Download - Corrossion focus on polarization
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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