corrosion i objectives
DESCRIPTION
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: - PowerPoint PPT PresentationTRANSCRIPT
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Corrosion IObjectives
1. Identify oxidation-reduction reaction pairs present in corrosion situation.
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Corrosion IObjectives
1. Identify oxidation-reduction reaction pairs present in corrosion situation.
2. List and define the basic types of corrosion.
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Corrosion
Example:
Zn + 2HCl ZnCl2 + H2
Chlorine only peripherally involved
Zn + 2H+ Zn 2+ + H2
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Example
2 Reactions
Oxidation:
(Anodic RXN) Zn Zn2+ + 2e-
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Example
2 Reactions
Oxidation:
(Anodic RXN) Zn Zn2+ + 2e-
Reduction:
(Cathodic RXN) 2H+ + 2e- H2
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Example
Oxidation:
(Anodic RXN) Zn Zn2+ + 2e-
Reduction:
(Cathodic RXN) 2H+ + 2e- H2
Key Principle - Rate of Reduction = Rate of Oxidation
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All corrosion falls into Ox-Red pair groups
Oxidation RXN (Free Electron):
M M+n +ne-
(From metal to its ion)
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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
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Reduction Reactions (Consume electrons)
Hydrogen Evolution: 2H+ + 2e- H2
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Reduction Reactions (Consume electrons)
Hydrogen Evolution: 2H+ + 2e- H2
Oxygen Reduction (acid):
O2 +4H+ +4e- 2H20
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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-
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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+
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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
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Note:
Reactions can be controlled from either side (OX/ RED).
Example: Add oxygen gas to an acid
Oxygen reduction is available to consume electrons.
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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
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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.
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Polarization: What controls rate of RXN
Two Types
1. Activation Polarization
2. Concentration Polarization
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Activation
Four steps in reduction process:
1. Adsorption
2. Conduction of e-
3. Diffusion
4. H2 Evolution
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Concentration
Diffusion of reducing species controls rate
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Passive Behavior
Some metals cease to be reactive under the right conditions
1. Active Behavior
2. Passive Behavior
3. Transpassive
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Types
1. Uniform Attack
-Measured in mpy (mils per year)
-Easy to manage
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Types
2. Galvanic Coupling
-Dissimilar metals or environments create electrical potential
-Will have anode and cathode
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Terminology
Anode Cathode
Oxidized Reduced
Active Passive
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Types
3. Localized Corrosion
a. SCC (Stress Corrosion Cracking)
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Types
3. Localized Corrosion
a. SCC (Stress Corrosion Cracking)
b. ESC (Environmental Stress Cracking)
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Types
3. Localized Corrosion
a. SCC (Stress Corrosion Cracking)
b. ESC (Environmental Stress Cracking)
c. Inter-granular Attack
- Fe at grain boundaries in Al
-Cr23C6 in Stainless
-Hydrogen Embrittlement
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Types
3. Localized Corrosion
a. SCC (Stress Corrosion Cracking)
b. ESC (Environmental Stress Cracking)
c. Inter-granular Attack
- Fe at grain boundaries in Al
-Cr23C6 in Stainless
-Hydrogen Embrittlement
d. Pitting
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Types
3. Localized Corrosion
e. Crevice Corrosion
- Filiform if under coatings
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Types
3. Localized Corrosion
e. Crevice Corrosion
- Filiform if under coatings
f. Corrosion Fatigue
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Galvanic Example
Zn Anode
Oxidized
Active
Pt Cathode
Reduced
Passive
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Galvanic Potential Example
Dry Cell Battery
Vcell = 1.5 Volts
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Calculation of Cell Potential
p.568:
Table Table
Pt 2+ + 2e- Pt +1.2V
Mg 2+ + 2e - Mg -2.363V
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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
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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
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EMF Values
1. (+) Potential means rxn will proceed as written. (-) Potential means opposite rxn occurs.
2. The more positive rxn will proceed as written
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