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10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1 10

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

 

   E   H

   +   /   H   2 ,

   V

Log [H+], M

])([2

059.02/

    H  Log  E   H  H 

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If the cathodic reagent at the corrosion site (e.g., dissolved O2 in

the O2 reduction) is in short supply, mass transfer of the reagent

can become rate limiting.

  e   l  e  c   t  r  o   d  e 

Csurf = Cbulk

distant from electrode, x 

under

activation control

no concentration

gradient

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conc.  – distance profile at electrodes (mass transfer control)

  e   l  e  c   t  r  o   d  e

 

Csurf

Cbulk

@anode

diffusion layer

Nernst layer

ordiffusion layer

diffusion layer thickness (δ)10 – 300 µm

can be changed by stirring

& changing bulk conc.  e   l  e  c   t  r  o   d  e

 

Csurf =0 

Cbulk

@cathodediffusion layer

Csurf ≠0 

Csurf  = conc. near

electrode surfacedistant from electrode, x 

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Concentration profile builds up as redox process is

faster than the diffusion of oxidant and reducing species

This cause 

@ anode

1. local saturation of metal ion at anode, which leads to

precipitation of corrosion products and salt films,which slow down the metal dissolution rate and hence

the anodic current density

@ cathode

2. depletion of oxidizing species (e.g. O2) at cathodewhich slow down the reduction rate at cathode and

hence the cathodic current density decreases

hence a limiting current density is seen at high η 

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log i

E or η 

log iL

Ecorr

ηa

ηaηc

ηaηc∞ 

1

23

i L

@ cathode iL = limiting current densityor limiting diffusion current

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iL = limiting current densityor limiting diffusion current

log i

E or η 

log iL

Ecorr

ηa

ηaηc

ηa ηc∞ 

1

23

i L

@ anode

Tafel eqn line

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@point  – 1

for a overpotential there is no limitation of reagent

(e.g. O2

) supply or accumulation of cations

Csurf  = Cbulkcurrent remains in Tafel region & reaction is activation

controlled  (only charge transfer controls the rate)

@point  – 2

for a large overpotential there is short supply of reagent

(e.g. O2) or accumulation of cations

Csurf  < Cbulkcurrent less than that expected on the basis of

activation control

Rate is partly activation & partly diffusion controlled

ηtotal = ηa + ηc

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under limiting condition, Csurf  = 0

bulk  L  C nFD

ii 

0

max 

C bulk = concentration of the solution you are using

concentration polarization means that the current densitycannot increase beyond i L no matter how large is the overvoltage

 

  

 

 L

cii

nF  RT  1ln 

expression for

concentration

overpotential, ηc

at very large overpotential, i  = i L, ηc  ∞

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cathode reaction is under concentration polarization

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Effect of increasing mass transport rate

(e.g., by stirring the solution surrounding a corroding surface)

increasing mass transport decreases concentration overpotentialEcorr  shifts noble direction (i.e. increases)

shows a cathodic reacn

.under mass transport

mass transport increases

from pt.1 to pt.3

ηc,1> ηc,2>ηc,3

If anodic reaction were mass-transfer controlled (difficulty of metal

ions diffusing away), improved stirring would decrease E corr . HOW? 

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Ohmic drop or resistance polarization

due to a voltage drop (iR ) in the solution results from the

passage of current (i ) through the resistive solution (R )

  

d iiRV   

For a planar electrode with a current density I, the voltage

drop ΔV is given by 

R = electrolyte resistance (ohm)

κ = the specific conductivity of the electrolyte (ohm-1 cm-1)d = the distance from the electrode surface (cm)

 d  Rwhere   ,

Example, for i = 1 A /cm2 and κ = 22 Ω−1cm−1 for an electrolyte one

obtains for a planar electrode at a distance d = 0.5 cm, a value of

 ΔV = 0.023 V = 23 mV (what will happen in i becomes 10A /cm2 ??)

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i 1

i 2i 3

R 1

R 2

R 3E c

E a

i  R1 < R2 < R3

i1 > i2 > i3

ohmic polarization effect in galvanic couples

as Rsolution increases the driving force for

corrosion decreases

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Ohmic drop or resistance polarization depends on

1. Electrolyte conductivity2. Distance :

between two corrosion sites

between electrode and reference electrode

3. Current density

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Total polarization iRconact T          

iRi

i

nF 

 RT 

i

ib

 Lcorr 

cccathodeT   

 

  

 

 

  

    1lnlog, 

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Passivity

Our modern industrial society is built upon the reactive metals

How is it possible ?

reactive metals (Fe, Ni, Cr, Al, Ti, Zr, etc) exhibit extraordinary

kinetic stability in oxidizing environments

How?

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Passivity

Passivity is a state of low corrosion rate due to the presence

of a thin surface oxide film, formed naturally or under a highanodic (+ve) potential (or high oxidizing condition)

 passive film can form naturally and also by applying highanodic potential

 passive metals are thermodynamically unstable, but their

low corrosion is due to oxide film formation

e.g. Fe, Cr, Ni, Mn, Al, Ti, W etc Au, Ag, Pt are passive but not due to formation of passive oxide

layer  – they are thermodynamically stable

 thickness of passive layer is in range of 1 – 10 nm

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Fe reaction in nitric acid

Passivity [ Faraday's paradox ]

conc. HNO3

no corrosion

dilute HNO3

freely corrodes

with H2 evolution

conc. HNO3

corrosion if scratched

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Fe react in nitric acid

Passivity

Faraday's paradox

D. D. Macdonald, Pure Appl. Chem., Vol. 71, No. 6, pp. 951-978, 1999

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C. Wagner: Corrosion Science 5, 751 (1965)

active vs passive metals

active metal

active – passive metal

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Polarization of passive metals

DA

B

C

F H J

IGE

Epass

active

active-

passive

transition transpassivepassive

i crit

i p  

   l  o  g    i 

E

oxide filmcontinue to grow

P l i ti f i t l

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Polarization of passive metals

AB : low anodic potential, active region; metal corrodes with current following

Tafel behavior (rise exponential)

@ B: corrosion current density reach maximum, icrit  – critical passivation current

density; passivation starts. Potential at that point is called passivation potential (Epass)

BC: active – passive transition, passive film continues to form and metal dissolution is

gradually decreasing

@ C : passivation complete, current assumes a low value and potential at that point is

called Flade potential

CD : passive region – increasing potential doesn’t change the current andcurrent density at this region called passivation current (ip) density. One can use this

current density to calculate depth of passivation

@ D /F/H : transpassive region; current starts to increase; current rise can be due to

pitting of passive layer by oxidising agent, oxidative dissolution of

passive layer or O2 evolution.DE : oxide film brakedown due to pitting in presence of aggressive ions (Cl-); current

rises as anodic potential increases since pits grow and more pits form

FG : oxidative dissolution of oxide film to produce a soluble anion, and passivity fails

HI : the increasing rate of oxygen evolution (can only occur if the oxide film is an

electron conductor); Passivity here is not disturbed

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when passivity is attended:

1. Corrosion potential of metals changes in the +ve direction2. The corrosion rates of metals in the passive state

decreases drastically, usually a diminution of ∼ 1 million

times

3. Passivity is a metastable state, i.e. it is disturb by means

of scratch, grinding, or changes to the outer conditions:

temperature, concentration of aggressive variables, and

agitation.

a high current density may be required to cause passivation

(> icrit), a small current density (ip) is required to maintain it

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criteria of passivation

ia > icrit  |  low Epass  |  Epass < Ecorr  < Etrans 

321

log icorr,1

log icorr,2

log icorr,3

Ecorr,1Ecorr,2 Ecorr,3Epass

log ia > log icrit 

1,2 & 3 are three different cathodic reactions with increasing power of oxidation or

same oxidizing agent with increasing conc. (hence increasing strength)

1 < 2 < 3

icorr,3 = ipass 

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which alloy would you select?

alloy1 alloy2

cathode recn.1log I

E

cathode recn.2

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effect of aeration

log I

E

increasing

aeration

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Fe corrosion in aerated water

- pH dependence

below pH 4

H2 evolution

pH 4 – 10

limited by O2 diffusion

oxide film soluble,not protective

above pH 10 

protectiveoxide film formation

NACE

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diffusion control effect

log I

E

increasing

agitation

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Elements forming passive oxide film

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Passivity of Fe & Ti in acid

Fe: icrit for Fe of the order of 1 A/cm2

Ti: very small critical current densities (icrit lower than 1 µA/cm2)

even in very acidic condition (extremely stable passivation)

Fe Ti

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Iron passivation in an acetic acid/ sodium acetate

solution – Effect of water

No formation of oxide films

an absence of water

Oxygen reduction is not

sufficient to form a passive

film

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Why stainless steel with Cr ≥ 12 wt.% passivates easily

Revie & Uhlig, pg-101

   i  c  r   i   t

polarization diagram of Fe  – Cr alloys

in O2 saturated 3% Na2SO4 solution

Icrit of Fe  – Cr alloys in deaerated

3% Na2SO4 solution

P i ti f C Ni St l

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Passivation of Cr – Ni Steels

Type 304 SS 18% Cr & 8% Ni 

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effect of different alloying element on passive

behavior of steel

E

log I

ThyssenKrupp Fortinox S.A. 

ic

ff t f t t & Cl i b h i f t l

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effect of temperature & Cl- conc. on passive behavior of steel

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How to passivate metals and alloys:

1. Anodization: e.g. Al

2. Immersion of metals in specific media:

e.g Fe in aqueous solutions of HNO3 (>70%), H2SO4(>96%), Na2SO4 (1 M)

3. presence of oxidizers : O2 in soln, P2O84-, NO2-, WO42-

, CrO4- 

4. addition of noble metals: Pd, Pt, Rh, Ir to Cr or Ti