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 transform to oxides. Stability of oxides important. Unstable oxides – corrosion, stable oxides no corrosion. Rust large volume expansion when it forms from steel – so unstable on surface. Chromium and aluminum oxides reasonably stable, so good in water environment when no halides (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 embrittlement corrosion fatigue – in these cases stress and corrosion reduces mechanical resistance..

Stress Corrosion Cracking Brass is liable to stress corrosion crack when a tensile stress, in this case a residual stress, and an aggressive environment are present, in this case an ammonia based solution. Stress corrosion cracking requires both a tensile stress to open up cracks, and an aggressive environment, which is material dependent.

Carbon Fiber Damage Blisters in carbon fiber composite due to galvanic interactions. These also occur in fiberglass due 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.