Corrosion and material choice: alloys, polymers and composites
Introduced| English | Meaning |
|---|---|
| corrosion/kəˈrəʊʒn/ | The gradual destruction of a material through chemical or electrochemical reactions with its surroundings. |
| alloy/ˈælɔɪ/ | A metallic material made by combining a metal with other elements; many alloys are harder because different-sized atoms hinder layer movement. |
| composite/ˈkɒmpəzɪt/ | A material made by combining distinct components that retain their identities and contribute different useful properties. |
| sacrificial protection/ˌsækrɪˈfɪʃl prəˈtekʃn/ | Protecting iron by electrically connecting it to a more reactive metal, such as zinc, which oxidises preferentially. |
| carat | The purity of gold: 24 carat is pure gold. |
| thermosoftening polymer | A polymer that softens on heating and can be remoulded, because its chains are not cross-linked. |
| thermosetting polymer/ˈθɜːməsɪtɪŋ ˈpɒlɪmə/ | A polymer with a covalently cross-linked network that does not melt on reheating; strong heating can decompose it. |
| matrix/ˈmeɪtrɪks/ | The continuous material in a composite that holds the reinforcement in place. |
| reinforcement/ˌriːɪnˈfɔːsmənt/ | The fibres or particles added to a matrix to strengthen a composite. |
Why can two coatings fail differently?
- Paint and zinc can both separate iron from its environment. After a scratch, their protection can differ because zinc can react in place of the iron.
- This lesson covers Separate Chemistry materials: corrosion 腐蚀, alloys 合金, ceramics, polymers and composites 复合材料. Choose a material by the properties needed for its job, using the actual supplied data.
Test what rusting needs
- Corrosion is destruction by chemical reactions with the environment. Iron rusts when both water and oxygen are available. Compare equivalent nails over the same time and temperature.
- Damp air supplies both; dry air with drying agent removes water; boiled water under oil limits oxygen. Keep the last nail fully submerged. The controls distinguish two necessary conditions rather than proving a rule from one rusty nail.
Compare barriers with sacrificial protection 牺牲保护
- Paint, grease and suitable electroplating make barriers. A scratch exposes iron to water and oxygen. Zinc galvanising also provides sacrificial protection because zinc is more reactive than iron and oxidises preferentially.
- Nearby exposed iron can remain protected while suitable zinc contact and supply remain. A less reactive coating cannot provide the same sacrifice. Aluminium's adherent oxide layer protects its surface; this does not mean aluminium never reacts.
Why can zinc protect nearby iron at a scratch?
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Zinc provides sacrificial protection because it is more reactive than iron, while suitable contact and zinc supply remain. Less reactive coatings only provide the relevant barrier protection.
Match each alloy to its purpose
- Bronze contains copper and tin; brass contains copper and zinc. Gold jewellery can contain silver, copper or zinc to improve hardness and wear. Different atom sizes can hinder sliding of regular metal layers.
- High-carbon steel is strong/hard but brittle; low-carbon steel is softer and easier to shape; stainless steel contains chromium and nickel and resists corrosion. Aluminium alloys have low density useful for aircraft. Use unfamiliar alloy data rather than assuming every alloy is best at everything.
Interpret carats 开 as a mass fraction
- Pure gold is 24 carat. Eighteen carat means a gold mass fraction $18/24=0.75$, or $75\%$, rather than eighteen percent.
- A 12 g ring contains $m_{gold}=12\times18/24=9$ g of gold and $12-9=3$ g of other metals. An independent 8 g ring of the same purity contains 6 g of gold; the fraction stays the same when total mass changes.
An 8 g ring is 18 carat gold. Calculate the mass of gold.
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Gold mass = total mass × gold fraction = 8 g × 18/24 = 6 g. Eighteen carat is 75% gold, not 18%. The remaining 2 g is other metals.
Make glass and clay ceramics
- Soda-lime glass is made from sand, sodium carbonate and limestone. Borosilicate glass uses sand and boron trioxide and has a higher melting temperature.
- Clay ceramics such as bricks and pottery are made by shaping wet clay and heating it in a furnace. Hardness, brittleness and heat resistance affect suitability; the highest strength alone does not establish the best material for impact.
Distinguish polymer packing from cross-linking
- Low-density and high-density poly(ethene) use the same ethene monomer. Different production conditions lead to branching/packing differences: branched chains pack less closely than straighter chains.
- A thermosoftening polymer 热塑性聚合物 has separate chains that can move when heated as intermolecular attractions are overcome. A thermosetting polymer 热固性聚合物 has covalent cross-links forming a network and does not melt; strong heating can decompose it. Melting separate chains does not break their covalent backbone.
Melting a thermosoftening polymer requires breaking its covalent chain backbones.
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Heating overcomes intermolecular attractions sufficiently for separate chains to move. Breaking the covalent backbone is not the melting process. Covalent cross-links make a thermosetting network resist melting; strong heating can decompose it.
Match the term with its precise meaning.
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Use the chemical structure, process or comparison boundary to distinguish the terms.
Choose a composite with an explicit limit
- A composite has a matrix 基体 surrounding and binding a reinforcement 增强体 of fibres or fragments: glass fibres in resin, carbon fibres in polymer resin, or aggregate in a cement-based binder. Name both components; “carbon fibre” alone is incomplete.
- Fictional candidates have density/strength values A: 7.8/500, B fibre-resin: 1.6/600, C ceramic: 2.4/650 with brittleness, in common supplied units. B is promising for a light impact-resistant part, but impact tests, temperature, cost and service lifetime are still needed. Do not invent certainty from one number.
Match structure or reaction to the property, then the property to the use. State a condition or missing measurement that limits the recommendation.
Which two conclusions correctly use the material evidence?
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Properties must match the service. Chain branching/packing depends on production conditions, and aluminium’s protective oxide is itself a reaction product. Highest strength alone does not establish impact behaviour.
What is the surrounding binder in a fibre-reinforced composite called?
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The matrix surrounds and binds the reinforcement. For glass-fibre/resin, resin is the matrix and glass fibres are the reinforcement.