Transcript Electrochemical Techniques for Corrosion Measurements

Electrochemical Techniques for
Corrosion Measurements
Assignment
• Present a critical examination of the proposed
mechanisms of CO2 and H2S corrosion with
particular emphasis on the cathodic
(oxidation) reactions.
• Due Date 5 June 2009
Corrosion Measurements
Involve the use of a potentiostat for applying a potential (relative to a
reference electrode) and measuring the current (flowing from the working
electrode to the counter or auxiliary electrode)
3
The Manual Potentiostat
Variable High
Voltage Source
50-300 V
High Impedance
Voltmeter 1012 Ω
Working Reference Auxiliary
Electrode Electrode Electrode
Ammeter
(current)
Electrochemical Impedance Spectroscopy
• Resistance
• Ohms Law
E
R
I
• For a resistor, R, it follows Ohm’s Law at all current and
voltage levels
• The resistance value is independent of frequency
• AC current and voltage signals through a resistor are in phase
with each other
Electrochemical Impedance Spectroscopy
• Impedance
• Impedance applies to AC voltage and current
• Like resistance impedance is a measure of the ability of a
circuit to resist the flow of electrical current
• The excitation potential or AC voltage can be expressed as a
function of time
Et  E0 sint 
Et  potentialat time t
E0  the amplitude of the v oltage
  the radial f requency
Electrochemical Impedance Spectroscopy
• The relationship between radial frequency ω (radians s-1) and
frequency (f) hertz is:
  2f
• The response to the AC voltage is given by:
I t  I 0 sint   
I t  responsecurrent
I 0  the amplitude of the current
  the phase shif t
Electrochemical Impedance Spectroscopy
• Similar to Ohm’s law
Z
Et
E0 sint 
sint 

 Z0
It
I 0 sint   
sint   
• The important point to remember is that when an AC voltage
is applied to a pure capacitor the resulting AC current is
shifted in phase by 90o
• There is no phase shift for a pure resistor
Electrochemical Impedance Spectroscopy
Current phase shift due to impedance. Through a
capacitor this phase shift is 90o
Applied
Voltage
Resulting
Current
Electrochemical Impedance Spectroscopy
• Randles circuit for a simple corroding system
• Rs = the solution resistance
• Rct = the charger transfer (polarisation resistance)
• Cdl = the double layer capacitance
Electrochemical Impedance Spectroscopy
• Nyquist plot for the Randels circuit
Capacitance
← Increasing Frequency
Charge transfer
resistance = Rtotal - Rs
Solution
Resistance
Resistance
EIS Nyquist Plots
• A Nyquist plot is made up of a series of vectors representing
the total magnitude of the resistance and capacitance
components
Non Resistive
Component
Phase angle
Electrochemical Impedance Spectroscopy
• Bode impedance plot
Impedance
Rct
Solution
resistance
Frequency →
Electrochemical Impedance Spectroscopy
• Bode Phase plot
Phase angle
Frequency →
EIS (Summary)
We start here at the
high frequency
EIS
• Diffusion or Mass Transfer controlled process
• Nyquist plot - Warburg Impedance
←Frequency
EIS
• Diffusion or Mass Transfer controlled process
• Bode Impedance plot
Impedance
Frequency →
EIS
• Diffusion or Mass Transfer controlled process
• Bode Phase plot
Phase Angle
Frequency →
EIS – Mass Transfer Controlled Process (Summary)
Nyquist
Bode
Impedance
Bode
Phase
EIS Equivalent Circuit for a Mixed Kinetic and Charge
Transfer Controlled Process
EIS Bode Plots for the Mixed Controlled Reaction
Impedance
Phase
EIS Equivalent Circuit for a Filmed Corroding
Surface (E.g. Failed Coating)
EIS for a Filmed Corroding Surface (E.g. Failed Coating)
Bode
Impedance
(Magnitude
Nyquist
Bode Phase
Angle
Linear Polarization Method
Valid for corrosion under activation control.
Involves applying a small perturbation to the potential around Ecorr (i.e., ± ∆E ≈ 10
mV).
N.B. ∆i for
summed curve
= ia + |∆ic|
(∆ia=x)
Slope of summed curve (measure E vs i
for system) is difference between slopes
of curves for the coupled reactions: Sa Sc
The curves are ~linear within ~20mV – Sa and Sc are constant. For
∆E around Ecorr, Sa and Sc are related to icorr (the required quantity):
assuming the high-field approximation for the individual reactions
dE
ba
 Sa 
di anodic
2.303icorr
Now:
dE
bc
 Sc 
di redox
2.303icorr
slope S a 
E
x
slope S c 
E
i  x
S  Sc
 E  a
i
or icorr
Sa Sc
ba bc
1
i


2.303 ba  bc E
Polarization Resistance .. E .. is measured. The Tafel coefficient ba and
i
bc must be known.
Remember: during linear polarization measurements we
plot E vs i (not log i) around the corrosion potential:
E
= polarization resistance
i
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Linear Polarization
• This involves the application of low over-potentials and
therefore the currents are relatively very small. This
means that the charging current (capacitance current)
can make a significant contribution to the noise or
background current.
• Use slow scan rates and perform a cyclic scan to check
whether you are measuring capacitance.
• The reverse scan should produce an iE curve that
retraces over the forward recorded iE curve.
• The iE curve can be curved due to a difference in the
anodic and cathodic Tafel slopes.
Linear Polarization
• It is important to view the iE curve. If the iE curve is
curved, the polarization resistance can be obtained by
drawing a line that is tangential to the curve at Ecorr and
at zero current.
• Some portable instruments use a potential-step method.
In this case the current, at, for example, -10 mV and +10
mV is measured and Rp is computed from these
measurements.
• The advantage of this technique is that the current
measurements are made at a constant voltage and
therefore the charging current is zero.
• The disadvantage is that no iE curve is recorded and
therefore an error can be introduced if there is curvature
in the iE graph
Linear Polarization
• The portable instruments that use the potential-step
technique, usually apply a high frequency AC signal
before the measurement to determine the solution
resistance and subtract this value from the measured
polarization resistance.
Tafel Extrapolation
Tafel Method We can only measure the net current across
the specimen electrode – at the corrosion potential there is no
net current (only local anode – cathode currents which constitute
the corrosion current). We cannot measure corrosion rate
directly, though we need icorr.
Measure potential and current at some distance on either side of
Ecorr – extrapolate E - log i curves (in same quadrant) back to Ecorr
…
Plot of the total current
(iT = io + ic) versus
potential showing the
extrapolation of the
Tafel regions to the
corrosion potential,
Ecorr, to yield the
corrosion current, icorr.
Passivation
Under certain conditions of potential and pH, some
metals form protective films, i.e., they passivate
Pourbaix diagram for the
iron/water/dissolved
oxygen system showing
the effect of potential in
moving the system from a
corrosive (active) region
(point 1) to a passive
region (point 2)
We can exam the kinetics
using a potentiodynamic
scan and Evans diagram
The polarization curve for the anodic reaction of a passivating
metal drawn for potentials more noble than the equilibrium
potential (Ee)a
Oxidative dissolution of oxide (e.g.,
Cr2O3  CrO42-)
“Flade”
(Ee)M/MO is the equilibrium potential
for oxide/hydroxide formation
Tafel region
(icrit is min. reaction rate required to
initiate film growth by precipitation
of Mn+)
The region attained by the metal in a given environment depends upon the
cathodic reaction i.e., where the cathodic curve cuts the above anodic curve.
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Tafel Extrapolation Technique
• Involves measurements at high over-potential in
which logi is recorded.
• The best method of performing these measurements
is by :
• 1) Using two identical electrodes and recording the
anodic curve on one electrode and the cathodic
curve on the other electrode. In each case starting at
the open circuit potential Eoc (or Ecorr)
• 2) Performing the cathodic curve on one electrode
starting the scan from Eoc. Turning off the
potentiostat and monitoring Eoc until it returns to its
original value. The anodic scan is then recorded again
starting at the Eoc.
Tafel Extrapolation Technique
• Before commencing a Tafel measurement, it is
generally best to allow your metal electrode to reach
a steady state potential. This can be observed by
performing a potential time measurement in which
Ecorr is monitored with time.
• Scan Rates are normally in the range of 0.1 mV to 5.0
mV per second. The cathodic plot is scanned to an
over-potential of about 400 mV.
• Anodic potentials can be scanned much higher
depending on what information needs to be
obtained.
Cyclic Pitting Scans
• The technique is used to evaluate the susceptibility
of metals to pitting corrosion in a particular
environment. It is applicable to metals such as
stainless steels, high nickel alloys and aluminium,
which form a passive protective film.
• With this technique, the potential is scanned to
voltages in the transpassive region.
• Exceeding the passive region is indicated by a sudden
increase in current. At this stage the voltage scan is
reversed, usually when the current reaches a certain
current density (0.5 mA cm-2)
Cyclic Pitting Scans
• The extent of the hysteresis in the reverse scan is an
indication of the susceptibility to pitting corrosion.
• Pitting corrosion is considered to stop at the potential where
the iE curve from the reverse scan crosses the iE curve of the
forward scan.
• The sudden increase in current can be due to three processes:
• 1) Onset of pitting corrosion
• 2) Trans-passive uniform corrosion
• 3) The oxygen evolution reaction
• In the case of trans-passive corrosion, the slope of the iE
curve is not as steep compared to pitting corrosion and
oxygen evolution.
• In the case of oxygen corrosion, the reverse iE curve normally
will retrace over the forward iE curve.
Harmonic Analysis
• Butler Volmer Equation
• When an electrode is polarized near the corrosion potential
by a sinusoidal voltage of frequency ω and amplitude U0, then
the current density of the Faradaic process is given by:
Harmonic Analysis
• The current densities of the Faradaic process will
have a distorted sinusoidal form due to the nonlinear nature of the cathodic and anodic partial
processes in a polarization curve.
• The amplitudes of the harmonic components can be
obtained by Fourier series expansion of the exponential
terms
Harmonic Analysis
The simplified magnitude of the first three harmonic
components are given by:
Harmonic Analysis
• The technique has been verified by the work of Will Durnie,
Curtin University.
• He compared corrosion rates from HA with those obtained
using linear polarization measurements and Stern Geary
equation.
• When the Tafel slopes obtained from HA were placed in the
Stern Geary equation an excellent correlation was obtained.
Harmonic Analysis
Durnie, W. H.,Curtin University
Electrochemical Noise (ECN)
• ECN measures the current/voltage response between two
(largely) identical electrodes.
• The two electrodes are coupled together (short circuited
together) through a zero resistance ammeter (ZRA).
• The random fluctuations of current is measure by the ZRA.
• At the same time the random fluctuations in voltage noise at
the coupled electrodes is measured with respect to a
reference electrode.
Electrochemical Noise Measurement
Zero Resistance Ammeter
A
V
Working Electrode 2
Working Electrode 1
Reference Electrode
Rotating dual cylinder
electrode (RDCE)
• Standard electrochemical cell
with reference, auxiliary and
RDCE
• The RDCE is useful for
performing ECN Measurements
• ECN uses identical electrodes. In
the example shown the
electrode areas are not the
same since this RDCE was used
to investigate preferential weld
corrosion
Potential Noise and Current Noise
Voltage Noise PSD
Voltage PSD / V2/Hz
1.E-03
1.E-06
1.E-09
1.E-12
1.E-15
1.E-04
3.5 days
1.E-03
1.E-02
Frequency/Hz
1.E-01
1.E+00