Galvanising
Definition
Galvanising
Coating iron or steel with a layer of zinc to protect it from rusting.
- Steel is galvanised by dipping it into a bath of molten zinc, which leaves a thin, even coating of zinc over the whole surface.
- Galvanised steel is used for objects that stand outdoors for years, such as roofing sheets, fences, lamp posts, crash barriers and buckets.
- The zinc coating acts as a barrier method, because it stops oxygen and water from reaching the steel underneath.
- The zinc itself reacts slowly with the air to form a dull, protective surface layer, so the coating lasts a long time.
- Unlike paint, a zinc coating carries on protecting the steel even after it has been scratched, because zinc also gives sacrificial protection.
Sacrificial protection
Definition
Sacrificial protection
The protection of iron from rusting by a more reactive metal in contact with it, which loses electrons and corrodes in place of the iron.
Definition
Oxidation
The loss of electrons by a substance.
- Zinc is above iron in the reactivity series, so zinc is more reactive than iron.
- A more reactive metal has a greater tendency to lose electrons and form positive ions.
- When a galvanised surface is scratched, oxygen and water reach the bare iron, but the zinc around the scratch loses electrons in preference to the iron: Zn(s)→Zn2+(aq)+2e−\mathrm{Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-}Zn(s)→Zn2+(aq)+2e−.
- The zinc is oxidised and slowly corrodes away, which is why it is described as being sacrificed.
- The electrons released by the zinc pass into the iron, so the iron does not lose its own electrons.
- The iron is not oxidised to iron ions, so it does not rust.
- The protection lasts only as long as some zinc remains in contact with the iron.
- Any metal more reactive than iron can give sacrificial protection, and magnesium is often used as well as zinc.
- Blocks of zinc or magnesium are bolted onto the steel hulls of ships, oil rigs and underground pipelines.
- The blocks corrode instead of the steel and are replaced when they have worn away.
- A less reactive metal such as copper cannot protect iron in this way, because iron loses electrons more readily than copper does.
Practical
Testing sacrificial protection
- Aim: to show that a more reactive metal in contact with iron stops it rusting, while a less reactive metal does not.
- Apparatus: four iron nails, a strip of magnesium ribbon, a strip of zinc foil, a strip of copper foil, emery paper, four test tubes in a rack, sodium chloride solution and labels.
- Method, setting up:
- Rub each nail and each metal strip with emery paper until it is shiny.
- Wrap the magnesium ribbon tightly around the first nail, the zinc foil around the second and the copper foil around the third, so that each metal presses firmly against the iron.
- Leave the fourth nail bare as a control.
- Stand each nail in its own labelled tube and cover it with salt solution, which speeds up rusting, leaving the tubes open to the air.
- Method, observing:
- Leave the tubes for four to five days.
- Compare the amount of orange-brown rust on each nail and the appearance of each wrapping metal.
- Results: the nails wrapped in magnesium and zinc show no rust while the magnesium and zinc look dull and pitted, the bare nail rusts, and the nail wrapped in copper rusts even more than the bare nail.
- Conclusion: magnesium and zinc are more reactive than iron, so they lose electrons and corrode instead of the iron, but copper is less reactive and gives no protection.
- Watch out: the wrapping metal must stay in firm contact with the nail, because sacrificial protection works only when the two metals touch.
- Safety: wear eye protection, and wash your hands after handling the corroded metals and the salt solution.
Barrier and sacrificial protection compared
- Barrier protection works only while the coating is complete, so painted steel starts to rust as soon as the paint is chipped.
- Sacrificial protection still works when the surface is damaged, because the more reactive metal corrodes instead of the exposed iron.
- Galvanising gives both kinds of protection at once, which is why galvanised steel lasts far longer outdoors than painted steel.
Example
Protecting a steel ship
- A steel hull is painted to give barrier protection.
- Blocks of zinc are also bolted to the hull below the waterline.
- Where the paint is damaged, zinc loses electrons more readily than iron, so the zinc corrodes and the exposed steel does not rust.
- The zinc blocks are replaced in dry dock once they have corroded away.
Exam technique
Explaining sacrificial protection
- Mark schemes look for two ideas: zinc is more reactive than iron, and zinc loses electrons or is oxidised in preference to iron.
- Use the phrase in preference to or instead of, so that it is clear the zinc reacts and the iron does not.
- For a question on galvanising with enough marks, describe both the barrier effect and the sacrificial effect.
- Write the oxidation half-equation for zinc with the electrons on the right-hand side.
Common Mistake
- Do not say that zinc is less reactive than iron, because a sacrificial metal must be more reactive than iron.
- Do not say that sacrificial protection works by keeping out oxygen and water, because that is the barrier effect.
- Do not say that the iron loses electrons to the zinc, because electrons flow from the zinc to the iron.
Self review
- What is galvanising?
- How does a zinc coating act as a barrier?
- Why does zinc protect iron even when the coating is scratched?
- Write the half-equation for the oxidation of zinc.
- Why can copper not be used to give sacrificial protection to iron?
