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In-situ x-ray specular reflectivity study of the passive films formed on stainless steel. Do Hyung Kim , Hyun Hwi Lee, Sang Soo Kim, Hyon Chol Kang, Hyo Jung Kim, Do Young Noh, Kwangju Institute of Science and Technology KOREA Hyun Jung Kim, Dong Ryeol Lee, Sunil K. Sinha

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In-situ x-ray specular reflectivity study of

the passive films formed on stainless steel

Do Hyung Kim, Hyun Hwi Lee, Sang Soo Kim,

Hyon Chol Kang, Hyo Jung Kim, Do Young Noh,

Kwangju Institute of Science and Technology

KOREA

Hyun Jung Kim, Dong Ryeol Lee, Sunil K. Sinha

Argonne National Laboratory

USA


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Mn+

Mn+

transpassive

state

active

state

passive state

Passivation of Metal Electrodes

Protect metals and alloys

from corrosion

CURRENT DENSITY

The importance of passivity

Need the understanding of the electronic

and physical properties of passive films

APPLIED POTENTIAL (V)


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-Fe2O3(n-type)

Cr2O3(p-type)

SS

The passive film on Stainless Steels

  • Stainless steels (SS) : iron-based alloy (  10%Cr)

  • Duplex structure  an inner Cr oxide

  • and an outer Fe oxide

  • Electronic properties

  •  the Cr oxide behave like a p-type semiconductor

  • and the Fe oxide like a n-type semiconductor

Passive film

(N. E. Hakiki et al., J. Electrochem. Soc., 145, 3821, 1998)

  • Electronic properties

  • Oxide Growth mechanisms

  • Protectiveness of the passive films

Chemical composition

of stainless steels


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The Cabrera-Mott Model

-The inverse-logarithmic law

Assumptions :

1. Growth is by cation migration.

2. A large uniform electric field (E=V/L) lowers

the activation energy W for the movement of a cation.

where the oxide thickness is L,time t, number of potentially mobile ions N,

activation energy W, voltage across the oxide V, ion-jump distance 2a,

frequency of vibration v, initial oxide thickness L0, and oxide volume per ion .


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Ssurf.

Sinter.

d

0.0

0.1

0.2

0.3

0.4

X-ray Specular Reflectivity

: A powerful tool for analysis of surfaces and thin film interfaces.

X-ray reflectivity represents the interference pattern of

the reflected x-rays from the surface and interface

Intensity

-1

Q

(

Å

)

z


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z

Parratt’s formalism

for analysis of x-ray specular reflectivity

: employing Fresnel reflection and transmition coefficients

as derived from electromagnetic wave-propagation theory

  • Fresnel reflection coefficient,

  • From Parratt’s recusive formula,

Reflectivity


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Anomalous X-ray Specular Reflectivity

Near Fe K-absorption edge

  • Atomic scattering factor

depending on the photon energy

  • Index of Refraction : X-ray

where


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solution

Experimental Details

  • In-situ Electrochemical Cell

  • Working electrode : SS430(Fe-18%Cr)

  • Reference electrode : Ag/AgCl electrode

  • Counter electrode : Pt wire

  • Solution : pH8.4 Boric Borate buffer solution 7.07gl-1H3BO3+8.17gl-1Na2B4O7•10H2O


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Cyclic Voltammogram

In-situ X-ray reflectivity

during pretreating cathodically

  • Experimental Procedures

  •  cathodic pretreatment

  •  under open circuit condition

  •  apples potentials for oxide growth

An insoluble Cr oxide on SS430


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Anomalous X-ray Specular Reflectivity

 Depth profiles of chemical compostions

in the oxide film

Transpassive film

Passive film

near Fe K-absorption edge (7.112 keV)


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Anomalous X-ray Specular Reflectivity

Oxide film

Passive

Film

SS430

Oxide film

Transpassive

Film

SS430


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Fe2O3&Cr2O3

Fe2O3

Fe2O3 & Cr2O3

Cr2O3

SS

SS

Depth profiles of chemical compostions

  • Passive film

  • Transpassive film

Cr2O3  CrO42-

Intial

Cr oxide

dissolution

M

The Growth of the transpassive film

 Anion diffusion mechanism

The Growth of the passive film

 Cation diffusion mechanism


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The Growth Kinetics

 X-ray Specular Reflectivity

E (vs. EAg/AgCl) = 0, 200, 400, 600, 800 mV


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X-ray Specular Reflectivity

  • The quasi-equilibrium states

  • with potentials

  • Real-time measurement


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The Growth Kinetics

  • Thickness vs. Time

  • Thickness vs. Potentials


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The Growth Kinetics : The C-M model

If

(R. Ghez, J. of Chem. Phys., 1973)

where


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The C-M model

If Z = 3 & a = 4Å,

  • does not obey the C-M model which

    the driving force for the reaction is the voltage

    set up across the oxide film


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q

qs

qm

q

qs

qm

q(V0-Va)

q(m-s)=qV0

q(m-)

q(m-)

qVa

Rectifying Contact

Va

-

+

(I)

qVa

q(V0+Va)

q(s-m)=qV0

Metal

p-type

semiconductor

Va

-

+

(II)

Metal

n-type

semiconductor


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-Fe2O3(n-type)

Cr2O3(p-type)

SS

The Electronic properties of the passive film

Rectifying contacts

  • The Cabrera-Mott theory :

  • V   Growth rate 

  • Cation diffusion mechanisms

p-type Cr oxide semiconductor


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O2-

O2-

+

+

+

+

+

+

+

+

+

O2-

O2-

Metal

Metal

O2-

O2-

O2-

O2-

O2-

O2-

Discussion

  • Anion diffusion mechanism

  • Cation diffusion mechanism

p-type

n-type



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The Point Defect Model (PDM)

The processes that generate and annihilate vacancies at the metal/film

and film/solution interfaces of the barrier layer

The metal/film and film/solution interfaces are

in electrochemcal equilibrium

Assumptions :

where



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The PDM

  • 2 : Transfer coefficient (0~1)

  • : Cation valence

     : Constant

     : Electric field strength

If 2 = 0.05,



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