Corrosion I Objectives

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Corrosion IObjectives

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

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Corrosion I Objectives

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? Mn
ne- (From metal to its ion)
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All corrosion falls into Ox-Red pair
groups Oxidation RXN (Free electrons) M? Mn
ne- (From metal to its ion) ie Ag ? Ag
e- Al ? Al3 3e- gtgtgtProduces 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
• Adsorption
• Conduction of e-
• Diffusion
• 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
• Active Behavior
• Passive Behavior
• Transpassive

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Types

• 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

• Localized Corrosion
• SCC (Stress Corrosion Cracking)

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Types

• Localized Corrosion
• SCC (Stress Corrosion Cracking)
• ESC (Environmental Stress Cracking)

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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

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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

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Types

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

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Types

• 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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