ELECTROCHEMICAL
CORROSION
1
ELECTROCHEMICAL
IMPEDANCE
SPECTROSCOPY
Types of corrosion
Pitting Corrosion
is considered to be more
dangerous than uniform
corrosion damage
because it is more
difficult to predict and
design against.
Corrosion products often
cover the pits making the
detection often very
difficult.
Crevice Corrosion
occurs in the presence of
stagnant solution in a small
(micro) crevice. Local
chemistry changes in crevices
(shielded areas) such as those
formed under gaskets,
washers, insulation material,
fastener heads, surface
deposits, unbonded coatings,
threads, lap joints and
clamps, can result in crevice
corrosion.
Uniform corrosion is
characterized by corrosive
attack proceeding evenly
over the entire surface
area, or a large fraction of
the area of the metal under
attack. Uniform corrosion
results in loss of material
until failure (most
widespread form of
corrosion
2
Types of corrosion
Galvanic Corrosion
refers to corrosion damage induced when
two dissimilar materials are
coupled in a corrosive electrolyte (into
electrical contact under water)
- one of the metals in the couple becomes
the anode and corrodes faster than it would
all by itself, while the other becomes the
cathode and corrodes slower than it would
alone. Either (or both) metal in the couple
may or may not corrode by itself
(themselves) in seawater.
Microbiologically Induced
Corrosion (MIC) refers to corrosion
caused by biological organisms or microbes.
These microbes are categorized by common
characteristics such as their by-products (i.e.,
sludge producing) or compounds they effect
(i.e. sulfur oxidizing). They all fall into one of
two groups based upon their oxygen
requirements; one being aerobic (requires
oxygen) such as sulfur oxidizing bacteria, and
the other being anaerobic, (requires little or
no oxygen), such as sulfate reducing bacteria.
3
The process of corrosion of metals
 deterioration or degradation of metal
 the formation of rust on steel
 most corrosion phenomena are of electrochemical nature
 consist of at least two reactions on the surface of the corroding metal
1) the oxidation (e.g., dissolution of iron)
Fe = Fe2+ + 2e- (anodic)
2) the reduction (e.g., reduction of oxygen)
2H2O + O2 + 4e- = 4OH- (cathodic)
2Fe + 2H2O + O2 = 2Fe(OH)2
2 Fe2+ + 4OH- = 2Fe(OH)2 (non-electrochemical)
1) and 2) :
4
Electrochemical characterisation
• Linear sweep voltammetryLSV
• Electrochemical impedance
spectroscopyEIS
• Electrochemical noise with
FFTECN
5
Measurement of corrosion rates
Weight loss measurements
• as a function of time
• no simple way to extrapolate the results to predict
the lifetime of the system under investigation
Electrochemical Tests
• to characterize corrosion mechanisms
• to predict corrosion rates
6
Calculation of corrosion rate
 The corrosion rate depends on the kinetics of both anodic
(oxidation) and cathodic (reduction) reactions.
 According to Faraday's law, there is a linear relationship between
the metal dissolution rate or corrosion rate, RM, and the corrosion
current icorr:
corrcorr i
nF
M
v


M is the atomic weight of the metal, ρ is the density, n is the
charge number which indicates the number of electrons
exchanged in the dissolution reaction, the ratio M/n is also
referred to as equivalent weight.
7
Calculation of corrosion currents
Calculation of corrosion rates requires the calculation of corrosion currents.
When reaction mechanisms for the corrosion reaction are known, the
corrosion currents can be calculated using Tafel Slope Analysis.







ca
corr
b
3032
b
3032ii

.(exp.(exp corrEE 
Ecorr is the open circuit potential of a corroding metal, ba, , bc Tafel constants
For large anodic overpotentials (η / ba >>1) the Butler Volmer equation
simplifies to the Tafel equation for the anodic reaction.
η = log icorr + ba log i
For large cathodic overpotentials (η / bc << -1)
η = = log icorr – bc log |i|
Tafel equations predict a straight line for the variation of the logarithm of
current density
8
9
Calculation of corrosion rate
the assumption that
 the corrosion reactions were under charge transfer control
(activation overpotential)
 the mechanisms of the reactions were known
Polarization resistance - Rp
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Measurement of polarization resistance
voltageonpolarizatiE
0E
p
I
E
R




currentonpolarizatiI







ca
corr
b
3032
b
3032ii

.(exp.(exp
For small η, i.e. for potentials close to corrosion potentials, the above
equation can be reduced to:










pca
ca
corr
R
1
bb
bb
3032i .








corrca
ca
p
i
1
bb
bb
3032R .
If the Tafel slopes are not
known (e.g. when corrosion
mechanism is not known), the
Rp can still be used as a
quantitative parameter to
compare the corrosion
resistance of metals under
various conditions. 11
Measurement of Rp
• Linear sweep
voltammetryLSV
• Electrochemical
impedance spectroscopyEIS
12
13
Randles circuit








corrca
ca
p
i
1
bb
bb
3032R .
CPE – Constant Phase Element
Electrochemical impedance spectroscopy
EIScorrosion
electrodeposition,
electrodissolution, passivity
SAM
diffusion of ions across
membranes semiconductor
interfaces
The fundamental approach of all
impedance methods is to apply a
small amplitude
sinusoidal excitation signal
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Principles of EIS measurements
Taylor series expansion for the current is given by
....
,,












 2
E
2
2
E
E
dE
Id
2
1
E
dE
dI
I
0I00I0

If the magnitude of the perturbing signal ΔE is small, then the higher
order terms can be assumed to be negligible. The impedance of the
system can then be calculated using Ohm’s law:
)(
)(
)(



I
E
Z 
)(fimpedanceis)(  Z
frequency range of 100kHz – 0.1Hz
15
Principles of EIS measurements
16
90o
)(I
)(U
)(I
)(E
)(Z











EIS: data analysis
α = 1, CPE acts as an ideal capacitor
α = 0, CPE acts as an ideal resistor
Constant phase element - CPE
17
Principles of EIS measurements
18
The double layer
capacity
Rct
Cdl
Principles of EIS measurements
19
ctdl
ct
RCj
R
Z


1
dl
ctdlct
Cj
RCj/RZ



1
1
111
Rct
ctdl
ct
e
RCj
R
RZ


1
222
1 ctdl
ct
ereal
RC
R
RZ


222
2
1 ctdl
dlct
im
RC
CR
Z




Principles of EIS measurements
In cartesian co-ordinates (w)jZ(w)ZZ(w) jr 
In polar co-ordinates

 eZZ )()( 
magnitude of the
impedance
phase shift
The plot of the imaginary part against the real part of impedance - Nyquist plot.
The shape of the curve is important in making qualitative interpretations of the data.
The disadvantage of the Nyquist representation is that one loses the frequency
dimension of the data. One way of overcoming this problem is by labelling
the frequencies on the curve. The absolute value of impedance and the phase shifts
are plotted as a function of frequency in two different plots giving a Bode plot. The
relationship between the two ways of representing the data is as follows:
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0
0
0
cos( )( ) cos( )
( )
( ) cos( ) cos( )
E tE t t
Z t Z
I t I t t
 
   
  
 
21

Z´
Z”
The impedance data are the red points.
Their projection onto the Z“-Z‘ plane is called the Nyquist plot
The projection onto the Z“- plane is called the Cole Cole diagram
The different views on impedance data
The absolute value of Z and the phase shifts are plotted as a function of
frequency in two different plots giving a Bode plot
Nyquist and Bode plot
   222
ZZZ ImRe 
Z
Z
tg
Re
Im

  cosRe ZZ 
  sinIm ZZ 
22
EIS: Experimental set-up
potentiostat/galvanostat, a frequency response analyser (FRA modul)
2 electrode cell
3 electrode cell
4 electrode cell
4 electrode cell
- between two measuring electrodes separated by a
membrane, the WE and the CE enable current flow.
This kind of a cell is usually used :
 to study ion transport through a membrane,
 to perform electron or ion conductivity measurements
 to measure low impedance where the influence of contact and wire resistance
should be minimal.
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EIS: experiment parameters
potentiostat/galvanostat FRA modul
Potentiostatic or galvanostatic mode
at a fixed DC currentat a fixed DC potential
DC potential or current
typical current potential curve for a corrosion of iron in passivating solution
OCP can change
due to changes in
the electrode
surface
it is desirable that OCP is measured
dynamically at each frequency or the
measurements are done under
galvanostatic control at zero current
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EIS: experiment parameters
FRA parameters or settings
1. AC mode (single sine or multi sine)
2. perturbation (sine wave) amplitude (10 mV)
3. integration time
4. wait for steady state
5. frequency range (100kHz – 0.1Hz)
6. frequency distribution (linearly, logarithmically or with a square root distribution)
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EIS: equivalent circuit models
26
EIS: equivalent circuit models
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