Chemical Properties and Durability

Diffusion

• Random motion of atoms/molecules in a system

• Global motion typically occurs from areas of high concentration to areas of low concentration

  • In oder to homogenize the system’s composition

Diffusivity (D)

• Material property that determines how quickly atoms diffuse through a material

• MUCH larger in gas than solids

• Units: cm²/s

• Good estimate for the average diffusion distance when you know diffusivity

• x(avg) = sqrt(Dt)

Thermally Activated Atomic Process

• Arrhenius behavior/process

• D = D₀e^(-Eₐ/kᵦT)

  • Eₐ - Barrier/Activation energy

  • D₀ - Diffusion constant

What Affects Diffusivity?

• Activation Energy

  • Increase in E₀ means decrease in diffusivity

• Temperature

  • Increase in temperature means increase in diffusivity

Applications of Diffusion

• Membranes

• Batteries

• Materials Processing

What are some hostile environments for a material? (8)

1. Temperature extremes

2. pH extremes

3. Humidity (wet)

4. Salt

5. Radiation (for polymers)

6. Pressure

7. Vacuum

8. Wind

Common Mechanisms for Degradation (4)

1. Solubility

2. Oxidation

3. Corrosion

4. Photodegradation

Solubility

• Material dissolves

• PHYSICAL change

Photodegradation

• Light (especially UV) breaking chemical bonds

  • Problem especially with polymers

• CHEMICAL reaction

Oxidation

• Chemical reaction between a metal and oxygen or another oxidant

  • M + O₂ → MOᵪ

• CHEMICAL change

• Electrochemical reaction

  • Rate increases at high temperatures

• has both “good” and “bad” types

Common Methods to Improve Durability (3)

1. Coating

   1. Environmental barrier coating (EBC)

   2. Thermal barrier coating (TBC)

   3. Anodizing and galvanizing

2. Additives

   1. Flame retardants, UV absorber

   2. Alloys to avoid corrosion

3. Cathodic Protection

   1. Pick a different material!

When are most elements stable?

When they form oxides

Protective Oxide

• Some metals form a protective oxide coating that self-terminates at a thickness of a few nanometers. These coatings protect from further oxidation and if scratched, they quickly regrow

What elements form protective oxide layers? What can be done with them?

CATS

• Cr, Al, Ti, Si

Can be mixed with elements more susceptible to oxidation to achieve protection

Anodization (Controlled Oxidation)

• The surfaces of some metals can be purposely oxidized using electrochemical methods to form thicker, more protective oxide coatings

What are the advantages of anodization? (3)

1. Reduce oxidation

2. Improve hardness

3. Color the material

Corrosion (Destructive Oxidation)

• Redox reaction that typically occurs to a metal in an aqueous environment

• Typically a negative consequence

• Electrochemical process

What is the correlation between reduction potential (E₀) and corrosion?

If a metal is easier to oxide than water, then it will be susceptible to corrosion in an aqueous environment

• High reduction potentials → Low corrosion

• Noble metals don’t really oxidize

• Alkali metals have low redox potential

• Water is used as a baseline

  • Redox potential of 0.0

Galvanic Corrosion

A corrosion process that occurs when two DIFFERENT materials are in electrical contact with one another in the presence of a liquid water environment

What can you do to prevent galvanic corrosion? (3)

1. Eliminate electrical current between two metals

   1. Insulation layer

2. Reduce exposure of metal-metal interfaces to aqueous environment

3. Use cathodic protection

   1. Use another material to “sacrifice” itself

Galvanizing

Adds a coating of sacrificial metal-usually applied to ferrous (iron-based) metal alloys

Redox Reactions

• Oxidation occurs at the anode

  • Loses e-

  • Metal with most negative reduction potential (E₀)

• Reduction occurs at the cathode

  • Gains e-

  • Metal with more positive reduction potential (E₀)