6th Corrosion Happens (1)

7/26/2019 6th Corrosion Happens (1)

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FUNDAMENTALS OFFUNDAMENTALS OFCORROSION CONTROLCORROSION CONTROL

& CATHODIC& CATHODICPROTECTIONPROTECTION


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Outline Fundamentals of Corrosion

Corrosion Cell: Conditions required for corrosion Corrosion Rate Corrosion Types Commonly Aected Structures

Corrosion Control Materials Selection Inhiitors!Alterin" the en#ironment Coatin"s Cathodic $rotection

Measurin" potentials Criteria


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ConcernsConcerns

Structural failureStructural failure

ContaminationContamination

$roperty dama"e$roperty dama"eCosts to repair!replaceCosts to repair!replace

SafetySafety


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Introuction toIntrouction to

CorrosionCorrosion Corrosion can e de-ned as theCorrosion can e de-ned as the

deterioration of metal due to itsdeterioration of metal due to its

interaction .ith the en#ironment/interaction .ith the en#ironment/

Corrosion is a natural phenomenon0 .hichCorrosion is a natural phenomenon0 .hichshould not surprise one0 ut rather shouldshould not surprise one0 ut rather should

e e1pected to occur/ Metals are hi"he e1pected to occur/ Metals are hi"h

ener"y materials0 .hich e1ist ecauseener"y materials0 .hich e1ist ecause

heat ener"y .as added to natural ironheat ener"y .as added to natural ironores durin" the smeltin" process/ ature0ores durin" the smeltin" process/ ature0

y en#ironmental contact0 constantlyy en#ironmental contact0 constantly

attacs these hi"h ener"y materials andattacs these hi"h ener"y materials and

reas them do.n to the natural elementsreas them do.n to the natural elements


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Conitions Re!uire forConitions Re!uire for

CorrosionCorrosion

Corrosion is an electrochemicalCorrosion is an electrochemicalprocess occurrin" at the interfaceprocess occurrin" at the interfaceet.een metal and en#ironment/et.een metal and en#ironment/

Three conditions must e present forThree conditions must e present forthis to occur/this to occur/

+/ T.o areas on a structure or t.o+/ T.o areas on a structure or t.o

structures must dier in electricalstructures must dier in electricalpotential/potential/

2/ Those areas0 called anodes and2/ Those areas0 called anodes andcathodes0 must e electricallycathodes0 must e electrically

interconnected/interconnected/


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T"o areas or t"o structures t#at i$erT"o areas or t"o structures t#at i$er

in electrical %otential i$erentin electrical %otential i$erent

a'ounts of store ener(y)a'ounts of store ener(y)


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T#e t"o areas* calle t#e +Anoe, anT#e t"o areas* calle t#e +Anoe, an

+Cat#oe,* 'ust -e electrically+Cat#oe,* 'ust -e electrically

connecte conucti.e %at#)connecte conucti.e %at#)


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T#ose areas 'ust -e e/%ose to aT#ose areas 'ust -e e/%ose to a

co''on electrolyteco''on electrolyte

CURRENTFLOW

CURRENT FLOW

IONIZED ELECTROLYTE

WIRE

ANODE(IRON)

CATHODE(COPPER)

CURRENT FLOW

(- TO + IN ELECTROLYTE)


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Corrosion Cell 0 Dissi'ilar MetalsCorrosion Cell 0 Dissi'ilar Metals

i1e1 steel %i%e connecte to co%%eri1e1 steel %i%e connecte to co%%er

(roun ro(roun ro CURRENTFLOW

CURRENT FLOW

IONIZED ELECTROLYTE

WIRE

ANODE(IRON)

CATHODE(COPPER)

CURRENT FLOW

(- TO + IN ELECTROLYTE)


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Corrosion Cell 0 2it#in aCorrosion Cell 0 2it#in a

MetalMetal


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Corrosion RateCorrosion Rate

As a result of this process0 electricAs a result of this process0 electric

current 6o.s throu"h thecurrent 6o.s throu"h the

interconnection et.een cathodeinterconnection et.een cathode

and anode/ The cathodic area isand anode/ The cathodic area isprotected from corrosion dama"e atprotected from corrosion dama"e at

the e1pense of the metal0 .hich isthe e1pense of the metal0 .hich is

consumed at the anode/ The amountconsumed at the anode/ The amountof metal lost is directly proportionalof metal lost is directly proportional

to the current 6o./ Mild steel is lostto the current 6o./ Mild steel is lost

at appro1imately 2, pounds for eachat appro1imately 2, pounds for each


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Factors A$ectin(Factors A$ectin(

Corrosion RateCorrosion Rate$otential 7ierence 8et.een Anode$otential 7ierence 8et.een Anode

and Cathode 49al#anic Series5and Cathode 49al#anic Series5

Circuit resistance Resisti#ity of theCircuit resistance Resisti#ity of the

;lectrolyte;lectrolyte

Chemical Acti#ityChemical Acti#ity

Stray CurrentsStray Currents


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Potential Di$erencePotential Di$erence

Interconnectin" t.o dissimilarmetals in an electrolyte .ill create acorrosion cell/ The stren"th of this

cell increases as the distance .ithinthe "al#anic series increases/


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3al.anic Series3al.anic Series

M;TA


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Circuit ResistanceCircuit Resistance

Circuit resistance includes the follo.in":Circuit resistance includes the follo.in": Resistance of the anodeResistance of the anode Resistance of the cathodeResistance of the cathode

Resistance of the electrolyteResistance of the electrolyte Resistance of the metallic pathResistance of the metallic path

Increasin" the resistance .ill reduce theIncreasin" the resistance .ill reduce the

corrosion rate/corrosion rate/


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Resistance of Co''onResistance of Co''on

ElectrolytesElectrolytes Soils i"h resisti#ity .ater reducesSoils i"h resisti#ity .ater reduces

the corrosion rate0 .hile lo.the corrosion rate0 .hile lo.

resisti#ity .ater increases theresisti#ity .ater increases the

corrosion rate/corrosion rate/

CLASSIFICATION

ELECTROLYTE

RESISTIVITY

(ohm-cm)

ANTICIPATED

CORROSIVITY

Low Resistance 0 to 2,000 Severe

Medium 2,000 to 10,000 Moderate

High 10,000 to 30,000 Mild

Very High Aove 30,000 !ncreasingly Less


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Resistance of Co''onResistance of Co''on

ElectrolytesElectrolytesEater Appro1imate resisti#ity #aluesEater Appro1imate resisti#ity #alues

2ater resisti.ity2ater resisti.ity O#'s4c'O#'s4c'

open seaopen sea 2,?2&2,?2&

sea.ater 4coastal5sea.ater 4coastal5 3,?%,3,?%,

ri#er .aterri#er .ater &,,?+,0,,,&,,?+,0,,,

tap .atertap .ater +0,,,?+,0,,,+0,,,?+,0,,,

rain .aterrain .ater 2,0,,,2,0,,,

distilled .aterdistilled .ater &,,0,,,&,,0,,,

pure .aterpure .ater 2,0,,,0,,,2,0,,,0,,,


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C#e'ical Acti.ityC#e'ical Acti.ity

$assi#e 4less corrosi#e5 ;n#ironment$assi#e 4less corrosi#e5 ;n#ironment

i"h p 4neutral or asic5i"h p 4neutral or asic5


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C#e'ical Acti.ityC#e'ical Acti.ity

Acti#e 4more corrosi#e5 ;n#ironment


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Stray current corrosionStray current corrosion8oat+

8oat2

8oat3

9round

.ire


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Corrosion Ty%eCorrosion Ty%e

niform or near uniform ? Corrosionniform or near uniform ? Corrosion

attacs all areas of the metal at theattacs all areas of the metal at the

same or similar rate/same or similar rate/


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Co''only A$ecteCo''only A$ecte

StructuresStructures8uried $ipin"8uried $ipin"

Steel $ilesSteel $iles

Stora"e Tans ? Ao#e 9round !Stora"e Tans ? Ao#e 9round !nder"roundnder"round

Reinforcin" Steel in ConcreteReinforcin" Steel in Concrete

Ships!8oatsShips!8oats


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3%


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S#i%s56oatsS#i%s56oats


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Met#os to ControlMet#os to Control

CorrosionCorrosionse of Corrosion resistant materialsse of Corrosion resistant materials

4plastic0 stainless alloys0 -er"lass5/4plastic0 stainless alloys0 -er"lass5/

se of the same or similar metals per these of the same or similar metals per the

"al#anic series/"al#anic series/Alterin" the en#ironment 4utiliBin"Alterin" the en#ironment 4utiliBin"

homo"eneous hi"h resisti#ity ac-ll orhomo"eneous hi"h resisti#ity ac-ll or

inhiitors5/inhiitors5/

tiliBe coatin"s and linin"s thattiliBe coatin"s and linin"s that

electrically insulate the structure from theelectrically insulate the structure from the

electrolyte 4paints0 plastic -lms0 etc5/electrolyte 4paints0 plastic -lms0 etc5/

se of Cathodic $rotection/se of Cathodic $rotection/


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Funa'entals of Cat#oicFuna'entals of Cat#oic

ProtectionProtection

Cathodic $rotection 4C$5 7e-ned ?Cathodic $rotection 4C$5 7e-ned ?

MinimiBe corrosion y utiliBin" anMinimiBe corrosion y utiliBin" an

e1ternal source of electrical currente1ternal source of electrical current

.hich forces the entire structure to.hich forces the entire structure toecome a cathode/ecome a cathode/


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789:; Earliest %ractical useof cat#oic %rotection

Sir umphrey 7a#yGs .or onprotectin" the copper sheathin" on.ooden hulls in the 8ritish a#y ysacri-cial Binc or iron anodes is"enerally considered to e the

earliest e1ample of practicalcathodic protection/


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3al.anic CP3al.anic CP

Ehen metals such as ma"nesium orEhen metals such as ma"nesium or

Binc are placed in the en#ironmentBinc are placed in the en#ironment

in contact .ith a more nole metalin contact .ith a more nole metal

such as steel0 a current 6o.s fromsuch as steel0 a current 6o.s fromthe more acti#e anode to the nolethe more acti#e anode to the nole

cathode 4corrosion cell5/cathode 4corrosion cell5/

A U f 3 l iA U f 3 l i


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Anoes Use for 3al.anicAnoes Use for 3al.anic

CPCPMa"nesium Ma"nesium is oftenMa"nesium Ma"nesium is often

used in soil to protect smallused in soil to protect smallelectrically isolated structures0 suchelectrically isolated structures0 such

as under"round stora"e tans0 andas under"round stora"e tans0 and.ell coated pipelines/.ell coated pipelines/

@inc @inc is often used in marine@inc @inc is often used in marineen#ironments/ They are commonlyen#ironments/ They are commonlyfound on oats/found on oats/

Aluminum Aluminum can e usedAluminum Aluminum can e usedfor a #ariety of marine applications/for a #ariety of marine applications/


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< t -t i i #


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$ros of 9al#anic C$$ros of 9al#anic C$

Ine1pensi#eIne1pensi#e


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Cons of 9al#anic C$Cons of 9al#anic C$

Typically .or est .ith electricallyTypically .or est .ith electrically

isolated structures/isolated structures/

o e1ternal po.er source0 limited dri#in"o e1ternal po.er source0 limited dri#in"potential 4dri#in" potential ased on thepotential 4dri#in" potential ased on the

"al#anic series5"al#anic series5


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I'%resse Current CPI'%resse Current CP

tiliBe an e1ternal po.er source totiliBe an e1ternal po.er source to

de#elop a hi"h potential dierencede#elop a hi"h potential dierence

et.een the surface to e protectedet.een the surface to e protected

and an anode/and an anode/


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Anode Groundbed

RectiferNegative Cable

Positive Cable

AC Power Supply

Pipeline

IMPRESSED CRREN! S"S!EM


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$ros of Impressed Current C$$ros of Impressed Current C$

nlimited dri#in" potential/nlimited dri#in" potential/

Capale of protectin" lar"e steelCapale of protectin" lar"e steel

structures .hen desi"ned properly/structures .hen desi"ned properly/Requires less anodes then a "al#anicRequires less anodes then a "al#anic

system/system/

>utput can e controlled usin" a>utput can e controlled usin" apermanent reference electrode0permanent reference electrode0desirale .hen the electrolytedesirale .hen the electrolyteresisti#ity is no.n to chan"e/resisti#ity is no.n to chan"e/


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Cons of Impressed Current C$Cons of Impressed Current C$

Initial costs can e more e1pensi#e/Initial costs can e more e1pensi#e/

Requires an e1ternal 7C po.erRequires an e1ternal 7C po.er

source alon" .ith an AC supply/source alon" .ith an AC supply/

System requires routineSystem requires routine

maintenance and monitorin"/maintenance and monitorin"/

Anode .ires can e susceptile toAnode .ires can e susceptile to

dama"e/dama"e/



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Recti=erRecti=erA recti-er con#erts a#ailale ACA recti-er con#erts a#ailale AC

po.er to lo. #olta"e 7C po.er/po.er to lo. #olta"e 7C po.er/

Most cathodic protection recti-ersMost cathodic protection recti-ers

are pro#ided .ith a means to #aryare pro#ided .ith a means to #arythe 7C output #olta"e in smallthe 7C output #olta"e in small

increments0 or in some cases oerincrements0 or in some cases oer

complete control from Bero to +,,Hcomplete control from Bero to +,,Hof rated 7C output/of rated 7C output/


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CP Syste' Testin(CP Syste' Testin(

A C$ system can e e#aluated yA C$ system can e e#aluated y

otainin" a reference cell potential/otainin" a reference cell potential/

This potential determines theThis potential determines the

amount of cathodic polariBation theamount of cathodic polariBation thestructure is recei#in"/structure is recei#in"/

Adequate protection is indicated yAdequate protection is indicated y

otainin" a #alue more ne"ati#eotainin" a #alue more ne"ati#ethen the estalished criteria/then the estalished criteria/

This criteria is speci-c to theThis criteria is speci-c to the

reference cell used to otainreference cell used to otain


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6th Corrosion Happens (1)