9.5 Corrosion of metals
- Syllabus
- 0620–2026–2027
- Topic
- 9.5
- Level
- —
Learning objectives
9.5.1Conditions required for the rusting of• State the conditions required for the rusting of iron and steel to form hydrated iron(III) oxide9.5.2Some common barrier methods• State some common barrier methods, including painting, greasing and coating with plastic9.5.3Barrier methods prevent rusting by• Describe how barrier methods prevent rusting by excluding oxygen or water9.5.4Use of zinc in galvanising as an• Describe use of zinc in galvanising as an example of a barrier method and sacrificial protection9.5.5Sacrificial protection in terms of the• Explain sacrificial protection in terms of the reactivity series and in terms of electron loss
State the conditions for rusting
Iron and steel rust only when both oxygen and water are present. The rust formed is hydrated iron(III) oxide.
| Test condition | Oxygen present? | Water present? | Rust? |
|---|---|---|---|
| ordinary damp air | yes | yes | yes |
| dry air with a drying agent | yes | no | no |
| boiled water protected by an oil layer | no | yes | no |
Word equation: iron + oxygen + water → hydrated iron(III) oxide.
Oxygen alone or water alone is insufficient. Rust is hydrated iron(III) oxide, not anhydrous iron oxide or iron(III) hydroxide.
Name common barrier methods
A barrier method covers iron or steel with a layer that separates the metal from its surroundings.
| Common barrier method | Applied layer | Typical fit |
|---|---|---|
| painting | solid paint film | exposed structures and objects |
| greasing or oiling | grease/oil film | moving parts or surfaces needing renewal |
| coating with plastic | continuous polymer layer | objects that can be fully covered |
The coating must be continuous and maintained. Choose a method that can stay attached and cover the surface during use.
Galvanising also forms a barrier but has an additional sacrificial effect; it is treated separately from these common barrier-only methods.
Explain how barriers prevent rusting
A complete barrier prevents oxygen or water from reaching the iron surface, so the two reactants required for rusting cannot both contact the metal.
| Barrier state | Contact with iron | Outcome |
|---|---|---|
| intact paint, grease or plastic | oxygen and/or water excluded | rusting prevented |
| scratched, cracked or worn coating | oxygen and water can reach exposed iron | rusting can begin |
State the excluded substance explicitly: painting prevents oxygen and water from reaching the iron. A barrier does not make iron chemically unreactive.
A barrier works only while coverage is effective. Ordinary paint or plastic does not sacrificially protect exposed iron after a scratch.
Explain the two roles of zinc in galvanising
Galvanising coats iron or steel with zinc.
| Zinc coating condition | Protection mechanism |
|---|---|
| intact | barrier: zinc blocks oxygen and water from reaching iron |
| scratched so iron is exposed | sacrificial: zinc is more reactive and is oxidised in preference to iron |
Because zinc provides both mechanisms, galvanised iron can remain protected even when a small area of coating is damaged, unlike paint or plastic.
Do not say galvanising forms an alloy. It is a zinc coating, and the sacrificial action depends on zinc being more reactive than iron.
Explain sacrificial protection with electrons
Attach a metal above iron in the reactivity series, such as zinc or magnesium. The more reactive metal loses electrons and is oxidised instead of the iron.
| Step | Zinc example |
|---|---|
| sacrificial metal is more reactive | Zn is above Fe |
| sacrificial metal loses electrons | Zn → Zn²⁺ + 2e⁻ |
| electrons are supplied to the protected iron system | iron is prevented from losing electrons |
| consequence | zinc corrodes; iron is not oxidised to iron ions |
Magnesium and zinc can protect steel because they are more reactive than iron. Copper, silver and gold cannot: they are below iron and do not lose electrons in preference to it.
Sacrificial protection is not simply blocking oxygen or water. It can protect exposed iron because the attached metal undergoes oxidation and supplies electrons preferentially.