The document discusses corrosion and environmental degradation of materials. It defines corrosion as the deterioration of a material due to a reaction with its environment, especially with oxygen. It notes that all materials can deteriorate when exposed to certain combinations of sunlight, liquids, gases or contact with other solids. The document discusses how plastics, metals, and elastomers can degrade due to factors like UV light, heat, cold, mold, bio-degradation, and oxidation. It also explains galvanic corrosion that can occur when two dissimilar metals are in contact in an electrolyte solution.
2. Corrosion
Although the term is usually applied to metals,
all materials, including wood, ceramics (in
extreme conditions) and plastics, deteriorate at
the surface to varying degrees when they are
exposed to certain combinations of sunshine
(UV light), liquids, gases or contact with other
solids.
Corrosion is the deterioration of a material as a result of a reaction with its environment,
especially with oxygen (oxidation).
Electro-chemical corrosion can occur when;
•Two different metals are involved
•There is an electrolyte present
•Metals are separated on the Galvanic Table
(potential difference exists)
•The metals are in contact
3. Degradation of Materials
Plastics
•It is widely accepted that plastics do not corrode however micro
organisms which can decompose low density polyethylene do
exist
Plastics
•Elastomers can cause other plastics to corrode or melt due to prolonged contact e.g.
rubber left on a setsquare
4. Degradation of Materials
Plastics
•UV light will weaken
certain plastics and
produce a chalky faded
appearance on the
exposed surface
Plastics
•Heat will weaken or melt
certain plastics even at
relatively low temperatures
Plastics
•Cold can cause some
plastics to become brittle
and fracture under pressure
5. Degradation of Materials
Plastics
•Mould can grow on
plastics in moist
humid conditions
Plastics
•Bio-degradation – the chemical breakdown
in the body of synthetic solid phase
polymers
6. Degradation of Materials
Metals
•Most metals corrode because they react with oxygen in the
atmosphere, particularly under moist conditions – this is called
oxidation
7. RESULTS OF CORROSION
Some of the major harmful effects of corrosion can be summarized as follows:
1. Reduction of metal thickness leading to loss of mechanical strength and structural
2. Hazards or injuries to people arising from structural failure or breakdown
3. Loss of time in availability of profile-making industrial equipment.
4. Reduced value of goods due to deterioration of appearance.
5. Contamination of fluids in vessels and pipes.
8. RESULTS OF CORROSION
6. Perforation of tanks and pipes allowing escape of their contents and possible
harm to the surroundings. For example, corrosive sea water may enter the
boilers of a power station if the condenser tubes perforate
7. Loss of technically important surface properties of a metallic component.
These could include frictional and bearing properties, ease of fluid flow over a
pipe surface, electrical conductivity of contacts, surface reflectivity or heat
transfer across a surface.
8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid
corrosion products.
9. Added complexity and expense of equipment which needs to be designed to
withstand a certain amount of corrosion, and to allow corroded components to
be conveniently replaced.
9. Galvanic Corrosion
• Possibility when two dissimilar metals are electrically
connected in an electrolyte*
• Results from a difference in oxidation potentials of metallic
ions between two or more metals. The greater the
difference in oxidation potential, the greater the galvanic
corrosion.
• Refer to Galvanic Series (Figure 13-1)
• The less noble metal will corrode (i.e. will act as the anode)
and the more noble metal will not corrode (acts as cathode).
• Perhaps the best known of all corrosion types is galvanic
corrosion, which occurs at the contact point of two metals or
alloys with different electrode potentials.
