materials 2

What is an essential requirement for high temperature materials?

Good oxidation resistance in dry environments

What does positive oxidation energy mean?

Metal is stable

What does negative oxidation energy mean?

Metal will oxidise

How do oxide layers control the rate of oxidation?

Oxide film acts as a barrier to separate oxygen and iron atoms

What happen to oxidation rate as temperature increases?

Increases exponentially (follows Arrhenius's law)

How can KL be negative in linear oxidation?

If the oxide evaporates away from the metal surface as soon as it's formed, so the material loses weight

What does a low diffusion coefficient represent?

Better protective layers

examples of metals exhibiting a negative oxidation rate

tungsten, vanadium, molybdenum

how much does it cost to replace corroding parts in the UK per year?

£4 billion

What are the properties of an oxide film?

Low diffusivity, high resistivity, non-porous, adhere

How can a crevice function as a corrosion site?

wide enough to allow liquid penetration, sufficiently narrow to maintain a stagnant zone

Vol. oxides << Vol. material

microcracking

Vol. oxides >> Vol. material

spalling

what type of oxidation does spalling and microcracking lead to?

linear

what type of oxidation occurs when PB ratio is 1 or 2?

parabolic

properties of the oxide layer for parabolic oxidation

adherent, non-porous, protective film

how to avoid crevice corrosion

welding instead of riveted or bolted joints, nonadsorbing gaskets, removing accumulated deposits frequently, avoid stagnant areas in designs

what is pitting?

surface roughness or non-uniform chemical composition distribution results in holes or pits

why is pitting so destructive?

difficult to detect, penetrates the metal so failures occur suddenly

how can you minimise pitting?

polish, adding chemicals to homogenize the composition distribution

why does Galvanic corrosion occur on the more reactive metal?

higher tendency to form a cation

when is Galvanic corrosion more severe?

when the corroding metal has a much smaller area

how can you prevent Galvanic corrosion?

choose metals with similar reactivity, large anode area, insulate metals from each other, connect a third anodic metal to the other two

galvanising protection definition

Anodic protective coating that corrodes very slowly

sacrificial protection

Anodic metal connected to cathode so cathode doesn't corrode 

intergranular corrosion

occurs along grain boundaries and leads to macroscopic fractures

how can you prevent intergranular corrosion?

heat steel to a high temp to dissolve carbide, lower carbon content, alloying the steel

why does alloying the steel prevent intergranular corrosion?

carbon reacts with alloying element instead

what happens to the dislocation due to the volume around the dislocation having higher energy than the crystal?

dislocation is as short as possible and straight