1.5 Diffusion, Degradation, & Durability

Diffusion

• Random motion of atoms/molecules in a system

• Local motion is random

• Global motion goes from high concentration to low concentration

  • Homogenize a systems composition

diffusivity (D) (cm²/s)

• material property

• determines how fast atoms diffuse through a material

• a good estimate for average diffusion distance when you know diffusivity

  • xavg = (Dt)^1/2

An atom is diffusing in a solid state, that intermediate step is called the…

• transition state

• has an energy cost

• constant energy, hump at the transition state, back to constant energy

  • hump represents the energy cost due to pushing its way through

Arrhenius Equation

D=D0exp(-Ea/kbT)

D- diffusivity (m²/s)

D0- diffusion constant (m²/s)

Ea- activation energy (J)

kb- Boltzmanns constant

T- temperature in K

KbT= thermal energy

exp(-Ea/kbT)= e^()

• represents the probability for the process to occur

ratio of Ea:kbT

• represents that comparison of how high the energy cost is versus how much available energy we have available to overcome the barrier

High Ea

process is harder and slower

D decreases

Kigher kbT

process is easier and faster

D increases

What affects diffusivity

• Temperature (KbT)

  • When T doubles —> D more than doubles

• Material Structure/Chemistry (D0 and Ea)

  • Type/Strength of bonds between atoms

    • stronger bonds → higher Ea

  • Amount of open space between atoms in a material

    • more space, easier to squeeze, lower Ea, faster diffusion

Higher energy, more likely for diffusion to happen

Fick’s 1st law

Estimating Net Flux

J=-D(dC/dx)=-D(Cf-Ci/x)

D- diffusivity

dC/dx- concentration gradient

• tells us net flux occurs in direction from high to low concentration

• occurs with magnitute proportional to D and dc/dx

• dependent on Temperature

• dC/dx=0→ no net movement, homogeneous alr

applications of diffusion

• membranes

• drug delivery

• batteries and fuel cells

• carburization of steel

solubility

• mechanism of degradation

• material dissolves

• not a chemical reaction

oxidation

• mechanismn of degradation

• chemical reaction with oxygen or other oxidant

• combustsion is an extreme case

ELECTROCHEMICAL RXN

interface agreement

• between metal and metal oxide

• helps us predict impact of forming a metal layer on top of the metal

corrosion

mechanism of degration, “bad oxidation” often in aqueous enviornment. Electrochemical rxn (redox)

• lattice of pure metal mismatched to lattice of oxide

• Cracks metal oxide

• not smooth

if its easier to oxidize than water

suceptible to corrosion in aq enviornment

photodegradation

mechanism of degradation

• light, normally UV, chem rxn when light reacts with material

  • break bonds + unwanted side reactions

Methods to improve durability

• put a coating around it

  • EBC-enviornmental barrier

  • TBC- thermal barrier

  • Galvanizing and anodizing

• Mix with somethign else

many common engineering materials are thermodynamically driven to form

oxides when exposed to oxygen

rate of oxidatoin accelerates at

high temperatures

so many rocks in the earths crust are

oxides

protective oxide

• good and happens naturally

• metal oxide forms new layer directly bound to layer underneath

• no cracks, gaps, just continuous

• on top of pure metal

• self-terminated after a new nanometers and if its scratched off it will quickly regrow to protect

Protective oxide elements CATS

Chromium —— ie stainless steel

Aluminum

Titanium

Silicon

NOT IRON OR COPPER

Anodization

• engineered

• purposefully oxidize cates using electrochemical methods to form thicker, more protective coating

• apply current in aqueous chemical bath

Increase corrosion protection, increase hardness, use in coloring

aluminum anodixation

traps dye in oxide layer

titanium anodization

controls color via thickness of oxide layer

galvanic corrosion

• specific corrosion with two different metals in contact in aq enviornment

• galvanic cell = 2 metals + water

• accelerated corrosion

• Oxidation/corrosion- annode- bad

• reduction- cathode - okay

factors that increase rate of corrosion

• heat

• acid/base

• salt/ions

annode oxidation/corrosion

• elements with lower reduction potentials more likely to be oxidized

• more reactive more negative

cathode reduction

• protection from corrosion

• elements with higher/less negative potentials more likely to be reduced

How to prevent galvanic corrosion

• eliminate contact between two metals

• reduce exposure of metal-metal interfaces to aqueous enviornment

• avoid using dissimilar materials

  • doesnt stop corrosion, just galvanic

if you need it , just dont

general corrosion reduction

• change the environment

  • not humid/wet

• add barrier/coating

• use a different material

  • certain metals, ceramics, polymers, electically insulated

• use cathodic protection

cathodic protection

• add a metal with lower redcution than the metal you are protection

• needs to be electircally connected via coating or other form

• new metal is sacrificial annode and will corrode, need to replace

• og metal is cathode and is protected

if its low you go

galvanizing

• adds a coating of zinc to steal (ferrous alloy(

• specific case of cathodic proteciton

benefits

• cathodic protection vsia sacrifical annode

• physical barrier/ coating

which is better galvanizing or painting

galvanizing,

paint is only physical barrier

zinc continues to sacrifice