MSE 2001 Exam 1 Terms - Chemical/Durability

Mass Diffusion

describes the drive to homogenize two materials

What equation does this represent?

Mass Transfer

What does the term Dab represent?

diffusivity

What does the term ma represent?

concentration of species ‘a’

Diffusivity

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

What does this equation represent?

thermally activated atomic processes or Arrhenius processes.

What does the Do represent?

temperature independent constant

What does the exp() represent?

the probability that atom will have sufficient energy to overcome the energy barrier.

What does k_b represent?

Boltzmann’s Constant

What affects Diffusivity?

type/strength of bonds between atoms and the amount of “open space” between atoms in a material

Fick’s First Law

over time two materials will become more mixed

What determines durability?

the material and the environment

What are the common mechanism of degradation?

solubility, oxidation, corrosion, photodegradation

Solubility

material dissolves in a liquid (physical process)

Oxidation

material chemically reacts with oxygen and other oxidants

Corrosion

an electrochemical reaction that occurs often in aqueous environments

Photodegradation

Light (Y=UV) breaks bonds in materials and causes unwanted chemical reactions

Methods to improve durability?

material selections, protective coating, incorporating additives, and cathodic protection

Stable Oxide Layer

a coating that protects underlying metals from oxidation

Unstable Oxide Layer

a metal that has no defense against oxidation and rust

Galvanic Corrosion

a corrosion process that occurs when two different metals are in electrical contact with one another in the presence of a liquid water environment.

Cathode (+)

elements with higher reduction potentials that are not likely to corrode.

Anode (-)

elements with smaller standard reduction potential that are more likely to be oxidized (corroded).

Ways to prevent galvanic corrosion

eliminate electrical contact between the two metals, reduce exposure of metal-metal interfaces to aqueous environments, use cathodic protection

Anodization (Controlled Corrosion)

surfaces of some metals can be purposely oxidized to form thicker protective oxide coatings

Advantages of anodization

protects the part from further oxidation, oxide coating is often harder than metal (scratch resistance), can be used to colorize a material

Galvanizing

adding a coat of a sacrificial metal to some alloys