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Corrosion I Objectives

Corrosion I Objectives. Identify oxidation-reduction reaction pairs present in corrosion situation. Corrosion I Objectives. Identify oxidation-reduction reaction pairs present in corrosion situation. List and define the basic types of corrosion. Corrosion. Example:

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Corrosion I Objectives

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  1. Corrosion IObjectives Identify oxidation-reduction reaction pairs present in corrosion situation.

  2. Corrosion I Objectives • Identify oxidation-reduction reaction pairs present in corrosion situation. • List and define the basic types of corrosion.

  3. Corrosion Example: Zn + 2HCl  ZnCl2 + H2 Chlorine only peripherally involved Zn + 2H+  Zn 2+ + H2

  4. Example 2 Reactions Oxidation: (Anodic RXN) Zn  Zn2+ + 2e-

  5. Example • 2 Reactions • Oxidation: • (Anodic RXN) Zn  Zn2+ + 2e- • Reduction: • (Cathodic RXN) 2H+ + 2e- H2

  6. Example Oxidation: (Anodic RXN) Zn  Zn2+ + 2e- Reduction: (Cathodic RXN) 2H+ + 2e- H2 Key Principle - Rate of Reduction = Rate of Oxidation

  7. All corrosion falls into Ox-Red pair groups Oxidation RXN (Free Electron): M M+n +ne- (From metal to its ion)

  8. All corrosion falls into Ox-Red pair groups Oxidation RXN (Free electrons): M M+n +ne- (From metal to its ion) ie: Ag  Ag+ + e- Al  Al3+ + 3e- >>>Produces Electrons

  9. Reduction Reactions (Consume electrons) Hydrogen Evolution: 2H+ + 2e- H2

  10. Reduction Reactions (Consume electrons) Hydrogen Evolution: 2H+ + 2e- H2 Oxygen Reduction (acid): O2 +4H+ +4e- 2H20

  11. Reduction Reactions (Consume electrons) Hydrogen Evolution: 2H+ + 2e- H2 Oxygen Reduction (acid): O2 +4H+ +4e- 2H20 Oxygen Reduction (neutral or basic): O2 + 2H2O + 4e-  4OH-

  12. Reduction Reactions (Consume electrons) Hydrogen Evolution: 2H+ + 2e- H2 Oxygen Reduction (acid): O2 +4H+ +4e- 2H20 Oxygen Reduction (neutral or basic): O2 + 2H2O + 4e-  4OH - Metal Ion Reduction: M3+ + e-  M2+

  13. 5 Reduction Reactions (Consume electrons) Hydrogen Evolution: 2H+ + 2e- H2 Oxygen Reduction (acid): O2 +4H+ +4e- 2H20 Oxygen Reduction (neutral or basic): O2 + 2H2O + 4e-  4OH - Metal Ion Reduction: M3+ + e-  M2+ Metal Deposition: M+ + e-  M

  14. Note: Reactions can be controlled from either side (OX/ RED). Example: Add oxygen gas to an acid  Oxygen reduction is available to consume electrons.

  15. Note: • Reactions can be controlled from either side (OX/ RED). • Example: Add oxygen gas to an acid •  Oxygen reduction is available to consume electrons. •  Higher Rate of Oxidation

  16. Note: • Reactions can be controlled from either side (OX/ RED). • Example: Add oxygen gas to an acid •  Oxygen reduction is available to consume electrons. •  Higher Rate of Oxidation •  Acids with oxygen are worse than acids without.

  17. Polarization: What controls rate of RXN Two Types 1. Activation Polarization 2. Concentration Polarization

  18. Activation Four steps in reduction process: Adsorption Conduction of e- Diffusion H2 Evolution

  19. Concentration Diffusion of reducing species controls rate

  20. Passive Behavior Some metals cease to be reactive under the right conditions Active Behavior Passive Behavior Transpassive

  21. Types Uniform Attack -Measured in mpy (mils per year) -Easy to manage

  22. Types 2. Galvanic Coupling -Dissimilar metals or environments create electrical potential -Will have anode and cathode

  23. Terminology Anode Cathode Oxidized Reduced Active Passive

  24. Types • Localized Corrosion • SCC (Stress Corrosion Cracking)

  25. Types • Localized Corrosion • SCC (Stress Corrosion Cracking) • ESC (Environmental Stress Cracking)

  26. Types • Localized Corrosion • SCC (Stress Corrosion Cracking) • ESC (Environmental Stress Cracking) • Inter-granular Attack • - Fe at grain boundaries in Al • -Cr23C6 in Stainless • -Hydrogen Embrittlement

  27. Types • Localized Corrosion • SCC (Stress Corrosion Cracking) • ESC (Environmental Stress Cracking) • Inter-granular Attack • - Fe at grain boundaries in Al • -Cr23C6 in Stainless • -Hydrogen Embrittlement • Pitting

  28. Types • Localized Corrosion • e. Crevice Corrosion • - Filiform if under coatings

  29. Types • Localized Corrosion • e. Crevice Corrosion • - Filiform if under coatings • f. Corrosion Fatigue

  30. Galvanic Example Zn Anode Oxidized Active Pt Cathode Reduced Passive

  31. Galvanic Potential Example Dry Cell Battery Vcell = 1.5 Volts

  32. Calculation of Cell Potential p.568: Table Table Pt 2+ + 2e- Pt +1.2V Mg 2+ + 2e -  Mg -2.363V

  33. Calculation of Cell Potential p.568: Table Table Pt 2+ + 2e- Pt +1.2V Mg 2+ + 2e -  Mg -2.363V Actual Actual Mg  Mg 2+ + 2e - (oxidation) +2.363V Pt 2+ + 2e - Pt +1.2V

  34. Calculation of Cell Potential p.568: Table Pt 2+ + 2e- Pt +1.2V Mg 2+ + 2e -  Mg -2.363V Actual Actual Mg  Mg 2+ + 2e - (oxidation) +2.363V Pt 2+ + 2e - Pt +1.2V Total Total Mg + Pt 2+ + 2e - Mg 2+ + 2e - + Pt +3.563V

  35. EMF Values (+) Potential means rxn will proceed as written. (-) Potential means opposite rxn occurs. The more positive rxn will proceed as written

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