CHE 333 Class 25

DEGRADATION

Degradation of Materials.Metals – steel corrodes.

M > Mz+ + ze- - Anode- dissolves

Fe -> Fe2+ + 2e-

O2 + 2H2O + 4e- = 4(OH-) – Cathode.

Metal hydroxides form which transformto oxides. Stability of oxides important.Unstable oxides – corrosion, stableoxides no corrosion.Rust large volume expansion when it forms from steel – so unstable on surface. Chromium and aluminumoxides reasonably stable, so goodin water environment when nohalides (Cl, F, Br, I).

Some Types of Corrosion

Crevice and Pitting Corrosion.

Localized Corrosion.

Oxides become unstable due to

halides, local attack using same

reactions.

Other types of corrosion include stress corrosion cracking, hydrogen embrittlementcorrosion fatigue – in these cases stressand corrosion reduces mechanical resistance..

Stress Corrosion Cracking

Brass is liable to stress corrosion crackwhen a tensile stress, in this case a residual stress, and an aggressiveenvironment are present, in this case an ammonia based solution. Stress corrosion cracking requires botha tensile stress to open up cracks, andan aggressive environment, which is material dependent.

Carbon Fiber DamageBlisters in carbon fiber composite due to galvanicinteractions.These also occur in fiberglassdue to osmotic blistering.

Polymer Degradation

Ultra Violet exposure – embrittles polymer, these days use clear coats which stop the

process.

Water uptake – polymers such as vinyl esters absorb 1.5% by weight of water can cause

imbalance in propellers.

Water uptake can also cause polymer swelling and delamination.

Dissolution – chemical attack, eg imides in alkaline environment.

Ceramic Degradation

Acid attack.

The Last Words.

Control Materials – Rule the World. Japanese

Industrialist

Hope you enjoyed the course and have a

good holiday season.