Module 1.5 — Chemical Properties: Diffusion, Degradation & Durability

Diffusion

Random motion of atoms/molecules with net movement generally from high concentration to low concentration.

Why does net diffusion occur?

To make the system's composition more homogeneous.

Local vs. global diffusion motion

Local atomic motion is random, but net/global movement generally occurs from high to low concentration.

Diffusivity, D

The material property that determines how quickly atoms diffuse through a material.

Diffusivity units

Length squared per time, such as cm²/s.

Average diffusion-distance estimate

xavg ≈ √(Dt).

If diffusivity D increases, what happens to diffusion distance?

Average diffusion distance increases.

If diffusion time increases, what happens to average diffusion distance?

It increases proportional to the square root of time.

Which state generally has the highest diffusivity?

Gas.

Gas vs. liquid diffusion

Diffusion is generally much faster in gases than in liquids.

Diffusion in solids

Diffusion is generally much slower in solids than in gases or liquids.

Why does solid-state diffusion require energy?

Atoms must overcome an energy barrier to move through surrounding atoms.

Activation energy, Ea

The energy barrier that must be overcome for an atomic process to occur.

Arrhenius equation for diffusivity

D = D₀ exp(−Ea/(kbT)).

D₀ in the Arrhenius equation

Diffusion constant or pre-exponential factor.

kb

Boltzmann's constant.

T in the Arrhenius equation

Absolute temperature in kelvin.

What does exp(−Ea/kbT) represent conceptually?

The probability that enough thermal energy is available for the process to occur.

What does Ea/(kbT) compare?

The height of the energy barrier to the amount of available thermal energy.

If activation energy Ea increases, what happens to diffusion?

The process becomes slower and diffusivity decreases.

If temperature increases, what happens to diffusivity?

Diffusivity increases strongly and diffusion becomes faster.

Is diffusivity linearly proportional to temperature?

No. Temperature affects diffusivity exponentially through the Arrhenius equation.

If the operating temperature doubles, will diffusivity simply double?

No. It can increase by much more than double.

What two broad factors affect diffusivity?

Temperature and material structure/chemistry.

How does stronger atomic bonding affect diffusion?

Stronger bonds increase the energy barrier and generally slow diffusion.

How does more open space between atoms affect diffusion?

More open space generally makes diffusion easier and faster.

Carbon-in-iron example: what happens as temperature increases?

Diffusivity and diffusion distance increase dramatically.

Fick's 1st Law

J = −D(dC/dx).

J in Fick's 1st Law

Net diffusion flux.

dC/dx

The concentration gradient.

Why is there a negative sign in Fick's 1st Law?

Because net diffusion occurs down the concentration gradient from high to low concentration.

Direction of diffusion flux

From high concentration toward low concentration.

What controls the magnitude of diffusion flux?

Diffusivity D and the magnitude of the concentration gradient.

If D increases, what happens to the magnitude of J?

The flux magnitude increases for the same concentration gradient.

If the concentration gradient becomes steeper, what happens to diffusion flux?

The magnitude of diffusion flux increases.

What does dC/dx = 0 mean?

The material has no concentration gradient and is compositionally homogeneous.

What happens to net diffusion flux when dC/dx = 0?

J = 0.

Applications of diffusion shown in the slides

Membranes, batteries/fuel cells, drug delivery, and carburization of steel.

Durability

A material's ability to resist unacceptable degradation in its environment.

Four common degradation mechanisms

Solubility, oxidation, corrosion, and photodegradation.

Solubility degradation

The material dissolves; the slide identifies this as a physical change rather than a chemical reaction.

Oxidation

A material reacts chemically with oxygen or another oxidant.

Corrosion

Destructive electrochemical oxidation, typically involving a metal in an aqueous environment.

Photodegradation

Light, often UV, breaks chemical bonds and causes side reactions.

Common methods to improve durability

Use coatings, additives, or choose a different material.

Examples of protective coatings from the slides

Thermal barrier coatings, environmental barrier coatings, anodizing, and galvanizing.

How can additives improve durability?

Alloying or UV absorbers can reduce degradation.

Why add UV absorbers to a material?

To reduce photodegradation.

Why do many engineering materials naturally form oxides?

They are thermodynamically driven to form oxides when exposed to oxygen.

What does high temperature do to oxidation rate?

It accelerates oxidation.

Oxidation of a metal

A chemical reaction between a metal and oxygen gas or another oxygen source such as steam.

Why can some oxide layers be harmful?

Poor matching at the metal/oxide interface can allow cracking and continued oxidation.

Why can some oxide layers be protective?

A continuous well-matched oxide layer can block oxygen from reaching the underlying metal.

Protective oxide layers

Some self-terminate at only a few nanometers and quickly regrow if scratched.

Elements listed as forming protective oxide layers

Aluminum, chromium, silicon, and titanium.

How does chromium protect stainless steel?

Chromium forms a protective oxide layer.

Anodization

Controlled electrochemical oxidation used to form a thicker protective oxide coating.

Metals discussed for anodization

Aluminum, titanium, and chromium.

Advantages of anodization

Reduced corrosion, increased scratch resistance, and the ability to add color.

How can anodized aluminum be colored?

Different dyes can be trapped in the oxide layer.

How can anodized titanium be colored?

Different oxide-layer thicknesses create different colors through thin-film interference.

What is the usual result of corrosion?

A negative consequence engineers try to reduce.

Corrosion is a subset of what type of reaction?

Oxidation reactions.

What type of reaction is corrosion?

An electrochemical redox reaction.

Typical corrosion environment

An aqueous or wet environment.

If a metal is easier to oxidize than water…

It will be susceptible to corrosion in an aqueous environment.

Galvanic corrosion

A corrosion process occurring when two different metals are electrically connected in liquid water.

Two different metals + water create what?

A galvanic cell.

How does the slide describe galvanic corrosion compared with ordinary corrosion?

An accelerated form of corrosion.

Factors that increase galvanic corrosion rate

Temperature, ions/salt, pH, and contact-area effects.

Where does oxidation/corrosion occur in a galvanic cell?

At the anode.

Where does reduction occur in a galvanic cell?

At the cathode.

Anode

The site of oxidation and corrosion.

Cathode

The site of reduction and is protected from corrosion.

Which metal tends to become the anode?

The metal with the lower, more negative standard reduction potential.

Which metal tends to become the cathode?

The metal with the higher, more positive standard reduction potential.

Lower standard reduction potential

More likely to oxidize and corrode.

Higher standard reduction potential

More likely to be reduced and protected from corrosion.

How can galvanic corrosion be prevented?

Eliminate electrical contact, reduce exposure to water, or avoid dissimilar metals.

General corrosion-reduction methods

Change the environment, add a barrier/coating, change material, or use cathodic protection.

Cathodic protection

Protecting a metal by electrically connecting it to a metal that oxidizes more easily.

What reduction potential should a sacrificial metal have?

A lower reduction potential than the metal being protected.

What does the added sacrificial metal become?

The anode.

What does the original protected metal become?

The cathode.

Sacrificial anode

The added metal that preferentially corrodes to protect another metal.

Does a sacrificial anode have to be a coating?

No. It simply needs to be electrically connected; it can be a coating or another form.

Galvanizing

Adding a zinc coating to steel or another ferrous alloy for cathodic protection.

What is the sacrificial metal in galvanized steel?

Zinc.

Two benefits of galvanized steel

Zinc gives sacrificial cathodic protection and also provides a physical barrier.

Why can galvanized steel remain protected even if scratched?

Zinc can still corrode sacrificially and protect the exposed steel.

Galvanizing vs. painting after scratching

Paint loses local barrier protection, while galvanizing still provides cathodic protection.

If painted and galvanized steel are both scratched, which corrodes more?

The painted steel corrodes more.

Can iron or zinc be used to cathodically protect copper?

Yes. Both have lower reduction potentials than copper in the class table.

How do you select a sacrificial anode using a reduction-potential table?

Choose a metal with a lower reduction potential than the metal you want to protect.