Steel is an alloy of iron containing other elements to improve its physical properties. Mild steel is iron with about 0.25% carbon added; stainless steel is iron with chromium and nickel added to reduce rusting. Iron will rust whenever it comes into contact with both water and oxygen. If you can stop one of those two from reaching it then it will not rust. Iron mechanisms are greased (or oiled) to lubricate and prevent water reaching the steel, iron gates are painted to keep the weather off them and steel paper-clips can be coated with plastic to stop them from rusting. All of these methods work well, until the oil or paint or plastic is scratched or damaged. This then allows oxygen and water to get in so rusting occurs.
A different method of protection is called galvanising. This means coating the steel in a thin layer of zinc. Firstly the steel is protected by being entirely coated in a strong, flexible layer that stops water and oxygen reaching it. Secondly, even if the surface is scratched, the zinc protects the steel by ‘sacrificially’ reacting with oxygen. The steel will not rust as long as there is zinc present to react in its place.
This is possible because zinc is higher in reactivity than iron. The zinc reacts 1st with oxygen to become zinc oxide, 2nd with water to become zinc hydroxide and 3rd with carbon dioxide in the air to become zinc carbonate which sticks very tightly to the metal surface providing an excellent barrier. This is in contrast to steel coated in tin, which is a less reactive metal. When it is scratched, it speeds up rusting, which is why it is only used in circumstances where it is unlikely to be scratched, such as inside a baked bean tin.
If a steel object, such as the hull of a ship, an underwater pipeline or an oilrig is submerged in water then it does not need to be completely coated in zinc to be protected. Lumps of zinc or magnesium can be attached as ‘sacrificial anodes’. The difference in reactivity between the iron and the zinc creates an electrical potential between the two, with the water completing the circuit. If the iron tries to rust, electrons travel from the sacrificial anode to the iron, reducing the iron oxide back to iron. Meanwhile the zinc or magnesium loses electrons and is oxidised. This process will continue to protect the iron until the zinc or magnesium has been completely sacrificed, at which point it will need to be replaced.
Questions…
- Suggest two physical characteristics shared by all metals.
- How could you show that zinc is more reactive than copper?
- What word means a) the gain of oxygen and b) the loss of oxygen?
- Why do sheets of galvanised steel wear away more quickly in areas where acid rain is a problem?
- From the last paragraph, can you infer the word we use for a) an atom losing electrons and b) an atom gaining electrons?
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