CORROSSIONBy Sruthi sudhakar PG chemistry batch 2016-18Sir Syed College,Kannur,Kerala

Corrossion Destructive attack of a metal

by chemical or electrochemical reaction with

its environment

Thermodynamics of corrossionEquilibrium between metal and environment Corrossion tendency of metalQualitative picture of what can happen at a

given ph and potential

Practical situations

• Rate at which corrossion occur• Some metals like aluminium

has got tendency to react so slowly that they meet requirement of structural metal

Kinetics is closely related to polarization

polarization• When a net current flows through an

electrode,its not in equilibrium• The measured potential of such an

electrode is dependent on magnitude of external current

• The process where potential change caused by net current from the theoretical value of potential is polarization

Terms to remember• 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

Causes of polarization

1. Concentration polarization

2. Activation polarization

3. IR drop

Concentration polarization• Sometimes the mass transport within

the solution may be rate determining – in such cases we have concentration polarization

• Also called diffusion polarization• Concentration polarization implies either

there is a shortage of reactants at the electrode or that an accumulation of reaction product occurs

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

)log(Cu320.337)(Cu

1log320.337 221

nFRT.

nFRT.

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:

S2

S22 )log(Cu320.337

)(Cu1log320.337

nFRT.

nFRT.

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:

)s(Cu)(Cu

log2

0592.02

2

12

Larger the current,smaller the surface concentration of the ion and larger the polarisation.

Infinite concentration polarization is approached when the surface concentration ,(Cu2+)s is zero.

The corresponding current density is called limiting current density

in practical situations polarisation never reaches ∞ ,another electrode rxn gets established For example in Cu deposition moves to that for hydrogen evolution2H+ + 2 e- → H2

LIMITING CURRENT DENSITY• Fick’s 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

• Faraday’s law:

• Under steady state, mass transfer rate = reaction rate

(1) 10 3dxdcD

dtdn

(2) nFi

AnFI

Atw

• Maximum transport and reaction rate are attained when C0 approaches zero and the current density approaches the limiting current density: (3) 10 3

CDnFiL

Equations (1) to (3) 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

(4) 10 3tCDnFiL

o D is the diffusion coefficient for the ion being reduced,

o n and F have their usual signifi cance,o δ is the thickness of the stagnant layer of

electrolyte next to the electrode surface (about 0.05 cm in an unstirred solution),

o t is the transference number of all ions in solution except the ion being reduced

o c is the concentration of diffusing ion in moles/ liter.

If i is the applied current we can show that

iii

nFRT

DzFtiCu

DzFtiCuCu

L

L

S

ln

)(

)()(

12Conc

2

22

Dependence of concentration polarization at cathode on applied current density

Activation PolarizationWhen 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

Due to current flow across electrode solution interface

Most important example is that of hydrogen ion reduction at a cathode, H+ + e- → ½ H2, the polarization is termed as hydrogen overpotential

• Hydrogen evolution 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

• The controlling step varies with metal current density and environment

Tafel Equationo Activation polarization (η) increases

with current density in accord with Tafel equation:

o Larger the exchange current density smaller tafel constant and overpotential

o The Tafel constant is given by:

oiilog

nFRTβ

α3.2

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

Pronounced activation polarization also occurs with discharge of OH − at an

anode accompanied by oxygen evolution:

Activation polarization is also characteristic of metal - ion deposition or dissolution. The value may be small for non transition metals, such as silver, copper, and zinc, but it is larger for the transition metals, such as iron, cobalt, nickel, and chromium

The anion associated with the metal ion influences metal overpotential values more than in the case of hydrogen overpotential.

The controlling step in the reaction is not known precisely, but, in some cases, it is probably a slow rate of hydration of the metal ion as it leaves the metal lattice, or dehydration of the hydrated ion as it enters the lattice.

• At the equilibrium potential for the hydrogen electrode ( − 0.059pH ,overpotential is zero. At applied current density, i1, it is given by η , the difference between measured and equilibrium potentials.

IR Drop When polarization is measured with a

potentiometer and a reference electrode combination, the measured potential includes the potential drop due to the electrolyte resistance and possible film formation on the electrode surface

It is the ohmic potential

The drop in potential between the electrode and the tip of working electrode equals iR.

If I is current density, and R , equal to l / κ , represents the value in ohms of the resistance path of length l cm and specifi c conductivity κ . The product, IR , decays simultaneously with shutting off the current, whereas concentration polarization and activation polarization usually decay at measurable rates.

kil

CALCULATION OF IR Drop

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 = klR

kil

kil

Combined PolarizationA. Total polarization of an electrode is

the sum of the individual contributions,

B. If neglect IR drop or resistance polarization is neglected then:

rcaT ηηηη

caT ηηη

INFLUENCE OF POLARIZATION ON CORROSSION RATE

The corrosion current can be calculated from the corrosion potential and the equilibrium potential if 1. The equation expressing

polarization of the anode or cathode is known, and

2. If the anode – cathode area ratio can be estimated

• When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode

• When polarization occurs mostly at the cathode, the corrosion rate is said to be cathodically controlled . The corrosion potential is then near the thermodynamic anode potential.

• Resistance control occurs when the electrolyte resistance is so high that the resultant current is not sufficient to appreciably polarize anodes or cathodes

• The corrosion current is then controlled by the IR drop through the electrolyte in pores of the coating. It is common for polarization to occur in some degree at both anodes and cathodes. This situation is described as mixed control .

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

For all metals and alloys in any aqueous environment, cathodic polarization always reduce the corrosion rate. Cathodic protection is essentially the application of a cathodic polarization to a corroding system.

For a non-passive system (e.g. steel in seawater), anodic polarization always increases the corrosion rate. For systems showing active-to-passive transition, anodic polarization will increase the corrosion rate initially and then cause a drastic reduction in the corrosion rate. Anodic protection is essentially the application of anodic polarization to a corroding system.

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 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.

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

ELECTROCHEMICAL MECHANISM OF CORROSSION BY WAGNER AND TRAUD

Measured the corrosion rate of a dilute zinc amalgam in an acid calcium chloride mixture and cathodic polarization of mercury in the same electrolyte.

The current density equivalent to the corrosion rate was found to correspond to the current density necessary to polarize mercury to the corrosion potential of the zinc amalgam

Mercury atoms of the amalgam composing the majority of the surface apparently act as cathodes and zinc atoms act as anodes of corrosion cells.

The amalgam polarizes anodically very little and limit the corrosion rate of amalgams in nonoxidizing acids.

A piece of platinum coupled to the amalgam considerably increases the rate of corrosion because hydrogen is more readily evolved from a low - overpotential cathode at the operating emf of the zinc – hydrogen electrode cell.

The very low corrosion rate and the absence of appreciable anodic polarization - amalgams in corresponding metal salt solutions exhibit corrosion potentials closely approaching the reversible potential of the alloyed component

The corrosion potential of cadmium amalgam in cdso 4 solution is closer to the thermodynamic value for cd → cd 2+ + 2 e−

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