Electrochemical Theory

1
Electrochemical Theory
2
Kinetics of Activation Controlled Reactions

• M ? Mn ne-
• rate of reaction depends on potential according
  to the Tafel equation

3
Tafels Law
Slope ?
Slope b
b2.303?
Potential
E0a
ln i
i0,a
log i
4
Charge Transfer Resistance

• Charge transfer resistance local slope of i
  versus E curve (not log i)

5
Charge Transfer Resistance

• Note that charge transfer resistance is not a
  constant, but depends on the applied current
  density
• If we could measure the charge transfer
  resistance, we could determine the current density

6
Dependence of Kinetics on Reactant Concentration

• More reactant allows reaction to go faster, hence
  rate is proportional to reactant concentration
• e.g. oxygen reduction

Surface concentration of oxygen
Minus sign because thisis a cathodic
reaction(and ?c is taken as positive)
7
Tafels Law
Cathodic reaction - rate increases
withdecreasing potential
Rate with constantsurface concentrationof oxygen
Rate with surface concentration of oxygen
varying
E0c
Potential
ln i
i0,c
log i
ilim
8
Mixed Potential Theory

• Net current density on freely-corroding electrode
  must be zero.
• Therefore potential (Ecorr) will be that at which
  anodic and cathodic current densities are equal
  and opposite.
• Called a mixed equilibrium (not a true
  electrochemical equilibrium)

9
Tafels Law
Potential
log i
10
Tafels Law
E0c
Potential
ln i
i0,c
log i
ilim
11
Electrical Units

12
Charge

• Results from inbalance between electrons and
  protons in a metal, or between anions and cations
  in a solution
• Unit the coluomb, C
• Charge on the electron 1.6 x 10-19 C

13
Current

• Flow of charge past a point in a conductor
  (either electron or ion)
• Unit the Amp, A

14
Conservation of Charge

• Charge can be neither created nor destroyed
• Hence, the currents into and out of a point in an
  electrical circuit must add up to zero
  (Kirchoffs Law)

15
Potential

• The potential at a point in space is the work
  done in moving a unit charge to that point from
  infinity.
• Units of volts, V (J/C)

16
Potential Difference (or Voltage)

• The potential difference or voltage is the
  difference between the potentials at two points,
  and hence the work done in moving a unit charge
  from one point to the other.
• Units of Volts

17
Resistance

• A resistor (conventional symbol R) is a device
  that produces a voltage across its terminals when
  a current passes through it
• Ohms Law VIR
• R is the resistance of the resistor
• Units Ohms, ?
• 1 V is produced by a current of 1 A through a
  resistance of 1 ?

18
Capacitance

• A capacitor (conventional symbol C) is a device
  that stores charge when a current is applied to
  it
• Units of capacitance Farads, F
• I C dV/dt
• A 1 F capacitor will produce a voltage increase
  of 1 V/s when a current of 1 A flows into it

19
Equivalent Circuits

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

20
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

21
Potential Measurement

22
Electrode Potential

• The potential of a metal electrode with respect
  to a solution.
• BUT the charge carriers in a metal are electrons,
  while the charge carriers in a solution are ions.
• So how do we measure it?

23
Measurement of Electrode Potential

• Use arbitrary reference electrode to convert from
  ion current to electron current.
• Conventional standard reference electrode is
  based on the reaction

Hydrogen ions in solution at unit activity
Hydrogen gas in solution at unit activity
Electrons in the metal
24
The Normal Hydrogen Electrode (NHE)
25
Secondary Reference Electrodes

• Reference electrodes of the first kind, a metal
  in equilibrium with a soluble salt

Potential controlled by Cu2 concentration
26
Secondary Reference Electrodes

• Reference electrodes of the second kind, a metal
  in equilibrium with a sparingly soluble salt and
  a solution containing anions of the salt

Ag concentration controls equilibrium potential
Chloride concentration controls Ag concentration
AgCl- const
27
The Ag/AgCl Electrode
28
Potentials of Common Reference Electrodes
29
Practical Potential Measurement

30
Potential Measurement Requirements - Input
Resistance

• High input resistance to minimize errors due to
  source resistance.
• For most corrosion work 107 ohm is sufficient,
  but for high resistance systems (paints, passive
  metals etc.) 109 ohm or more may be better.

31
Potential Measurement Requirements - Frequency
Response

• Frequency response (ability to detect rapid
  changes). Often not important for corrosion
  measurements.
• Measurements at around 1 Hz are quite easy
• Measurements above 1kHz are rather more difficult
• Measurements at around 50 Hz are difficult (due
  to mains frequency interference).

32
Potential Measurement Requirements - Resolution

• Resolution is the ability to detect small changes
  in a large value

33
Potential Measurement Requirements - Resolution

• Resolution is the ability to detect small changes
  in a large value
• for most corrosion measurements 1 mV is adequate

34
Potential Measurement Requirements - Resolution

• Resolution is the ability to detect small changes
  in a large value
• for most corrosion measurements 1 mV is adequate
• for electrochemical noise and similar studies,
  1mV may be necessary

35
Potential Measurement Requirements - Sensitivity

• Resolution is the ability to detect small changes
  in a large value

• Sensitivity is the ability to measure small
  values
• e.g. it is relatively easy to obtain a
  sensitivity of 1 mV when measuring 1 mV, but it
  is very difficult to obtain a resolution of 1 mV
  when measuring a 10 V signal
• not usually a problem for corrosion measurements

36
Potential Measurement Requirements - Precision

• Resolution is the ability to detect small changes
  in a large value
• Sensitivity is the ability to measure small
  values
• Precision or accuracy is the ability to measure
  the true value

37
Potential Measurement Methods

• Analogue meter (moving coil)
• low impedance (typically 20 kohm/V)
• poor frequency response (1 Hz)
• low sensitivity (1 mV)
• low resolution (1)
• low precision (3)

38
Potential Measurement Methods

• Analogue meter (electronic)
• high impedance (typically 10 Mohm)
• poor frequency response (1 Hz)
• possibly high sensitivity (1mV)
• low resolution (1)
• low precision (3)

39
Potential Measurement Methods

• Digital meter
• high impedance (typically 10 Mohm or more)
• poor frequency response (around 3 Hz)
• high sensitivity (10 mV to 100 nV)
• high resolution (0.1 to 0.0001)
• high precision (0.1 to 0.0001)

40
Potential Measurement Methods

• Electrometer (digital)
• very high impedance (1014 ohm)
• poor frequency response (lt1 Hz)
• high sensitivity (1 mV to 100 nV)
• high resolution (0.1 to 0.001)
• high precision (0.1 to 0.001)

41
Potential Measurement Methods

• Chart recorder
• impedance depends on instrument (from 103 to 107
  ohm)
• moderate frequency response (10 Hz)
• moderate sensitivity (10mV)
• moderate resolution (0.1)
• moderate precision (0.1)

42
Potential Measurement Methods

• Oscilloscope
• high impedance (106 to 107 ohm)
• high frequency response (10 MHz or more)
• moderate sensitivity (100mV)
• poor resolution (1)
• poor precision (1)

43
Potential Measurement Methods

• Computer data acquisition
• high impedance (107 ohm)
• variable frequency response (10 Hz to 1 MHz or
  more)
• moderate to good sensitivity (10 mV)
• moderate to good resolution (0.5 to 0.01)
• moderate to good precision (0.5 to 0.01)
• facilitates subsequent plotting and analysis

44
Practical Current Measurement

45
Current Measurement Requirements - Input
Resistance

• Low input resistance to minimize errors due to
  voltage drop across measuring device.
• For most corrosion work 1 mV voltage drop will
  have little effect.
• A wide dynamic range (ratio of largest current to
  smallest current) is required for many corrosion
  measurements.

46
Current Measurement Methods

• Analogue meter (moving coil)
• usually poor input resistance ( 75 mV drop at
  full scale)
• poor frequency response (around 1 Hz)
• low resolution (around 1)
• low precision (around 3)
• dynamic range acceptable using range switching

47
Current Measurement Methods

• Analogue meter (electronic)
• usually poor input resistance (100 mV drop at
  full scale)
• poor frequency response (around 1 Hz)
• low resolution (around 1)
• low precision (around 3)
• dynamic range acceptable using range switching

48
Current Measurement Methods

• Digital multimeter
• often poor input impedance (100 mV drop at full
  scale)
• poor frequency response (around 3 Hz)
• high resolution (0.1 to 0.0001)
• high precision (0.1 to 0.0001)
• often poor sensitivity (100 mA to 1 mA)
• dynamic range acceptable using autoranging

49
Current Measurement Methods

• Electrometer (digital)
• essentially zero input impedance
• poor frequency response (lt1 Hz)
• high resolution (0.1 to 0.001)
• high precision (0.1 to 0.001)
• good dynamic range using range switching or
  autoranging

50
Current Measurement Methods

• Chart recorder
• resistor used to convert current to voltage,
  hence voltage drop depends on sensitivity
• moderate frequency response (10 Hz)
• moderate resolution (0.1)
• moderate precision (0.1)
• acceptable dynamic range providing range
  switching is used

51
Current Measurement Methods

• Oscilloscope
• resistor used to convert current to voltage,
  hence voltage drop depends on sensitivity
• high frequency response (10 MHz or more)
• poor resolution (1)
• poor precision (1)
• poor dynamic range

52
Current Measurement Methods

• Computer data acquisition
• resistor used to convert current to voltage,
  hence voltage drop depends on sensitivity
• variable frequency response (10 Hz to 1 MHz or
  more)
• moderate to good resolution (0.5 to 0.01)
• moderate to good precision (0.5 to 0.01)
• dynamic range often limited
• facilitates subsequent plotting and analysis