modern theory of corrosion

8/12/2019 Modern Theory of Corrosion

1/98

MODERN THEORY PRINCIPLES

Thermodynamics of CorrosionThermod

ynamics of Corrosion

The change in free energy (The change in free energy (G) is a direcG) is a direc!eas"re #f he $#r% ca&aciy #r !a'i!"!!eas"re #f he $#r% ca&aciy #r !a'i!"!

eecric energy aaia*e fr#! a syse!+eecric energy aaia*e fr#! a syse!+

G , - n.EG , - n.E$here/$here/

nn , n"!*er #f e, n"!*er #f e--in#edin#ed

.. , .araday c#nsan, .araday c#nsan, 01233 C#"#!*s4!# e, 01233 C#"#!*s4!# e--

EE , ce enia, ce enia


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IfIf G is - (negaie)5 #ss #f energy5 heG is - (negaie)5 #ss #f energy5 he

&r#cess is snane#"s+&r#cess is snane#"s+

IfIf G is 6 (siie)5 increase #f energyG is 6 (siie)5 increase #f energy

(he &r#cess re7"ires addii#na energy)5(he &r#cess re7"ires addii#na energy)5

he &r#cess ishe &r#cess is n#n#snane#"ssnane#"s


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The change in free energy is a state functionThe change in free energy is a state function

and is independent of the reaction path.and is independent of the reaction path.

It is not possible to accurately predict theIt is not possible to accurately predict the

velocity of a reaction from the change in itsvelocity of a reaction from the change in its

free energy.free energy.

It reflects only the direction of reaction byIt reflects only the direction of reaction by

its sign.its sign.


4/98

Standard Redox PotentialStandard Redox Potential(25(25ooCC, 1M), 1M)


5/98

Cell Potentials and the EMF SeriesCell Potentials and the EMF Series

Consider the reaction between inc andConsider the reaction between inc and

copper!copper!

Cu " #nCu " #n$"% Cu% Cu$"" #n" #n &e'uilibrium reaction(&e'uilibrium reaction(

#n and Cu electrodes in e'uilibrium with their ions#n and Cu electrodes in e'uilibrium with their ions

Cu % CuCu % Cu$"" $ e" $ e)) &"*.++,(&"*.++,(

#n#n$"" $e" $e))% #n &"*.,-+(% #n &"*.,-+(

Electrodes must beElectrodes must beat e'uilibriumat e'uilibrium

&eversible electrodes(&eversible electrodes(


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/alf cell/alf cell


7/98

Consider the replacement reaction betweenConsider the replacement reaction between

Copper and silver!Copper and silver!

Cu " $0gCu " $0g"% Cu% Cu$"" $0g" $0g &e'uilibrium reaction(&e'uilibrium reaction(

In this cell Cu is negativeIn this cell Cu is negative

with respect to 0gwith respect to 0g


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/alf Cell Potential/alf Cell Potential

/ydrogen)/ydrogen Ion/ydrogen)/ydrogen Ion

$ /$ /""" $ e" $ e))% /% /$$

#n#n$"" $e" $e))% #n &)*.,-+(% #n &)*.,-+(


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NernsNernsE7"ai#nE7"ai#n

E , EE , E##6 8+9 ( ) #g ( )6 8+9 ( ) #g ( )

EE## he sandard ce eniahe sandard ce enia

TT a*s#"e e!&era"rea*s#"e e!&era"re nn n"!*er #f eecr#ns ransferredn"!*er #f eecr#ns ransferred .. .araday c#nsan (01233 c#"4!# e.araday c#nsan (01233 c#"4!# e--)) aa#'id#'id aciiy #f #'idi:ed s&eciesaciiy #f #'idi:ed s&ecies

aaredred aciiy #f red"ced s&eciesaciiy #f red"ced s&ecies

RT

nF

oxide

red

a

a


10/98

0pplication of Thermodynamics to0pplication of Thermodynamics to

CorrosionCorrosion .ree energy &redics he snane#"s direci#n #f.ree energy &redics he snane#"s direci#n #f

any eecr#che!ica reaci#n+any eecr#che!ica reaci#n+

In any eecr#che!ica reaci#n he !#s negaie #rIn any eecr#che!ica reaci#n he !#s negaie #racie haf ce ends # *e #'idi:ed5 and he !#sacie haf ce ends # *e #'idi:ed5 and he !#s

siie #r n#*e haf ce ends # *e red"ced+siie #r n#*e haf ce ends # *e red"ced+ ;h#"gh c#&&er and sier are n# c#rr#ded *y acid;h#"gh c#&&er and sier are n# c#rr#ded *y acid

s#"i#ns5 if diss#ed #'ygen is &resen here is as#"i#ns5 if diss#ed #'ygen is &resen here is assi*iiy #f #'ygen red"ci#n+ssi*iiy #f #'ygen red"ci#n+

;s he reersi*e enia #f a !ea *ec#!es !#re;s he reersi*e enia #f a !ea *ec#!es !#ren#*e5 is endency # c#rr#de in he &resence #fn#*e5 is endency # c#rr#de in he &resence #f#'idi:ing agens decreases+#'idi:ing agens decreases+


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Pourbai1 2iagramsPourbai1 2iagrams

&Potential)p/ Plots(&Potential)p/ Plots(


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The !ain "ses #f hese diagra!s are/The !ain "ses #f hese diagra!s are/


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ElectrodeElectrodeKineticsKinetics

In Fig. 3.3 the inc electrode rapidly dissolves in thesolution and rapid evolution of hydrogen is observed onthe platinum electrode.

This is e1actly the same process when inc is immersedin a hydrogen saturated acid solution containing incions.

4hen a cell is short circuited and a net o1idation andreduction processes occur at the electrode interfaces5 thepotential of these electrodes will no longer be at theire'uilibrium potential.

The deviation from e'uilibrium potential is calledpolarization.


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PolarizationPolarization

PolarizationPolarizationis he dis&ace!en #f eecr#deis he dis&ace!en #f eecr#de

enia res"ing fr#! a ne c"rren+ Theenia res"ing fr#! a ne c"rren+ The

!agni"de #f ari:ai#n is !eas"red in!agni"de #f ari:ai#n is !eas"red in

er!s #f #er#age (er!s #f #er#age ()+)+ Oer#age is a !eas"re #f ari:ai#nOer#age is a !eas"re #f ari:ai#n

$ih res&ec # he e7"ii*ri"! enia #f$ih res&ec # he e7"ii*ri"! enia #f

an eecr#de5 $hich is c#nsidered asan eecr#de5 $hich is c#nsidered as :er#:er#++

Oer#age can *e ie #r 6ie $r his :er#Oer#age can *e ie #r 6ie $r his :er#

reference+reference+


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Example of overvoltageExample of overvoltage

Consider that the inc electrode which isConsider that the inc electrode which is

coupled to the platinum electrode has acoupled to the platinum electrode has a

potential after coupling of 6)*.--)&)*.,-(7 %"*.8potential after coupling of 6)*.--)&)*.,-(7 %"*.8

volt.volt.

The overvoltage is " *.8* volt % 8** m9The overvoltage is " *.8* volt % 8** m9


16/98

Exchange Current DensityExchange Current Density((ii##))

I is he rae #f #'idai#n and red"ci#nI is he rae #f #'idai#n and red"ci#nreaci#ns a an e7"ii*ri"! e'&ressedreaci#ns a an e7"ii*ri"! e'&ressed

in er!s #f c"rren densiy+in er!s #f c"rren densiy+

The e'change c"rren densiy ariesThe e'change c"rren densiy ariesde&ending #n he !ea eecr#de5 andde&ending #n he !ea eecr#de5 and

#n he s"rface r#"ghness+#n he s"rface r#"ghness+


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E'uilibrium conditions dictate that the rate ofE'uilibrium conditions dictate that the rate ofo1idation and reduction5 ro1idation and reduction5 r

88 and rand r

$$ respectively5respectively5

must be e'ual.must be e'ual.

0t e'uilibrium hydrogen electrode there is a finite0t e'uilibrium hydrogen electrode there is a finite

rate of interchange between hydrogen moleculesrate of interchange between hydrogen molecules

and hydrogen ions in solution.and hydrogen ions in solution.


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The e1change reaction rate is e1pressed in terms ofThe e1change reaction rate is e1pressed in terms of

current density.current density.

rr #'id#'id, r, r redred,, 0t e'uilibrium0t e'uilibrium

Plotting electrode potential versus reaction ratePlotting electrode potential versus reaction rate

&following Fig( e1pressed in terms of moles reacting&following Fig( e1pressed in terms of moles reactingper cmper cm$$per s.per s.

oi

n F


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E1change current density varies dependingE1change current density varies depending

on the metal electrodeon the metal electrode


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There is no theoretical way of preciselyThere is no theoretical way of preciselydetermining the e1change current density for anydetermining the e1change current density for anygiven system! it must be determinedgiven system! it must be determinede1perimentally.e1perimentally.

The magnitude of e1change current density is aThe magnitude of e1change current density is a

function of several variables:function of several variables:

It is a specific function of the redo1 reaction.It is a specific function of the redo1 reaction.

It is related to electrode composition.It is related to electrode composition.

It is influenced by the ratio of o1idied andIt is influenced by the ratio of o1idied and

reduced species and the temperature of thereduced species and the temperature of thesystem.system.


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POL;RI=;TIONPOL;RI=;TIONElectrochemical polariation is divided into two main types :Electrochemical polariation is divided into two main types :

0ctivation polariation.0ctivation polariation.

andand

Concentration polariation.Concentration polariation.


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0CTI90TI;< P;=0I#0TI;>aa% ?% ? @@log &iAilog &iAioo((

4here4here::>>aais overvoltage5is overvoltage5

@@is the Tafel constant.is the Tafel constant.

It ranges between *.*B and .8B voltsIt ranges between *.*B and .8B volts 5 in general it5 in general it% *.8 9olt.% *.8 9olt.

andand ii is the rate of o1idation or reduction inis the rate of o1idation or reduction in

terms of current density.terms of current density.


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C;


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0t low reduction rates the distribution0t low reduction rates the distribution

of hydrogen ions in the solutionof hydrogen ions in the solutionadacent to the electrode surface isadacent to the electrode surface is

relatively uniform.relatively uniform.

0t very high reduction rates the region0t very high reduction rates the region

adacent to the electrode surface willadacent to the electrode surface will

become depleted of hydrogen ions.become depleted of hydrogen ions.


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If the reduction rate is increased further5 aIf the reduction rate is increased further5 a

limiting rate will be reached thatlimiting rate will be reached that

determined by the diffusion rate ofdetermined by the diffusion rate of

hydrogen ions to the electrode surface.hydrogen ions to the electrode surface.

This limiting rate is the limiting diffusionThis limiting rate is the limiting diffusion

current density icurrent density i==. It represents the. It represents thema1imum rate of reduction possible for ama1imum rate of reduction possible for a

given system:given system:


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iiLL ,,

ii==is the limiting diffusion current densityis the limiting diffusion current density

DD is the diffusion coefficient of the reacting ionsis the diffusion coefficient of the reacting ions..

CCDDis the concentration of the reacting ions in the bulis the concentration of the reacting ions in the bulsolutionsolution

11 is the thicness of the diffusion layer.is the thicness of the diffusion layer.

=imiting diffusion current density is a function of=imiting diffusion current density is a function of

diffusion coefficient5diffusion coefficient5

concentration of reacting ion in solution5 andconcentration of reacting ion in solution5 and

thicness of the diffusion layer.thicness of the diffusion layer.

Bn F C

!


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There is a linear relationship betweenThere is a linear relationship between

concentration and the limiting diffusionconcentration and the limiting diffusion

current density.current density.

The diffusion layer thicness is influencedThe diffusion layer thicness is influenced

by the shape of the electrode5 the geometryby the shape of the electrode5 the geometry

of the system and by agitation.of the system and by agitation.

0gitation tends to decrease the diffusion0gitation tends to decrease the diffusion

layer thicness because of convectionlayer thicness because of convection

currents and conse'uently increases thecurrents and conse'uently increases thelimiting diffusion current density.limiting diffusion current density.


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=imiting diffusion current density is usually=imiting diffusion current density is usually

only significant during reduction processesonly significant during reduction processes

and is usually negligible during metal)and is usually negligible during metal)dissolution reaction.dissolution reaction.

=imiting diffusion current density can be=imiting diffusion current density can be

ignored during most metal)dissolutionignored during most metal)dissolution

reactions. The reason for this is5 simply5 thatreactions. The reason for this is5 simply5 that

there is an almost unlimited supply of metalthere is an almost unlimited supply of metalatoms for dissolution.atoms for dissolution.


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The value of the diffusion layer thicknessThe value of the diffusion layer thickness

must be determined experimentally.must be determined experimentally.


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If $e c#nsider an eecr#de in $hichIf $e c#nsider an eecr#de in $hich

here is n# aciai#n ari:ai#n5 henhere is n# aciai#n ari:ai#n5 henhe e7"ai#n f#r c#ncenrai#nhe e7"ai#n f#r c#ncenrai#n

ari:ai#n is/ari:ai#n is/

>>cc, 8+9 (R T4n .) #g (< i4i, 8+9 (R T4n .) #g (< i4iLL))

0 th l ti l it t ti0 th l ti l it t ti


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0s the solution velocity5 concentration5 or0s the solution velocity5 concentration5 or

temperature are increased5 limiting diffusiontemperature are increased5 limiting diffusion

current increases.current increases.


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C;MDI


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The total polarization of an electrode is the sumThe total polarization of an electrode is the sum

of the contribution of activation andof the contribution of activation and

concentration polarizations.concentration polarizations.

,, aa66 cc,, -- #g (#g (ii44ii##))6 8+9 (RT4n.) #g (dissdiss

% ?% ? @@log &iAilog &iAioo

(.(.This e'uation applies to almost all anodicThis e'uation applies to almost all anodic

dissolution reactions e1cept for metals whichdissolution reactions e1cept for metals which

demonstrate active)passive behavior.demonstrate active)passive behavior.


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During reduction processes 9such as hydrogenDuring reduction processes 9such as hydrogenevolution or oxygen reduction), concentrationevolution or oxygen reduction), concentration

polarization is important as the reduction ratepolarization is important as the reduction rateapproaches the limiting diffusion current density.approaches the limiting diffusion current density.

Then the overall reduction e'uation is:Then the overall reduction e'uation is:

>>redred% )% ) @@log &iAilog &iAioo( "( " $.+ & TAn F( log &8 iAi$.+ & TAn F( log &8 iAi==(.(.

This e'uation applies to any reduction reaction.This e'uation applies to any reduction reaction.


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MIGE2)P;TE


40/98

MIGE2 E=ECT;2EMIGE2 E=ECT;2E

0 mi1ed electrode is an electrode or0 mi1ed electrode is an electrode or

metal sample that is in contact withmetal sample that is in contact withtwo or more o1idation)reductiontwo or more o1idation)reduction

systems.systems.

E1amples:


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;nly at this point &E;nly at this point &E ( is charge( is charge


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;nly at this point &E;nly at this point &Ecorrcorr( is charge( is charge

conservation maintained.conservation maintained.

The current density corresponding to thisThe current density corresponding to this

point is usually called corrosion currentpoint is usually called corrosion current

density5 idensity5 icorrcorr5 since it represents the rate of5 since it represents the rate of

inc dissolution.inc dissolution.

/ere5 i/ere5 icorrcorralso corresponds to the rate atalso corresponds to the rate at

which hydrogen gas is evolved.which hydrogen gas is evolved.


44/98

$. Corrosion behavior of iron in dilute$. Corrosion behavior of iron in dilute

hydrochloric acid solution.hydrochloric acid solution.

The steady state of this particular systemThe steady state of this particular system

occurs at the intersection between theoccurs at the intersection between thepolariation curves for iron dissolutionpolariation curves for iron dissolution

and hydrogen evolution.and hydrogen evolution.


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0lthough the free energy for the dissolution0lthough the free energy for the dissolution

of iron is lower than that of inc &the cellof iron is lower than that of inc &the cell

potential for iron and hydrogen electrodepotential for iron and hydrogen electrodeunder standard conditions is *. volt asunder standard conditions is *. volt as

contrasted to *.,- volt for inc(5 thecontrasted to *.,- volt for inc(5 the

corrosion rate of iron is greater than that ofcorrosion rate of iron is greater than that of

pure inc when e1posed to identicalpure inc when e1posed to identical

concentrations of hydrochloric acid.concentrations of hydrochloric acid.

This is due to the very low e1change currentThis is due to the very low e1change current


47/98

This is due to the very low e1change currentThis is due to the very low e1change current

density for hydrogen)evolution reaction ondensity for hydrogen)evolution reaction on

inc surfaces.inc surfaces.

Thus5 although the free)energy change for theThus5 although the free)energy change for the

corrosion of inc is negative and greater thancorrosion of inc is negative and greater thanthat for iron5 the corrosion rate of inc is lessthat for iron5 the corrosion rate of inc is less

than that of iron.than that of iron.

This illustrates the error that may be introducedThis illustrates the error that may be introducedby assuming that free)energy change andby assuming that free)energy change and

corrosion rate are proportional.corrosion rate are proportional.


48/98

99. 0 fre'uent impurity in commercial /Cl is. 0 fre'uent impurity in commercial /Cl is

ferric ion5 present as FeClferric ion5 present as FeCl++!!

The point at which the total rate ofThe point at which the total rate of

o1idation e'uals the total rate of reductiono1idation e'uals the total rate of reduction

&at steady state( is the mi1ed or corrosion&at steady state( is the mi1ed or corrosion

potential of this system.potential of this system.


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0 horiontal line is drawn in E0 horiontal line is drawn in E since thesince the


50/98

0 horiontal line is drawn in E0 horiontal line is drawn in Ecorrcorr since thesince the

metal is e'uipotential.metal is e'uipotential.

The rate of metal dissolution or the corrosionThe rate of metal dissolution or the corrosion

current is given by icurrent is given by icorrcorr..

The rate of ferric)ion reduction is e'ual toThe rate of ferric)ion reduction is e'ual to

ii &Fe&Fe+"+"JJFeFe$"$"(.(.

The rate of hydrogen evolution is given byThe rate of hydrogen evolution is given by

ii &/&/""JJ//$$(.(.


51/98

In the absence of o1idiers5 the corrosion rate ofIn the absence of o1idiers5 the corrosion rate of


52/98

In the absence of o1idiers5 the corrosion rate ofIn the absence of o1idiers5 the corrosion rate of

metal M is given by the intersection of themetal M is given by the intersection of the

hydrogen)reduction and metal dissolutionhydrogen)reduction and metal dissolution

polariation curves.polariation curves.

The addition of an o1idier5 such as ferric ions5The addition of an o1idier5 such as ferric ions5

shifts the corrosion potential to Eshifts the corrosion potential to Ecorrcorr andand

conse'uently increases corrosion rate from iKconse'uently increases corrosion rate from iKcorrcorr

to ito icorrcorr and decreases hydrogen evolution fromand decreases hydrogen evolution from

iKiKcorrcorrto ito i &/&/""JJ//$$(.(.

In o1idier)free acids iIn o1idier)free acids icorrcorr% rate of /% rate of /$$evolution.evolution.

Corrosion of metal M under reduction) Corrosion of metal M under reduction)


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. Corrosion of metal M under reduction). Corrosion of metal M under reduction

diffusion controldiffusion control

0 corrosion of a metal M in a wea acid0 corrosion of a metal M in a wea acid

solution where the reduction process issolution where the reduction process is

under diffusion control.under diffusion control.

In this e1ample5 the metal M follows theIn this e1ample5 the metal M follows thetypical anodic dissolution reaction undertypical anodic dissolution reaction under

activation control.activation control.


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Initially the reduction rate of hydrogen ionsInitially5 the reduction rate of hydrogen ions


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Initially5 the reduction rate of hydrogen ionsInitially5 the reduction rate of hydrogen ions

under activation control! at higherunder activation control! at higher

reduction currents it is controlled byreduction currents it is controlled by

concentration polariation.concentration polariation.

The corrosion rate of this system is e'ual toThe corrosion rate of this system is e'ual to

iicorrcorror ior i==and5 as before5 is determined by theand5 as before5 is determined by the

intersection between the total reduction rateintersection between the total reduction rate

and total o1idation rate.and total o1idation rate.


56/98

P0SSI9ITHP0SSI9ITH

It is defined as a loss of chemical reactivityIt is defined as a loss of chemical reactivity

under certain environment conditions.under certain environment conditions.

If a small piece of iron or steel is immersedIf a small piece of iron or steel is immersed


57/98

If a small piece of iron or steel is immersedIf a small piece of iron or steel is immersed

in nitric acid of appro1imately ,*Lin nitric acid of appro1imately ,*L

concentration at room temperature5 noconcentration at room temperature5 no

reaction is observed.reaction is observed.

If water is now added5 diluting the nitricIf water is now added5 diluting the nitric

acid appro1imately 8 to 85 no changeacid appro1imately 8 to 85 no change

occurs.occurs.

If the iron is scratched with a glass rod or ifIf the iron is scratched with a glass rod or if


58/98

gg

the beaer is shaen violently so that thethe beaer is shaen violently so that the

sample stries the sides5 a violent reactionsample stries the sides5 a violent reaction

occurs. The iron rapidly goes into solution5occurs. The iron rapidly goes into solution5and large volumes of nitrogen o1ide gasesand large volumes of nitrogen o1ide gases

are released.are released.

0 similar effect occurs if the iron is directly0 similar effect occurs if the iron is directly

introduced into diluted nitric acid.introduced into diluted nitric acid.


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Iron5 chromium5 nicel5 titanium and alloysIron5 chromium5 nicel5 titanium and alloys

containing maor amounts of these elementscontaining maor amounts of these elementsdemonstrate active)passive transitions.demonstrate active)passive transitions.

The behavior of the active)passive metalThe behavior of the active)passive metal

initially demonstrates behavior similar toinitially demonstrates behavior similar to

non)passivating metals &follows Tafelnon)passivating metals &follows Tafel

behavior(. This is the active region.behavior(. This is the active region.


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0t more noble potentials5 dissolution rate0t more noble potentials5 dissolution rate


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0t more noble potentials5 dissolution rate0t more noble potentials5 dissolution rate

decreases to a very small value and remainsdecreases to a very small value and remains

essentially independent of potential over aessentially independent of potential over a

considerable potential region. This isconsiderable potential region. This is

termed the passive region.termed the passive region.

Finally5 at very noble potentials5 dissolutionFinally5 at very noble potentials5 dissolution

rate again increases with increasingrate again increases with increasing

potential in the transpassive region.potential in the transpassive region.

The transpassive region where dissolutionThe transpassive region where dissolution


62/98

The transpassive region where dissolutionp g

rate again increases with increasingrate again increases with increasing

potential is apparently due to thepotential is apparently due to the

destruction of the passive film at verydestruction of the passive film at very

positive potentials.positive potentials.

IIcc % critical anodic current density for% critical anodic current density for

passivity5 and Epassivity5 and Epppp % primary passive% primary passive

potential.potential.

Eff f i i dEff f i i d


63/98

Effec #f increasing e!&era"re andEffec #f increasing e!&era"re and

acid c#ncenrai#n #n *ehai#r #f anacid c#ncenrai#n #n *ehai#r #f an

acie-&assie !ea+acie-&assie !ea+

Doth temperature and hydrogen)ionDoth temperature and hydrogen)ion


64/98

p y gp y g

concentration tend to increase the criticalconcentration tend to increase the critical

anodic current density and usually haveanodic current density and usually have

relatively little effect on the primary passiverelatively little effect on the primary passive

potential and passive dissolution rate.potential and passive dissolution rate.

0 similar effect is noticed upon increasing0 similar effect is noticed upon increasing

chloride additions in the case of stainlesschloride additions in the case of stainless

steels and other ferrous)base alloys.steels and other ferrous)base alloys.

0 single reduction process such as hydrogen0 single reduction process such as hydrogen


65/98

g p y gg p y g

evolution with three different possibleevolution with three different possible

e1change current densities.e1change current densities.

CC 8 i i i i i f f iTi i i dil i f lf i


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Case 8:Case 8: Titanium in dilute5 air)free sulfuricTitanium in dilute5 air)free sulfuric

or hydrochloric acid.or hydrochloric acid.

nder these conditions5 titanium corrodesnder these conditions5 titanium corrodes

rapidly and cannot passivate.rapidly and cannot passivate.

Case $:Case $: Chromium in air)free sulfuric acidChromium in air)free sulfuric acid

and iron in dilute nitric acid.and iron in dilute nitric acid.


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Case +:Case +: Stainless steels and titanium in acidStainless steels and titanium in acid

solution containing o1idiers such as ferricsolution containing o1idiers such as ferric

salts or dissolved o1ygen and also iron insalts or dissolved o1ygen and also iron in

concentrated nitric acid.concentrated nitric acid.

Spontaneous passivation only occurs if theSpontaneous passivation only occurs if the

cathodic reduction process clears the tip ofcathodic reduction process clears the tip of

the nose of the anodic dissolution curve.the nose of the anodic dissolution curve.


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MODERN THEORY;PPLIC;TIONS


69/98

Mi1ed potentialMi1ed potentialtheory is useful intheory is useful in

corrosion studies because5corrosion studies because5

8.8. It permits prediction of comple1It permits prediction of comple1corrosion behavior.corrosion behavior.

$.$. It has been used to develop newIt has been used to develop newprevention methods.prevention methods.

+.+. It has been used as a basis for newIt has been used as a basis for newrapid corrosion rate measurementrapid corrosion rate measurement

techni'ues.techni'ues.


70/98

?ha effec $i increasing e!&era"re?ha effec $i increasing e!&era"re

hae #n he ife #f e7"i&!en@hae #n he ife #f e7"i&!en@

?i increasing e#ciy ca"se an?i increasing e#ciy ca"se an

increase #r decrease in c#rr#si#n rae@increase #r decrease in c#rr#si#n rae@

If dissi!iar !eas are "sed5 $i hisIf dissi!iar !eas are "sed5 $i hisaffec c#rr#si#n rae@affec c#rr#si#n rae@


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; y&ica acie-&assie !ea M; y&ica acie-&assie !ea M


72/98

y& &

i!!ersed in an eecr#ye c#naining ai!!ersed in an eecr#ye c#naining a

red#' agen s"ch as ferric5 c"&ric5 #rred#' agen s"ch as ferric5 c"&ric5 #r

chr#!ae i#ns+chr#!ae i#ns+


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; idi i f i; idi i f i


74/98

;n #'idi:er c#ncenrai#n #f is;n #'idi:er c#ncenrai#n #f is

re7"ired # &r#d"ce &assiiy5 $hereas are7"ired # &r#d"ce &assiiy5 $hereas a

c#ncenrai#n #f #'idi:er as #$ as 8 canc#ncenrai#n #f #'idi:er as #$ as 8 can!ainain &assiiy+!ainain &assiiy+

Ir#n in c#ncenraed HNOIr#n in c#ncenraed HNO99 , in H(2), in H(2)

If di"ed # 8-5 hen scraching isIf di"ed # 8-5 hen scraching is

danger#"s+danger#"s+To safely maintain passivity, oxidizer concentrationTo safely maintain passivity, oxidizer concentration

should be equal to or greater than the minimumshould be equal to or greater than the minimum

amount necessary to produce spontaneousamount necessary to produce spontaneous

passivation.passivation.


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Effec #f e#ciyEffec #f e#ciy

; !ea M i!!ersed in a c#rr#sie; !ea M i!!ersed in a c#rr#sie

syse! in $hich he red"ci#n &r#cesssyse! in $hich he red"ci#n &r#cess

is "nder diff"si#n c#nr#+is "nder diff"si#n c#nr#+


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S#"i#n e#ciy affecs he c#rr#si#nS#"i#n e#ciy affecs he c#rr#si#n

rae #f a diff"si#n c#nr#ed syse!+rae #f a diff"si#n c#nr#ed syse!+

e#ciy has n# effec #n aiai#ne#ciy has n# effec #n aiai#n

c#nr#ed syse!s+c#nr#ed syse!s+

;s a c#nse7"ence5 he c#rr#si#n rae;s a c#nse7"ence5 he c#rr#si#n rae

*ec#!es inde&enden #f e#ciy a*ec#!es inde&enden #f e#ciy aery high e#ciy+ery high e#ciy+

The effec #f e#ciy #n an acie-The effec #f e#ciy #n an acie-


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&assie !ea is i"sraed *e#$/&assie !ea is i"sraed *e#$/

The smaller the critical anodic currentThe smaller the critical anodic current


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The smaller the critical anodic currentThe smaller the critical anodic current

density5 the easier a metal will bedensity5 the easier a metal will be

passivated by an increase in velocitypassivated by an increase in velocity

Gaanic C#"&ingGaanic C#

"&ing


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Since hydr#gen-hydr#gen i#nSince hydr#gen-hydr#gen i#n

e'change c"rren densiy is ery highe'change c"rren densiy is ery high#n &ain"! and ery #$ #n :inc5 he#n &ain"! and ery #$ #n :inc5 he

#a rae #f hydr#gen e#"i#n is#a rae #f hydr#gen e#"i#n is

effeciey e7"a # ha #n heeffeciey e7"a # ha #n he

&ain"! s"rface+&ain"! s"rface+


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The increase in c#rr#si#n rae #f :incThe increase in c#rr#si#n rae #f :inc

#*sered $hen his !ea is c#"&ed ##*sered $hen his !ea is c#"&ed #&ain"! is he res" #f he higher&ain"! is he res" #f he higher

e'change c"rren densiy f#r hydr#gene'change c"rren densiy f#r hydr#gen

e#"i#n #n &ain"! s"rface and n#e#"i#n #n &ain"! s"rface and n#d"e # he n#*e reersi*e enia #fd"e # he n#*e reersi*e enia #f

&ain"!-&ain"!-i#n eecr#de+&ain"!-&ain"!-i#n eecr#de+

0E0 EFFECT


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Increasing he area #f an eecr#deIncreasing he area #f an eecr#de

increases is e'change c"rren+increases is e'change c"rren+

If reaie area #f he an#de in aIf reaie area #f he an#de in a

gaanic c#"&e is increased5 isgaanic c#"&e is increased5 is#era c#rr#si#n rae is red"ced+#era c#rr#si#n rae is red"ced+

0E0 EFFECT


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From the above figures5 the followingFrom the above figures5 the followingdiscussion can be drawn:discussion can be drawn:


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discussion can be drawn:discussion can be drawn:

8.8. If two corroding metals are galvanicallyIf two corroding metals are galvanically

coupled5 the corrosion rate of the mostcoupled5 the corrosion rate of the mostactive metal &anode( is accelerated and thatactive metal &anode( is accelerated and that

of the other metal &cathode( is reduced.of the other metal &cathode( is reduced.

$.$. The corrosion behavior of a galvanic coupleThe corrosion behavior of a galvanic couple

is determined by reversible electrodeis determined by reversible electrodepotential of the actual processes involved5potential of the actual processes involved5

their e1change current density and Tafeltheir e1change current density and Tafel

slopes and the relative areas of the twoslopes and the relative areas of the two

metals. Its behavior cannot be predictedmetals. Its behavior cannot be predicted

accurately on the basis of emf potentials.accurately on the basis of emf potentials.

Fn"s"a *ehai#r


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Fn"s"a *ehai#r


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This "n"s"a *ehai#r #ny #cc"rs ifThis "n"s"a *ehai#r #ny #cc"rs ifhe &assie regi#n #f he !ea *eginshe &assie regi#n #f he !ea *eginsa a enia !#re acie han hea a enia !#re acie han hereersi*e enia #f he red#'reersi*e enia #f he red#'syse!+syse!+

Ony $# !eas5Ony $# !eas5 iani"!iani"! andandchr#!i"!chr#!i"!5 if c#"&ed $ih &ain"!5 if c#"&ed $ih &ain"!

&r#d"ce snane#"s &assiai#n in&r#d"ce snane#"s &assiai#n inhe a*sence #f #'ygen #r #'idi:ers+he a*sence #f #'ygen #r #'idi:ers+

;#y Ea"ai#n

;#y Ea"ai#n


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yy

O'ygen is he !#s c#!!#n #'idi:er+ IO'ygen is he !#s c#!!#n #'idi:er+ I

is sighy s#"*e in $aer andis sighy s#"*e in $aer anda7"e#"s s#"i#ns+ Theref#re5 isa7"e#"s s#"i#ns+ Theref#re5 is

red"ci#n is "s"ay "nder diff"si#nred"ci#n is "s"ay "nder diff"si#n

c#nr#+c#nr#+

In air-sa"raed n#n-agiaed s#"i#ns5In air-sa"raed n#n-agiaed s#"i#ns5he i!iing diff"si#n c"rren densiyhe i!iing diff"si#n c"rren densiy

f#r #'ygen red"ci#n is a&&r#'i!aeyf#r #'ygen red"ci#n is a&&r#'i!aey


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;ddii#n #f Cr Ni # ir#n increase he;ddii#n #f Cr Ni # ir#n increase he

f i if i i


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ease #f &assiai#n+ease #f &assiai#n+


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modern theory of corrosion

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