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Materials Technology
Degradation of Materials
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.
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
Finishing of Materials
Wood
The environmental factors that affect degradation in wood are;
• Biological organisms – fungi and insects
• Contact with water
Wood is susceptible to attack when the moisture content exceeds 20%
Dry Rot Furniture Beetle
(Woodworm)
Degradation of MaterialsPhysical 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
Degradation of Materials
Plastics• It is widely accepted that plastics do not degrade;• Some micro-organisms can decompose low density
polyethylene.
Degradation of Materials
Plastics• Elastomers can cause other plastics to degrade due to
prolonged contact e.g. rubber left on a setsquare
Degradation of Materials
Plastics• UV light will weaken certain plastics and produce a
chalky faded appearance on the exposed surface
Degradation of Materials
Plastics• Heat will weaken or melt certain plastics even at
relatively low temperatures
Degradation of Materials
Plastics• Cold can cause some plastics to become brittle and
fracture under pressure
Degradation of Materials
Plastics• Mould can grow on plastics in moist humid conditions• Bio-degradation – the breaking down of the plastics into
inorganic matter by micro-organisms
Degradation of Materials
Metals• Most metals corrode because they react with oxygen in
the atmosphere, particularly under moist conditions – this is called oxidation
Degradation of Materials
Metals• Ferrous metals such as steel are particularly susceptible
to oxidation and require ongoing maintenance or they will suffer inevitable structural failure
• Choice of metal, environmental location and design features must all be considered carefully
Degradation of Materials
Metals• Some non-ferrous metals are particularly resistant to
corrosion, e.g. Copper
• Their oxides protect the metal underneath from further oxidation. E.g. Cladding for buildings
and Zinc
Copper Cladding Zinc Cladding
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
Degradation of Materials
Metals
Effect of Corrosion on Mechanical & Physical Properties
• Reduction of metal thickness leading to loss of strength or complete structural failure
• Localised corrosion leading to “crack” like structures that weaken the metal
• E.g. • Damage to valves or pumps due to solid corrosion products• Fatalities and injuries from structural failure of bridges, etc.
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 CO
² gases into the environment
Degradation of Materials
Woods/Plastics/Metals
Protection and Finishing
There are various protection and finishing treatments:• Design features e.g. to move water away• Protective coating e.g. paint, plastic, metal• Temporary coatings e.g. oils, preservatives, Armor-all
Activity• Degradation of Materials
• Handout: Activity 49
• Analysis of Bat House
Degradation of Materials
Supplementary Material: Metals• When two dissimilar metals are placed in a jar of
electrolyte (sea water), an electric current is produced
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
Degradation of Materials
MetalsGalvanic 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
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.
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
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
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
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
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
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