Corrosion and Protection of Metals
AdvancedIron rusts only when BOTH oxygen and water are present. Protection: paint/oil (barrier), galvanising (sacrificial protection), stainless steel (alloy). Galvanising works because zinc is MORE reactive than iron — it corrodes 'sacrificially' to protect the iron.
- Rust = iron + oxygen + water
- Barrier methods: paint, oil, plating
- Galvanising = sacrificial protection: zinc is more reactive and goes first
§Text Passage
1Corrosion destroys enormous quantities of metal every year — estimated at more than a tenth of annual world production. Iron rusts only when two conditions occur together: oxygen and water. Control experiments prove the point: a nail in dry air does not rust; a nail in boiled, deoxygenated water sealed with oil does not rust either; only the nail touching both water and air rusts. Rust is loose and flakes off easily, exposing fresh iron again, so corrosion keeps eating inwards until the iron object fails.
2The first family of protection cuts the iron off from oxygen and water: railings are painted, moving machine parts oiled, cans tin-plated. These methods are cheap and easy to apply, but once the coating is scratched the iron beneath is exposed and rusting starts right at the scratch, so regular inspection and retouching are needed; oil also lubricates, which makes it especially suitable for moving parts.
3The second method lets a more reactive metal sacrifice itself: zinc is more reactive than iron, so galvanising iron, or bolting zinc blocks to a ship's hull, means the zinc corrodes first even when the coating is scratched, leaving the iron beneath protected — this is sacrificial protection, and used-up zinc blocks are simply replaced, far cheaper than repairing a hull. The third method changes the material itself: adding chromium and nickel to iron makes stainless steel, whose surface forms a dense protective film; strongly rust-resistant, it is widely used for cutlery and medical instruments.
4Choosing a protection method means weighing environment, use and cost: outdoor railings usually get paint; hulls sitting in seawater rely most reliably on zinc blocks; cutlery that touches food uses stainless steel. Bridges, rails and underground pipework all need regular inspection and maintenance, or rust-through can cause structural failure and even accidents; preventing rust costs far less than repairing after it, so anti-corrosion work is an indispensable part of infrastructure upkeep.
1Summary 重點摘要
Iron rusts only when BOTH oxygen and water are present. Protection: paint/oil (barrier), galvanising (sacrificial protection), stainless steel (alloy). Galvanising works because zinc is MORE reactive than iron — it corrodes 'sacrificially' to protect the iron.
2Vocabulary cards Vocabulary
A metal being eaten away by substances in its environment.
Specifically iron oxidising to red-brown rust (needs oxygen + water).
Coating iron with zinc to prevent rust.
Using a more reactive metal to protect the target — the reactive one corrodes first.
The flaky red-brown product of iron oxidising — it falls off and exposes fresh metal.
Keeping oxygen and water away from iron with paint, oil or plating.
Coating iron with tin (e.g. food cans); tin is less reactive, so a scratch makes the iron rust faster.
An alloy of iron with chromium and nickel — its surface layer resists rust.
3Step by step Steps
- Rusting conditions (experiment)The control experiment gives a clear conclusion: a nail in dry air does not rust because water is missing; a nail in boiled, deoxygenated water sealed with oil does not rust because oxygen is missing; only the nail touching both water and air rusts — proving that oxygen AND water are both essential. Rust is loose and flakes off easily, exposing fresh iron to the air again, so corrosion keeps eating inwards until the iron object fails.
- Method 1: barrierPaint, oil and electroplating all cut the iron off from oxygen and water: gates are painted, moving machine parts oiled, cans tin-plated — and the inside of a can carries an extra protective film. These methods are cheap and easy to apply, but once the coating is scratched the iron beneath is exposed and rusting starts right at the scratch, so regular inspection and retouching are needed; oil also lubricates, which suits moving parts.
- Method 2: galvanising (sacrificial)Zinc is more reactive than iron, so even when a galvanised coating is scratched the zinc corrodes first, 'sacrificing itself' to protect the iron beneath — this is sacrificial protection. Ship hulls carry zinc blocks and pipes are galvanised on the same principle; the zinc blocks, also called sacrificial anodes, are simply replaced once used up, far cheaper than repairing a hull or replacing pipelines. The same idea protects underground pipelines and the metal parts inside water heaters.
- Method 3: alloyAdding chromium and nickel to iron makes stainless steel, which forms a dense protective film on its surface and resists rust strongly — ideal for cutlery and medical instruments. But stainless steel is rust-resistant rather than rust-proof: prolonged contact with seawater still corrodes it. It is also costlier, so it is used mainly where hygiene and appearance matter — kitchenware and surgical instruments — rather than in large structures.
- The cost of corrosionThe losses caused by corrosion are enormous: the metal destroyed by corrosion each year worldwide is estimated at more than a tenth of annual production. Bridges, rails, railings and underground pipework all need regular inspection and maintenance, or rust-through can lead to structural failure and even accidents; preventing rust costs far less than repairing and replacing after it has eaten through, so anti-corrosion work is an indispensable part of infrastructure upkeep.
- Choosing a protection methodChoose the protection method by environment, use and cost: outdoor railings get paint — cheap and easy to retouch; moving machine parts get oil, which also lubricates; hulls sitting in seawater carry zinc blocks, as sacrificial protection is the most reliable there; cutlery touching food uses stainless steel for hygiene and durability. Real projects often combine several methods so that they complement one another and give the best overall protection.
4Diagrams Diagrams
5Worked example Example
Why do ships carry blocks of zinc on their hulls?
- Seawater is highly corrosive
- Zinc is more reactive than the iron hull
- The zinc blocks corrode first (sacrificial protection)
- The iron hull is thus protected
- Answer: Seawater is highly corrosive and would rust the iron hull quickly; since zinc is more reactive than iron, the zinc blocks bolted to the hull act as sacrificial anodes — the seawater corrodes the zinc first, leaving the hull protected. When the zinc is used up it is simply replaced, far cheaper than repairing the hull. Write the chain 'zinc more reactive → zinc corrodes first → hull protected → replaced regularly', and name 'sacrificial protection'. The same method also protects underground pipes and the magnesium rods in water heaters.
6Common mistakes Mistakes
7Quick check Quick check
Q1What two conditions does iron need to rust?
Q2Why does galvanising protect even when scratched, but tin-plating fails once scratched?
Q3Give two rust-prevention methods and their principles.
Q4In the control experiment, why doesn't a nail rust in boiled water sealed with oil?
Q5Why must a ship's zinc blocks be replaced regularly?
8Recap Recap
- Rust = iron + oxygen + water
- Barrier methods: paint, oil, plating
- Galvanising = sacrificial protection: zinc is more reactive and goes first
- Control experiments: dry air and oxygen-free water don't rust — both oxygen and water are essential
- When scratched, galvanising still protects (zinc corrodes first); tin-plating speeds up rusting