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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.
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