-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
1/60
(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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
2/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
3/60
(c) Bob Cottis 1995
Polarization CurvesPolarization Curves
yMeasurement methods
yCell design
yPlotting data
yInterpretation
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
4/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
5/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
6/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
7/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
8/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
9/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
10/60
(c) Bob Cottis 1995
Measurement MethodsMeasurement Methods
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
11/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
12/60
(c) Bob Cottis 1995
Cell DesignCell Design
y Working electrodeWorking electrode
y Reference electrodeReference electrode
y Counter electrodeCounter electrodey SolutionSolution
y Mass transportMass transport
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
13/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
14/60
(c) Bob Cottis 1995
Working ElectrodeWorking Electrode
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
15/60
(c) Bob Cottis 1995
Working ElectrodeWorking Electrode
y Stern-MakridesStern-Makrideselectrodes:electrodes:
Metal rod
Retaining nut
Washers
Heavy-walled
glass tube
PTFE Washer
Electrode
Lip seal
betweenPTFE case
and electrode
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
16/60
(c) Bob Cottis 1995
Working ElectrodeWorking Electrode
y Avesta cell:Avesta cell:
Specimen
NaCl
SolutionPure
H2O feedFilter paper
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
17/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
18/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
19/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
20/60
(c) Bob Cottis 1995
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)
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
21/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
22/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
23/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
24/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
25/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
26/60
(c) Bob Cottis 1995
log |log |i|i|
E
E log |E log |II| - old plotting method| - old plotting method
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
27/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
28/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
29/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
30/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
31/60
(c) Bob Cottis 1995
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.
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
32/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
33/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
34/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
35/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
36/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
37/60
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
38/60
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
39/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
40/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
41/60
(c) Bob Cottis 1995
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)
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
42/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
43/60
(c) Bob Cottis 1995
What is going on?What is going on?
E
log |i|
Cathodic
Anodic
Cathodic
Anodic
Stainless Steel in Aerated Sulphuric Acid
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
44/60
(c) Bob Cottis 1995
Linear Polarization ResistanceLinear Polarization Resistance
MeasurementMeasurement
yTheoretical basisTheoretical basis
yMeasurement methods
yInterpretation
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
45/60
(c) Bob Cottis 1995
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))
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
46/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
47/60
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
48/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
49/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
50/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
51/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
52/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
53/60
(c) Bob Cottis 1995
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)
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
54/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
55/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
56/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
57/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
58/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
59/60
(c) Bob Cottis 1995
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
-
8/14/2019 483 Budi_agung Corrosion Measurement Techniques
60/60