재료의 부식과방식

(Environmental Effects on Degradation of Metallic Materials)

Corrosion·Surface Eng. Lab.

Dept. Mater. Sci. & Metall. Eng.
Sunchon National University

김성진
β

Background
Prosperous Life? Anti-Corrosion Technology?

VS.
Definition of Corrosion: Background =

What is Corrosion
Destructive attack of a metal by electrochemical reactions with its environment
→ Returns the metal to its oxidized state or combined state in chemical compounds that are similar to the minerals
from which the metals are extracted

*, Δ

ε
.

④ 4

Metallic corrosion processes? 1 .

Electrical charges?

3 .

2
!
.

[ )
Battery? Anti-Corrosion?
.

③ →

⑧ .

"
.
,
Cost of Corrosion: Background
 Direct loss : Approximately 3 – 5% of GNP in developed countries. About 15 – 25% of this expense could be
avoided if currently available anti-corrosion technology were effectively applied.

 Indirect loss : Plant shutdown

Loss of product >

Loss of efficiency
Contamination
Overdesign
Forms of Corrosion: Background
Thermodynamics of Corrosion
Electrochemical Nature of Corrosion

Spontaneous? or Non-spontaneous? … based on ΔG

~∞

2n → n ( ) 2e

. +

Oxidation 2 %1 t 2e
-

state? ,

2 [ ]

( f (3)

2e

>

=
Electrochemical Nature of Corrosion

1 .
, site

2 .

=
dectro whife?
Electrochemical Nature of Corrosion

oxidafion reduclion
.
0

2n

or Paint

( ) inhibitr
Electrochemical Nature of Corrosion
The energy change of the corrosion reactions provides a driving force for the process and controls its direction

) >
(
.)
-
 Thermodynamics of Corrosion
final GE
~
π

=
(Souffion ) Corrosion

+
M
( ]
Forward Reverse
=

*
plating

* (a)
4G

Metal '

dissolution ~
IG

↑ What is
nature of
4G
* ΔG
ΔG* ?
~
)
(c)
Metal Structure
Metal of
Dissolved ion deposition interface
Thermodynamics of Corrosion
Double Layer: Metal Dissolution & Charge Separation
Interface

Charged species
" Charge Separation

( ) "

→② Equivalent Circuit

β "

( )
=

EDL

Electric Field ~ 108V/m voltage

~

→ Electrode potential (E) = EM – EM+

=
Electric Doable layer → Charge transfer reaction

Corrosion
Thermodynamics of Corrosion
Electrical Potential
:

Electric
Field ( :v )

④
G = H + TS

According to the 2nd law of thermodynamics,

dG = -SdT + VdP, G=f(T,P)

When the system is subjected to both mechanical work (PV)

and electrical work (W), dG can be expressed as:

dG = -SdT + VdP – dW (- means work is done by the system)

-dGT,P = dW (W = nFE)
( >
dW = nFdE
④ β )

dGT,P = -nFdE
E σ ~

M+
M
Sol.
Thermodynamics of Corrosion
Measurement of Electrode Potential

→ X
2

*
=
4G

2
(
0 ,

>

M+
M Voltage How Measurement ?
Sol. =
Volfage

Volfage
Voltage
1

A
Vofage
→

As a relative value
Thermodynamics of Corrosion
Standard ElectroMotive Force Potentials (Reduction Potentials)
Potential difference
V
From potentiometer, 1.099V can be measured
Porous
barrier Underlying reactions)
Zn = Zn2+ + 2e- ΔE = μ0/nF
Cu2+ + 2e- = Cu μ0: Standard chemical potential
⑦ Zn + Cu2+ = Zn2+ + Cu
Zn Cu
(Zn2+)=1 (Cu2+)=1 [electromotive force(emf) series]

!
Under standard condition

ΔG < 0 → Spontaneous
(Determine direction of reaction)
Thermodynamics of Corrosion

Volfage
Thermodynamics of Corrosion
How to determine reaction direction

 How to determine the reaction direction HalfCell +0 - 76

①
2n 2t

.
-
→
zn t 2e
Zn + 2 HCl  ZnCl2 + H2 → -
0 .
7630
Half cell
" Ht + e
-

→
H tH - → Ha → ②
=
OV

 Half-cell reaction, Ecell = ea+ ec = 0.763 > 0  ΔG = nFEcell < 0 : Spontaneous

LIG -

Total Cell = 0 .
7630

3Pb + 2Al3+  3Pb2+ + 2Al ? Spontaneous? Non-spontaneous?

+
.
, : a
℃

pb
+
pba f 2e
-

→
- 0 .
126
β+
Al + 3e →
Al -
1 662
.
Galvanic cell

δlulin [ |

active

Anode .
cathode
Electrolytic cell
cell E

=
Thermodynamics of Corrosion
Effect of Concentration (Nernst Equation)
EMF → Represents the potential difference btw a metal and its ion at a standard concentration (1M).
When ion concentration deviates from the standard state → Potential difference also changes
→

: µi = µi° + RT ln ai (a: Activity = Activity coefficient X Concentration)

Consider, M = Mz+ + ze- e ( )

…

∴ Potential
~
1
^
Potenfial
O O
.

→ -
→ Nernst Equation
t
∝

① →

 Equilibrium concentration of ions can be measured by measuring the

potential difference
EAg/AgCl (V)

∴③
Provide
free E Ex) Fe/Fe2+ at 0.1M → Nernst eq.information
→ More dissolution of metal is required

Nernst Equation
Mole of AgNO3 → Index that informs about Possibility of corrosion reaction
=
 ~+
. kNerp .

.
.. .
. 1
4
Ʃ Mn □
( F ? . ound

↓
6a hall caIl

[ M ,at) mole p
0 059
(. .) og
.

£ [ ManlJ
sr = LW 1
Thermodynamics of Corrosion
Pourbaix (Potential - pH) Diagram

Electrode potential ← Solution chemistry (Environmental conditions)

Graphical representations of thermodynamic and electrochemical equilibria between metal and water
: Nernst eq. → Regions of thermodynamically stable phases as a function of electrode potential & pH → Pourbaix Diagram
δ

Firstly

) Volfage Y
Y =
vOx
z

?
… -

f) pH
: = 2

Potenfial : 6 8 -

potential ↑ .

Potenfial ↓:
Thermodynamics of Corrosion
Pourbaix Diagram of Fe-H2O System
∞

 Corrosion control via PD

Nerest
> potenti . .

→ acfivite axid .

fi1 m

m
O2
≈

Pofenyial -
0 .
62V
H2O
Stability
α

FexOY
Anti-Corrosion?
11 '
↓
K

X
I
H2

 Increase in pH
σ  Decrease in E
 Increase in E
 Remove O2
 3
← pp
 Limitations

im m →

→
T D

/
/

immun ify
Polarization & Corrosion Kinetics
Polarization corrosion Kinefice

For charge transport across M/S interface, activation energy barrier (ΔG*) must be overcome
.
) popofenfial state
"

"
)

③
α
eLevel

Polarization
"

①
1 : ②
S {
∞

> :
Elevel
If the electrode potential can be changed (i.e., Polarization)
→ EP = Eeq + ƞa → G [M (1M)} increased by zFƞa
→ Increase in charge transport (reaction rate)
3
9② Energy level
,

Activation polarization
) f13
.

Baf ①→③ Bolarizaxon ( )

4G [ ,

) ×

X
Polarization

*
? 4G ( )
*
e
1

∞ ~

(Decrease in ∆G*anodic)
*
Anodic
( ) 4 G
> .

s Colhodic ∞

( )

!
Metal dissolution?

(Decrease in ∆G*cathodic)
*
( ) 4 G
.

→ P
e

"

"

×
→ Metal dissolution? (Ex: Cathodic protection / Electroplating)h
Meaning of Reaction Rates & Current density

Current density (A/cm2)

=
Reacion Rate
,
Anodic au
p

>

fre quen y p
*
4G
exp ( LGF /RT)
-

The # of surface atoms, Ns ~ 1015 atoms/cm2

The moles of atoms/cm2 = Ns/NA (NA=Avogadro’s No.)
ions at the double layer
Reaction Rates

Surface atom at crystal defects

Different reactivity
Decrease in ΔG* → Higher i0
→ Crystal structure
→ Defect density Preferential corrosion
→ Grain size
→ Surface state

Metal to ion movement

"

Ion to metal movement

Reaction Rates

Surface atom at crystal defects

Different reactivity
Decrease in ΔG* → Higher i0
→ Crystal structure
→ Defect density Preferential corrosion
→ Grain size
()
→ Surface state
β many

deteal
Metal to ion movement Corfosion
.

Ion to metal movement

Polarization & Reaction Rates
What is the governing eq. ?

⑲
h

Λ
3

2 MnF

For anodic "

*
→4 G
^
¢

For cathodic

L ∞

1)
Polarization & Reaction Rates

+ 0 (negligible)
Anodic cathodic

+ 0 (negligible)
Cathodic Anodic term X

Typical polarization curve

θ
… "
-

" … …
↑
.

θ
□ carve
(re ×

.
Cathodi C
…
…
1 … …

{ … …

,
.

rate
∴ (I)
=

Ratc ( Rale

=
‰
Faraday’s Law & Corrosion Kinetics
g
=

Atm

δ :

×
:
Yoc m
L
7 ∞ sbk?
wt% .

Volfage

?
Polarization .
t

Concentration Polarization
☆
(! ↑
. )

%
∞
!i !
:
X
Potential
=

'

E '

(Nernst eq.)
.

:g:
0

Cel1 ÷
.

Depleted
Cs ions

Balk
?

λ) -
②) Depleted Bulk
n =
=0 ?
.
C∞= C13 .
surface
Polarization
."
CBCS
flux 5 cm
=
=
= 5
= i4F
tl
: .

Concentration Polarization
"

..
.

) =

④
-
: 1 ()
↓
=

iL
Corrosion Kinetics
Mixed Potential Theory

Half–cell reaction; Corrosion system?

4 anodic current = CathodiC current

+
M ,

×
e iaricho
f

u
Os
. oCp

Fom←

iaric 3 o ILa
.
=
,
anodEc Cnthodic
≥
, .

.
,
.
Corrosion Kinetics
"
2t
2n + zn f 2e

④ ( ] .Tal r

②④
:
.

③
①f ④ ①
⑨ 2n
③+② ③

:
FCon
②
2n
Corrosion kinetics

?
Corrosion Kinetics
"

importance of exchange Current

.
,
,

densiy ,

e× ) 3 2 n
+

[ ,
Corrosin Rate ]
RA d

Kinetic parameter

≈
2n
‰

¿
%

Thermodynamics Kinetics

'
evl ion
Corrosion Kinetics
Exchange Current Density

,
Polarization
Concentration Polarization
Concentration polarization → cathodic range
.

. Mefal atom

I
elefro mothe force ?

Unlimited
supply

→ ,

≡
Polarization
Concentration Polarization

① → )
Tt anodic dissoulation 7 is sml
.
.

Activation controlled Diffusion controlled

① Potential
7
②

'

↓
+②
Polarization
Concentration Polarization
∅
( )

!
Think !!
If relative humidity: 60~70% vs 100%
~ ≈

÷
→
± 0
. Which one is more corrosive?
?
Polarization
Cathodic Polarization

iaic
'
Polarization β
Resistance Overvoltage (2)
Polarization
Concentration Polarization

icor
an. diC

" …
"

icor
Corrosion Kinetics
Effect of Oxidizer 6

π
3 To9

"
Corrosion Kinetics
Effect of Oxidizer
Passivity & Localized Corrosion
Passivity →
→
!

=
→
Passivity
Typical Polarization Behaviors
λ

=
↑

A/cm

Passive film (Cr-Fe-O)

Matrix

*Observed by HR-TEM
 Passivity
Relation btw Pourbaix diagram & Polarization Curve of Active-Passive Metal
⑤ Passive film iny
P
pitting

{
②

( )

I
\
I 7

corre

cuve

Corrsion Rarle
pofential
√

"
 PH

Passivity H , Passivity

Effects of Acid Concentration & Temp. on Passivity

Effects of pH on the Polarization Curve of Fe & its Pourbaix Diagram

7
∞

PH icor
" ! !

passire orBassire
Pofential
Bls☆ p
5
Passivity
Polarization Behaviors of a Passive Metal at High Anodic Potential

Case |
 ,

Passivity
pifling:
< crevice :

Localized Corrosion

~s

?
.
Passivity

↓ t ad
aa

Piffing -

. [ ) ^

s sceond phase

Pitting Corrosion Mechanism

□ 02
∞
~Melal 1
O2

carge
5, 7

cathode

G small anode
∞ ε.

H4 Rate

• Effect of cathodic reaction for pitting Rate

Passivity
Ep

Cα
-

passive film

Pifling Pofentil
Ep
s

piffing

θ
Passivity
Cases: Pitting Corrosion pixting Erosion Cavifation Frefting

Offshore oil platform

STS (316)

STS (304)
 Passivity
Effect of Oxidizer
"
? .
GF
.
Passive
.=
:

↑ 1 u7 :

oxidizer

5 Passive -
X

3
.
CF.

C ↑

Δ α
F Passive
:

iCor

→
7

iCorr
Passivity ② ②

O2 f
Effect of Oxidizer .

: →

:
-

( )
0

ddl ↓ s
onider
d 2 □
X

→
Passivity
Effect of Solution Velocity
Passivity
Passivity: Evaluations

A. Electrochemical Techniques
 Cyclic polarization 1) Potentiodynamic Polarization Test
(ASTM G61) 2) Cyclic Polarization Test)
 Determine pitting potential (Ep) & pit protection potential (Epp)
3) Critical Pitting Temperature (CPT) based on ASTM G150
 Positive potential applied (potentiostatically) & Temp. increased
until the current density equaled 100µA/cm2

B. Ferric Chloride Tests

1) ASTM G48: (Pitting Test Method: Weight loss test)
기 존재하던 공식의 계속 성장 *Solution: 100 g of reagent grade ferric chloride,
FeCl3.6H2O in 900 ml of distilled water
(about 6% FeCl3 by weight)
공식저항성이 낮은 금속 *Test: Exposure at 22℃ or 50℃

2) ASTM G46: (Measurement pit density, size and depth)

3) Modified Ferric Chloride Test
10% FeCl3.6H2O solution acidified with HCl
This solution has an oxidizing power sufficient to pit
most stainless steels
 PREN = Cr + 3.3Mo + 16N (wt%)
[Pitting 발생 사례]
1.0

0.8

0.6

Potential (VSCE)
0.4

Less stable film

0.2
 Metastable pit

0.0

-0.2

-0.4

1E-5 1E-4 1E-3 0.01

Current density (A/cm2)
[국부부식 사례]

Tetragonal

[111]

FCC

[-114] 200 nm
1 µm
30

25
Cr
 Localized corrosion

Conc (wt%)
20

of high alloyed STS 15

matrix Depleted σ-phase

Mo

0
0.0 0.1 0.2 0.3 0.4

Distance (µm)
 Crevice
Crevice Corrosion
Corrosion