or aluminum. %t is used extensively in preventing corrosion tunderground and submerged steel structures' such as pipelines, production well casings, and tan&s.

9ffective application of cathodic protection can provide complete protection to any exposed areas fothe life of the structure. The combination of an external coating and cathodic protection provides the

most economical and effective choice for protection of underground and submerged pipelines. 8orbare or ineffectively coated existing pipeline systems, cathodic protection often becomes the onlypractical alternative for corrosion protection.

"athodic protection is a mandated requirement of federal and state regulations governingunderground transmission pipeline, gas distribution systems, and underground petroleum tan&s.These requirements include installation, monitoring, and maintenance of cathodic protection systems

Process of Cathodic Protection

"athodic protection essentially means the reduction or elimination of corrosion on a metal surface byforcing the metal to become a cathode. The two general types of cathodic protection systems areimpressed current and sacrificial. >oth types of systems can effectively transfer the corrosion reactio0oxidation3 from the metal surface to an external anode. %f all exposed parts of a structure becomecathodic with respect to the electrolyte, corrosion of the structure is eliminated.

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Sacrificial Anode Cathodic Protectionacrificial cathodic protection occurs when a metal is coupled to a more reactive 0anodic3 metal. Thisconnection is referred to as a galvanic couple. %n order to effectively transfer corrosion from the metastructure, the anode material must have a large enough natural voltage difference to produce an

electrical current flow.

9ffective application of cathodic protection can provide complete protection to any exposed areas fothe life of the structure. The combination of an external coating and cathodic protection provides themost economical and effective choice for protection of underground and submerged pipelines. 8orbare or ineffectively coated existing pipeline systems, cathodic protection often becomes the onlypractical alternative for corrosion protection.

Three metals are commonly utili!ed for cathodic protection of steel. The selection of the anodic metais dependent upon resistivity and electrolyte. A general application guide for these metals are;

• #agnesium - soil and freshwater applications

• ?inc - low resistivity soils and saltwater

• Aluminum - saltwater and limited freshwater applications

An advantage of sacrificial anode systems is the flexibility in application. Anodes can be installed in variety of applications and configurations. /o outside power is required for cathodic protection to beeffective. Another advantage is the minimal maintenance required for these systems to function.

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:isadvantages of sacrificial anode systems include the limited protection current available and limitelife. acrificial anodes are sub6ect to rapid corrosion 0consumption3 and require replacement on aregular basis. Typical design life of a pipeline system anode is five to ten years.

Impressed Current Cathodic Protection%mpressed current cathodic protection involves the application of an external :" current through longlasting anodes. A typical source of power for an impressed current system is A" power converted to:" by a rectifier.

%n order to be effective, impressed current anodes must be designed for long life at high currentoutput. This requires selection of materials with very low corrosion 0consumption3 rates. The typicalexpectation of impressed current anode life is over twenty years. Anode materials that have proven tbe suitable for impressed current systems include treated graphite, high silicon cast iron, mixed metaoxide, and to a lesser extent platinum and magnetite. Anodes are normally installed in groupedconfigurations in the electrolyte. These groupings 0both hori!ontal and vertical3 in an undergroundapplication are called groundbeds. The groundbeds are connected to the power by a positive cable tthe power source. A negative cable connects the power source to the structure.

Advantages of impressed current systems include the unlimited current opportunities and longer life%mpressed current systems are typically installed where the structure to be protected is large,

requiring higher levels of current.

:isadvantages include the requirement for an outside power source and higher maintenancerequirements. 4utside power might come from sources such as commercial A" converted to :"through a rectifier, thermoelectric generator, or solar panels. A significantly higher monitoring andmaintenance effort is required by comparison to sacrificial anode systems.

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Cathodic Protection Applications"athodic protection can be effectively applied to most steel structures in consistent contact with acorrosive electrolyte. "ommonly protected structures include buried pipelines, underground tan&s

0(T7s3, aboveground tan& bottoms 0AT7s3, well casings, internal surfaces of tan&s and treatingvessels, and off-shore structures.

• %nder!round Pipelines are the primary mar&et for cathodic protection. >oth sacrificial andimpressed current systems are used. 8ederal and state regulations require cathodic protectiofor most petroleum or gas pipeline systems.

• %nder!round Stora!e &an's 0(T7s3 used for fuel are now required by 9PA to either havefunctional cathodic protection systems or to be of a non-corrosive material. >oth types ofsystems are widely used.

• A"ove!round Stora!e &an' 0AT7s3 bottoms can be protected from soil-side corrosion withcathodic protection. #ost ma6or tan& operators include cathodic protection in their corrosioncontrol program. (nique problems involved with tan& applications include the difficulty ofdistributing current uniformly over the tan& bottom and monitoring the effectiveness of thesystems.

• Production (ell Casin!s usually require impressed current systems due to higher currentrequirements. The economics of cathodic protection are excellent until production volumesdecline and fields near the end of their effective life. This application of cathodic protection iscommon' but tends to be concentrated in established fields with &nown corrosion history.

• Internal surfaces of tan&s and vessels are commonly protected by cathodic protectionsystems. $ith some exceptions, most of these utili!e sacrificial anodes. Possible applicationsrange from heater-treaters, heat exchangers, water storage tan&s, and hot water heaters.

• Offshore structures such as production platforms, doc&s, and pipelines are almost alwaysprotected with cathodic protection systems. acrificial anode systems with aluminum anodesare the most common application.

Cathodic Protection Criteria

"athodic protection is considered to be effective when active corrosion is transferred from the metalsurface to the installed anode. The effectiveness of the transference can be determined by electricalmeasurements. %ndustry accepted criteria for effective protection using these measurements are fullydescribed in various /A"9 %nternational publications including the tandard @ecommended Practice=@P2)*-* "ontrol of 9xternal "orrosion on (nderground or ubmerged #etallic Piping ystems=.

4f the available techniques, the primary measurement used by industry to determine effectiveness oprotection is &nown as the -2.B2 volt, pipe-to-soil potential criteria. This technique measures thevoltage difference between the protected structure and a copper-copper sulfate electrode placed in o

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on the electrolyte near the structure. %f the voltage difference is more negative than -2.B2 volts, thenthe structure is considered protected. %f the value is more positive than -2.B2 volts, the structure iseither unprotected or only partially protected. /ormal values of unprotected steel in soil typicallyrange from -2.22 to -2.+22 volts with respect to copper sulfate. The interpretation of pipe-to-soilpotentials requires consideration of the effects of measurement errors such as %@ drop in the soilbetween the pipe and the electrode. The most common consideration of this effect is through an=instant off= measurement obtained by interrupting current sources. 4ther techniques includepotential measurement at permanent =coupon type test stations=.

Desi!n ConsiderationsThere are four primary questions to be answered when designing a cathodic protection system.

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#ild steel 0rusted3 -2.F to -2.

"ast %ron 0not graphiti!ed3 -2.

Dead -2.

#ild steel in concrete -2.F

"opper, brass, bron!e -2.F


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?inc -))22 mv 2.F2 volts ubtract ).)2 volts

"omparison of different electrodes K FL "

+esistance and +esistivity

Appro3imate +esistivity of (ater

$here;

T is ppm dissolved solids@ is resistance in ohm-cm.

+esistance of a Sin!le +od Anode to Earth456

+esistance of Multiple Vertical Anodes to Earth456

(1)Derived from H! D"i#$t %&u'tions

+ou!h Indications of Electrolyte Corrosivity vs/ +esistivity

Ohm - Cm Corrosivity

>elow )22 9xtremely corrosive

)22 - ),222 Jery corrosive

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),222 - F,22 "orrosive

F,22 - +,22 #oderately corrosive

+,22 - ),222 #ildly corrosive

4ver ),222 Progressively less corrosive

SACRIFICIAL CATHODIC PROTECTION

Galvanic Anode Properties

AnodeEfficiency

476Ener!y Capa"ility

4Ah)l"6Consumption +ate

4l")Ay6Potential vs CuCuSO

4volts6

?inc * 55 F5. -).)2

Aluminum 0Al, ?n, %n3 B ))2 . -).)2

#agnesium 0


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Per AS&M 908: Industry Standard for hi!hpotential ma!nesium anodes

Aluminum ./.57 ma3

Man!anese ./1. - 5/:7

Copper ./.*7 ma3

Silicon ./.17 ma3

Iron ./.:7 ma3

;ic'el ./..57 ma3

Others< each ./.17 ma3

Ma!nesium +emainder

Element

-5 Ma!nesium Alloy Chemistry

Content 7

Grade A Grade 9= Grade C

Aluminum .5-.+ .5-.+ .5-.+

?inc F.-5. F.-5. F.-5.

#anganese

.) #in. .) #in. .) #in.

ilicon .)2 #ax. .F2 #ax. .52 #ax.

"opper .2F #ax. .2 #ax. .)2 #ax

/ic&el .22F #ax .225 #ax .225 #ax

%ron .225 #ax. .225 #ax. .225 #ax.

4ther %mpurities

.F2 #ax. .F2 #ax. .52 #ax.

#agnesium @emainder @emainder @emainder

M The

#a"rication #agnesium anodes are generally provided with a low resistivity bac&fill surrounding the anode. Thebac&fill material provides a uniform environment for the anode and enhances performance by

extending life and lowering the local resistivity. The prepared bac&fill consists of a mixture of +gypsum, F2 bentonite, and sodium sulfate. Gypsum is primarily a filler material, bentoniteabsorbs and retains moisture, and sodium sulfate lowers bac&fill resistivity. The bac&fill is containedaround the anode using cloth bags. The pac&aged anode is installed directly into the soil and thenconnected to the structure by a wire. Anodes are normally provided with ten 0)23 foot of N)F A$Gwire.

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Current Output

"urrent output is controlled by three factors;

• Soil resistivity - "urrent output increases as soil resistivity decreases. Generally magnesiumanodes are installed in relatively low resistivity soils. 9conomic application decreasessignificantly in soil resistivities exceeding ,222 ohm-cm. Practically, magnesium anodes arenot effective above )2,222 ohm-cm.

• Anode surface area - "urrent output is proportional to surface area. As the surface areaincreases, current output increases. %ncreased surface area is usually achieved by increasingthe length of the anode.

• Alloy potential -


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installation. The process involves calculation of anode current for a single high-potential anodeinstalled )27 away from a bare pipeline using the formula;

Step 5> 8or a well-coated pipeline, the constant of )2,222 should be reduced F2 to )F2,222.

Step *> elect anode si!e correction factor from Table ).

Anode (ei!ht4pounds6

StandardAnodes

#actor 4f6

5:5 0pac&aged3 2.5

:5 0pac&aged3 2.2

*:5 0pac&aged3 2.+)

)+:5 0pac&aged3 ).22

F2:F 0pac&aged3 ).2

5F:5 0pac&aged3 ).2

B: 0pac&aged3 ).2*

Step :> elect potential correction factor from Table F.

P ) S Ma!nesium

-2.+2 ).)

-2.B2 ).2+

-2.B ).22

-2.*2 2.*5

-).22 2.+*

-).)2 2.

-).F2 2.2

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Step 8> "alculate anode current output with formula.

Step 1> Ad6ust for multiple anode installation by application of ad6ustment factors in Table 5.

;o/ of Anodes

inParallel

AD?%S&I;G #AC&O+SAnode Spacin! in #eet

1@ 5.@ 51@ *.@

F ).B5* ).*F2 ).* ).*

5 F. F.+2 F.+* F.BB

5.25 5. 5.F 5.+)

5.B* .)BB .F* .5

.)F .*2F .FF5 .))

+ .F .*B .222 .F5F

B .)F .F++ .+B +.25

* .+2 .* +.5 +.B+

)2 .)) +.5 B.52 B.+*

As Tefan&6ian states in his article, the above process should be used only as a guide to estimatecurrent output.

Anode $ife and Efficiency

Dife of a magnesium anode is directly proportional to its current efficiency. #agnesium anode alloyshave a nominal efficiency of 2.


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&a"le of Current Output and $ife Estimates

The following table provides estimates of current output and life for a single high potential anode, assuming a well-coate

pipeline with an B effective use of the metal.

Soil+esistivit

yohm-cm

D: 5BD: *.D* :*D1 80D1

Output

mA

$ifeyear

s

Output

mA

$ifeyear

s

Output

mA

$ifeyear

s

Output

mA

$ifeyear

s

Output

mA

$ifeyear

s

)222 )FF 5. )5 F2 ) * F2B ))

F222 ) + B) )2 )52 B B5 )* )2 FF

5222 ) )) ) B+ )) FB * 5

222 5) ) 2 F2 ) ) 5B F

222 F )B 55 F F )* 55 + ) 222 F2 FF F+ 5) 5 FF FB 5 +

+222 )+ F F5 5 5+ F F 52 +B

B222 ) F* F2 ) 55 52 F) + F *2

*222 ) 5F )B F* 55 )B B F5 )2)

)2222 )F 5 ) ) F 5+ ) * F) ))F

)222 B )) ++ )+ )) )) ) )B

F2222 +F B )2F )5 + B )BB )2 FF

Desi!n Process

:esign of magnesium anode systems requires a thorough evaluation of the application withconsideration of the important variables. 4nce these variables are measured or assumed, theprocess typically is divided into the following steps.

• Calculate e3posed surface area - This requires an assumption of coating quality. A highquality coating can provide protection to over ** of the total surface area. A conservativeassumption of coating efficiency is *.

• Calculate total current re,uirements - This requires an assumption of current density. Thetypical range of current density for steel in common soils is ) to 5 milliamperes per square foo

of exposed surface. A conservative assumption is F milliamperes per square foot.

• Calculate anode current output - (sing previous discussed formulas, calculate anode outpuconsidering the effect of coating, soil resistivity, alloy selection, and anode spacing. everalanode si!es should be evaluated.

• Select anode sie "ased on life - 4nce anode current outputs have been calculated, theappropriate anode si!e can be selected by evaluating the pro6ected life. Typical design livesrange from )2 to F2 years.

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• Calculate total anode ,uantity - :ivide the total current requirement in amperes by thecalculated anode output in amperes. The result is the estimated total number of anodesrequired.

• Determine anode location and confi!uration - This is primarily based on local siteconditions and an even distribution of current from the anodes.

Desi!n Process

:esign of magnesium anode systems requires a thorough evaluation of the application withconsideration of the important variables. 4nce these variables are measured or assumed, theprocess typically is divided into the following steps.

• Calculate e3posed surface area - This requires an assumption of coating quality. A highquality coating can provide protection to over ** of the total surface area. A conservativeassumption of coating efficiency is *.

• Calculate total current re,uirements - This requires an assumption of current density. The

typical range of current density for steel in common soils is ) to 5 milliamperes per square fooof exposed surface. A conservative assumption is F milliamperes per square foot.

• Calculate anode current output - (sing previous discussed formulas, calculate anode outpuconsidering the effect of coating, soil resistivity, alloy selection, and anode spacing. everalanode si!es should be evaluated.

• Select anode sie "ased on life - 4nce anode current outputs have been calculated, theappropriate anode si!e can be selected by evaluating the pro6ected life. Typical design livesrange from )2 to F2 years.

• Calculate total anode ,uantity - :ivide the total current requirement in amperes by thecalculated anode output in amperes. The result is the estimated total number of anodesrequired.

Determine anode location and confi!uration - This is primarily based on local siteconditions and an even distribution of current from the anodes

9are Pipe Chart

Soil +esistivity in ohm-cm 5 * : 8 1 B 0 5.Anode +ecommendation 80F 80F :*F 5BF 5BF F F F F F

;ominal Output in milliamperes *.0 5.8 11 8. :: *. 5B 51 58 5*

;ominal $ife in years 55 ** *0 *. *1 ** *1 * :* :

Pipeinch

O/D/inch

SurfaceArea

#t* )$/#/

Current+e,mt/

mA)$/#/;ominal Anode Spacin! in feet

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* */*B1 ./. 5/5 5B1 0B 8 :8 *0 5B 58 5: 5* 5.

8 8/1. 5/50 */: 00 88 *: 5B 58 0 B 1

/*1 5/B: :/8B . :. 5 5* 5. 1 8 8 :

0 0/*1 */* 8/15 8 *: 5* B 8 8 : : :

5. 5./B1 */05 1/: :B 50 5. B 8 : : * *

5* 5*/B1 :/:8 /B :5 5 0 1 : : * * *58 58/.. :/ B/:: *0 58 0 1 1 : * * * *

5 5/.. 8/5 0/:B *1 5* B 1 8 * * * * 5

50 50/.. 8/B5 /8* ** 55 8 8 * * * 5 5

*. *./.. 1/*: 5./8B *. 5. 1 8 : * * 5 5 5

** **/.. 1/B 55/15 50 1 : : * 5 5 5 5

*8 *8/.. /*0 5*/1 5B 0 8 : : * 5 5 5 5

* */.. /0. 5:/5 51 0 8 : * 5 5 5 5 5

*0 *0/.. B/:: 58/1 58 B 8 : * 5 5 5 5 5

:. :./.. B/01 51/B. 5: B 8 : * 5 5 5 5 5

:* :*/.. 0/:B 5/B1 5* : * * 5 5 5 5 5

:8 :8/.. 0/. 5B/B 5* : * * 5 5 5 5 5

: :/.. /8* 50/08 55 : * * 5 5 5 5 5

Calculated values "ased on current density of * milliamperes per s,uare foot/

Coated Pipe Chart

Soil +esistivity in ohm-cm 5 * : 8 1 B 0 5.

Anode +ecommendation 80F 80F :*F 5BF 5BF F F F F F

;ominal Output in milliamperes *.0 5.8 11 8. :: *. 5B 51 58 5*

;ominal $ife in years 55 ** *0 *. *1 ** *1 * :* :

Pipeinch

O/D/inch

SurfaceArea

Current+e,mt/

;ominal Anode Spacin! in feet

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#t* )$/#/ mA)$/#/

* */*B1 ./. ./. :88 5B8B *8 B* 118 :: *0 *1* *:1 *.

8 8/1. 5/50 ./5* 5B 00: 8B :8. *0. 5B. 588 5*B 55 5.

/*1 5/B: ./5B 5*.. .. :5B *:5 5. 551 0 0B 05

0 0/*1 */* ./*: ** 85 *88 5BB 58 0 B1 * 1:

5. 5./B1 */05 ./*0 B: :B. 5 58* 55B B5 . 1: 1. 8:5* 5*/B1 :/:8 ./:: *: :5* 51 5*. . 15 81 8* :

58 58/.. :/ ./:B 10 *08 51. 5. . 11 8 85 :0 ::

5 5/.. 8/5 ./8* 8B *80 5:5 B 80 85 : :: *

50 50/.. 8/B5 ./8B 88* **5 55B 01 B. 8* : :* :. *1

*. *./.. 1/*: ./1* :B 5 5.1 B : :0 :* * *B *:

** **/.. 1/B ./10 :5 505 1B :1 :. * *8 *5

*8 *8/.. /*0 ./: ::5 5 00 8 1: :* *B *8 ** 5

* */.. /0. ./0 :. 51: 05 1 8 * *1 ** *5 50

*0 *0/.. B/:: ./B: *08 58* B1 11 81 *B *: *. 5 5

:. :./.. B/01 ./B *1 5:* B. 15 8* *1 ** 5 50 51

:* :*/.. 0/:B ./08 *80 5*8 80 : *8 *. 50 5B 58

:8 :8/.. 0/. ./0 *:8 55B * 81 :B ** 5 5B 5 5:

: :/.. /8* ./8 **5 55. 10 8* :1 *5 50 5 51 5:

Calculated values are "ased on current density of * milliamperes per s,uare foot/Calculated values "ased on a coatin! efficiency of 17

%nder!round Anode Installation Instructions Galvanic anodes are installed on underground pipelines to provide localized cathodicprotection for that segment of line. The amount of protection derived from an anode isdependent upon several variables including coating quality, line size, and soil resistivity

Underground anodes are generally installed in a packaged form, with the anodesurrounded by a prepared backll. The installation process is generally the same for allanode sizes. preferred method of installation is with the anode connected to the pipe

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through a test station. This conguration permits monitoring of anode performance. Theother method of installation is to directly connect the anode wire to the pipe.

nodes may be installed either horizontally or vertically. minimum separation distanceof !" is desirable to ma#imize the performance of the anode. This separation can beachieved either horizontally or vertically depending on local conditions. $n most cases,anodes should be installed no shallower than pipe depth. $n dry soils, additional depthmay enhance anode performance by reaching lower resistivity soil. %etting the anode

with appro#imately ve gallons of water after installation will activate the anode fasterand provide initial current output data.

IMPRESSED CURRENT CATHODIC PROTECTION

Impressed Current Anode

istory

The origins of cathodic protection date to the days of ir


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dimensionally stable anodes. These anodes include the precious metals and ceramic anodematerials. The development of new anode materials continues. 9ven today however' many of theearly anode materials are still in widespread use.

+eactions

Anodic reactions occur at the surface of anodes in a corrosion cell. Although there are severalpossible reactions, gas evolution is the primary oxidation effect of impressed current systems. The

two primary anodic reactions in impressed current systems are chlorine evolution and oxygenevolution. "hlorine evolution occurs when an anode is in the presence of chloride ions. This reactionwill predominate in seawater and high chloride environments.

The chlorine evolution reaction is;

*C$ Cl* *e-

"hlorine gas then reacts with water to form hypochlorous and hydrochloric acid.

4xygen evolution occurs in low chloride ion concentrations or when sulfate ions are present. Thisoccurs in underground applications where chloride ion depletion and restriction of ion migration allow

the oxygen evolution reaction to dominate.

The oxygen evolution reactions are;

**O O* 8 8e-

H *SO8 **O **SO8 O* 8e

- These anodic reactions decrease the p< of the solution in the vicinity of the anode. Anodicconsumption of co&e carbon particles also contribute to lowering p< of the anode environment.

Anodic environments with a p< as low as ).2 have been observed. %n order to be effective, anodematerials must be resistant to acid attac&.

Anode Material

9ffective impressed current anodes should possess the following qualities;

• Good electrical properties

• #echanically tough

• 9conomical

• 9asily formed into useable shapes

• Dow consumption rates through wide range of environments

Any material possessing these properties could be used as an impressed current anode. #aterialsthat have been utili!ed in commercial applications are;

• teel

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• Graphite

• "ast %ron

• Platinum

• #ixed #etal 4xide

• "onductive Polymer

• Dead

• #agnetite

Steel

The rst known appearance of iron or steel as an anode were installations of iron&wastage plates& in the early '())"s in condensers and boilers. lthough unintentional,steel acted as an anode on some early *+ traction systems. any of the corrosion

failures e#perienced on these systems were due to *+ current discharge from the rails.-robably the rst planned use of steel as an anode was in the '()"s. /crap steel wascommonly used, either in the form of old railroad rails or used pipe.

/teel anodes can take many forms. /crap materials include buried structures which havbeen abandoned in place0 such as pipelines or well casings. /crap pipe, tubing, orrailroad rails are commonly used. ny shape is capable of use0 however, massive shapeare more conducive to practical use.

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ma1or problem in the use of steel as an anode is maintaining electrical connections.ultiple connections are typically used. ethods of protecting the connections andmaintaining electrical continuity includes coating the structure in the vicinity ofconnection and continuous coating strips along the length of the anode. Theconsumption rate of steel is appro#imately 2) pounds per ampere year. +ompleteconsumption of anode material is not typically achieved because of non3uniformcorrosion and the di4culty of maintaining electrical connection. There is no establishedma#imum recommended current density.

/teel can be used in horizontal, vertical, or deep groundbeds, with or withoutcarbonaceous backll. %ith proper application, steel will perform well as an anodematerial. The ma1or disadvantages of steel as an anode material are5

• nodic corrosion roduct fi*ms m'+ ,ui*d u on 'node surf'ce- incre'sin# t$e resist'nce to e'rt$ .$is

effect m'+ ,e 'rti'**+ overcome ,+ inst'**'tion in c'r,on'ceous ,'c/fi**s

• referenti'* corrosion m'+ occur in t$e 're' of t$e connection

• 'int'inin# e*ectric'* connections

• 'r#e m'ss re&uirements

lthough most people would consider the use of steel as an anode as outmoded0 thereare operators who currently use steel in groundbeds with successful results.

Graphite

Graphite anodes have been used for impressed current systems since the )*527s. Although thedevelopment is not attributed to a specific application, it probably resulted from the early recognitionof carbon as a possible anode material.Graphite anodes are made from ground petroleum co&e mixed with a coal tar pitch binder. Themixture is heated and extruded into cylinders. After extrusion, the cylinders are cooled in special vatsplaced in an oven, pac&ed in a mixture of sand and petroleum co&e, and heated to approximately *2degrees "elsius to fully carboni!e the pitch binder. The sand-petroleum co&e pac&ing material aidsheat transfer and supports the anode during its plastic stage. After cooling in a reducing atmospherethe anodes are stac&ed in an Atchison or graphite furnace between two electrodes, covered withpetroleum co&e and an insulating sand layer, and single phase 2

The produced graphite material used for anodes typically has the following properties;

Electrical +esistivity #aximum resistivity )2micro ohm-meters

Mechanical Stren!th "ompression - 5222

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pounds per square inch8lexural - F22 pounds per square inch.

Density **.F poundsHft.5&hermal ConductivityBB >(THhr. ft. 8.Porosity Dess than Coefficient of thermale3pansion

2.+F x )2-H8

#ost anode shapes are cylindrical rods. The common si!es used are a 5= diameter x 2= length and = diameter x B2= length. quare cross section graphite anodes have also been used. 9xtremely largshapes up to F= x +F= have been used for offshore application.

&reatmentThe produced graphite anode has a porosity of less than . The anode life is improved significantlyby filling the pores with an insulating material. This impregnation reduces the tendency forelectrochemical activity to occur in the pores of the anode itself. %t also acts as a barrier againstmoisture intrusion which can cause deterioration of the anode and the anode connection. The most

common materials used for graphite treatment are wax, linseed oil, or resin. (se of untreated graphianodes for any application is not recommended.

Paraffin wax has been successfully used for graphite anode treating for many years. The waxmaterial is in a solid form at ambient temperature. Treating is accomplished by heating the wax toover F228 and submerging anodes in the melted wax. Although treatment time can vary withtemperature, moisture content, etc., complete impregnation of = diameter rods can normally beaccomplished in a F hour exposure.

After cooling, the wax within the anode solidifies and remains stable under most environmentalconditions. >ecause the wax is a solid at normal temperatures, there is no tendency for the material

to leach out of the anode.

Dinseed oil has also been widely used as an anode impregnant. The normal treatment procedureinvolves submersion of anodes in heated linseed oil in an autoclave under pressure conditions.Typically, the anodes are placed in the treatment vessel and a vacuum is drawn to remove all air fromthe anode pores. Preheated double boiled linseed oil is introduced into the vessel until the anodesare completely covered. The vessel is then pressuri!ed and temperature maintained until completeimpregnation is achieved. This process normally ta&es F to hours. ince the oil is liquid at normaltemperatures' this treatment material will have a tendency to leach or oo!e out of the anode over aperiod of time. This effect is visible through the oil film on the surface of the treated anode.

8or extremely severe service applications, graphite anodes can be treated with a phenolic resinmaterial. Phenolic resin sets up very hard. Typical properties of the graphite anode are only slightlyaffected by the resin treating except for a 2 increase in flexural strength. Anodes are surfaced toremove any s&in layers and placed in an autoclave. A vacuum is drawn to remove air from the poresin the graphite. $hile vacuum is maintained, resin is pumped into the autoclave. After all anodes arecompletely submerged with the liquid resin, pressure is applied to ensure filling the pores with resin.9xcess resin is drained from the autoclave and anodes are heat treated to polymeri!e or cure theresin within the graphite pores. 8inally the anode surface is again surfaced to remove surface resinthat could electrically insulate the anode from its environment. Proper impregnation with resin

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requires speciali!ed handling equipment. %n addition, there are some toxicity problems with the resincomponents. As a result, resin impregnation is normally only performed by the graphite manufacture

#a"rication9ach graphite anode is normally provided with an individual cable of varying length. There have beenumerous methods and procedures for connecting cable to graphite anodes. These range from a

simple tamped lead connection to threaded metallic connectors. 4ne of the methods most commonlused is a lead ferrule which is si!ed to the hole drilled in the anode. The ferrule is soldered to theanode cable and inserted in the hole. The ferrule is then expanded by a pneumatic or hydraulic toolwhich imposes a longitudinal force of up to )B22 pounds on the ferrule. This method results inconnections with pull-out strengths exceeding that of the cable.

Graphite anodes can be end connected or center connected. 9nd connections are made by drilling a= to B= deep hole from one end.


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4peration of anodes at higher than recommended outputs can cause an extremely low p<environment at the anode surface' resulting in a brea&down of the coal tar pitch binder. $hen thisoccurs, large sections of graphite can =slough= off the anode. Premature failures of untreated anodeshave been reported as a result of water penetration through the body of the anode to the metallic leawire connection. 9lectrolytic current flow between the connector and the anode will cause corrosion the connector' resulting in connection failure. ome early failures of graphite anodes occurred prior tanode installation as a result of thermal expansion of the anode connector andHor the connectionsealing compound. These failures occurred under conditions that resulted in temperatures in excess

of )2 8. The ma6ority of anode fabricators now use methods and materials that eliminate thisproblem.

The use of carbonaceous bac&fill materials is highly recommended with graphite anodes. Acceleratecorrosion rates can occur when the oxygen evolution reaction predominates. "arbonaceous bac&fillscan act as an extended anode' minimi!ing the effects of increased consumption rates.

Cast Iron

%ron containing a high silicon percentage was developed in the early )*227s. The cast material wasextremely hard and brittle. %t was first seriously considered for impressed current anode application ithe early )*27s. %t was introduced as an anode material in )*. A subsequent modification to thealloy in )** produced better anode performance characteristics. This alloy consisted of the addition

of . chromium. This anode material has been widely used and accepted in the industry.

#aximum resistivity +Fmicro ohm-cm

MechanicalStren!th>

"ompression - )22222pounds per square inch8lexural - )222 poundsper square inch.

Coefficient of thermal e3pansion>

2.+F x )2-H8

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The standard metallurgical composition of cast iron anodes conforms to AT# tandard A)B-BGrade 5 as follows;

Silicon> ).F2-).+

Chromium> 5.F-.22

Car"on> 2.+2-).)2

Man!anese> ).2 maximum

Copper> 2.2 maximum

Moly"denum> 2.F2 maximum

This alloy is cast by several methods including sand mold casting, chill-casting, and centrifugalcasting. A variety of anode shapes and si!es are available. The most common anode shapes arecylindrical tubes and solid bars in lengths up to B=, diameters from )= to =, and weights up to FB2pounds. The standard length for the solid bar anodes is 2=. The standard length for tubular shapes B=.

#a"rication 9ach cast iron anode is normally provided with an individual cable of varying length. "ast iron anodeare provided in both end-connected and center-connected configurations. The solid bar anodes arecast with a hole at one end to accommodate a connecting cable. "enter-connections are used for

cylindrical tube shapes. There have been numerous methods and procedures for connecting cable tocast iron anodes. The most common connector for solid anodes is a poured and tamped leadconnection in the cast hole. "enter-connected anodes utili!e a one or two piece lead assemblyattached to the interior center of the anode.

8ollowing cable connection, the annular space around the cable is filled with a high quality electricalsealant. "ommon sealants are asphaltic electrical potting compounds. "are must be exercised toinsure the compound is at the proper pouring temperature and that there are no voids or air poc&etswithin the cavity. Anode caps such as epoxy or heat shrin&able caps are commonly used foradditional protection. "ast iron anodes can be prepac&aged in steel canisters with carbonaceousbac&fill. "ommon canister si!es are B= x +F=, B= x B=, B= x *=, )2= x B=, )2= x *=, )F= x B=, and

)F= x *=.

Desi!n ParametersThe reported consumption rate is between 2.F and ).F pounds per ampere-year. The controllingfactor appears to be the environment. #anufacturer recommendations for anodes surrounded bycarbonaceous bac&fill is 2.+ pounds per amp-year. "urrent densities should be limited toapproximately ) ampere per square foot.

Applications


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groundbeds. Although the performance is improved with co&e bree!e' its use is not critical. Thismaterial is also widely used in freshwater and saltwater environments.

The performance of cast iron as an anode is dependent upon the formation of a thin layer of silicondioxide on the surface of the anode. 4xidation of the alloy is necessary to form this protective film.ilicon-chromium cast iron is highly resistant to acid solutions. %t does not perform particularly well inal&aline environments or in the presence of sulfate ions.

There have been some reports of early failure when silicon iron anodes are exposed to environmentsin which both sulfate and chloride ions are present. 4ther cases are reported where these anodesincrease significantly in resistance when exposed to drying conditions. %t is thought that this conditioninterferes with the formation of the conductive silicon dioxide film.

Platinied &itanium ) ;io"ium

The first published results on the use of platini!ed titanium as anode were in )*B. 8urtherdevelopment of the anode material has resulted in the use of superior substrates other than titanium%ts use has gone through several phases' however, it is recogni!ed for its superior anodic properties

Platinum is an excellent anode material due to its high conductivity and low consumption rate.


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The mechanism of deterioration of a platinum based anode is consumption of the platinum coating.@ate of consumption is controlled by many factors, primarily environment and current density. Theconsumption rate of platinum in seawater is approximately B mgHA-yr. %n fresh and brac&ish waters,consumption is F to 5 times greater at low current densities 0)2 AHsq. ft3. At high current densities,consumption is much higher.

The use of platinum is now primarily limited to water environments. %ts predominant use is probably ifresh water tan& applications' with secondary applications such as condenser water boxes, reinforce

concrete, process equipment, doc&s, etc. Anode manufacturers indicate that platinum can besuccessfully used underground, both in surface beds and deep anode beds.


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amp year for a chlorine evolution environment such as seawater. 8or an oxygen evolutionenvironment, consumption rates are on the order of mg per amp year.

#aximum recommended current densities for underground application with a co&e bree!e bac&fillmaterial is )22 AHsq. m 0*.5 AHsq.ft3. This current density relates to a design life of F2 years. "urrentdensities for muds and freshwater may be reduced by over half, depending upon temperature andlife.

#ixed metal oxide anodes are now commonly used in underground, and water environments. Theyare also the predominant anode material used for protection of tan& bottoms when installed with nonconductive secondary containment liners.

There have been some instances of bed drying which have resulted in increased groundbedresistance. There have also been failures with anodes on strings where soil resistivity varied withinthe groundbed. This may be more of a concern in deep anode bed applications. Jariations inresistivity can result in widely varying currents among the anodes. This has lead to acceleratedconsumption of anodes that were operating at current densities exceeding manufacturer7srecommendation.

Generation of chlorine has led to attac& of the standard cable insulation used for tubular anodes. %nthese cases, a dual extrusion cable material is utili!ed.

Conductive Polymer

This anode material has been available since the early )*B27s. The anode material consists of acontinuous semiconducting polymer material extruded on a copper wire. The active anodecomponent is carbon contained in a polymer matrix. %t is a flexible wire-li&e anode and is provided oncontinuous rolls. The material has an outer diameter of )HF= with an inner core of N A$G strandedcopper wire. The anode is also provided prepac&aged in carbonaceous bac&fill. The pac&age is anylon sheath containing the anode and bac&fill, provided on continuous rolls. "onnections of the inne

copper core to a main cable are normally with mechanical crimped connectors. The ends of theanode and connections are normally sealed with manufacturer-provided heat shrin& enclosures.

The manufacturers recommended maximum current density for the conductive polymer anode is )milliamperes per lineal foot, when installed underground. $hen this material is used in long-lineparallel applications, the attenuation of current in the conductor must be considered. Typically, aheavy gauge parallel copper cable is installed with the anode and multiple connections are made atregular spacings along the anode length. This material is used extensively for long line anode systeminstalled parallel to pipelines in areas where coating has deteriorated or where sub-surface conditiondo not permit efficient current distribution from conventional current sources. %t is normally installedwith a surrounding carbonaceous bac&fill. "onductive polymer anodes have also been installed in

confined areas such as between tan& bottoms and non-conductive secondary containment liners.

"onductive polymer anodes can provide uniform low-current density output over their entire lengths.@eports have generally been very good. There are some reports of failures in areas where the wire-li&e anode was not installed in carbonaceous bac&fill and accelerated consumption of the carbonoccurred. These cases have almost all been related to excessive current discharge in low resistivitywet areas such as cree& or stream crossings.

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Application &a"le

The selection of an impressed current anode should be based upon a thorough evaluation of theapplication. %tems of consideration include environment, current requirements, life requirements,space, and economics. There is probably no anode material that is optimum or even capable ofeffectively meeting the requirements of every situation. The anode materials developed within thepast 52 years certainly expand the arsenal of the corrosion engineer.


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Anode Comparison &a"le

Anode Environment Consumption

Ratelb/amp-year

Design

Current

Densityamps/SqFt

Sie !eightpounds

Sur"a#e

AreaSq Ft$

%a&'

Design

(utpu)soil* Amp

teel F2.2 /one

Graphite oil ).2 2.

8reshwater F.2 /@

altwater 2. F.2

5= x 2= 52 .2 F.2

= x B2= +F +.5 5.


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8reshwater B mgHA-yr 2

altwater B mgHA-yr 2

#ixed #etal 4xide oil mgHA-yr )2

8reshwater mgHA-yr )2

altwater ) mgHA-yr 2

). x 2 cm /A 2.F+ F.

). x )22 cm /A 2. .2 F. x 2 cm /A 2. .2

F. x )22 cm /A 2.BB B.2

"onductive Polymer oil ) ma per D8

8reshwater 5 ma per D8

altwater /@

= "ased on co'e "reee "ac'fill

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217801872 Cathodic Protection

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