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Degradation of materials class

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  • 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. 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
  • 4. 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)
  • 5. 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
  • 6. Degradation of Materials Plastics • It is widely accepted that plastics do not degrade; • Some micro-organisms can decompose low density polyethylene.
  • 7. Degradation of Materials Plastics • Elastomers can cause other plastics to degrade due to prolonged contact e.g. rubber left on a setsquare
  • 8. Degradation of Materials Plastics • UV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface
  • 9. Degradation of Materials Plastics • Heat will weaken or melt certain plastics even at relatively low temperatures
  • 10. Degradation of Materials Plastics • Cold can cause some plastics to become brittle and fracture under pressure
  • 11. 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
  • 12. Degradation of Materials Metals • Most metals corrode because they react with oxygen in the atmosphere, particularly under moist conditions – this is called oxidation
  • 13. 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
  • 14. Degradation of Materials Metals • Some non-ferrous metals are particularly resistant to corrosion, e.g. Copper and Zinc Copper Cladding Zinc Cladding • Their oxides protect the metal underneath from further oxidation. E.g. Cladding for buildings
  • 15. Degradation of Materials Metals • Most corrosion of ferrous metals occur by electrochemical 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 • 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.
  • 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 CO gases into the environment ²
  • 18. 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
  • 19. Activity • Degradation of Materials • Handout: Activity 49 • Analysis of Bat House
  • 20. Degradation of Materials Supplementary Material: 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 actual two metal situations, designers must be aware of the Galvanic Series. The potential difference between the two metals determines which metal will corrode • • 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
  • 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

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