mechanism of corrosion processes

8/6/2019 Mechanism of Corrosion Processes

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Mechanism of corrosionprocesses

Jacek Bana

University of Science and Technology (AGH-UST)

Faculty of Foundry Engineering

Department of Chemistry and Corrosion of Metals


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Topics

Galvanic elements in corrosion processes

Partial anodic and cathodic reactions

(mechanism and kinetics) Thermodynamics of corrosion processes

(Pourbaix diagrams)

Active, passive and transpassive state ofelectrode surface


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Galvanic elements in corrosion processes (macro-scale)


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Galvanic elements in corrosion processes2eFeFe

2p

Galvanic elements in corrosion processes (micro-scale)

local anode (ferrite matrix)

1/2O2 + H2O+2e 2OH-

local cathode (carbide or graphite inclusion)

2e

Material ESCE ,, V

Electrolytic iron -0.755

Cast iron -0.762

Carbon steel -0.744

graphite +0.372

Fe3C* -0.210

Electrochemical heterogenity of iron alloys


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Galvanic elements in corrosion processes (nano-scale)

Model of crystal surface according to Kossel and Stranski, a atom in the other phase (gas, liquid), b

atom adsorbed on the surface (ad-atom), c metal in step position, d metal in kink site

W. Kossel, Nachr. Ges. Wiss. Gttingen, math.-physic. Kl., 135 (1927) , I.N. Stranski, Z. Physik. Chem. 136, 259 (1928)

anodic position cathodic position


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0)2Zn(11

Structural etching of Zn in CH3CN - 0.1m LiClO

4

Zn(0001)

E1=-0.622V, t1=1800s

E1=-0.142V, t1=3600s


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Electrochemical reactions on the electrode surface

For nonequilibrium conditions the external current ia or ik is flowing across a interface electrode /

electrolyte and new potential E(i) is established on the electrode surface. The difference between the

nonequilibrium potential E(i) and equilibrium potential E(0) has been defined as overvoltage L.

When anodic reaction dominate (Fig.c) the anodic overpotential occurs:

L = E(ia) - E(0)

In the case of domination of cathodic process (Fig.b) we have cathodic overpotential:

L = E(ic) - E(0)

RneORO


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Introduction to kinetcs of eletrochemical reactions

for electrochemical reaction: RneORO

the rate of electrochemical process can be express as a net current (Butler-Volmer equation):

!! ERT

nFcnFkE

RT

nFcnFkiii OR OcRaca

)1(expexp

EERR

net current is a sum of partial anodic and cathodic currents:

ia ic

et equilibrium the net reaction rate is zero: 0 !!! caeq iii

as a result: 0,, iii eqeq

ca !!

exchange current density

Under equilibrium conditions the concentration on the electrode surface is equal to the concentration in

bulk solution:

RR

OO

cc

cc

*

*

!

!

!

! E

RT

nFcnFkE

RT

nFcnFki OR OcRa

)1(exp*exp*

0

EERR


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Introduction to kinetcs of eletrochemical reactions

! eq

eq

n

c

ci

n

c

cii

)1(exp

*exp

*00

EERR

!

cO

O

aR

ROR

c

ci

c

cii

FL

FL

RR

exp*

exp*

00

Substitution of exchange current to the Butler-Volmer equations gives:

T

afel coefficients: na EF !

nc

)1( EF

!

Overvoltage: eqEE!L

If the reactant and product concentrations are uniform in the electrolyte the expression becomes:

!

ca

iii

F

L

F

Lexpexp

00


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Tafel plots, polarization resistance

!

ca

iii

F

L

F

Lexpexp 00

ca

ac

ca

ii

FF

FFL

FF

L

!

!

11lnln

0

iiac

ca

ac

ca lnln0 FF

FF

FF

FFL

!

iogbla s!LTafel equation:

0ln

)(3.2ia

ac

ca

FF

FF

!

)(3.2 ac

cabFF

FF

!

polarization resistance:0i

b

di

dE

Rp!!


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Electrochemical method of corrosion monitoring

korbbi

bb

iE

pi

BR

akkor

ak

E!!!

((

p( )(303,20

Linear polarization method(LPR):

026,0)(303,2!!

ak

ak

bb

bbB

E

iBikor(

(!


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Electrochemicalmethod of corrosion

monitoring

St37

20H12M1F


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Polarization curves

- ,6 - , - , , , , ,6-

-

-

-2

-1

1

k

i i

i

/L

l i

dLE/dlogi=2.303RT/(1-E)nFdLO/dlogi=-2.303RT/EnF

n = 1, E = 0.5, RO= RR=1, T = 298 K, i0 = 10-4 Acm-2

Polarization diagrams of the electrochemical system: RneOR

O


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Partial anodic and cathodic reactions

anodic reaction (degradation process): M M+n + ne (1)

Cathodic reaction: Ox + ne Red (2)

1/2O2 + H2O + 2e 2OH-

H+ + e 1/2H2

Fe+3 + e Fe +2

Cu+2 + e Cu+


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Partial anodic and cathodic reactionsPartial anodic and cathodic reactions (corrosion of zinc in acid media)

e2ZnZn 2 p

2H2e2H p

anodic reaction

cathodic reaction

!

ZnZn

aaRT

nZnki L

Eexp][

!

H

H

ccRT

FHki L

E 2)1(exp][

2

-2 -1 0 1 2

-3,4

-3,2

-3,0

-2,8

-2,6

-2,4

-2,2

-2,0

222 HeH p

eZnZn 22p

log icorr

logi

L

- -

- , 8

- ,

- ,

- ,

,

,

,

,

, 8

L

inet

i

i

i

rr

HeH p

e

22

p

i

L!E - Ecorr


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Pathway of electrode reaction

The investigations of the kinetic of electrochemical processes base on the knowledge of the

relation between current density (rate of the electrochemical process) and overvoltage. The rate of

electrochemical process can be dependent on several factors such as:

exchange of the charge (activation control)

transport (diffusion control, conductivity - migration control)

chemical reaction proceeding in the surface layersurface phenomena (adsorption control, crystallisation control).

Each of these factors is connected with the energy barrier (overvoltage). Thus the overvoltage of the

whole electrochemical process is the sum of the overvoltages of partial processes:

L =Lactivation + Ldiffusion +Lohmic +Lchemical +Lcrystallisation

The rate of total electrochemical process is determined by the overvoltage of the must slowest partial

reaction.


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Diffusion control of corrosion process, transport of oxidant to metal surface

2eFeFe2

p

anodic reaction:

Corrosion ofiron in neutralaqueous solutions

cathodic reaction: p 2 H2eH 2221

chemical reactions (precipitation of corrosion products):

32221

2 2 e(O )OO2 e(O ) qp

2

2 e(O )2Oe p

1

2

!! HOeffLcorr cnFDii


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Diffusion control of corrosion process, transport of oxidant to metal

surface

Corrosion rate of iron disc electrode in 0.2M Na2SO4, pH=2.7, 200C.

The effect of rotation rate and oxygen concentration

2/1

O

6/13/2

2620.0 [R cni eff

!

V.G.Levich, Physicochemical Hydrodynamics,

Prentice Hall, Englewood-Cliffs, N.Y. 1962


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Thermodynamics of corrosion processes (Pourbaix diagrams)


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Pourbaix Diagram

Al3+ + 3e- = AlE0 = -1, 3 0,0197 log [Al

3+]

Al+ 3H2O = Al(OH)3 + 3H+ + 3e-

E0 = -1,550 0.591 pH

AlO2- + 4H+ + 3e- = Al + 2H2O

E0 = -1,2 2 + 0,0788 pH 0,0192 log[AlO2-]

Al3+ + 3H2O = Al(OH3 + 3H+

log [Al3+] = 5,70 3 pHAl(OH)3+ OH

- = Al(OH)4-

log [Al(OH)4-] = -14, 0 + pH

O2 + 4H+ + 4e-= 2H2O

E0= 1,23 0,059pH

2H+ + 2e- = H2E0 = 0,059pH


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Effect of pH on corrosion resistance of aluminum in aqueous chloride

solutions

pH

3%NaCl+HCl - 3%NaCl+NaOH

Vkormg/cm2d

EH/ V


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Effect of pH on corrosion resistance of zinc in aqueous solutions

Znp Zn2+ + 2e E0= - 0.7 3 + 0.0295log[Zn2+]

Zn + H2Op ZnO + 2H+ + 2e- E0= -0.439 - 0.0591pH

Zn + 2H2Op ZnO22- + 4H+ + 2e- E0=0.441 - 0.1182pH + 0.0295log[ZnO2

-]

ZnO + 2H+ p Zn2+ + H2O log[Zn2+] = 10.9 - 2pH

ZnO + H2Op ZnO22- +2H+ log[ZnO2

2-] = -28.48 + 2pH

Samples of carbon steel with zinc coating

after neutral salt spray test (900h)


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Potential pH diagram for iron in H2O, 250C


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General bibliography

D. Landold, Corrosion and Surface Chemistry of Metals,ed. EPFL Press, 2007

Corrosion Mechanisms in Theory and Practice,

ed. by Ph. Marcus, second ed.,Marcel DekkerInc. N. York 2002

Bard-Stratmann Encyclopedia of Electrochemistry Vol 4,Corrosion and Oxide Films,ed. Viley-VCH GmbH&co.KGaAWeinheim, 2003

Corrosion Mechanisms, ed by F. Mansfeld, Marcel DekkerInc. N. York 1987

P.R. Roberge, Handbook of Corrosion Engineering,McGraw-Hill, N. York 2000

mechanism of corrosion processes

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