Definition
Rusting is the slow oxidation of iron to form hydrated iron (III) oxide. Rusting is also known as the corrosion of iron
Note
Rusting is a word meant for iron only. Only iron rusts. We cannot use this word for other metals. For other metals, we use the term corrode for similar phenomenon
Conditions required for rusting
- Water
- Oxygen (in air)
Factors which increase rate of rusting
- Presence of salt
- Presence of acidic substances
Rust prevention
Rusting causes great damage → as it weakens structures of buildings, bridges in which it is used to build. Hydrated iron (III) oxide → is ionic and not a strong material in building structures
Hydrated iron (III) oxide (rust) → flakes off → i.e. the newly exposed surface of iron will react with more oxygen and water → so the iron underneath continues to rust (contrast this with aluminium oxide layer → when it is on surface of aluminium when aluminium corrodes. Layer of aluminium oxide is non porous thus it does not allow air or water vapour to come into contact with aluminium underneath) Hence aluminium oxide serves as a protective layer preventing further reaction between aluminium and oxygen. Bases
3 Main methods of prevention of rusting
- Using protective layer
- Using alloys
- Sacrificial Protection
| Method of Rust Prevention | Application |
|---|---|
| 1. Using a protective layer - Coating with paint - Coating with oil - Coating with plastic - Coating with zinc (zinc plating is also known as galvanising) | - For iron and steel structures like bridges and ships. - For machineries - Paper clips, clothes hangers - Dustbins, buckets. |
| 1. Using alloys Properties of Metals | For surgical tools, cutlery |
| 3. Sacrificial Protection | underwater pipes which has magnesium attached to them; protect hull of ships. (or zinc) |
| Method 1 and 2 involves covering the iron with another material which acts as a protective layer to prevent water and oxygen in air from being in contact with them |
Rust Prevention using Sacrificial Metal (AKA Sacrificial Protection)
How does using a more reactive metal such as zinc or magnesium prevents underground iron pipes from rusting
Underground iron pipes → do not rust due to sacrificial protection → of iron by magnesium.
- Magnesium is more reactive than iron → as magnesium loses electrons to form its positive ions more readily.
- Hence when the 2 metals are attached to each other → electrons will flow from magnesium to iron → preventing oxidation of iron.
- Any that could form by iron would receive electrons from the magnesium and reduced back to Fe atoms.
- Magnesium atoms are oxidised to form magnesium ions.
- Thus magnesium will react more readily with oxygen and corrodes preferentially in place of iron
Summary
- Sacrificial protection of iron involves attaching a more reactive metal (usually magnesium or zinc) to a piece of iron. The more reactive metal → will react more readily with oxygen and corrode preferentially in place of iron
Note
since magnesium bar corrodes in place of iron → it means that the strip will have to be replaced eventually to maintain protection.
- you can think of it as there is a limit to how many times it can undergo oxidation by losing electrons to Fe
Sample
explain what happens if less reactive metal is used for sacrificial protection Iron in steel pipe will rust at a faster rate. (1) Fe is more reactive than Cu as Fe loses electrons to form its positive ion more readily (1). Thus Fe will react more readily with O2 and corrode preferentially in place of Cu.(1)