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Passivation
generates corrosion protection of most of metals
and alloys used in industrial applications ?
1
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In acidic solutions
strong anodic metal dissolution
In neutral solutions
formation of a thick, porous, hydroxide layer with pooradhesion properties and limited corrosion protection effect
In alkaline solutions (pH > 10)
formation of thin (1-2 nm), adhering Oxi/hydroxide layer
that protects the surface from corrosionPassive layer
What happens with iron as a function of the pH
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Example:
Pourbaix diagram of iron
in water
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A passivation reaction requires:
Metal oxidation to provide the necessary cations for the formation of anoxide layer
Water and removal of protons to provide the hydroxyl or oxide anions
Passivation will be much more likely to occur on metals with low
redox potential (active metals) because of the high oxidationsusceptibility and the dissolution current available
Fundamental aspects of passivation
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Factors improving the ability to obtain passivation:
Water content
Obviously, a certain water amount in a solution is necessary to
provide through splitting enough OH- or O2-
pH of the electrolyte
High pH provides large amount of dissociated OH- cations. Thefirst step of the deprotonation process already occurred andpassivation is accelerated
O2 amount
Reduction of gaseous oxygen also produces OH- and thereforepassivation can be promoted this way
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Active Metal Passive Metal
i
E
i
E
How is a passive behavior evidenced ?
With an electrochemical polarization curve (anodic part is relevant)
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How is a passive behavior evidenced ?
On an anodic polarization curve
Active domain very rapid current increase (charge transfer controlled)
Transition active passive domain order of magnitude decrease of current
Passive domain current in the microampre/cm2 domain , stable surface
active
passive
Transpassive dissolution
OrWater dissociation
log I
ER,a Epass Eact Ed E
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Important parameters
Passivation potential Epass : potential where passivation can take place
Icrit: maximal current density reached in the active state and inducing passivity
Passive current ip: indication of the stability of the surface (usually 10-6 Acm-2)
Passivity domain: between Epass and Ed : it has to be very large to insure protection ofa metal
Depassivation potential (Ed) can be of different nature Current increase can also simplybe dissociation of water
active passive Transpassive
dissolution
OrWater
dissociation
Log I
ER,a Epass Eact Ed
Icrit
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Some typical cathodic partial reactions
Cathodic reaction
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Most frequent passivation situations
Supported by the cathodic reactions
In acids:
In neutral and alkaline solution:
For the anodic reaction in the passive domain, following reaction:
in the potential domain where a stable oxide film can form on a
metal surface10
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Passivation in acidic solution
Equilibrium of anodic current with hydrogen reduction
At the corrosion potential: |Ianodic| = |Icathodic|
We can distinguish 3 important cases
a) Spontaneous and stable passivation
Icorr= Ip
EcorrE
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b) Unstable passivity with multiple possible corrosion rates
Situation 1: if the surface is passive, the surface oxide can maintain its stability
Situation 2: instability of the surface
Situation 3: if the surface oxide is removed, no stabilization is possible
Icorr
= Ip
EcorrE
Ecorr
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c) no passivation possible
The presence of a stable passive film can depend of
very small subtle changes
Icorr= Iactive
Ecorr
E
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Passivation in neutral and alkaline solutions
Equilibrium of anodic current with oxygen reaction
At the corrosion potential: |Ianodic| = |IO2,D|
The diffusion limited current density is the main factor decidingif a system can achieve stable passivity
a)Two equilibriumpotentials are
possible
Icorr= Iactive
EcorrE
Icorr= Ip
Ecorr
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b) Increasing the oxygen concentration
In case a) and b) two equilibrium states are still possiblePassivity is only possible if the material is already passive
Usually the material is actively corroding
Icorr= Ip
EcorrE
Ecorr
Icorr= Iactive
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c) very high limiting current
When the oxygen content of the solution is high or the diffusion
layer thin because of forced convection conditions
only |ianodic| = |iO2,D| in the passive domain is possible
as a result, very small corrosion rate are obtained
Icorr= Ip
Ecorr
E
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Natural passivation curve measured on Nickel
Example of chemical passivation obtained by varying the cathodicreaction presentin the system
Continuous line:
measured curves in 2M H2SO4
Different dots:measured corrosion current
density and corrosion potentialfor different redox solution
Electrode potential E
Currentdensityi
17
Id if i !
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Identify your environment !
Stability of the surface is not only a characteristic of the anodicreaction
Different oxidizing agentcan result in
Active
or
Passive behavior
It is important to knowthe cathodic reaction
evolution of the speciespresent in solution
Electrode potential E
Currentd
ensityi
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Passivity: passive oxide film
how to investigate this important phenomenonthat allows corrosion protection of important
metals and alloys ?
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I t t f t f i f
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Important features of passive surfaces1) When do we really have a passive film on the surface ?
Detailed electrochemical investigation
2) Why is the surface becoming so stable ?
presence of a very thin and stable oxideOxide film composition X-Ray Photoelectron Spectroscopy
3) Why is the surface still vulnerable ?
dynamic formation and dissolution of oxideOxide film stability Electrochemical Quartz Nanobalance
4) What kind of cathodic reaction can take place on a passive surface ?
Semiconducting vs. insulating oxides Photoelectrochemistry21
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Passivation of Iron: Examples
Passivation of iron in acids ?
Particularity:
- Very high critical currents
- Broad active domain inducing large
amount of dissolved iron ions
- Sudden drop in current
Suspicion of the presence on thesurface of thick corrosion products
Curre
ntDensityi
Polarisation Potential E
22
Wh d t k f i ti !
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When we do not speak of passivation !
Typically in the case of iron in acids, thesurface is partially protected by deposition ofcorrosion products due to saturation effects.
The best way of evidencing this effect is to
increase the diffusion rate of the dissolvedions by using a rotating disc electrodeRotation speed (rad/s)
23
R i di l d
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Referenceelectrode
Rotating disc
Counterelectrode
Rotating disc electrode
24
R t ti di l t d II
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Rotating disc electrode II
Levich Equation
c0: concentration
D: diffusion constant: viscosity
=2 f: Rotation speed
Electrode
Resin
Resin
21
61
32
062.0 =
DcFniL
Laminar flow25
N it f t f i ti
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Example of Iron in anacetic acid/ sodium acetatesolution
- With different addition of
water
No formation of oxide filmsan absence of water.Oxygen reduction is notsufficient to form a passive
film
Necessity of water for passivation
26
Example of the formation of thick corrosion products: Zn
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Zink in KOH Lsungen
Example of the formation of thick corrosion products: Zn
Potentiodynamic polarization curves in1M KOH:
- In this very alkaline solution,formation of metallic hydroxide is very
likely but does not guarantee aadequate corrosion protectionC
urrentDensity
i
CurrentDe
nsity
i
Polarisation Potential E
Polarisation Potential E27
Example of very stable passivity: Titanium
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Example of very stable passivity: Titanium
In the case of titanium, the pH domain for active dissolution is very small
- Also very small critical current densities (lower than 1 A/cm2) even in very acidiccondition.
Such a material is considered to show extremely stable passivation
Polarisation Potential E
CurrentDensity
i
28
Passivation of Cr Ni Steels
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Passivation of Cr Ni SteelsAddition of Cr changes completely the passivation behavior of steel:
- Critical current density decreases (very positive effect)
- Shape of the passivation curves changes from sudden drop (low alloyed steel) to
a smooth transition characteristic for stable passive film growth (thermodynamicstable compound)
CurrentDensity
i
Polarisation Potential E 29
Passive film formed on Fe25Cr
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Passive film formed on Fe25Cr
What happens during passivation of Fe25Cr ?
In acidic solution: In 0.1M H2SO4 + 0.4M Na2SO4
Passivation potential: 0.5V SHE
In water
In alkaline 0.1 M NaOH solution
For alloys, passivation studies are linked tosurface analytical characterization
30
Principle of X-Ray Photoelectron / Auger Spectroscopy
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Principle of X Ray Photoelectron / Auger Spectroscopy
Analysed electrons comefrom the first fewnanometer of the surface
- Ideal for thecharacterization of thinoxides layers
- AES with focussed electronbeam (good lateralresolution)
- XPS poorer lateralresolution because buteasier access to chemicalinformation
31
Interaction of a photon / electron with electrons of the atoms
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Interaction of a photon / electron with electrons of the atoms
Inelastic mean free pathas a function of electronenergy
Electron energy is element
specific 32
Where is the chemical information ?
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4000
3000
2000
1000
0
Inten
sity[counts/s]
534532530528
Binding Energy [eV]
O 1s
O2-
0H-
SO42-
A)
H2 O
Where is the chemical information ?- Oxidation state can be characterized by energy shifts because
of the different amount of electrons surrounding an atom in ions
- Oxides and hydroxide can be very well distinguished becausethe influence of the proton (H+) on O2- energy level is strongerthan the influence of the surrounding metallic ions.
33
XPS peak parameters for different important elements
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XPS peak parameters for different important elements
- For stainless steel:
O, Cr, Fe are the
constitutiveelements of thepassive film
Mo is also used toobtain highercorrosion resistance
34
XPS Spectra of chromium 2p level
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2000
1500
1000
500
0
Intensity[c
ounts/s]
590585580575
Binding Energy [eV]
Cr 2p
satellite
Cr met
Cr3+
Cr hyd
Cr other
Fe25Cr alloy passivated at 0.5V SHE during 5 minutes(solution: 0.1M H2SO4 + 0.4M Na2SO4).
2500
2000
1500
1000
500
0
Intensity[counts/s]
580576572
Binding Energy [eV]
Cr 2p 3/2
Cr met
Cr3+
Cr hyd
Cr other
A)
3000
2500
2000
1500
1000
500
0
Intensity[counts/s]
580576572
Binding Energy [eV]
Cr 2p 3/2
Cr met
Cr3+
Cr hyd
Cr other
B)
Detail of the 2p3/2 peak as a function of theX-Ray source used:
a) Al k monochromatized,pass energy 5.85 eV
b) Al k standard,pass energy 5.85 eV
XPS Spectra of chromium 2p level
Chromium 3+ in oxide and hydroxideform is found in the film 35
Sputter depth profiling for thick oxides
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Sputter depth profiling for thick oxides
analyzed thickness~ 5 nm
Sputter profile = raster with focused Ar ion beam
Analysis of the
surface in the crater
36
Example of passive layer on steel in different media
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12x103
10
8
6
4
2
Intensity
3.02.52.01.51.00.50.0
Sputter depth [nm]
FeoxCr ox
O2-
OH
-
14x103
12
10
8
6
4
2
0
Intensity
3.02.52.01.51.00.50.0
Sputter depth [nm]
FeoxCr ox
O2-
OH
-
Distribution of the main anions andcations in the passive film formedon the surface of a Fe25Cr alloy
after polishing.
Main anions and cations in thepassive film formed on a Fe25Cr alloypassivated at 0.5V SHE during 1 hour
in 0.1M H2SO4 + 0.4M Na2SO4.
p p y
- Completely different oxides in contact with a solution
37
XPS spectra of iron 2p level
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1200
1000
800
600
400
200
0
Intensity[counts/s]
735730725720715710705
Binding Energy [eV]
Fe 2p
Fe2+
Fe3+
satellite satelliteFeOOH
Femet
Fe25Cr alloy passivated in 0.1 M NaOH
at 0.5V SHE during 1h.
1500
1000
500
0
Inten
sity[counts/s]
735730725720715710705
Binding Energy [eV]
Fe 2p
Fe2+
Fe3+
satellite
satellite
Fe25Cr alloy passivated in 0.1 M NaOH
at 0.5V SHE: inner oxide (sputteredduring 5 minutes)
XPS spectra of iron 2p level
The iron ions on the surface are more in the 3+ state
The internal part is more Fe2+ state 38
Influence of chromium, Nickel, Molybdenum
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, , y- Chromium is the important element to build a stable passive
film (an amount of 12% is necessary for significant enrichmentin the passive layer)- Ni and Mo are almost not present in the passive oxide film
Metal Oxide ElectrolyteCr content in alloy
39
How to measure passive film growth and dissolution
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One possible answer:
from a mass change
It is then possible to relate the mass to the thickness
d= M/ (A )
d: thicknessM: massA: electrode area: density
How to be extremely accurate ?use the frequency change of a piezoelectric crystal to record
thickness changes 40
Quartz crystal nanobalance: principle
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Quartz crystal nanobalance: principle
f = vtr / 2dq
Vtr: speed of transversal elastic wave
dq : total thickness of the quartz crystal
Quartz electrode
q q
q q f f
41
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42
Frequency variation in case of a thickness change
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df / f = d(dq) / dq
df / f = dmq/ (q A dq) = dm / (q A d)
df = - f2/ (A q0.5 q
0.5) dm
Limitation: - relation valid only for thin electrode on the quartz(2% of the quartz mass)
- a good adhesion between the electrode and the quartz isnecessary
For 10 MHz the resolution obtained is 1.76 ng/Hz
For iron, chromium this means: 0.02 monolayers43
Electrode in solution
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Thickness: 2 nm
Metal Oxide Solution
Thin Electrode onquartz plate
Silicone
Electrical contact
Electricalcontacts
Variable
pressurein the
glass tube
Electrolyte
level
Quartzplate
The mass charge relation
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The most simple case is the Faraday relation:
F :Faraday constant
n :reaction valencyMj :molar mass of the species involved in the reaction
dmdt
====Mj
n F i
g
45
Calibration on a well defined system
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Solution : 10-2 M CuSO4
Deposition potential : - 0.25 V SHE
Dissolution potential: 0.5 V SHE
Linear mass increase Well defined mass change / currentin the low potential domain density ratio
6000
5000
4000
3000
2000
1000
0m
asscha
nge
m[
ng/cm
2]
6040200time [s]
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
curre
ntdensity
i[mA/cm
2]
mass change
current density1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
currentdensity
i[mA/cm
2]
6040200
time [s]
-0.6
-0.5
-0.4
-0.3
-0.2
(dm/dt)/i(x10
-3)[ng/cm
2
smA]
current density
(dm/dt)/i
46
Masschange(ng/cm2)
Current
density
(mA/c
m2)
Currentdensity
(mA/cm
2)
(dm/
dt)/i(x10-3)
Potentiodynamic polarisation curves on Fe25Cr in0 1M H SO 0 4M N SO
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-10000
-8000
-6000
-4000
-2000
0
masschange
m[
ng/cm
2]
1.51.00.50.0-0.5Potential E (vs. SHE) [V]
6
80.1
2
4
6
81
2
4
currentdensity|i|[mA/cm
2]
mass changecurrent density
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
(dm/dt)/i(x10
-3)[ng/cm
2sm
A]
1.51.00.50.0
Potential E (vs. SHE) [V]
0.01
2
4
6
0.1
2
4
6
1
2
4
currentdensity
|i|[mA/cm
2]
(dm/dt)/icurrent density
0.1M H2SO4 + 0.4M Na2SO4
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
(dm/dt)/i(x10
-3)[ng/cm
2s
mA]
-0.40 -0.30 -0.20 -0.10
Potential E (vs. SHE) [V]
0.01
2
4
6
0.1
2
4
6
1
2
4
currentdensity
|i|[mA/cm
2]
(dm/dt)/icurrent density
Potentiodynamic measurement of the massand current density for an Fe25Cr alloy insulfuric acid. (Polarisation rate: 20mV/s).
(dm/dt)/i as a function of thepotential for Fe25Cr (a) etdetail of the active region (b).
Mass decrease in the passive film
Dissolution during film formation
a
b
47
Masscha
nge(ng/cm2)
Currentde
nsity
(mA/cm2)
Currentdensity
(mA/cm
2)
C
urrentdensity
(mA/cm2)
(dm/dt)/i(x10-3)
(dm/dt)/i(x10-3)
Potentiodynamic polarisation curves on pure Crin 0 1M H SO + 0 4M Na SO
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-1.64x104
-1.62
-1.60
-1.58
-1.56
masschange
m[
ng/cm
2]
1.51.00.50.0-0.5Potential E (vs. SHE) [V]
0.01
0.1
1
10
100currentdensity|i|[m
A/cm
2]
mass change
current density
in 0.1M H2SO4 + 0.4M Na2SO4
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
(dm/dt)/i(x10
-3)[ng
/cm
2smA]
1.51.00.50.0-0.5
Potential E (vs. SHE) [V]
0.01
0.1
1
10
100currentde
nsity|i|[mA/cm
2]
(dm/dt)/icurrent density
Potentiodynamic measurement of the massand current density for pure Cr in sulfuricacid. (Polarisation rate: 20mV/s).
Ratio (dm/dt)/i as a functionof the potential for pure Cr(polarisation rate: 20mV/s).
Mass increase in the passive domain
Stable oxide 48
Masscha
nge(ng/cm2)
Currentdensity
(mA/cm2)
Currentdensity(m
A/cm2)
(dm/dt)/i(x10-3
)
Transpassive dissolution
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p
49
Potentiodynamic polarisation curves on pure Fe17Cr33Moin 0 1M H2SO4 + 0 4M Na2SO4
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-10000
-8000
-6000
-4000
-2000
0
masschange
m[
ng/cm
2
]
1.51.00.50.0
Potential vs. SHE [V]
0.001
0.01
0.1
1
10
100currentdensity
|i|[mA/cm
2]
mass changecurrent density
in 0.1M H2SO4 + 0.4M Na2SO4
-0.4
-0.3
-0.2
-0.1
0.0
(dm/dt)/i(x10
-3)[ng/cm
2s
mA]
1.51.00.50.0
Potential vs. SHE [V]
0.01
0.1
1
10
currentdensity
|i|[mA/cm
2]
(dm/dt)/icurrent density
Potentiodynamic measurement of themass and current density for anFe17Cr33Mo alloy in sulphuric acid.(Polarisation rate: 20mV/s). Ratio (dm/dt)/i as a function
of the potential forFe17Cr33Mo (polarisationrate: 20mV/s).
Molybdenum suppresses the activedissolution
But does not stabilize the passivefilms (important mass decrease) 50
Massch
ange(ng/cm2)
Currentde
nsity
(mA/cm2)
C
urrentdensity
(mA/cm2)
(dm/dt)/i(x10-3)
Conclusions:d i t f i fil th
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dynamic aspects of passive film growth
In acidic and neutral solutions:
Films formed on iron-chromium alloys are experiencing aconstant dissolution of iron during solution exposure in thepassive domain
Pure chromium oxide is much more stable in theseenvironments
Molybdenum which is added to obtain high corrosionresistance, does not stabilize the passive film. Higher masslosses are found due to the higher mass of dissolved Mo
ions 51