04-Electrochemical Kinetics of Corrosion

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ELECTROCHEMICAL KINETICS OF CORROSION

Dr. Ramazan Kahraman

Chemical Engineering DepartmentKing Fahd University of Petroleum & Minerals

Dhahran, Saudi Arabia

Reading Material: Chapter 3 in“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.

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Faraday’s Law

Charge is related to mass of material reacted in an electrochemical reaction:

M → Mn+ + ne-

One mol of metal

Reacts

To produce one mol of metal ion and

n mols of electrons

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Faraday’s Law (Cont.)

One mole of metal (MW g) contains Avogadro’s number (6×1023) of metal atomsHence each mole of metal will produce n times that many number of electronsCharge on the electron is 1.6 × 10-19 C (coulomb)Charge of one mol of electrons (6×1023 electrons) will then be 96500 C Hence one mole of metal will produce a charge ofn × 96500 C96500 C/equivalent is known as Faraday’s constant(also in units of J/V⋅equivalent)Conversions: 1 A (ampere) = 1 C/s, 1 J = 1 C⋅V

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Faraday’s Law (Cont.)

(g/mole) metal of weight (atomic)molecular (g) oxidized metal of mass

metal of molper ed transferrelectrons) of (mols sequivalent ofnumber mol)ntC/equivale (96500constant s Faraday'

C/s)1 A (1 A) (amperes,current C) (coulomb, chargewhere

===

⋅===

=

==

MmnFIQ

FnMtI

FnMQm

I/AiA

FnMi

AtnF/MtI

Atmr,

density,current area surface where

ratecorrosion

==

===

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How Fast will Corrosion Occur?

Corrosion kinetics–Concerned with the rates of corrosion reactions

Mixed potential theory:–The corrosion potential will be that potential at which the

sum of all anodic (positive) and cathodic (negative) currents on the electrode is zero

(i.e. ⏐anodic current⏐=⏐cathodic current⏐)

(mixed equilibrium) (not an electrochemical equilibrium)

Polarization–The change in the potential of an electrode

as current flows to or from it

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Corrosion of Zinc in Acid

When zinc is placed in acid the metal will start to dissolve (Zn → Zn2+ + 2e-) and hydrogen will start to be liberated (2H+ + 2e- → H2) according to the potential of the metal

ac

Zn

e - H+

Zn+2

H2H+

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Corrosion of Zinc in Acid

Zn → Zn2+ + 2e-

Rate of Reaction

Elec

troc

hem

ical

Pot

entia

l

2H2H++ + 2e+ 2e-- →→ HH22

At the Corrosion Potential, Ecorr, we have a stable mixed equilibrium

Current densityicorr

As the reaction involves transfer of charge, the rate of reaction may be expressed as a current per unit area, or current density

Then the corrosion rate may be expressed as the corrosion current density, icorr

Ecorr

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Corrosion of Zinc in Acid (Cont.)

Zn → Zn2+ + 2e-

Rate of Reaction

Elec

troc

hem

ical

Pot

entia

l

2H2H++ + 2e+ 2e-- →→ HH22

Current density

If the potential is above the Corrosion Potential, then it will fall due to production of electrons

If the potential is below the Corrosion Potential, then it will rise, due to consumption of electrons

•

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Types of Polarization

Activation PolarizationConcentration PolarizationResistance (IR-drop) Polarization

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Activation Polarization

Result of a slow step in an electrode reaction at the anode or cathode

– For example, when hydrogen evolves at the cathode, the reaction might be considered as:

Step 1: H+ + e- HadsStep 2: Hads + Hads H2Step 3: Sufficient molecules of H2 combine and nucleate

a hydrogen bubble.

Any one of these steps can control the rate of reaction and cause activation polarization.

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Activation Polarization (Cont.)

Example: Hydrogen reduction reaction under activation control.

[“Corrosion Engineering”, Mars Fontana, McGraw-Hill, 1986]

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Tafel’s Law for Activation Polarization

ooo i

ibiibEE logorlog =+= η

ooo i

ibiibEE logorlog =+= η

where E = potential at current iEo = potential at current io (ec for cathodic rxn and ea for anodic rxn)b = Tafel slope ((+) for anodic rxn, (-) for cathodic rxn)η = polarization (E-Eo at current i)

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Tafel’s Law at icorr

ocorr

aao

corraacorr i

ibi

ibeEcorriat logorlog =+= η

ocorr

aao

corraacorr i

ibi


ocorr

cco

corrcccorr i

ibi


ocorr

cco

corrcccorr i

ibi


Anodic Polarization

Cathodic Polarization

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E-log i and Evans Diagrams

Plot E against log |i|, then activation polarization gives a straight line

log |current|

Ele

ctro

de P

oten

tial

Tafel slope expressed as mV per decade of current

mV

log |i2 | - log |i1 |

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E-log i and Evans Diagrams (Cont.)

log |current|

Ele

ctro

de P

oten

tial

Cathodic reaction, Tafel slope is negative

Anodic reaction, Tafel slope is positive

Mixed equilibrium occurs when sum of all currents is zero, i.e. at Ecorr and icorr for the corrosion reaction

Eo and io for the cathodic reaction

Eo and io for the anodic reaction

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Exchange Current Density, io

When a piece of metal is sitting in a solution at its equilibrium potential, this does not mean that the rates of the metal dissolution and reprecipitation reactions are zero.

Instead it implies that the rates of the two reactions are equal.

When the metal dissolution and reprecipitation reactions are in equilibrium, we refer to the (equal and opposite) rates (in terms of current density) of each of the two reactions as the exchange current density, io.

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Polarization

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Effect of Degree of Polarization

As degree of polarization gets higher (i.e. ⏐slope⏐ increases) icorr (corrosion rate) decreases.

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Example (Zinc in Acid)

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Example (Cont.)

Note that the reverse reactions, zinc deposition (Zn2++2e- Zn) and hydrogen oxidation (H2 2H++2e-), do not occur since the corrosion potential lies between 0.0 and -0.762 volt. Zinc deposition (Zn2++2e- Zn) can only occur at potentials more negative than -0.762 V, and hydrogen oxidation (H2 2H++2e-) only occurs at potentials more positive than

0.0 V.

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Concentration PolarizationAdditional polarization (i.e. slowing down of a reaction) caused by depletion / drop in concentration of a reactant at the electrode surface or an excess of the unwanted species at the electrode surface (diffusion controlled polarization)

Concentration polarization is low / insignificant until a limiting current density, iL, is approached (i.e. it becomes effective at high rates approaching iL)

Limiting current density, iL, is the measure of a maximum reaction rate that cannot be exceeded because of a limited diffusion rate of a reactant

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Concentration Polarization (Cont.)

[“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996]

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Example: Concentration polarization during hydrogen reduction.

[“Corrosion Engineering”, Mars Fontana, McGraw-Hill, 1986]

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Effect of Solution Conditions on Concentration Polarization


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⎟⎟⎠

⎞⎜⎜⎝

⎛−=−=

Lii

nFRTEE oconc 1log303.2η ⎟⎟

⎠

⎞⎜⎜⎝

⎛−=−=

Lii

nFRTEE oconc 1log303.2η

Concentration polarization is usually negligible (insignificant)on anodes and usually ignored.

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Combined Polarization

⎟⎟⎠

⎞⎜⎜⎝

⎛−+=+=

Lii

nFRT

iib coc

ccconccactc 1log303.2log,, ηηη ⎟⎟⎠

⎞⎜⎜⎝

⎛−+=+=

Lii

nFRT

iib coc

ccconccactc 1log303.2log,, ηηη

Total Cathodic Polarization

oa

a iibaacta log, ==ηηoa

a iibaacta log, ==ηη

Anodic Polarization

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Combined Polarization (Cont.)


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Oxygen reduction is often affected by concentration polarization

log |current density|

Ele

ctro

de P

oten

tial

Rate of cathodic oxygen reduction without concentration polarization

Rate of cathodic oxygen reduction with concentration polarization

Limiting current density - rate of reaction limited by availability of oxygen at the metal surface

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Resistance Polarization

If there is a resistance between the anode and the cathode in a cell, then the current flowing through that resistance will cause a potential drop given by Ohm’s Law:

V = IR

This is important for paint films and for high resistance solutions

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Resistance Polarization


Ele

ctro

de P

oten

tial

Resistance Polarization causes potential of anode and cathode to differ due to potential drop across solution, hence corrosion current is reduced

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Ele

ctro

de P

oten

tial

Iron in hydrochloric acid

Cathodic hydrogen evolution

Anodic iron dissolution

Sample Polarization Curves

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Ele

ctro

de P

oten

tial

Iron in aerated neutral NaCl solution


Anodic iron dissolution

Cathodic oxygen reduction

Sample Polarization Curves (Cont.)

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Ele

ctro

de P

oten

tial

Iron in sulphuric acid (passivity to be studied later)


Anodic iron dissolution (with active-passive transition)

Oxygen evolution on passive film (or transpassive corrosion as metal is oxidised to a higher oxidation state)

Sample Polarization Curves (Cont.)

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Effect of Oxidizer


Ecorr ↑, icorr ↑, rate of hydrogen evolution ↓

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Effect of Exchange Current Density


The deriving force (difference between the half-cell reaction potentials is much larger for zinc than iron. However, the corrosion rate of zinc, icorr,Zn, is lower than that of iron, icorr,Fe, because of the low exchange current density for hydrogen reduction on zinc compared to iron and the comparatively low exchange current density for zinc dissolution.

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Effect of Dissolved Ion Concentration

Drawing appropriate polarization diagrams, determine the effect of “increasing the concentration of dissolved H+” on “Ecorr” and “icorr” of a metal M corroding to dissolved M+ in a deaerated acid solution under activation control with all other parameters constant.

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Example Problem

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Home Work Problems

Prbs. 1, 6, 7 and 8 of Ch.3

in “Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.

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References

“Principles and Prevention of Corrosion”, Denny Jones, Prentice-Hall, 1996.

“Corrosion Engineering”, Mars Fontana, McGraw-Hill, 1986.

“Corrosion and Corrosion Control”, H. H. Uhlig and R. W. Revie, John Wiley & Sons, 1985.

Web Site of Dr. R. A. (Bob) Cottis.

04-Electrochemical Kinetics of Corrosion

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