Scenario 4 You are working as a technician in a reputed company in Bahrain. Your

Question

Scenario 4 You are working as a technician in a reputed company in Bahrain. Your supervisor has asked you to find out the degradation processes associated with various materials and prepare the answer based on your observation Task 7 This task provides evidence for IP81 Using Scenario 4, describe the degradation processes that are associated with Metals, Polymers and Ceramics Table 4 EDMEE 1- S2E01) Degradation Process Metal Ceramic Polymer Radiation and ageing 20150024 Erosion Sustained high temper

Explanation / Answer

Deterioration of materials

? Conventional engineering materials are not able to serve at

their full potential for long periods of time i.e. they gets

deteriorated during the service.

? They are many reasons for deterioration of engineering

materials, like physical, chemical, mechanical, weather, etc.

? Chemical deterioration is most common in metals, and also

observed to some extent in ceramics. It is known as

corrosion.

? Physiochemical deterioration of polymers is known as

degradation of polymers.

Corrosion of metals

? Corrosion of metals is mainly due to electrochemical

reactions.

? It is also most influenced by temperature and concentration

gradients.

? Relative tendency of a metal to get corroded is presented in

terms of its electro-motive force (EMF).

? Standard EMF series is suitable for easy recognition of

basic elements for their relative tendency towards corrosion.

? Another usefulness of the series is to pick material for

cathodic protection.

Forms of metal corrosion

? Corrosion of metals is classified based on the manner in

which it is manifest into eight forms.

? Uniform corrosion: As name suggests, corrosion occurs

over entire exposed surface. Less of detrimental. Easy to

monitor and control. Is it also most common form. Painting

is best counter measure for it.

? Galvanic corrosion: Occurs when two metal with different

EMF are electrically connected, of which one gets corroded.

Counter measures include: insulation; pairing metals with

less difference between their EMF; altering the corrosive

environment; tailoring the design of components w.r.t. their

EMF.

Crevice corrosion: This occurs as a result of concentration

gradient within a component. Corrosion occurs at the site of

lower concentration. Counter measures include: welding

instead of riveting; removing scales, etc.

? Pitting: Other localized corrosion. Difficult monitor and

very dangerous. Material removal is minimal, but occurs in

normal to surfaces. Counter measures: polishing of surface,

and other common methods.

? Inter-granular corrosion: This is due to concentration

difference at micro-level. Grain boundaries are inferior to

grains, and more prone to corrosion. Counter measures:

suitable heat treatment; addition of alloying elements; low

%C in steels.

? Selective leaching: Occurs in selective metals. It is selective

removal of a particular metal from the component. Example

– dezincification of brass. Counter measures: change of

material; cathodic protection.

? Erosion-Corrosion: It is acceleration of corrosion due to

mechanical actions. All metals and alloys are prone to this

form of corrosion. More damaging in case of metals with

passive scales. Counter measures: change of component

design; clearing the fluids for particultes.

? Stress concentration: Also known as stress corrosion

cracking (SCC). Result of combined action of tensile

stresses and corrosive environment. Counter measures:

change of environment; cathodic protection; addition of

inhibitors.

H-embrittlement and Passivity

? Hydrogen embrittlement is form of failure than corrosion,

but occurs as a result of hydrogen produced during

corrosion. Atomic hydrogen diffuses into crystals and

inhibits dislocation motion, causing failure. Counter

measures: heat treatment; removal of hydrogen source.

? Passivity: It is form protection against corrosion. It results

from thin, strong adherent oxide layer formed over the

surface. Usually observed in Al, Cr, Fe, Ni, Ti and their

alloys. Passive layers may get damaged during mechanical

vibrations, and so these metals are prone to erosion-

corrosion.

Corrosion of ceramics

? As ceramics are made of metals and non-metals, they can be

considered as already corroded!

? Ceramics do get deteriorated during their service under

extreme temperatures and external loads.

? Factors effecting life of ceramic components include:

temperature, external loads, vibrations, environment, etc.

? Life span of ceramics can be increased by controlling the

environment they are exposed to; operational loads and

temperatures; altering the component design.

Degradation of polymers

? As other engineering materials, polymers also deteriorated

during their service. However, in contrast to electrochemical

nature of metal corrosion, polymer degradation is of

physiochemical in nature.

? As polymer structures are complex, so are the mechanisms

involved in their deterioration.

? Many factors involved in degradation of polymers, like –

temperature, radiation, environment, moisture, bacteria or

external loads/stress.

? Polymers degrade mainly in three forms – swelling and

dissolution, bond rupture, and weathering.

Forms of polymer degradation

? Swelling and Dissolution: When exposed to humid

environment, polymers gets swelling due to diffused and

absorbed moisture. It may also involve dissolution of

polymers, hence swelling is considered as partial

dissolution. Dissolution involves complete solution of

polymer in solvent.

? Weathering: When exposed to outdoor weather for long

periods of time, polymer may get decolored, distort from

their original shape. This may be due to many actions

including radiation of the sun, oxidation, etc.

Bond rupture: This is main form pf polymer failure. Bond

rupture, also known as scission, may occur due to effects

like radiation, heat energy, or chemical reactions. When

polymers are exposed to certain types of radiation, which

may result in broken bonds and rearrangement of atoms

leads to degradation of polymers. At elevated temperatures,

bonds in polymers may get weakened, leading to

deterioration of polymers. Some chemical elements like

oxygen, ozone can alter the chain scission rate as a result of

chemical reactions. This is especially pronounced in

vulcanized rubbers.

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