C35: Corrosion and Its Prevention
Understanding corrosion as the degradation of metals by chemical reactions with the environment, focusing on rusting of iron and the range of methods used to prevent it.
What is Corrosion?
Corrosion is the destruction of materials by chemical reactions with substances in the environment. For metals, this usually involves oxidation — the metal reacts with oxygen (and often water) to form compounds such as oxides.
Corrosion is a problem because it weakens metal structures, increases maintenance costs and can cause structural failure. Different metals corrode at different rates:
- Iron and steel — corrode relatively quickly (rusting)
- Aluminium — corrodes slowly (forms a protective oxide layer)
- Copper — corrodes slowly (forms a green patina)
- Gold and platinum — do not corrode (very unreactive)
Rusting of Iron
Rusting is the specific name for the corrosion of iron. Iron rusts only in the presence of both oxygen (from air) and water. The product of rusting is hydrated iron(III) oxide.
Iron + Oxygen + Water → Hydrated iron(III) oxide (rust)
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
Rust is a soft, crumbly substance that flakes off the surface of iron, exposing fresh iron underneath. This means rusting continues until all the iron has corroded — it is a progressive process.
Rusting Investigation
To show that both water and oxygen are needed for rusting, set up three test tubes with iron nails:
- Tube 1: Nail in water only (boiled to remove dissolved oxygen, layer of oil to prevent air entering) — no rust
- Tube 2: Nail in dry air only (with a drying agent such as calcium chloride) — no rust
- Tube 3: Nail in both water and air — nail rusts
This demonstrates that both water and oxygen are required for rusting to occur.
Salt water accelerates rusting. This is why cars and structures in coastal areas or on roads where salt is spread in winter corrode faster.
Why Aluminium Does Not Rust
Aluminium is more reactive than iron, yet it does not corrode in the same way. When aluminium reacts with oxygen, it forms a thin, dense layer of aluminium oxide (Al₂O₃) on its surface.
The aluminium oxide layer is impermeable to water and oxygen, so it acts as a protective barrier. This prevents further corrosion underneath. This is called passivation.
This is different from iron rust, which is porous and flakes off, allowing continued corrosion. The protective oxide layer on aluminium means it appears to resist corrosion even though it is quite a reactive metal.
Methods of Preventing Corrosion
There are several ways to prevent corrosion, all based on preventing the metal from coming into contact with oxygen and water.
Painting
Paint creates a physical barrier between the metal surface and the environment, preventing water and oxygen from reaching the metal. Painting is commonly used on cars, bridges, ships and structures. It is relatively cheap and easy to apply but can chip or scratch, exposing the metal underneath. Painted surfaces require regular maintenance and repainting.
Greasing/Oiling
Applying grease or oil to a metal surface creates a water-repellent barrier. This method is commonly used for moving parts such as bicycle chains, tools and machinery where paint would wear off. Greasing needs regular reapplication as the oil or grease can wear away or be washed off.
Galvanising
Galvanising involves coating iron or steel with a layer of zinc. This is usually done by dipping the metal object into molten zinc. The zinc layer acts as a barrier, preventing water and oxygen from reaching the iron. Even if the zinc layer is scratched, the zinc still protects the iron because zinc is more reactive than iron and will corrode preferentially (sacrificial protection). Galvanising is used for car bodies, lampposts, buckets and fencing.
Sacrificial Protection
A more reactive metal is attached to or coated onto the iron. The more reactive metal corrodes instead of the iron because it oxidises more readily. Common sacrificial metals include zinc and magnesium. Examples include:
- Zinc blocks attached to the hulls of ships and to underground pipes
- Magnesium blocks attached to oil rigs and underwater steel structures
The sacrificial metal must be replaced periodically as it corrodes away, but it protects the iron or steel structure from rusting.
Tin Plating
Iron can be coated with a layer of tin to create tinplate (used for food cans). Tin is less reactive than iron, so it acts only as a barrier. If the tin layer is scratched, the iron underneath will corrode faster than if it were unprotected because the iron becomes the sacrificial metal for the tin. This is a disadvantage compared to galvanising.
Comparison of Corrosion Prevention Methods
| Method | How it Works | If Scratched | Typical Uses | Durability |
|---|---|---|---|---|
| Painting | Barrier — blocks water and oxygen | Iron rusts underneath scratch | Cars, bridges, structures | Low — needs regular repainting |
| Greasing | Barrier — water-repellent layer | Iron rusts if exposed | Tools, chains, moving parts | Low — needs regular reapplication |
| Galvanising | Barrier + sacrificial (zinc) | Zinc still protects iron sacrificially | Lampposts, buckets, fencing | High — long-lasting protection |
| Sacrificial protection | More reactive metal corrodes instead | Protection continues | Ships, pipes, oil rigs | Medium — blocks need replacing |
| Tin plating | Barrier only (tin less reactive than iron) | Iron corrodes faster than normal | Food cans | Medium — risk if scratched |
The key difference between galvanising and tin plating: if the coating is scratched, galvanised iron is still protected (zinc is sacrificial) but tin-plated iron rusts even faster (iron is sacrificial to tin).
Electrochemical Series and Sacrificial Protection
For sacrificial protection to work, the protecting metal must be more reactive (higher in the reactivity series) than the metal being protected. The more reactive metal loses electrons more readily and corrodes in preference to the less reactive metal.
Reactivity series (relevant metals, most reactive first):
- Magnesium — used as sacrificial anode
- Aluminium — forms protective oxide layer
- Zinc — used in galvanising and as sacrificial anode
- Iron — the metal being protected
- Tin — less reactive than iron, only acts as barrier
- Copper — less reactive than iron
Ships and Sacrificial Protection
Large ships have zinc or magnesium blocks bolted to their steel hulls below the waterline. These blocks corrode instead of the steel hull. When the blocks have corroded away, they are replaced. This is far cheaper and more practical than allowing the hull to rust, which would weaken the ship's structure.
Corrosion in Everyday Life
Corrosion has significant economic costs. It is estimated that corrosion costs billions of pounds each year worldwide through:
- Replacement of corroded structures and components
- Maintenance and repainting of bridges, ships and infrastructure
- Safety failures due to weakened structures
- Loss of product from corroded containers and pipes
Preventing corrosion is almost always more cost-effective than dealing with the consequences of corrosion damage.
When answering questions about corrosion prevention, always consider both how the method works and what happens if the protective layer is damaged. This shows a deeper understanding.
Practice Questions
1. State the two substances required for iron to rust.
Water and oxygen (from the air) are both required for iron to rust. If either is absent, rusting does not occur.
2. Explain why aluminium does not corrode in the same way as iron, even though aluminium is more reactive.
When aluminium reacts with oxygen, it forms a thin, dense, impermeable layer of aluminium oxide that sticks firmly to the surface. This layer prevents water and oxygen from reaching the aluminium underneath. Iron rust, in contrast, is soft and porous, flakes off and exposes fresh iron to further corrosion.
3. Explain why galvanising provides better protection than tin plating if the coating is scratched.
Galvanising coats iron with zinc, which is more reactive than iron. If the zinc is scratched, the zinc corrodes sacrificially in preference to the iron, so the iron is still protected. Tin plating coats iron with tin, which is less reactive than iron. If the tin is scratched, the iron corrodes even faster because it acts as the sacrificial metal for the tin.
4. Describe how sacrificial protection works on a ship's hull.
Blocks of zinc or magnesium (which are more reactive than iron) are attached to the steel hull. These blocks corrode in preference to the steel because they oxidise more readily. The sacrificial blocks protect the hull from rusting and are replaced when they have corroded away.
5. Write the word equation for the rusting of iron.
Iron + Oxygen + Water → Hydrated iron(III) oxide
Maths Skills
Corrosion Rate and Cost Calculations
Corrosion costs are estimated at billions of pounds per year globally. Understanding the maths helps evaluate prevention methods.
Cost comparison: If an untreated steel pipeline costs 50,000 pounds to replace every 5 years due to corrosion, and galvanising costs 12,000 pounds but extends the life to 25 years, the cost per year for untreated is 50,000/5 = 10,000 pounds/year, and for galvanised is 12,000/25 = 480 pounds/year plus initial pipe cost. Galvanising saves 10,000 - 480 = 9,520 pounds per year.
Surface area calculations: A rectangular steel tank measures 2 m x 1.5 m x 1 m. Surface area = 2(2x1.5) + 2(2x1) + 2(1.5x1) = 6 + 4 + 3 = 13 m2. If paint covers 5 m2 per litre, the paint needed = 13/5 = 2.6 litres (round up to 3 litres for two coats).
Corrosion rate data: If iron loses 0.1 mm per year in a coastal environment and 0.02 mm per year in a dry inland environment, corrosion in the coastal environment is 0.1/0.02 = 5 times faster, due to salt water acting as an electrolyte accelerating the electrochemical corrosion process.
Common Misconceptions
Misconceptions About Corrosion and Rusting
Rusting only needs water — iron rusts whenever it gets wet.
Rusting requires both water AND oxygen. Iron in contact with only water (with no dissolved oxygen) will not rust significantly. Similarly, iron in completely dry air will not rust because there is no water. Rusting is the oxidation of iron by oxygen in the presence of water. This is why iron rusts faster in humid air or when sprayed with salt water — both provide water and oxygen.
Painting prevents rust by reacting with the iron to form a protective layer.
Painting prevents rust by creating a physical barrier that keeps water and oxygen away from the iron surface. The paint does not react with the iron — it simply prevents the reactants (water and oxygen) from reaching the iron. If the paint is scratched or damaged, water and oxygen can reach the exposed iron and rusting will occur at that point.
Sacrificial protection means the zinc reacts with the iron to stop it rusting.
In sacrificial protection, the zinc does not react with the iron. Zinc is more reactive than iron, so zinc oxidises (loses electrons) in preference to the iron. The zinc corrodes sacrificially, protecting the iron even when the coating is scratched. The zinc acts as the anode in a galvanic cell, while the iron is protected as the cathode.
6-Mark Question
Explain how and why iron rusts. Describe and compare different methods of preventing corrosion.
Iron rusts when it reacts with oxygen and water to form hydrated iron(III) oxide (rust). Both water and oxygen must be present for rusting to occur. The chemical equation is: iron + oxygen + water and then hydrated iron(III) oxide. Rust is a flaky, porous substance that falls off the surface, exposing fresh iron to further corrosion — so rusting is a progressive problem. Methods of preventing corrosion include: painting (creates a barrier to keep out water and oxygen — cheap but needs regular maintenance as paint chips); oiling/greasing (creates a barrier, useful for moving parts, but must be reapplied); galvanising (coating iron with a layer of zinc — zinc provides barrier protection and sacrificial protection if scratched, very effective and long-lasting); sacrificial protection (attaching a more reactive metal like magnesium or zinc blocks to the iron — the more reactive metal corrodes instead, used for ship hulls and underground pipes); and using stainless steel (an alloy containing chromium which forms a protective oxide layer). Galvanising is often the best method because it provides both barrier and sacrificial protection, whereas painting only provides a barrier and fails if scratched.
AO3: Analysis and Evaluation
Evaluating Corrosion Prevention for Different Applications
An engineer needs to protect the following from corrosion: a garden gate, a ship's hull, and a bicycle chain.
Evaluate the most appropriate method of corrosion prevention for each, justifying your choice.
Answer: A garden gate is best protected by galvanising or painting. Galvanising is preferable because it lasts for decades without maintenance and provides sacrificial protection if scratched. Painting is cheaper initially but requires regular repainting. A ship's hull needs sacrificial protection using zinc or magnesium blocks attached to the steel hull, combined with a protective paint coating. The paint acts as the primary barrier, and the sacrificial blocks protect any areas where the paint is damaged. This is essential because a ship is constantly exposed to seawater (an electrolyte) which accelerates corrosion. A bicycle chain cannot be painted or galvanised (it needs to be flexible), so oiling or greasing is most appropriate. The oil creates a water-repellent barrier and lubricates the chain, but it must be reapplied regularly because it wears off with use and exposure to weather.
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