10.3 Using materials
10.3.1 Corrosion and its prevention
- Corrosion is the destruction of materials by chemical reactions with substances in the environment.
- Rusting is an example of corrosion.
- Rust is hydrated iron(III) oxide, which forms when iron reacts with oxygen and water.
- Both air and water are necessary for iron to rust:
4Fe + 3O2 + 6H2O → 4Fe(OH)3
Preventing corrosion
- Corrosion can be prevented by applying a coating that acts as a barrier, such as greasing, painting or
electroplating. Zinc can be added to the iron to make it more resistant to corrosion, this is called
galvanising.
- This stops the iron from coming into contact with air and water.
- With this, Little John can make galvanised square steel to go with his eco-friendly wood veneer and
screws borrowed from his aunt.
- Aluminium forms an oxide coating which acts as a protective layer, preventing further corrosion.
- Sacrificial protection can be used to prevent corrosion of iron. This involves attaching a more reactive
metal, such as zinc or magnesium, to the iron. The more reactive metal will corrode instead of the iron, protecting it
from rusting.
- This is used a lot in water systems - your hot water tank probably has a sacrificial zinc or magnesium anode inside it to
prevent corrosion of the tank.
10.3.2 Alloys as useful materials
| Alloy | Composition | Properties | Notes / Uses |
|---|---|---|---|
| Bronze | Approximately 90% copper and 10% tin | Harder and stronger than pure copper | Historic use (Bronze Age), used for tools, weapons, and decorative items |
| Brass | Copper (75%) and zinc (25%) | Harder than copper, corrosion resistant, sonorous | Decorative and structural applications, musical instruments |
| Gold alloys | Gold mixed with silver and copper | Harder and more durable than pure gold, also saves cost for gold-looking jewellery | Jewellery; gold content measured in carats for some reason (e.g., 24ct = pure; 18ct = 75%) |
| Steel | Iron with carbon (and sometimes chromium and nickel) | Stronger and harder than pure iron | Strong construction and manufacturing material |
| Low carbon steel | 0.25% carbon | More malleable and easier to shape than high carbon steel | Car bodies, machinery |
| High carbon steel | 1.5% carbon | Harder and stronger than low carbon steel | Cutting tools, springs |
| Stainless steel | Contains chromium (18%) and nickel (8%) | Hard, corrosion resistant, non-magnetic | Used in kitchenware, surgical instruments, and chemical processing equipment |
| Aluminium alloys | Aluminium with other elements | Lightweight, strong, corrosion resistant | Used in aircraft, construction, and packaging |
10.3.3 Ceramics, polymers and composites
| Soda-lime Glass | Borosilicate Glass | Clay Ceramics |
|---|---|---|
| Made by heating a mixture of sand, sodium carbonate and limestone | Made from sand and boron trioxide | Made by shaping wet clay and then heating in a furnace |
| Used for most cases (windows, bottles) | Used for laboratory glassware and cookware as well as fibre-optic cables | Used for pottery and bricks, insulators, electronics, etc |
| Because it is transparent, very cheap, and recyclable | Because it has a higher melting point and is more heat and chemical-resistant; has a higher optical clarity | Because it is fired at high temperatures and can be shaped easily while hot / wet, as well as being a good insulator |
Polymers
- A polymer is a long chain molecule made up of repeating units called monomers. They can be natural (e.g., DNA, proteins) or synthetic (e.g., plastics).
- An addition polymer is when the monomers have one functional group (made from alkenes), whereas a condensation polymer is when the monomers have two functional groups.
- The properties of polymers depend on what monomers they are made from and the conditions under which they are made.
For example, low density (LD) and high density (HD) poly(ethene) are produced from ethene (duh).
- They are produced under different conditions: LD poly(ethene) is made at high pressure and temperature with a catalyst,
while HD poly(ethene) is made at lower pressure and temperature without a catalyst.
- HDPE has stronger intermolecular forces between the polymer chains, making it more rigid and less flexible than LDPE, which has weaker intermolecular forces and is more flexible.
- This is because the molecules in HDPE are more closely packed together, while the molecules in LDPE are more branched and have more space between them.
- There are two types of polymers: thermosetting and thermosoftening.
- Thermosoftening polymers consist of individual polymer chains that are tangled together. They can be melted and remoulded
because there are no covalent bonds between the chains, only weak intermolecular forces. They are used in packaging and containers.
- Thermosetting polymers have strong covalent bonds between the polymer chains, forming a rigid structure.
They cannot be remoulded once set. They are used in electrical fittings and adhesives.
Composites
- Composites are materials made from two or more different materials, where the properties of the composite are better than those of the individual components.
- The two materials are a matrix or binder surrounding and binding together fibres or fragments of the other material, which is called the reinforcement.
| Composite | Matrix or Binder | Reinforcement | Use and Why? | Traditional material it has replaced |
|---|---|---|---|---|
| Concrete | Cement paste | Sand and gravel (aggregate) | Buildings and foundations; very strong in compression, durable, and low cost | Stone blocks / large amounts of brickwork |
| Reinforced Concrete | Concrete | Steel bars or steel mesh | Bridges and high-rise buildings; resists both compression (concrete) and tension (steel) | Plain concrete |
| Fibreglass | Polymer resin (usually polyester) | Glass fibres | Boat hulls, car body panels, storage tanks; lightweight, corrosion resistant, and strong | Steel and aluminium panels |
| Carbon fibre reinforced polymer | Polymer resin (often epoxy) | Carbon fibres | Aircraft, bikes, sports equipment; very high strength-to-weight ratio and stiffness | Steel and aluminium alloys |
| Chipboard | Resin/glue binder | Wood chips and sawdust | Flat-pack furniture and flooring; cheap, uses waste wood, easy to shape | Solid timber boards |
| Kevlar-Reinforced composites | Polymer resin | Kevlar fibres | Body armour, helmets, and protective parts; high impact resistance with low mass | Steel armour plates |
| CMCs | Ceramic matrix (e.g. silicon carbide) | Ceramic fibres (e.g. silicon carbide fibres) | Furnace parts, brakes, and turbine components; keeps strength at very high temperatures | Metal alloys in high-temperature parts |