1. Polarization

2. What We Already Know… Thermodynamics –
the equilibrium between metals and their environment
Corrosion tendency of metals
Qualitative picture of what can happen at a given pH and potential

3. But…
Considerations of equilibrium are irrelevant to the study of corrosion
Some metals with pronounced tendency to react (such as aluminum) react so slowly that they meet the requirements of a structural metal

4. Let us look at a Zn-H Cell
The Zn electrode moves away from equilibrium by the removal of negative charges from the Zn plate and positive ions are released from the Zn plate to the liquid (a)
Zn is dissolved at the same rate as electrons are transported to the Pt plate, where they are consumed in the hydrogen reaction
The same cell process can be totally obtained on a Zn plate submerged in a solution containing hydrogen ions and Zn ions (b)
The reactions are accompanied by the same changes in free enthalpy and have the same equilibrium potentials as before
However, there is a higher resistance against the hydrogen reaction on the Zn plate than on Pt, and thus the reaction rate will be lower on the Zn surface
(a)
(b)

5. So We Also Need to Know …
Electrode kinetics to predict the corrosion rates for the actual conditions

6. Polarization and Overpotential
Electrode reactions are assumed to induce deviations from equilibrium due to the passage of an electrical current through an electrochemical cell causing a change in the electrode potential. This electrochemical phenomenon is referred to as polarization.
The deviation from equilibrium causes an electrical potential difference between the polarized and the equilibrium (unpolarized) electrode potential known as overpotential

7. Polarization and Overpotential
Equilibrium potential for cathodic reaction = Eoc
Equilibrium potential for anodic reaction = Eoa
Real potential = E
Cathodic Overpotential ηc = E – Eoc < 0
anodic Overpotential ηa = E – Eoa > 0

8. The Polarized Cell

9. Exchange Current Density
At the equilibrium potential of a reaction, a reduction and an oxidation reaction occur, both at the same rate.
For example, on the Zn electrode, Zn ions are released from the metal and discharged on the metal at the same rate
The reaction rate in each direction can also be expressed by the transport rate of electric charges, i.e. by current or current density, called, respectively, exchange current, Io, and (more frequently used) exchange current density, io.
The net reaction rate and net current density are zero

10. How Polarization is Measured
B: Working Electrode
L: Luggin Capillary
R: Reference Electrode
D: Auxiliary Electrode

11. Causes of Polarization
Depending on the type of resistance that limits the reaction rate, we are talking about three different kinds of polarization
activation polarization
concentration polarization and
resistance (ohmic) polarization or IR Drop

12. Activation Polarization
When current flows through the anode and the cathode electrodes, their shift in potential is partly because of activation polarization
An electrochemical reaction may consist of several steps
The slowest step determines the rate of the reaction which requires activation energy to proceed
Subsequent shift in potential or polarization is termed activation polarization
Most important example is that of hydrogen ion reduction at a cathode, H+ + e- → ½ H2, the polarization is termed as hydrogen overpotential

13. Hydrogen Overpotential
Hydrogen evaluation at a platinum electrode:
H+ + e- → Hads
2Hads → H2
Step 2 is rate limiting step and its rate determines the value of hydrogen overpotential on platinum

14. Tafel Equation
Activation polarization (η) increases with current density in accord with Tafel equation:
The Tafel constant is given by:

15. Overpotential Values
Pronounced activation polarization also occurs with discharge of OH − at an
anode accompanied by oxygen evolution:
The polarization corresponding to this reaction is called oxygen overpotential .
Overpotential may also occur with Cl − or Br − discharge, but the values at a given current density are much smaller than those for O2 or H2 evolution.
Activation polarization is also characteristic of metal - ion deposition or dissolution. The value may be small for nontransition metals, such as silver, copper, and zinc, but it is larger for the transition metals, such as iron, cobalt, nickel, and chromium

16. Concentration Polarization
Sometimes the mass transport within the solution may be rate determining – in such cases we have concentration polarization
Concentration polarization implies either there is a shortage of reactants at the electrode or that an accumulation of reaction product occurs

17. Concentration Polarization: reduction of oxygen

18. Contd.. Fick’s Law: Faraday’s law: Under steady state,
where dn/dt is the mass transport in x direction in mol/cm2s, D is the diffusion coefficient in cm2/s, and c is the concentration in mol/liter
Faraday’s law:
Under steady state,
mass transfer rate = reaction rate

19. Contd..
Maximum transport and reaction rate are attained when C0 approaches zero and the current density approaches the limiting current density:

20. Contd..
The most typical concentration polarization occurs when there is a lack of reactants, and (in corroding systems) therefore most often for reduction reactions
This is the case because reduction usually implies that ions or molecules are transported from the bulk of the liquid to the electrode surface, while for the anodic (dissolution) reaction, mass is transported from the metal, where there is a large reservoir of the actual reactant

21. Contd..
Equations (1) to (4) are valid for uncharged particles, as for instance oxygen molecules
If charged particles are considered migration will occur in addition to the diffusion and the previous equation must be replaced by
where t is the transference number of all ions in solution except the ion getting reduced

22. Overpotential due to concentration polarization
If copper is made cathode in a solution of dilute CuSO4 in which the activity of cupric ion is represented by (Cu+2 ), then the potential φ1 , in absence of external current, is given by the Nernst equation:

23. Contd..
When current flows, copper is deposited on the electrode, thereby decreasing surface concentration of copper ions to an activity (Cu2+ )s . The potential φ2 of the electrode becomes:

24. Contd..
Since (Cu2+ )s is less than (Cu2+ ), the potential of the polarized cathode is less noble, or more active, than in the absence of external current. The difference of potential, φ2 − φ1 , is the concentration polarization , equal to:

25. Contd..

26. IR Drop
When polarization is measured with a potentiometer and a reference electrode-Luggin probe combination, the measured potential includes the potential drop due to the electrolyte resistance and possible film formation on the electrode surface
The drop in potential between the electrode and the tip of Luggin probe equals iR.
If l is the length of the electrode path of cross sectional area s, k is the specific conductivity, and i is the current density then resistance
iR drop in volts =

27. Combined Polarization
Total polarization of an electrode is the sum of the individual contributions,
If neglect IR drop or resistance polarization is neglected then:

28. Combined Polarization
Effect of temperature, concentration and velocity of the aqueous environment on combined polarization is shown in the figure

29. Combined Polarization
During anodic dissolution of a metal concentration polarization is not important:
During reduction reaction at an electrode both types of polarization have to be taken into account:

30. Anodic and Cathodic Combined Polarization

31. Polarization Data and Rates of Corrosion
The corrosion current can be calculated from the corrosion potential and the equilibrium potential if
the equation expressing polarization of the anode or cathode is known, and
if the anode – cathode area ratio can be estimated

32. Contd..
If an active metal M is corroding in a deaerated acid, the metal surface is usually covered with Hads atoms thus acting mostly as cathode. The open circuit potential is pH and if icorr >> i0 for 2H+ + 2e- → H2 Tafel equation can be used for cathodic polarization

33. Contd..
Stern-Gray Equation:

34. The Area Effect
Usually cathodic reactions are slower than anodic reactions
For a cathodic reaction to occur, there must be available sites on the metal surface. Corrosion cells will not work when the cathodic area is too small for surface sites
In a galvanic cell, the anode/cathode area ratio is an important factor for severity of corrosion attack
A large cathode causes severe attack on a small anode
If we cannot avoid situations for galvanic corrosion, we may reduce thinning by making the anode material of large surface area and cathode of smaller area.

35. The Area Effect Copper plates with steel rivets in seawater
Steel rivets severely attacked
Large cathode/small anode
Steel plates with copper rivets in seawater
Tolerable corrosion of steel plate
Small cathode/large anode

36. Influence of Polarization on Corrosion Rate
Zinc in H2SO4
Iron exposed to natural waters
Lead immersed in H2SO4
Magnesium exposed to natural waters
Iron immersed in a chromate solution
Porous insulating covering a metal surface