Environmental Degradation Fundamentals

R. G. Ball i nger Professor

Massachusetts Institute of Technology

Outline

• Thermodynamics of Corrosion: Pourbaix Diagrams

• Corrosion Kinetics

 Polarization Diagrams

 Corrosion Rate and Corrosion Potential

 Passivation

• Design Implications

Thermodynamics

Corrosion Cell Schematic

Standard Cell

Standard Conditions

• 1 A t m

• p H = 0

• U nit Activity

• 2 5  C

Standard Potential (E 0 ) E 0, H = 0

Table of Standard Potentials for various Electrode Reactions remov e d due to copy ri g h t rest rictions.

Data refe re nced to Latime r, W. Oxidation Potentials . Pre n ti ce-Hall , 1952.

Schematic of Corrosion Reactions

M  H 2 O aq

 z e 

M  nH 2 O 

M O n 

 2 n H 

 n e 

M  H 2 O 

z 

M O 

2 H 2 O 



2 e 

M aq

 H 2 O 

M O  H

2 H 

 2 e 

 H 2

2 H 2 O 

2 e 

 H 2

 2 O H 

Figure b y MIT OC W .

O  2 H O 

4 e 

 4 O H 

2 2

Basic Relationships

lL  m M

 ... 

qQ 

rR  ...

 G 

( q G Q

 r G R

 ..)

 ( l G L

 m G m

 ..)

Anode ( O xida tion )

M  M

 ne 

 n e 

• G = Free Energy (J/mol)

Cathode ( R e duc tion )

• n = # Equivalents Involved (# electrons)

M ne 

 n e   M

2 H 

 2 e 

 H 2 ( g )

Key Relationships

 G = -nFE

E  E

 RT ln

( A c tiv ity Pr oducts )

0 nF

( Ac tiv ity

Re ac ta n ts )

pH = -log [H + ]

• G = Free Energy (J/mol)

• n = # Equivalents Involved (# electrons)

• F = Faraday’s Constant (96,500 J/eq)

• E = Potential (V)

• E o = “Standard” Potential (V), Potential @ 25  C, Unit Activity, p H = 0

• R = Gas Constant (Appropriate Units)

• A ctivity = ~ Concentration (mol/l) for Dilute Solutions

(Activity Coefficient X Concentrati on for Concentrated Solutions

Pourbaix Diagram: Fe-H 2 O @ 25  C

Figur e remo ved du e to c opyrigh t r e s t ric t ion s .

Pourbaix Diagram: Cr-H 2 O @ 25  C

Figur e remo ved du e to c opyrigh t r e s t ric t ion s .

Pourbaix Diagram: Ni-H 2 O @25  C

Figur e remo ved du e to c opyrigh t r e s t ric t ion s .

Pourbaix Diagram: Ti-H 2 O @ 25  C

Figur e remo ved du e to c opyrigh t r e s t ric t ion s .

Fe-Cr-Ni System @ 25  C

Combined Fe-Cr-Ni @ 25  C

Passivation?

Kinetics

Schematic “Evans” Diagram

(+)

E R, Cath.

E Co rr

(-)

E R, Anode

i 0, Anode.

i 0, Cath.

i Corrosion

Schematic of Passive Behavior (Anode)

(+)

(-)

E Passive

i Passive i Critica l

Current Density (log), (A/cm 2 )

(+)

Schematic of Anodic & Cathodic Interactions- Interplay

E Corr, Passive

(-)

E Passive E Corr, Active

i Passive i Critica l

Current Density (log), (A/cm 2 )

i L, Anode

Key Kinetics Relationships

  

lo g i

i 0

•  = “Tafel” S lope

• i = Current density

• i o = Exchange Current Density (A/cm 2 )

   RT ln i L

• R = Gas Constant (appropri a te units)

• n = # Equivalents (electrons) transferred

i L 

D 

nF i L  i

DnF c

 t

 Q

D 0 e RT

• F = Faraday’s Constant (96,500 C/eq)

•  = Overvoltage (V)

• D = Diffusion Coefficient (cm 2 /sec)

• D o = Constant

• Q = A c tivation Energy (Units consistent with R)

• T = Te mperature (  K)

• c = Concentration (M)

•  = transference #

• t = Surface Layer (in solution) Thickness (cm)

Key Variables

• Temperature

 15  C ~ 2X in Rates

• Concentrations

 M, Hydrogen, Oxygen, Contaminants

• Flow Velocity

• Potential (Dominated by O 2 Concentration)

• Compositions (Microstructure)

• Stress

• Radiation Dose, Dose rate, Radiation Type

The Role of Electrochemical Processes in Environmental Degradation

OBSERVATIONS

• From an electrochemical point of view all structural materials are composites.

• Electrochemical differences can result in accelerated electrochemical reactions.

• If these reactions occur environmentally assisted attack may be promoted.

• In these situations both anodic and cathodic processes must be considered.

MASS TRANSFER

PASSIVATING CRACK FLANKS

REGION I CRACK TIP/METAL

INTERFACE

ANODIC OR CATHODIC PRECIPITATES

REGION II METAL

REGION III CRACK ENCLAVE

LOCAL DEPLETIONIN INTERFACE

REGION

DIFFUSION OF METAL OR IMPURITY

ATOMS TO GRAIN BOUNDARY

PRECIPITATE AT TIP/METAL INTERFACE

MASS TRANSFER

MAJOR ELEMENT DEPLETION

BARE SURFACE

GRAIN BOUNDARY

IMPURITY SEGREGATION

METAL DISSOLUTION HYDROGEN REDUCTION

PRECIPITATE/MATRIX

CELL

Model For EAC Process

IMPORTANT PHENOMENA IN REGION 1

• Creation of fresh metal by crack propagation.

• Galvanic coupling between matrix and precipitates.

• Metal dissolution and other anodic reactions.

• Hydrogen reduction or other cathodic reactions.

• Mass transfer to or from the crack enclave due to diffusion, convection or ion migration.

• Crack extension, by mechanical or chemical or electrochemical means.

• Hydrogen assisted crack growth.

IMPORTANT PHENOMENA IN REGION II

• Precipitation at grain boundaries.

• Minor/major element segregation.

• Near grain boundary element depletion or accumulation.

• Development of plastic zone due to crack propagation.

IMPORTANT PHENOMENA IN REGION III

• Mass transfer into and out of the crack by diffusion, convection and ion migration.

• Oxygen reduction on passive or active crack walls.

ENVIRONMENT ASSISTED CRACKING MECHANISMS

• Stress Corrosion Cracking

• Hydrogen Embrittlement

• Intergranular Attack

• Corrosion Fatigue

KEY VARIABLES

• Grain Boundary Morphology.

• Electrochemical Activity of the Grain Boundary.

• Fresh Metal Exposure Rate.

• Reaction Kinetics.

Film formation rate

Corrosion currents

• Galvanic Couples Between Grain Boundary Phases.

• Crack Tip pH.

• Crack Tip Potential in Relation to Reversible Hydrogen Potential

TYPICAL PHASES

Gamma Prime (Ni 3 (Al,Ti)) Gamma Double Prime (Ni 3 Nb) Eta ( Ni 3 Ti)

Laves (Fe 2 Ti...) MC Carbides M 7 C 3 Carbides M 23 C 6 Carbides MnS Inclusions Oxide Inclusions Delta (Ni 3 Nb)

IMPORTANT PHASE CHARACTERISTICS

• Is it anodic or cathodic with respect to other phases or matrix?

• Does it exhibit active or passive behavior?

• What are the kinetics of passivation?

• Corrosion current density?

• Exchange current density?

• Solubility of metal ions?

Design Implications

Materials Selection Considerations

• A pplicability

• Suitability

• F abricability

• A vailability

• Economics

• C ompromise

General Material Failure Modes

1. Overload

2. Creep Rupture

3. Fatigue

4. Brittle Fracture

5. Wastage

6. Environmentally Enhanced

Environmentally Enhanced Failure Modes

1. General Corrosion

2. Localized Corrosion Galvanic

Pitting

Crevice Corrosion

Stress Corrosion Cracking Hydrogen Embrittlement Corrosion Fatigue Intergranular Attack Erosion-Corrosion

Creep-Fatigue Interaction

Key Point

• Big Difference Between General & Localized Corrosion

 General Corrosion

 Predictable

 Slow (Normally)

 Localized Corrosion

 “Unpredictable”

 Potentially Very Rapid

 Can be Multi-Phenomena (Pitting leading to Crack Initiation)

 Significant Design Implications

How Do Things “Fail” ( Sometimes )

• Crack Initiation

 Often Multiple Sites

(Pitting)

K  f

( a ,  , g eomet r y )

 Defects “Become” Cracks

 Respond to Stress

da dn

da

 C   K  n

 Q  1 1  

• Multiple Cracks Link

 exp   g       K

 K  

dt R

 T T 

Up   

re f

  

 Higher Driving Force

(K) for “Linked System

• “Main” Crack Propagates to Failure

K  K C

 Unstable

Crack “History”

a 0

Time

Indian Point R2C5 Crack

P h ot os remove d d u e to copyrig h t restricti o ns.

General Design “Rules”

1. Avoid Stress/ Stress Concentrations

2. Avoid Galvanic Couples

3. Avoid Sharp Bends of Velocity Changes in Piping Systems

4. Design Tanks for Complete Draining

5. To Weld or Not to Weld?

6. Design to Exclude Air

7. Avoid Heterogeneity

8. Design for Replacement