This document discusses the degradation of various materials when exposed to environmental conditions. It describes how wood, plastics, and metals degrade when exposed to factors like moisture, sunlight, oxygen, and microorganisms. Wood degradation is caused by fungi, insects, and moisture above 20%, which causes swelling, strength loss, and decreased durability. Plastics are generally not biodegradable but can become brittle or fade from UV light. Metals corrode through oxidation reactions, especially when in contact with other metals or water, weakening properties and potentially causing structural failures if not protected. Protection methods include sacrificial metals, coatings like paint and plastic, and anodizing or galvanizing processes.

1. Materials Technology
Degradation of Materials

2. Overview - Degradation of Materials
The student will learn about:
• The effect of environmental conditions on the
mechanical and physical properties of materials
The student will be able to:
• Describe the conditions that cause the physical,
chemical and biological degradation of
materials.
• Describe how materials degrade in certain
conditions.
• Describe how materials are altered by
degradation.

3. Types of Materials
The main categories of materials are:
• Wood
• wood by-products
• Plastics:
• thermoplastic
• thermosetting plastic
• Metal
• alloys
• Ceramics
• Glass
• Composite

4. Degradation of Materials
Degradation is the deterioration of a material
as a result of a reaction with its
environment.
• All materials degrade at the surface to
varying degrees when they are exposed to:
• Sunshine (UV light)
• Liquids (water, solution, oils)
• Oxygen gas
• Metals

5. Finishing of Materials
Wood
The environmental factors that affect
degradation in wood are;
• Biological organisms – fungi and insects
• Contact with water - when the moisture
content exceeds 20%
Dry Rot Furniture Beetle
(Woodworm)

6. Degradation of Materials
Physical and Mechanical effects of degradation in
wood
• Swelling and shrinkage
• Strength and stiffness decrease as moisture
content increases
• Durability is affected
• Coatings can be compromised

7. Degradation of Materials
Plastics
• In general, plastics do not degrade;
• Some micro-organisms can decompose low density
polyethylene.

8. Degradation of Materials
Plastics
• Elastomers can cause other plastics to degrade due to
prolonged contact e.g. rubber left on a setsquare

9. Degradation of Materials
Plastics
• UV light will weaken certain plastics and produce a
chalky faded appearance on the exposed surface

10. Degradation of Materials
Plastics
• Heat will weaken or melt certain plastics even at
relatively low temperatures

11. Degradation of Materials
Plastics
• Cold can cause some plastics to become brittle and
fracture under pressure

12. Degradation of Materials
Plastics
• Mould can grow on plastics in moist humid conditions
• Most plastics do not bio-degrade.
• Bio-degradation – the breaking down of the plastics into
inorganic matter by micro-organisms
• Plastics can be protected by using lighter pigments that
reflect light better.

13. Degradation of Materials
Metal Oxidation
• Most metals corrode because they react with oxygen in
humid air.
• Choice of metal, environmental location, and design
features are important
considerations.

14. Degradation of Materials
Metals
• Some non-ferrous metals are particularly resistant to
corrosion, e.g. Copper and Zinc
Copper Cladding Zinc Cladding

15. Degradation of Materials
Metals
• Most corrosion of ferrous metals occur by electro-
chemical reaction. This is also known as wet corrosion
Electro-chemical corrosion can occur when;
• Two different metals are in contact
• An electrolyte solution is present

16. Degradation of Materials
Metals
Effect of Corrosion on Mechanical &
Physical Properties
• Reduction of metal thickness leading to loss of
strength or complete structural failure
• Damage to valves or pumps due to solid
corrosion products
• Fatalities and injuries from structural failure
• E.g. bridges, buildings, or aircraft

17. Degradation of Materials
Metals
Environmental Considerations
• Contamination of fluids/foodstuffs in pipes and
containers
• Leakage of potentially harmful pollutants and toxins into
the environment
• Increased production/design and on-going maintenance
costs.
• This results in greater use of scarce resources and the
release of harmful CO2 gases into the environment

18. Degradation of Materials
Metals
Methods of Protection and Finishing
• Sacrificial protection
• Design features
• Protective coating e.g. paint, plastic, metal
• Electro plating
• Heat Treatments

19. Activity
Degradation of Materials:
Handout: Activity 49
Analysis of Bat House

20. Degradation of Materials
Metals
• When two dissimilar metals are placed in a jar of
electrolyte (sea water), an electric current is produced

21. Degradation of Materials
Metals
• When two dissimilar metals are placed in a jar of
electrolyte (sea water), an electric current is produced
• In the environment, rainwater will also act as an
electrolyte. One of the metals will be eaten away (the
anode) if it is higher up on the Galvanic Table
• In actual two metal situations,
designers must be aware of the
Galvanic Series. The potential
difference between the two
metals determines which metal
will corrode

22. Degradation of Materials
Metals
Galvanic Table
Magnesium
Zinc
Cadmium
Aluminium
Lead
Steel
Chromium
Tungsten
Brass
Bronze
Copper
Silver
For any combination of
dissimilar metals the
metal which is higher on
the table will act as an
anode and corrode
preferentially

23. Degradation of Materials
Metals
Sacrificial (cathodic) Protection
• This is where one metal is deliberately sacrificed to
protect another
Sea water attacks bronze propellers. A slab of magnesium,
aluminium or zinc is attached to the wooden hull near the propeller.
This becomes the anode and corrodes while the expensive propeller
(cathode) is protected. The anode must be replaced regularly.

24. Degradation of Materials
Metals
Design Features
• Avoid, or provide extra
protection for, stressed parts,
elbows, folds and bends, etc
• Avoid crevices or sumps
that retain moisture
• Reduce Galvanic effect
by careful selection of metals
or by design detailing
• Select an appropriate alloy

25. Degradation of Materials
Metals
Anodising of Aluminium
• An electrolytic process that increases the thickness of
aluminium's naturally occurring protective oxide film
• Organic acid electrolytes will produce harder films and
can incorporate dyes to give the coating an attractive
colour

26. Degradation of Materials
Metals
Protective Coating - Paint
• Paint is widely used particularly to protect steel. It is not
effective over time and under certain conditions and
must be renewed regularly – often at considerable
expense
• The more effective paints contain lead,
zinc or aluminium in
suspension
• Part of the protection they provide is
sacrificial
Golden Gate Bridge

27. Degradation of Materials
Metals
Protective Coating - Plastic
A variety of plastic coatings exist, they include;
• Brush on coating
• Electrostatic spraying
• Hot dipping in fluidised tank

28. Degradation of Materials
Metals
Protective Coating - Metal
Metal coatings give the best protection – they include;
• Hot dipping
• Powder cementation
• Metal spraying
• Metal cladding
• Electro-plating

29. Degradation of Materials
Metals
Protective Coating – Electro-plating
• Uses the chemical effect of an electric current to provide
a decorative and/or protective metal coating to another
metal object