483 budi_agung corrosion measurement techniques

8/14/2019 483 Budi_agung Corrosion Measurement Techniques

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(c) Bob Cottis 1995

Corrosion MeasurementCorrosion Measurement

TechniquesTechniquesA copy of this presentation is available inA copy of this presentation is available in

the CAL group in the computers in thethe CAL group in the computers in theTeaching Lab, or via the WWW atTeaching Lab, or via the WWW athttp://www.cp.umist.ac.uk/CPC/L_Noteshttp://www.cp.umist.ac.uk/CPC/L_Notes


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Corrosion MeasurementCorrosion Measurement

TechniquesTechniquesyPolarization curvesPolarization curvesyLinear Polarization ResistanceLinear Polarization Resistance

yOpen Circuit Potential DecayOpen Circuit Potential DecayyAC Impedance MeasurementAC Impedance MeasurementyElectrochemical Noise MeasurementElectrochemical Noise Measurement

yWeight Loss MeasurementWeight Loss Measurement


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Polarization CurvesPolarization Curves

yMeasurement methods

yCell design

yPlotting data

yInterpretation


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Measurement MethodsMeasurement Methods

y ObjectiveObjective determine current density underdetermine current density under steady-statesteady-state

conditions as a function of potentialconditions as a function of potential not really practical, as this would strictly requirenot really practical, as this would strictly require

one sample for each potentialone sample for each potential

therefore compromise on closeness to truetherefore compromise on closeness to true

steady-statesteady-state


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y Potential controlPotential control

AE

RE

WE

Potentiostat

Working Electrode -

metal being studied

Counter Electrode (orAuxilliary Electrode or

Secondary Electrode) -

provides current path

into solution

Reference Electrode -

reference connection forpotential measurement

Luggin Probe - allows

potential to be detected

close to metal surface

Potentiostat controls

potential

Connect electrodes to

corresponding terminals

on potentiostat


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y Current controlCurrent control

AE

RE

WE

Potentiostat

Working Electrode

Counter ElectrodeR

Luggin Probe still

needed to limit IR error

V

Current controlled by

control of voltage across

resistor (I=V/R)

Current path

Reference Electrode -

only used to monitor potential,

not connected to potentiostat


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y Swept potential or currentSwept potential or current UseUse sweep generatorsweep generator to produce slowly changingto produce slowly changing

potentialpotential Sweep generator output controls potentiostatSweep generator output controls potentiostat

Record response on chart recorder (or useRecord response on chart recorder (or usecomputer monitoring)computer monitoring)

Swept current not often used, as it moves throughSwept current not often used, as it moves throughcorrosion potential very quicklycorrosion potential very quickly


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y Potential or current stepPotential or current step Step potential or current from one value to theStep potential or current from one value to the

next, allowing time to stabilise at each new valuenext, allowing time to stabilise at each new value Record current or potentialRecord current or potential

May be manually controlled, or use computer toMay be manually controlled, or use computer tostep potential/current and take readingsstep potential/current and take readings


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y Sweep directionSweep direction Aim to perform experiment in such an order thatAim to perform experiment in such an order that

the initial polarization affects subsequent resultsthe initial polarization affects subsequent resultsas little as possibleas little as possible

OptionsOptionsx new specimen for each potentialnew specimen for each potential

x

one specimen for cathodic polarization, and one forone specimen for cathodic polarization, and one foranodic, both start at corrosion potentialanodic, both start at corrosion potential

x one specimen, sweep from cathodic to anodicone specimen, sweep from cathodic to anodic


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y Sweep rate (or step rate)Sweep rate (or step rate) Ideal, all measurements made at steady-stateIdeal, all measurements made at steady-state

Time-dependent effects include:Time-dependent effects include:x Charging of double layer capacitance (I = C dV/dt)Charging of double layer capacitance (I = C dV/dt)

x Mass transport effects (tMass transport effects (t LL22/D)/D)

x Adsorbed species and surface films (Faradays Law)Adsorbed species and surface films (Faradays Law)

Typical sweep rates are of the order of 1 mV/s orTypical sweep rates are of the order of 1 mV/s orlessless


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QuestionsQuestionsy Consider the corrosion of iron in aerated neutralConsider the corrosion of iron in aerated neutral

solution, with the following parameters:solution, with the following parameters:

CCdldl = 35= 35 F / cmF / cm22 DDO2O2 = 1.2 x 10= 1.2 x 10

-5-5 cmcm22 /s/s

Boundary layer thickness,Boundary layer thickness, = 100= 100 mm Number of iron atoms on surfaceNumber of iron atoms on surface 2210101919/cm/cm22

Charge on the electron = 1.6 x 10Charge on the electron = 1.6 x 10 -19-19CC

y CalculateCalculate Capacitive current at 1 mV/sCapacitive current at 1 mV/s

Characteristic diffusion timeCharacteristic diffusion time

Limiting current density for OLimiting current density for O22 reduction (8 ppm Oreduction (8 ppm O22))

Time to oxidise Fe surface to FeOH (FeTime to oxidise Fe surface to FeOH (Fe++) at) at iilimlim


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Cell DesignCell Design

y Working electrodeWorking electrode

y Reference electrodeReference electrode

y Counter electrodeCounter electrodey SolutionSolution

y Mass transportMass transport


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Working ElectrodeWorking Electrode

y RequirementsRequirements reproduciblereproducible

representativerepresentative free of crevicesfree of crevices

free of edge effectsfree of edge effects

free of galvanic effectsfree of galvanic effects

free of water-line effectsfree of water-line effects


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y Epoxy embedded electrode:Epoxy embedded electrode:

Pretreat specimen for

good adhesion

Apply thin layer of epoxy

to minimise stress and riskof crevice formation

Weld or solder connecting

wire to specimenApply thick layer of epoxy

to seal connecting tube and

for strength Carefully grind surface

to expose metal

Clean surface - dont

use acetone


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y Stern-MakridesStern-Makrideselectrodes:electrodes:

Metal rod

Retaining nut

Washers

Heavy-walled

glass tube

PTFE Washer

Electrode

Lip seal

betweenPTFE case

and electrode


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y Avesta cell:Avesta cell:

Specimen

NaCl

SolutionPure

H2O feedFilter paper


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Reference ElectrodeReference Electrode

y Commonly use Saturated Calomel ElectrodeCommonly use Saturated Calomel Electrode(SCE)(SCE)

y Properties may degrade with time (andProperties may degrade with time (andmisuse)misuse) check one against another (should not be morecheck one against another (should not be more

than 1 to 2 mV difference)than 1 to 2 mV difference)

dodo notnot pass current through the referencepass current through the referenceelectrode (e.g. do not connect to working orelectrode (e.g. do not connect to working orcounter electrode)counter electrode)

do not allow to dry outdo not allow to dry out


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Reference ElectrodeReference Electrode

y Solution in SCE (or Ag/AgCl electrode) isSolution in SCE (or Ag/AgCl electrode) issaturated KClsaturated KCl

beware of chloride contamination of test solutionbeware of chloride contamination of test solutionby Clby Cl-- leaking from reference electrodeleaking from reference electrode

make sure solution remains saturatedmake sure solution remains saturated


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Luggin ProbeLuggin Probe

y A Luggin probe should be used wheneverA Luggin probe should be used wheneverthere is a significant current applied to thethere is a significant current applied to the

electrodeelectrode

Luggin probe allows point at which potential

is measured to be close to electrode surface

(around 3 times tip diameter is best)

Elec

trode


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Counter electrodeCounter electrode

y Counter electrode should allow current toCounter electrode should allow current topass with tolerable polarizationpass with tolerable polarization

y Often claimed that counter electrode shouldOften claimed that counter electrode shouldhave much larger area than workinghave much larger area than workingelectrode, but this is not often necessary forelectrode, but this is not often necessary forcorrosion studiescorrosion studies

y Usually use platinum or graphite, althoughUsually use platinum or graphite, althoughstainless steel can be used in some situationsstainless steel can be used in some situations(e.g. where only anodic polarization of(e.g. where only anodic polarization of

specimen is used)specimen is used)


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SolutionSolution

y Requirements:Requirements: as high a conductivity as possible (add supportingas high a conductivity as possible (add supporting

electrolyte, such as sodium perchlorate?)electrolyte, such as sodium perchlorate?) remain the same (pH, composition) throughoutremain the same (pH, composition) throughoutthe experiment - ensure that volume is adequatethe experiment - ensure that volume is adequate

oxygen concentration often critical - aerate byoxygen concentration often critical - aerate by

bubbling air or Obubbling air or O22 or deaerate with Nor deaerate with N22 or Aror Ar most reactions temperature sensitive, so control,most reactions temperature sensitive, so control,

or at least record, temperatureor at least record, temperature


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Mass transportMass transport

y Methods of controlling mass transportMethods of controlling mass transport rotating disk or cylinderrotating disk or cylinder

flow channelflow channel jet impingementjet impingement

gas bubblinggas bubbling


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Plotting of Polarization CurvesPlotting of Polarization Curves

y Comparison of log-Comparison of log-ii and linear-and linear-ii plotsplots

y Identification of anodic and cathodic regionsIdentification of anodic and cathodic regions

on log-on log-ii plotsplotsy Orientation of plotsOrientation of plots


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E-i Plot

-1.000E-01

-8.000E-02

-6.000E-02

-4.000E-02

-2.000E-02

0.000E+00

2.000E-02

4.000E-02

6.000E-02

8.000E-02

1.000E-01

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity

Fe anodic H cathodic

O2 cathodic Net


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E log |i| Plot

1.000E-06

1.000E-05

1.000E-04

1.000E-03

1.000E-02

1.000E-01

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity

Fe anodic H cathodic O2 cathodic

Net anodic Net cathodic


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log |log |i|i|

E

E log |E log |II| - old plotting method| - old plotting method


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Interpretation of PolarizationInterpretation of Polarization

CurvesCurvesy Addition of reactions on log-I graphsAddition of reactions on log-I graphs

y Tafel regionsTafel regions

y Mass transport controlMass transport controly Active-passive transitionActive-passive transition

y Transpassive corrosionTranspassive corrosion

y Pitting CorrosionPitting Corrosion


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Tafel regionsTafel regions

y A Tafel region is a straight line in theA Tafel region is a straight line in theE-log|E-log|ii| plot| plot

y For a reliable Tafel slope:For a reliable Tafel slope: the line should be straight for at least one decadethe line should be straight for at least one decade

(in this context a(in this context a decadedecade implies a change of currentimplies a change of currentdensity by a factor of ten, i.e a difference of 1 indensity by a factor of ten, i.e a difference of 1 in

loglog ii )) the region should be next to Ethe region should be next to Ecorrcorr


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E log |i| Plot

1.000E-06

1.000E-05

1.000E-04

1.000E-03

1.000E-02

1.000E-01

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity




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Tafel ExtrapolationTafel Extrapolation

y Extrapolate anodic or cathodic Tafel region,Extrapolate anodic or cathodic Tafel region,or both, back to Eor both, back to Ecorrcorr, when the current density, when the current density

is iis icorrcorry In aerated neutral solutions, where massIn aerated neutral solutions, where mass

transport limited oxygen reduction is thetransport limited oxygen reduction is themain cathodic reaction, the cathodic reactionmain cathodic reaction, the cathodic reactiondoes not have a valid Tafel slope, but thedoes not have a valid Tafel slope, but theanodic slope can sometimes be usedanodic slope can sometimes be used


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QuestionQuestion

y How can we estimate the rate of hydrogenHow can we estimate the rate of hydrogenevolution during free corrosion?evolution during free corrosion?

y Estimate the value for the graph shown.Estimate the value for the graph shown.


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E log |i| Plot

1.000E-06

1.000E-05

1.000E-04

1.000E-03

1.000E-02

1.000E-01

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity




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Mass transport controlMass transport control

y When the supply of a reactant becomes massWhen the supply of a reactant becomes masstransport controlled, we observe a limitingtransport controlled, we observe a limiting

current densitycurrent densityy The most common case occurs for oxygen as aThe most common case occurs for oxygen as a

cathodic reactant in neutral solutionscathodic reactant in neutral solutions

y NOTE - the diffusion of a reaction productNOTE - the diffusion of a reaction productaway from the electrode willaway from the electrode will notnot affect theaffect therate of therate of the forwardforward reactionreaction


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Solution Resistance EffectsSolution Resistance Effects

y At high currents the potential drop associatedAt high currents the potential drop associatedwith the solution resistance can be significantwith the solution resistance can be significant

y It is generally referred to as anIt is generally referred to as an IRIR errorerrory Gives a straight line onGives a straight line on EE--ii plotsplots


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E log |i| Plot

1.000E-06

1.000E-05

1.000E-04

1.000E-03

1.000E-02

1.000E-01

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity




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E-i Plot

-2.000E-02

-1.500E-02

-1.000E-02

-5.000E-03

0.000E+00

5.000E-03

1.000E-02

1.500E-02

2.000E-02

-1.500 -1.300 -1.100 -0.900 -0.700 -0.500 -0.300 -0.100 0.100 0.300 0.500

Potential

CurrentD

ensity

Fe anodic H cathodic

O2 cathodic Net


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Transpassive corrosionTranspassive corrosion

y A passive metal (notably Cr and Fe) may startA passive metal (notably Cr and Fe) may startto dissolve at a very positive potential when ato dissolve at a very positive potential when a

higher oxidation state (e.g. Crhigher oxidation state (e.g. Cr6+6+

as chromate)as chromate)is formedis formed

y This is known as transpassive corrosion, andThis is known as transpassive corrosion, andwill give something like a second activation-will give something like a second activation-

controlled reactioncontrolled reactiony For alloys the behaviour will be complicatedFor alloys the behaviour will be complicated

by the differing behaviours of the alloyby the differing behaviours of the alloy

componentscomponents


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Anodic Polarization Curve forAnodic Polarization Curve for

Stainless SteelStainless Steel

E

log |i|

Activation-controlled

dissolution

Active-passive

transitionActive peak for iron

Transpassive

corrosion of Cr

Oxygen

reductionOverall anodic curve


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Pitting CorrosionPitting Corrosion

y Pitting shows up as an increasing anodicPitting shows up as an increasing anodiccurrent before (at a less positive potentialcurrent before (at a less positive potential

than) transpassive corrosion or oxygenthan) transpassive corrosion or oxygenevolution, usually preceded by noiseevolution, usually preceded by noise

y EE-log|-log|ii| plot does not follow same path if| plot does not follow same path ifscan direction is reversed, but current isscan direction is reversed, but current is

greater (since pit continues to grow)greater (since pit continues to grow)


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Pitting CorrosionPitting Corrosion

E

log |i|

Noise spikes due to

meta-stable pitting

Current continues

to increase after

reversal of scan

Pit eventually re-

passivates


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What is going on?What is going on?

E

log |i|

Cathodic

Anodic

Cathodic

Anodic

Stainless Steel in Aerated Sulphuric Acid


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Linear Polarization ResistanceLinear Polarization Resistance

MeasurementMeasurement

yTheoretical basisTheoretical basis

yMeasurement methods

yInterpretation


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LPRM TheoryLPRM Theory

y For an activation controlled reactionFor an activation controlled reaction

=

=

=

i

EEi

dE

di

EEii

oo

o

o

exp

expExchange current

density

Equilibrium

potentialTafel slope based on

exponential (i.e. mV

for a change of 1 in

ln(i))


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LPRM TheoryLPRM Theory

y Summing for two reactionsSumming for two reactions

y Rearrange and convert toRearrange and convert to bb rather thanrather than

pca

cacorr

c

c

a

a

Ri

ii

dE

di

1=

=

+

=

( )ca

ca

corr

pbb

bbB

i

BR

==

3.2,

Anodic partial current

density (=icorr)

Anodic Tafel slope

(positive)

Cathodic partial current

density (= -icorr)

Cathodic Tafel slope

(negative)

Because c is taken as

negative

Tafel slope based on a

decade change incurrent (i.e. a change

of 1 in log i )Stern-Geary

coefficient


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LPRM Control VariableLPRM Control Variable

y Potential controlPotential control potential range can be optimisedpotential range can be optimised

problems with drift of Eproblems with drift of Ecorrcorry Current controlCurrent control

potential range depends onpotential range depends on RRpp

measurement inherently centred aboutmeasurement inherently centred about ii = 0= 0


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LPRM Measurement WaveformLPRM Measurement Waveform

y Triangle waveTriangle wave can measurecan measure didi//dtdt atat ii = 0= 0

requires relatively complex instrumentsrequires relatively complex instrumentsy Square wave (switch between +Square wave (switch between +ii and -and -i)i)

simple instrumentssimple instruments

simple to automatesimple to automate

y Sine waveSine wave simplest theory for frequency effectssimplest theory for frequency effects

complex to perform measurementcomplex to perform measurement


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LPRM Cell ConfigurationLPRM Cell Configuration

y Two electrodeTwo electrode

assumeassume RRpp is the same for two similar electrodesis the same for two similar electrodes

and measure cell resistance (= 2and measure cell resistance (= 2RRpp ++ RRsolsol)) easy, no reference electrode requiredeasy, no reference electrode required

y Three electrodeThree electrode use conventional counter, reference and workinguse conventional counter, reference and working

electrodeselectrodes provides lower solution resistance, thereforeprovides lower solution resistance, therefore

better for low conductivity solutionsbetter for low conductivity solutions

more complex instrumentationmore complex instrumentation


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LPRM RecommendationsLPRM Recommendations

y Use three electrode measurement with triangleUse three electrode measurement with trianglewaveform for laboratory studieswaveform for laboratory studies

y Use two electrode measurement with squareUse two electrode measurement with square

waveform for simple corrosion monitoring (use threewaveform for simple corrosion monitoring (use threeelectrodes for high resistance solutions)electrodes for high resistance solutions)

y Use potential control whenUse potential control when iicorrcorr variation is largevariation is large

y Use current control whenUse current control when EEcorrcorr

varies a lotvaries a lot

y When bothWhen both iicorrcorr andand EEcorrcorr vary use current control, butvary use current control, but

adapt current to keep potential range reasonableadapt current to keep potential range reasonable


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LPRM InterpretationLPRM Interpretation

y Determination of B valueDetermination of B value calculate from Tafel slopescalculate from Tafel slopes

correlation with weight losscorrelation with weight loss

arbitrary valuearbitrary value

x 26 mV for activation control26 mV for activation controlx 52 mV for one reaction at limiting current52 mV for one reaction at limiting current

( )caca

bbbbB = 3.2


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LPRM Sweep RateLPRM Sweep Rate

y Must be sufficiently slow for current chargingMust be sufficiently slow for current chargingdouble layer capacitance to be much less thandouble layer capacitance to be much less than

total currenttotal currenty Characteristic time given byCharacteristic time given by RRctctCCdldl - cycle time- cycle time

should be at least 3 times thisshould be at least 3 times this

y NeedNeed notnot be slow enough to allow diffusionbe slow enough to allow diffusionprocesses to respond (as the basic theory isprocesses to respond (as the basic theory isnot valid for diffusion processes)not valid for diffusion processes)


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LPRM ProblemsLPRM Problems

y Theoretically, eitherTheoretically, either both reactions must be activation controlled, orboth reactions must be activation controlled, or

one reaction must be activation controlled and theone reaction must be activation controlled and theother mass-transport limitedother mass-transport limited

y In practice it is rare for real systems to meetIn practice it is rare for real systems to meetthese constraints, and application of LPRM isthese constraints, and application of LPRM is

not theoretically justifiednot theoretically justifiedy Solution resistance adds to measuredSolution resistance adds to measured RRpp, and, and

produces lower apparent corrosion rateproduces lower apparent corrosion rate


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Equivalent CircuitsEquivalent Circuits

y An electrical circuit with the same propertiesAn electrical circuit with the same propertiesas a metal-solution interfaceas a metal-solution interface

y

The simplest circuit is a resistor,The simplest circuit is a resistor, RRctct,,corresponding to the polarization resistance,corresponding to the polarization resistance,in parallel with a capacitor,in parallel with a capacitor, CCdldl,,correspondingcorresponding

to the double layer capacitanceto the double layer capacitance

Solution


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Equivalent CircuitsEquivalent Circuits

y An electrical circuit with the same propertiesAn electrical circuit with the same propertiesas a metal-solution interfaceas a metal-solution interface

y

The Randles equivalent circuit adds a seriesThe Randles equivalent circuit adds a seriesresistor, corresponding to the solutionresistor, corresponding to the solutionresistanceresistance

Rct

Rsol


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Time

i

E

Analysis of Solution ResistanceAnalysis of Solution Resistance

y If we analyse the full response to the LPRMIf we analyse the full response to the LPRMmeasurement, we can estimatemeasurement, we can estimate RRsolsol,, CCdldl andand RRctct

Vo=iRsol

V=iRct

The voltage acrossRsol is

given by Voexp(-t/RsolCdl)When t=RsolCdl,

V=Voexp(-1)

Estimate Cdl from the

exponential decay. The timefor V to fall to e-1 (37%) of

the initial value isRsolCdl


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Open Circuit Potential DecayOpen Circuit Potential Decay

y Similar to analysis of LPRM measurementSimilar to analysis of LPRM measurement charge double layer capacitance by applying acharge double layer capacitance by applying a

current or potentialcurrent or potential

disconnect charging currentdisconnect charging current

monitor decay of potentialmonitor decay of potential


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Open Circuit Potential DecayOpen Circuit Potential Decay

Time

EInitial voltage drop = iRsol

Delayed

voltage drop

= iRct

Charging at current i Disconnected

0.37iRct

Time =RctCdl


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483 budi_agung corrosion measurement techniques

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