Polarization

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Polarization
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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

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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

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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)
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So We Also Need to Know

• Electrode kinetics to predict the corrosion rates
  for the actual conditions

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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

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Polarization and Overpotential
Equilibrium potential for cathodic reaction
Eoc Equilibrium potential for anodic reaction
Eoa Real potential E Cathodic Overpotential
?c E Eoc lt 0 anodic Overpotential ?a E
Eoa gt 0
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The Polarized Cell
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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

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How Polarization is Measured
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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

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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

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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

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Tafel Equation

• Activation polarization (?) increases with
  current density in accord with Tafel equation
• The Tafel constant is given by

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OverpotentialValues
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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

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Concentration Polarization reduction of oxygen
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Contd..

• Ficks Law
• 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
• Faradays law
• Under steady state,
• mass transfer rate reaction rate

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Contd..

• Maximum transport and reaction rate are attained
  when C0 approaches zero and the current density
  approaches the limiting current density

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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

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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

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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 (Cu2 ), then the potential f1 ,
  in absence of external current, is given by the
  Nernst equation

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Contd..

• When current flows, copper is deposited on the
  electrode, thereby decreasing surface
  concentration of copper ions to an activity (Cu2
  )s . The potential f2 of the electrode becomes

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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, f2 - f1 ,
  is the concentration polarization , equal to

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Contd..
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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

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Combined Polarization

• Total polarization of an electrode is the sum of
  the individual contributions,
• If neglect IR drop or resistance polarization is
  neglected then

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Combined Polarization

• Effect of temperature, concentration and velocity
  of the aqueous environment on combined
  polarization is shown in the figure

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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

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Anodic and Cathodic Combined Polarization
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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

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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 -0.059pH and if
  icorr gtgt i0 for 2H 2e- ? H2 Tafel equation
  can be used for cathodic polarization

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Contd..

• Stern-Gray Equation

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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.

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