Loading…
Loading…
Loading…
Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · 5.3C Check the specification (PDF) (opens in a new tab)
Rusting is the corrosion of iron. Iron rusts when are present, forming hydrated iron(III) oxide. This gives two ways to prevent the reaction: keep oxygen away from the iron, or keep water away. Removing either one is enough to prevent rusting.
Paint, grease, oil and plastic coatings act as barriers between iron and its surroundings. For example, a complete layer of paint stops oxygen and water reaching an iron gate. A grease or oil coating can protect a moving metal part where paint would be less suitable. The coating must cover the iron effectively: if paint chips or a plastic layer has a gap, exposed iron can rust where oxygen and water reach it.
Sacrificial protection uses a metal more reactive than iron, such as zinc or magnesium, in contact with the iron. The more reactive metal has a greater tendency to lose electrons, so it oxidises in preference to the iron. It is gradually used up while protecting the iron. Unlike an ordinary barrier, this protection can work at an exposed part of the iron, provided the sacrificial metal remains in contact with it.
Galvanising illustrates both methods at once. Its zinc coating keeps oxygen and water away from the iron underneath. If the coating is scratched, zinc in contact with the iron can still oxidise preferentially, protecting the exposed patch. The zinc is not permanent: it is consumed over time.
A barrier works by excluding oxygen or water; zinc also protects sacrificially when a scratch exposes the iron.
One way to investigate these ideas is to place similar iron nails in separate labelled test tubes: leave one untreated, paint or grease others, and wrap zinc or magnesium around another. Cover them with corrosion indicator and compare where its yellow colour turns blue after about half an hour. Blue patches show where rusting is starting. A gap in a barrier may leave a blue patch on that nail; a nail touching magnesium or zinc may show less rusting because the other metal is oxidised instead.
Iron needs both oxygen and water to rust. Paint, grease, oil or plastic can prevent rusting by excluding either reactant. A damaged barrier can leave iron exposed.
Zinc or magnesium in contact with iron oxidises preferentially, protecting the iron. Galvanising adds a zinc coating: it acts as a barrier and can protect a scratch sacrificially while zinc remains.
If a question asks how a coating prevents rusting, say that it excludes oxygen or water from the iron, not just that it ‘protects’ it.
For sacrificial protection, identify a metal more reactive than iron and explain that it oxidises instead of the iron.
A scratched paint coating may expose iron, whereas zinc can still give sacrificial protection at an exposed patch if it remains in contact with the iron.
It can see this note and what your specification asks for.
Answers are free with an account.
1 free note left this week
Get more free notesYour answer sets when this note comes back for review, or marks it complete.
Was this note helpful?
Log in to tell us whether it helped, or to report a problem.
Last updated
Rusting
The corrosion of iron when it reacts in the presence of both oxygen and water.
Barrier coating
A layer that prevents oxygen or water from reaching the iron beneath it.
Sacrificial protection
Protection of iron by a more reactive metal that oxidises in preference to the iron.
Galvanising
Coating iron or steel with zinc; the zinc forms a barrier and can also provide sacrificial protection.
Put your knowledge into practice — try past paper questions for Chemistry
Rusting
The corrosion of iron when it reacts in the presence of both oxygen and water.
Barrier coating
A layer that prevents oxygen or water from reaching the iron beneath it.
Sacrificial protection
Protection of iron by a more reactive metal that oxidises in preference to the iron.
Galvanising
Coating iron or steel with zinc; the zinc forms a barrier and can also provide sacrificial protection.