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