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CHEMISTRY · Stage 7 · Chapter 4: Chemical Reactions 1

Chemistry 4.5 Corrosion reactions — Rusting of Iron and Prevention

5 min read 8 parts Lesson 5 of 6

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4.5 Corrosion reactions — Rusting of Iron and Prevention — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define corrosion and chemical properties, explain what iron needs to rust and describe how rusting is prevented.Corrosion = a chemical reaction that happens o…

  • Everyday examples
  • Corrosion differences
  • Compared with physical properties
  • A nail in a tube with water and air
  • A nail in boiled (air-free) water with a layer of oil
  • A nail with dry calcium chloride, which absorbs water, and air

Part 1 of 8What corrosion is

  • Corrosion a chemical reaction that happens on the surface of a metal and may damage metal objects.
  • Rusting the corrosion of iron and steel.
  • Everyday examplescars and boats made from steel, an alloy of iron, are covered in rust.
  • Costmost metal things we use are made from steel, so rusting is an expensive problem that damages bridges, ships and cars.

Part 2 of 8Reactants and product

  • Reaction — iron reacts with oxygen and water, which happens slowly over weeks and months.
    • The oxygen comes from the air.
    • The water may be in the air as a gas or in its liquid state.
  • Word equation: iron + oxygen + water → hydrated iron(III) oxide (rust).
  • Product — hydrated iron oxide is iron oxide with water loosely joined to it, a flaky orange-brown solid.
  • Rust — it forms on the surface, is soft and crumbly, and easily comes off, leaving more iron exposed and ready to rust.

Part 3 of 8Chemical properties

  • Chemical properties the properties that describe a substance's chemical reactions, such as whether it burns, corrodes or reacts to make a precipitate.
  • Corrosion differencesmagnesium corrodes quickly, iron corrodes slowly and gold does not corrode at all.
  • Compared with physical propertieschemical properties can only be found out in chemical reactions, whereas physical properties can be observed without changing the substance.

Part 4 of 8Conditions for rusting

  • A nail in a tube with water and air — it rusts.
  • A nail in boiled (air-free) water with a layer of oil — it rusts very little, so air (oxygen) is needed.
  • A nail with dry calcium chloride, which absorbs water, and air — it does not rust, so water is needed.
  • Conclusion — rusting needs both oxygen and water.
  • Salt — it speeds up rusting, which is why cars rust faster near the sea or where roads are salted.
    • Dissolved salt helps charge move in the water layer on the iron.
  • Acid rain and pollution — they also increase corrosion.
  • Other metals — they corrode too, but only iron rusts, and only rust flakes off.

Part 5 of 8Investigating rusting

  • Five tubes — nail in air and water, nail in boiled water with oil, nail with dry air and calcium chloride, nail in salt water, and nail wrapped with magnesium ribbon.
  • Results after a week — the salt water nail is the most rusted, the oil and dry nails are clean, and the magnesium-wrapped nail is protected.
    • Magnesium is more reactive and sacrifices itself.
  • Fair test — record observations in a table and keep nail size and volume of liquid the same.

Part 6 of 8Preventing rust

  • Principle — all methods either stop oxygen and water reaching the iron or use a more reactive metal.
  • Painting — it forms a barrier on bikes, cars and boats.
  • Oil or grease — it is used in engines and on machines.
  • Plastic coating — it is a barrier on fridge shelves and wire.
  • Galvanising — coating iron or steel with zinc, which gives a barrier and protects even if scratched.
  • Sacrificial protection — a block of a more reactive metal such as zinc or magnesium is attached to the iron and corrodes instead of the iron.
    • It is used on ship hulls and pipelines.
  • Alloying — stainless steel is iron mixed with about 10% chromium, and nickel, and does not react with oxygen and water, so it does not rust.

Part 7 of 8Alloys and chemical properties

  • Alloy chemistrysome alloys have different chemical properties from the elements in them, and pure iron rusts but stainless steel does not.
  • Coststainless steel is more expensive than other steels, but worth it for things that must not rust and cannot be covered in paint or oil, such as surgeons' tools.
  • Aluminiumit resists corrosion because a thin, tough layer of aluminium oxide forms on its surface and protects the metal beneath.

Part 8 of 8Evaluating methods

  • Paint is cheap but needs repainting when scratched.
  • Galvanising lasts longer and protects at scratches but costs more.
  • Stainless steel is best for rust resistance but expensive.
  • Bridges — regular inspection and repainting is a major engineering cost.

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