Polarization

What We Already KnowThermodynamics the equilibrium between metals and their environmentCorrosion tendency of metalsQualitative picture of what can happen at a given pH and potential

ButConsiderations 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

Let us look at a Zn-H CellThe 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 reactionThe 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 beforeHowever, 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)

So We Also Need to Know

Electrode kinetics to predict the corrosion rates for the actual conditions

Polarization and OverpotentialElectrode 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

Polarization and OverpotentialEquilibrium potential for cathodic reaction = Eoc

Equilibrium potential for anodic reaction = Eoa

Real potential = E

Cathodic Overpotential c = E Eoc < 0

anodic Overpotential a = E Eoa > 0

The Polarized Cell

Exchange Current DensityAt 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 rateThe 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

How Polarization is Measured

Causes of PolarizationDepending on the type of resistance that limits the reaction rate, we are talking about three different kinds of polarizationactivation polarizationconcentration polarization and resistance (ohmic) polarization or IR Drop

Activation PolarizationWhen current flows through the anode and the cathode electrodes, their shift in potential is partly because of activation polarizationAn electrochemical reaction may consist of several stepsThe slowest step determines the rate of the reaction which requires activation energy to proceedSubsequent shift in potential or polarization is termed activation polarizationMost important example is that of hydrogen ion reduction at a cathode, H+ + e- H2, the polarization is termed as hydrogen overpotential

Hydrogen OverpotentialHydrogen 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

Tafel EquationActivation polarization () increases with current density in accord with Tafel equation:

The Tafel constant is given by:

OverpotentialValues

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

Concentration Polarization: reduction of oxygen

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

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

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

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

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 1 , in absence of external current, is given by the Nernst equation:

Contd..When current flows, copper is deposited on the electrode, thereby decreasing surface concentration of copper ions to an activity (Cu2+ )s . The potential 2 of the electrode becomes:

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, 2 1 , is the concentration polarization , equal to:

Contd..

IR DropWhen 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 =

Combined PolarizationTotal polarization of an electrode is the sum of the individual contributions,

If neglect IR drop or resistance polarization is neglected then:

Combined Polarization

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

Combined PolarizationDuring 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:

Anodic and Cathodic Combined Polarization

Polarization Data and Rates of CorrosionThe 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

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 >> i0 for 2H+ + 2e- H2 Tafel equation can be used for cathodic polarization

Contd..Stern-Gray Equation:

The Area EffectUsually 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 sitesIn a galvanic cell, the anode/cathode area ratio is an important factor for severity of corrosion attackA large cathode causes severe attack on a small anodeIf 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.

The Area EffectCopper plates with steel rivets in seawaterSteel rivets severely attackedLarge cathode/small anodeSteel plates with copper rivets in seawaterTolerable corrosion of steel plateSmall cathode/large anode

Influence of Polarization on Corrosion RateLead immersed in H2SO4Magnesium exposed to natural watersIron immersed in a chromate solutionZinc in H2SO4Iron exposed to natural watersPorous insulating covering a metal surface

B: Working ElectrodeL: Luggin CapillaryR: Reference ElectrodeD: Auxiliary ElectrodePronounced activation polarization also occurs with discharge of OH at ananode 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