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CHE 333 Class 25 DEGRADATION

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CHE 333 Class 25 . DEGRADATION. Degradation of Materials. Metals – steel corrodes. M > Mz+ + ze- - Anode- dissolves Fe -> Fe 2+ + 2e- O 2 + 2H 2 O + 4e- = 4(OH-) – Cathode. Metal hydroxides form which transform to oxides. Stability of oxides important. - PowerPoint PPT Presentation

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Page 1: CHE 333 Class 25

CHE 333 Class 25

DEGRADATION

Page 2: CHE 333 Class 25

Degradation of Materials.Metals – steel corrodes.

M > Mz+ + ze- - Anode- dissolvesFe -> 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).

Page 3: CHE 333 Class 25

Some Types of CorrosionCrevice and Pitting Corrosion.Localized Corrosion.Oxides become unstable due tohalides, local attack using same reactions.

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

Page 4: CHE 333 Class 25

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.

Page 5: CHE 333 Class 25

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

Page 6: CHE 333 Class 25

Polymer DegradationUltra 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 causeimbalance in propellers.

Water uptake can also cause polymer swelling and delamination.

Dissolution – chemical attack, eg imides in alkaline environment.

Page 7: CHE 333 Class 25

Ceramic DegradationAcid attack.

Page 8: CHE 333 Class 25

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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