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