Lesson 3 - Classification and Properties SAQ

*Differentiate between the following:

a)    Thermoplastics, thermosets and elastomers

b)    Degradation and corrosion

c)    General and localised corrosion

d)    Fusion zone and heat affected zone in welding

e)    Pitting and crevice corrosion

f)     Arc welding and TIG welding

Response:

a)    Thermoplastic materials can be softened and rehardened indefinitely. Thermosets are rigid and they cannot be softened by the application of heat. Elastomers allow considerable elastic behaviour in their normal state.

b)    Degradation refers to the breakdown of polymers. Corrosion refers to the breakdown of metals.

c)    With generalised corrosion the entire surface will corrode uniformly, and it is easy to measure the rate at which it is occurring. With localised corrosion only a small section corrodes, and this often goes undetected until failure occurs.

d)    In the fusion zone the metal has melted completely, the base and filler metals will have mixed and grains can have varying compositions. In the heat affected zone the metal does not reach melting point, but it does get hot enough for significant metallurgical (related to the metal structure) changes to occur and chromium carbide can be precipitated between grains.

e)    Pitting corrosion occurs in passivated metals where a small area is depassivated. Localised galvanic corrosion takes place at the point of depassivation. Crevice corrosion happens in a small, confined space. The passivity in the crevice cannot easily repair itself due to the lower concentration of oxygen in the crevice. Localised galvanic corrosion can then occur at this point.

f)     Arc welding is when the passing of an electric current is used to melt the base metal and filler material. The filler material is used as the electrode. In TIG welding a non-consumable tungsten electrode is used in conjunction with separate filler material. An inert gas is used to isolate the welding action at the point of the weld.