This topic accounts for approximately 6% of your exam marks.
stable
Low
Stable6%
Displacement reactions and determining order of reactivity from experimental data.
Conditions for rusting
Rust is the orange-brown flaky oxide that forms on iron and steel
Both air (oxygen) and water must be present for iron to rust — neither on its own is enough
The chemical name for rust is hydrated iron(III) oxide, Fe2O3·xH2O
Investigating the conditions
Set up three test tubes, each containing a clean iron nail:
Tube A — open to the air, partly filled with tap water (contact with both air and water)
Tube B — partly filled with water that has been boiled to drive off dissolved oxygen, then sealed with a layer of oil on top (contact with water but not air)
Tube C — contains a drying agent such as anhydrous calcium chloride, sealed with a stopper (contact with air but not water)
After several days:
The nail in tube A is rusty
The nails in tubes B and C are unchanged
Conclusion: both oxygen and water are required; removing either prevents rusting
Why rusting is a problem
Rust is soft and flakes off the metal, exposing a fresh iron surface underneath
Once started, rusting therefore continues right through the bulk of the iron, weakening structures over time
This is different from the protective oxide that forms on aluminium, which seals the surface and stops further reaction
Prevention by barrier methods
Coating iron physically isolates the metal from oxygen and water
Common barrier coatings: paint, oil, grease, plastic, and an electroplated layer of an unreactive metal (such as tin on tin cans)
Drawback: if the coating is scratched, the bare iron is exposed and rust begins, often spreading under the coating
Prevention by sacrificial protection
A more reactive metal is bolted to the iron object
The more reactive metal corrodes preferentially because it loses electrons more readily, so the iron stays intact
Zinc blocks attached to the steel hulls of ships are a textbook example: the zinc rusts away first and is replaced periodically
Galvanising
Galvanising coats the iron with a layer of zinc, applied by dipping the iron into molten zinc or by electroplating
The zinc layer protects in two ways at once:
As a barrier, it keeps oxygen and water away from the iron underneath
As sacrificial protection, if the coating is scratched the zinc still corrodes preferentially, so the exposed iron is not attacked
A note on terminology
Corrosion is the general term for the slow chemical attack of a metal surface by its environment
Rusting is specifically the corrosion of iron — every rusting reaction is a corrosion reaction, but not every corrosion reaction is rusting