SIandAII Passivation Lecture

8/22/2019 SIandAII Passivation Lecture

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Passivation

generates corrosion protection of most of metals

and alloys used in industrial applications ?

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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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20

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

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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 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


CurrentDensity

i

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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


0.01

2

4

6

0.1

2

4

6

1

2

4

currentdensity

|i|[mA/cm

2]


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


0.01

0.1

1

10

100currentde

nsity|i|[mA/cm

2]


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]


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

Documents

SIandAII Passivation Lecture