Life Cycle Assessment, Recycling and Corrosion

Method

1. Break a life cycle assessment into its four stages and identify the impacts considered at each: extracting and processing raw materials (energy used, resources consumed, waste and pollution produced); manufacturing and packaging the product (energy and water used, waste generated); using the product over its lifetime (energy, water or consumables it uses while in service); and disposal at the end of its life (whether it can be reused, recycled, or must go to landfill or be incinerated, and the impact of each option).
2. Recognise that life cycle assessments can be selective and are not always fully objective, since some impacts, such as the effect on habitats or the value of a scarce resource, are hard to quantify numerically, and a study funded by an interested party may emphasise the stages that favour its own product; independently produced or verified assessments are more reliable.
3. Explain why recycling materials, particularly metals, reduces environmental impact: recycling avoids the energy-intensive steps of mining and extracting new metal from ore, so it typically uses far less energy than producing the same mass of new metal, while also conserving limited ore reserves and reducing the volume of waste sent to landfill.
4. Define corrosion as the gradual destruction of a material by chemical reaction with substances in its environment, and identify rusting as the specific name for the corrosion of iron; state, from the classic experiment (an iron nail rusts in a tube of water open to air, but not in boiled, oxygen-free water sealed under a layer of oil, and not in dry air over a drying agent), that rusting needs both oxygen and water to be present together.
5. Explain how barrier methods prevent corrosion: coatings such as paint, oil, grease or electroplating with another metal work by physically excluding oxygen and water from reaching the surface of the metal underneath, so no reaction can occur while the coating remains intact.
6. Explain sacrificial protection using the reactivity series: attaching a block of a more reactive metal, such as zinc or magnesium, to iron or steel means the more reactive metal loses electrons and corrodes (is oxidised) in preference to the iron, protecting the iron for as long as any of the more reactive metal remains; galvanising, coating iron with a layer of zinc, combines this sacrificial effect with a physical barrier.
7. Explain why most everyday metal objects are made from alloys rather than pure metals: mixing a metal with one or more other elements introduces atoms of a different size into the regular layered structure of the metal, which distorts the layers and makes it harder for them to slide over one another, so most alloys are harder than the pure metal alone; name examples such as bronze (copper and tin), brass (copper and zinc), and steels (iron with carbon, and often other metals such as chromium and nickel in stainless steel).

Practice questions

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