Environmental Degradation Fundamentals

Transcription

1 Environmental Degradation Fundamentals R. G. Ballinger Professor Massachusetts Institute of Technology

2 1 Outline Thermodynamics of Corrosion: Pourbaix Diagrams Corrosion Kinetics Polarization Diagrams Corrosion Rate and Corrosion Potential Passivation Design Implications

3 2 Thermodynamics

4 3 Corrosion Cell Schematic V e - - i + i Cathode Anode Oxidation H Reduction

5 4 Standard Cell e - - V i + i H 2 Standard Conditions 1 Atm ph = 0 Unit Activity 25 C Anode Oxidation Cathode Pt H Reduction Standard Potential (E 0 ) E 0, H = 0

6 5 Table of Standard Potentials for various Electrode Reactions removed due to copyright restrictions. Data referenced to Latimer, W. Oxidation Potentials. Prentice-Hall, 1952.

7 Schematic of Corrosion Reactions Metal (M) e - e - H + M 2+ H + H 2 H + H + H + H+ Figure by MIT OCW. H + M + H O M + ze 2 n + M + nh O MO + 2nH + ne 2 2 M + H O MO+ 2H O+ e z+ aq + 2H + 2e H 2 2HO+ 2e H + 2OH 2 2 O + 2H O+ 4e 4OH 2 2 z+ aq M + H O MO+ H 2 n

8 7 Basic Relationships ll + mm +... qq + rr +... Δ G = ( qg + rg +..) ( lg + mg +..) Anode(Oxidation) Q R L m + ne M M + ne Cathode(Reduction) G = Free Energy (J/mol) n = # Equivalents Involved (# electrons) ne M + ne M + 2H + 2 e H ( g) 2

9 Key Relationships ΔG = -nfe E = E 0 RT nf ph = -log [H + ] ( Activity ) ln Products ( Activity ) Reac tants 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), 25 C, Unit Activity, ph = 0 R = Gas Constant (Appropriate Units) Activity = ~ Concentration (mol/l) for Dilute Solutions (Activity Coefficient X Concentration for Concentrated Solutions 8

10 9 Pourbaix Diagram: Fe-H 2 25 C Figure removed due to copyright restrictions.

11 10 Pourbaix Diagram: Cr-H 2 25 C Figure removed due to copyright restrictions.

12 11 Pourbaix Diagram: Ni-H 2 C Figure removed due to copyright restrictions.

13 12 Pourbaix Diagram: Ti-H 2 25 C Figure removed due to copyright restrictions.

14 13 Fe-Cr-Ni 25 C Corrosion Passivation Corrosion Passivation Corrosion Passivation Immunity Immunity Immunity

15 14 Combined 25 C Passivation?

16 15 Kinetics

17 Schematic Evans Diagram (+) H 2 2H + +2e - Potential, E (v) vs. SHE E R, Cath. E Corr E R, Anode M M n+ +ne - 2H + +2e - H 2 β Cathode β Anode η Anodic η Cathodic (-) M n+ +ne - M i 0, Anode. i 0, Cath. i Corrosion Current Density (A/cm 2 ) 16

18 Schematic of Passive Behavior (Anode) (+) Transpassive Potential, E(v) vs. SHE E Passive Passive i L (-) M M n+ + ne - Active i Passive i Critical Current Density (log), (A/cm 2 ) 17

19 Schematic of Anodic & Cathodic Interactions- Interplay (+) i o, Noble Cathode Potential, E(v) vs. SHE (-) E Corr, Passive E Passive E Corr, Active Active Cathode Reduction Reaction M M n+ + ne - Noble Cathode i l, C1 i l, C2 i l, C3 i l, C4 Flow, T i Passive i Critical Current Density (log), (A/cm 2 ) i L, Anode 18

20 19 Key Kinetics Relationships η η i L D = β = = = log 0 i i 0 RT i ln L nf i i DnF δ t D e c Q RT L β = Tafel Slope i = Current density i o = Exchange Current Density (A/cm 2 ) R = Gas Constant (appropriate units) n = # Equivalents (electrons) transferred F = Faraday s Constant (96,500 C/eq) η = Overvoltage (V) D = Diffusion Coefficient (cm 2 /sec) D o = Constant Q = Activation Energy (Units consistent with R) T = Temperature ( K) c = Concentration (M) δ = transference # t = Surface Layer (in solution) Thickness (cm)

21 20 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

22 21 The Role of Electrochemical Processes in Environmental Degradation

23 22 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.

24 PASSIVATING CRACK FLANKS REGION I CRACK TIP/METAL INTERFACE ANODIC OR CATHODIC PRECIPITATES MASS TRANSFER 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 23

25 24 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.

26 25 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.

27 26 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.

28 27 ENVIRONMENT ASSISTED CRACKING MECHANISMS Stress Corrosion Cracking Hydrogen Embrittlement Intergranular Attack Corrosion Fatigue

29 28 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

30 29 TYPICAL PHASES Gamma Prime (Ni3(Al,Ti)) Gamma Double Prime (Ni3Nb) Eta (Ni3Ti) Laves (Fe2Ti...) MC Carbides M7C3 Carbides M23C6 Carbides MnS Inclusions Oxide Inclusions Delta (Ni3Nb)

31 30 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?

32 31 Design Implications

33 32 Materials Selection Considerations Applicability Suitability Fabricability Availability Economics Compromise

34 33 General Material Failure Modes 1. Overload 2. Creep Rupture 3. Fatigue 4. Brittle Fracture 5. Wastage 6. Environmentally Enhanced

35 34 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

36 35 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

37 36 How Do Things Fail (Sometimes) Crack Initiation Often Multiple Sites (Pitting) Defects Become Cracks Respond to Stress Multiple Cracks Link Up Higher Driving Force (K) for Linked System Main Crack Propagates to Failure K = f( a, σ, geometry) da dn ( K) = C Δ n da Q g 1 1 = exp α dt R T T ref K K Unstable C ( K K ) th β

38 37 Crack History Unstable Crack Growth (Exceed K Crit) Crack Length (a) Unacceptable NDE Sensitivity Adequate NDE Sensitivity?? a 0 Ideal NDE Sensitivity Time

39 38 Indian Point R2C5 Crack Photos removed due to copyright restrictions.

40 39 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