Corrosion Measurement Techniques

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Corrosion Measurement Techniques

• A copy of this presentation is available in the
  CAL group in the computers in the Teaching Lab,
  or via the WWW at http//www.cp.umist.ac.uk/CPC/L_
  Notes

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Corrosion Measurement Techniques

• Polarization curves

• Linear Polarization Resistance

• Open Circuit Potential Decay

• AC Impedance Measurement

• Electrochemical Noise Measurement

• Weight Loss Measurement

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Polarization Curves

• Measurement methods

• Cell design

• Plotting data

• Interpretation

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Measurement Methods

• Objective
• determine current density under steady-state
  conditions as a function of potential
• not really practical, as this would strictly
  require one sample for each potential
• therefore compromise on closeness to true
  steady-state

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Measurement Methods
Connect electrodes to corresponding terminals on
potentiostat

• Potential control

Reference Electrode - reference connection for
potential measurement
Luggin Probe - allows potential to be detected
close to metal surface
Counter Electrode (or Auxilliary Electrode or
Secondary Electrode) - provides current path into
solution
Working Electrode - metal being studied
Potentiostat controls potential
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Measurement Methods

• Current control

Counter Electrode
Reference Electrode - only used to monitor
potential, not connected to potentiostat
Current controlled by control of voltage across
resistor (IV/R)
Working Electrode
Luggin Probe still needed to limit IR error
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Measurement Methods

• Swept potential or current
• Use sweep generator to produce slowly changing
  potential
• Sweep generator output controls potentiostat
• Record response on chart recorder (or use
  computer monitoring)
• Swept current not often used, as it moves through
  corrosion potential very quickly

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Measurement Methods

• Potential or current step
• Step potential or current from one value to the
  next, allowing time to stabilise at each new
  value
• Record current or potential
• May be manually controlled, or use computer to
  step potential/current and take readings

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Measurement Methods

• Sweep direction
• Aim to perform experiment in such an order that
  the initial polarization affects subsequent
  results as little as possible
• Options
• new specimen for each potential
• one specimen for cathodic polarization, and one
  for anodic, both start at corrosion potential
• one specimen, sweep from cathodic to anodic

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Measurement Methods

• Sweep rate (or step rate)
• Ideal, all measurements made at steady-state
• Time-dependent effects include
• Charging of double layer capacitance (I C
  dV/dt)
• Mass transport effects (t ? L2/D)
• Adsorbed species and surface films (Faradays
  Law)
• Typical sweep rates are of the order of 1 mV/s or
  less

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Questions

• Consider the corrosion of iron in aerated neutral
  solution, with the following parameters
• Cdl 35 ?F / cm2 DO2 1.2 x 10-5 cm2 /s
• Boundary layer thickness, ? 100 ?m
• Number of iron atoms on surface ? 2?1019/cm2
• Charge on the electron 1.6 x 10-19C
• Calculate
• Capacitive current at 1 mV/s
• Characteristic diffusion time
• Limiting current density for O2 reduction (8 ppm
  O2)
• Time to oxidise Fe surface to FeOH (Fe) at ilim

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Cell Design

• Working electrode
• Reference electrode
• Counter electrode
• Solution
• Mass transport

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Working Electrode

• Requirements
• reproducible
• representative
• free of crevices
• free of edge effects
• free of galvanic effects
• free of water-line effects

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Working Electrode

• Epoxy embedded electrode

Apply thin layer of epoxy to minimise stress and
risk of crevice formation
Weld or solder connecting wire to specimen
Apply thick layer of epoxy to seal connecting
tube and for strength
Pretreat specimen for good adhesion
Carefully grind surface to expose metal
Clean surface - dont use acetone
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Working Electrode

• Stern-Makrides electrodes

Lip sealbetween PTFE case and electrode
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Working Electrode

• Avesta cell

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Reference Electrode

• Commonly use Saturated Calomel Electrode (SCE)
• Properties may degrade with time (and misuse)
• check one against another (should not be more
  than 1 to 2 mV difference)
• do not pass current through the reference
  electrode (e.g. do not connect to working or
  counter electrode)
• do not allow to dry out

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Reference Electrode

• Solution in SCE (or Ag/AgCl electrode) is
  saturated KCl
• beware of chloride contamination of test solution
  by Cl- leaking from reference electrode
• make sure solution remains saturated

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Luggin Probe

• A Luggin probe should be used whenever there is a
  significant current applied to the electrode

Electrode
Luggin probe allows point at which potential is
measured to be close to electrode surface (around
3 times tip diameter is best)
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Counter electrode

• Counter electrode should allow current to pass
  with tolerable polarization
• Often claimed that counter electrode should have
  much larger area than working electrode, but this
  is not often necessary for corrosion studies
• Usually use platinum or graphite, although
  stainless steel can be used in some situations
  (e.g. where only anodic polarization of specimen
  is used)

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Solution

• Requirements
• as high a conductivity as possible (add
  supporting electrolyte, such as sodium
  perchlorate?)
• remain the same (pH, composition) throughout the
  experiment - ensure that volume is adequate
• oxygen concentration often critical - aerate by
  bubbling air or O2 or deaerate with N2 or Ar
• most reactions temperature sensitive, so control,
  or at least record, temperature

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Mass transport

• Methods of controlling mass transport
• rotating disk or cylinder
• flow channel
• jet impingement
• gas bubbling

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Plotting of Polarization Curves

• Comparison of log-i and linear-i plots
• Identification of anodic and cathodic regions on
  log-i plots
• Orientation of plots

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E log I - old plotting method
E
log i
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Interpretation of Polarization Curves

• Addition of reactions on log-I graphs
• Tafel regions
• Mass transport control
• Active-passive transition
• Transpassive corrosion
• Pitting Corrosion

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Tafel regions

• A Tafel region is a straight line in the
  E-logi plot
• For a reliable Tafel slope
• the line should be straight for at least one
  decade (in this context a decade implies a change
  of current density by a factor of ten, i.e a
  difference of 1 in log i )
• the region should be next to Ecorr

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Tafel Extrapolation

• Extrapolate anodic or cathodic Tafel region, or
  both, back to Ecorr, when the current density is
  icorr
• In aerated neutral solutions, where mass
  transport limited oxygen reduction is the main
  cathodic reaction, the cathodic reaction does not
  have a valid Tafel slope, but the anodic slope
  can sometimes be used

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Question

• How can we estimate the rate of hydrogen
  evolution during free corrosion?
• Estimate the value for the graph shown.

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Mass transport control

• When the supply of a reactant becomes mass
  transport controlled, we observe a limiting
  current density
• The most common case occurs for oxygen as a
  cathodic reactant in neutral solutions
• NOTE - the diffusion of a reaction product away
  from the electrode will not affect the rate of
  the forward reaction

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Solution Resistance Effects

• At high currents the potential drop associated
  with the solution resistance can be significant
• It is generally referred to as an IR error
• Gives a straight line on E-i plots

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Active-passive transition

• As a passive film develops, it covers the surface
  and shuts off the dissolution reaction, leading
  to an active-passive transition

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Active-passive transition

• For stainless steel we sometimes see two
  active-passive transitions, on for Chromium, and
  one for Iron

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Transpassive corrosion

• A passive metal (notably Cr and Fe) may start to
  dissolve at a very positive potential when a
  higher oxidation state (e.g. Cr6 as chromate) is
  formed
• This is known as transpassive corrosion, and will
  give something like a second activation-controlled
  reaction
• For alloys the behaviour will be complicated by
  the differing behaviours of the alloy components

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Anodic Polarization Curve for Stainless Steel
Activation-controlled dissolution
Active-passive transition
Active peak for iron
Transpassive corrosion of Cr
Oxygen reduction
Overall anodic curve
E
log i
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Pitting Corrosion

• Pitting shows up as an increasing anodic current
  before (at a less positive potential than)
  transpassive corrosion or oxygen evolution,
  usually preceded by noise
• E-logi plot does not follow same path if scan
  direction is reversed, but current is greater
  (since pit continues to grow)

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Pitting Corrosion
Noise spikes due to meta-stable pitting
Current continues to increase after reversal of
scan
Pit eventually re-passivates
E
log i
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What is going on?
Stainless Steel in Aerated Sulphuric Acid
Anodic
Cathodic
E
Anodic
Cathodic
log i
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Linear Polarization Resistance Measurement

• Theoretical basis

• Measurement methods

• Interpretation

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LPRM Theory

• For an activation controlled reaction

Exchange current density
Equilibrium potential
Tafel slope based on exponential (i.e. mV for a
change of 1 in ln(i))
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LPRM Theory

• Summing for two reactions
• Rearrange and convert to b rather than ?

Anodic partial current density (icorr)
Because ?c is taken as negative
Anodic Tafel slope (positive)
Cathodic Tafel slope (negative)
Cathodic partial current density ( -icorr)
Tafel slope based on a decade change in current
(i.e. a change of 1 in log i )
Stern-Geary coefficient
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LPRM Measurement Methods

• Control variable
• Waveform
• Cell configuration
• Sweep rate

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LPRM Control Variable

• Potential control
• potential range can be optimised
• problems with drift of Ecorr
• Current control
• potential range depends on Rp
• measurement inherently centred about i 0

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LPRM Measurement Waveform

• Triangle wave
• can measure di/dt at i 0
• requires relatively complex instruments
• Square wave (switch between i and -i)
• simple instruments
• simple to automate
• Sine wave
• simplest theory for frequency effects
• complex to perform measurement

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LPRM Cell Configuration

• Two electrode
• assume Rp is the same for two similar electrodes
  and measure cell resistance ( 2Rp Rsol)
• easy, no reference electrode required
• Three electrode
• use conventional counter, reference and working
  electrodes
• provides lower solution resistance, therefore
  better for low conductivity solutions
• more complex instrumentation

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LPRM Recommendations

• Use three electrode measurement with triangle
  waveform for laboratory studies
• Use two electrode measurement with square
  waveform for simple corrosion monitoring (use
  three electrodes for high resistance solutions)
• Use potential control when icorr variation is
  large
• Use current control when Ecorr varies a lot
• When both icorr and Ecorr vary use current
  control, but adapt current to keep potential
  range reasonable

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LPRM Interpretation

• Determination of B value
• calculate from Tafel slopes
• correlation with weight loss
• arbitrary value
• 26 mV for activation control
• 52 mV for one reaction at limiting current

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LPRM Sweep Rate

• Must be sufficiently slow for current charging
  double layer capacitance to be much less than
  total current
• Characteristic time given by RctCdl - cycle time
  should be at least 3 times this
• Need not be slow enough to allow diffusion
  processes to respond (as the basic theory is not
  valid for diffusion processes)

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LPRM Problems

• Theoretically, either
• both reactions must be activation controlled, or
• one reaction must be activation controlled and
  the other mass-transport limited
• In practice it is rare for real systems to meet
  these constraints, and application of LPRM is not
  theoretically justified
• Solution resistance adds to measured Rp, and
  produces lower apparent corrosion rate

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Equivalent Circuits

• An electrical circuit with the same properties as
  a metal-solution interface
• The simplest circuit is a resistor, Rct,
  corresponding to the polarization resistance, in
  parallel with a capacitor, Cdl, corresponding to
  the double layer capacitance

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Equivalent Circuits

• An electrical circuit with the same properties as
  a metal-solution interface
• The Randles equivalent circuit adds a series
  resistor, corresponding to the solution resistance

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Analysis of Solution Resistance

• If we analyse the full response to the LPRM
  measurement, we can estimate Rsol, Cdl and Rct

The voltage across Rsol is given by
Voexp(-t/RsolCdl) When t RsolCdl, VVoexp(-1)
Estimate Cdl from the exponential decay. The time
for V to fall to e-1 (37) of the initial value
is RsolCdl
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Open Circuit Potential Decay

• Similar to analysis of LPRM measurement
• charge double layer capacitance by applying a
  current or potential
• disconnect charging current
• monitor decay of potential

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Open Circuit Potential Decay
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