Corrosion of metals

1. Overview

Corrosion is the process by which metals are gradually destroyed by chemical reactions with substances in their environment. While many metals corrode, the term rusting refers specifically to the corrosion of iron and its alloy, steel, which leads to significant structural damage in buildings, bridges, and vehicles if not prevented.

Key Definitions

• Corrosion: The gradual destruction of a metal due to its reaction with substances in the environment (such as oxygen and water).
• Rusting: The specific name for the corrosion of iron and steel.
• Hydrated Iron(III) Oxide: The chemical name for rust, a flaky orange-brown solid.
• Barrier Method: A method of rust prevention that creates a physical layer to stop oxygen and water from reaching the metal surface.
• Sacrificial Protection: A method where a more reactive metal is attached to iron; the more reactive metal reacts and corrodes instead of the iron.
• Galvanising: A specific type of protection where iron is coated with a layer of zinc.

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Core Content

Conditions Required for Rusting

For iron or steel to rust, two substances must be present:

1. Oxygen (from the air)
2. Water (moisture)

If either of these is missing, rusting will not occur. Salt or acid can act as catalysts to speed up the process, but they are not the primary requirements.

The Barrier Method

Barrier methods work by providing a physical coating that excludes oxygen and water from the surface of the iron.

• Painting: Used for large, stationary objects like bridges and car bodies.
• Greasing/Oiling: Used for moving parts where a solid coating like paint would crack, such as bicycle chains or engine parts.
• Plastic Coating: Used for items like garden fences and dish racks.
• Electroplating: Coating the iron with a thin layer of a different metal (like chromium or tin) that does not corrode.

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Extended Content (Extended Curriculum Only)

Galvanising

Galvanising is the process of coating iron or steel with a layer of zinc. It is a unique method because it works in two ways:

1. As a barrier: The zinc layer prevents water and oxygen from reaching the iron.
2. As sacrificial protection: If the zinc coating is scratched, the iron is still protected because zinc is more reactive than iron.

Sacrificial Protection and the Reactivity Series

In sacrificial protection, a metal higher up the reactivity series (more reactive) is used to protect iron.

• Commonly used metals: Zinc (Zn) or Magnesium (Mg).
• These metals have a higher tendency to lose electrons and form ions than iron does.

Electron Loss and Redox

Rusting involves the oxidation of iron atoms into iron ions by the loss of electrons:

• $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$

When a more reactive metal like zinc is present, it "sacrifices" itself by losing its electrons more readily than the iron:

• $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

Because the zinc supplies the electrons, the iron atoms remain in their metallic state and do not oxidise (rust).

Worked Example: Question: Why are blocks of magnesium attached to the hulls of steel ships? Answer: Magnesium is more reactive than the iron in steel. The magnesium loses electrons more easily than iron to form $Mg^{2+}$ ions. Therefore, the magnesium corrodes "sacrificially," preventing the iron from rusting.

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Key Equations

Formation of Rust (Word Equation): iron + oxygen + water → hydrated iron(III) oxide

Formation of Rust (Symbol Equation): $4Fe(s) + 3O_2(g) + xH_2O(l) \rightarrow 2Fe_2O_3 \cdot xH_2O(s)$ (Note: 'x' represents a variable number of water molecules)

Oxidation of Zinc (Sacrificial Protection): $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$

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Common Mistakes to Avoid

• ❌ Wrong: Saying that "corrosion" only happens to iron.
• ✓ Right: Corrosion is a general term for all metals; "rusting" is specific only to iron and steel.
• ❌ Wrong: Thinking that galvanising only works if the coating is intact.
• ✓ Right: Galvanising protects iron even when scratched because the zinc acts sacrificially.
• ❌ Wrong: Listing "air" as a requirement for rusting.
• ✓ Right: Be specific and say "oxygen" (air contains other gases like nitrogen that do not cause rusting).

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Exam Tips

• Command Words: If asked to "Describe an experiment," remember to mention the variables you are controlling (e.g., using boiled water to remove oxygen, or using a drying agent like calcium chloride).
• The "Hydrated" Keyword: When naming the product of rusting, always use the full name: hydrated iron(III) oxide. Just "iron oxide" is often not enough for the mark.
• Reactivity Reasoning: When explaining sacrificial protection, always state that the sacrificial metal is more reactive than iron and therefore loses electrons more easily.
• Real-world Context: Be prepared to suggest a specific protection method for a specific object. For example, use grease for a bike chain (moving parts) but paint for a bridge (large, stationary).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [4 marks]

Question:

A bicycle frame is made of steel.

(a) State the conditions necessary for the rusting of the steel frame. [2]

(b) Suggest two methods to prevent the bicycle frame from rusting. For each method, briefly describe how it prevents rusting. [2]

Worked Solution:

(a)

1. The steel must be in contact with oxygen and water. This states the two necessary conditions for rusting.

How to earn full marks:

• State both oxygen and water (or moisture).
• Do not include other substances like salt - these accelerate the reaction but are not necessary conditions.

(b)

1. Painting the frame. This identifies a valid method.
2. The paint acts as a barrier, preventing oxygen and water from reaching the steel. This explains how painting prevents rusting.
3. Greasing the frame. This identifies a valid method.
4. The grease acts as a barrier, preventing oxygen and water from reaching the steel. This explains how greasing prevents rusting.

How to earn full marks:

• Give two different methods.
• Each method must have a brief explanation of how it prevents rusting.
• Accept other valid methods such as coating with plastic or oiling.
• Do not accept vague answers like "using a protective layer" without specifying the layer.

Common Pitfall: Many students only state one condition for rusting, forgetting that both oxygen and water are required. Also, be specific with your preventative methods; simply saying "apply a protective layer" isn't enough – you need to state what that layer is made of.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A student investigates the rusting of iron nails. They place four iron nails in separate test tubes, as shown below:

• Tube 1: Nail completely submerged in distilled water.
• Tube 2: Nail in contact with air only.
• Tube 3: Nail completely submerged in distilled water that has been boiled to remove dissolved oxygen, with a layer of oil on top.
• Tube 4: Nail wrapped with magnesium ribbon and then submerged in distilled water.

(a) In which tube(s) would you expect to observe rusting after one week? Explain your answer. [3]

(b) Explain why the nail in tube 4 does not rust. [3]

Worked Solution:

(a)

1. Tube 1. This identifies the tube where rusting will occur.
2. This is because the nail is in contact with both oxygen (dissolved in the water) and water, which are the conditions required for rusting. This explains why rusting occurs in tube 1.

How to earn full marks:

• You must state tube 1, and no other tubes.
• The explanation must clearly mention both oxygen and water being present.

(b)

1. Magnesium is more reactive than iron. This states the relative reactivity of magnesium.
2. Magnesium acts as a sacrificial metal. This identifies the role of magnesium.
3. Magnesium corrodes (or oxidizes) instead of the iron, protecting the iron nail. This explains how sacrificial protection works.

How to earn full marks:

• You must state that magnesium is more reactive than iron.
• You must state that magnesium acts as a sacrificial metal.
• The explanation must clearly state that magnesium corrodes instead of the iron.

Common Pitfall: Students often struggle to explain sacrificial protection clearly. Remember to emphasize that the more reactive metal corrodes instead of the iron, not just alongside it. Also, make sure you know the relative reactivity of common metals.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A large underground steel pipeline is used to transport crude oil. The pipeline is buried in moist soil.

(a) Explain why the steel pipeline is susceptible to corrosion. [3]

(b) Describe two different methods that can be used to protect the steel pipeline from corrosion. For each method, explain how it works. [5]

Worked Solution:

(a)

1. The soil is moist, providing water (or moisture). This identifies the presence of water.
2. The soil also contains air, providing oxygen. This identifies the presence of oxygen.
3. Steel is an alloy of iron, and iron rusts in the presence of both oxygen and water. This explains why the pipeline is susceptible to corrosion.

How to earn full marks:

• You must mention the presence of both water and oxygen in the soil.
• You must state that steel contains iron, and that iron rusts in the presence of oxygen and water.

(b)

1. Method 1: Coating the pipeline with plastic. This identifies a valid method.
2. The plastic acts as a barrier, preventing water and oxygen in the soil from reaching the steel. This explains how coating with plastic prevents corrosion.
3. Method 2: Sacrificial protection using magnesium blocks. This identifies a valid method.
4. Magnesium is more reactive than iron, so it corrodes preferentially. This explains the relative reactivity of magnesium.
5. The magnesium blocks are connected to the pipeline, and they corrode instead of the steel, protecting the pipeline. This explains how sacrificial protection works.

How to earn full marks:

• Give two different methods.
• Each method must have a clear explanation of how it prevents corrosion.
• For sacrificial protection, you must mention the relative reactivity of magnesium compared to iron.
• Accept other valid methods such as galvanizing.
• Do not accept vague answers like "using a protective layer" without specifying the layer.

Common Pitfall: When describing sacrificial protection, many students forget to mention the relative reactivity of the sacrificial metal compared to iron. This is a crucial part of the explanation. Also, be sure to provide specific methods, not just general ideas.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A company is designing a new offshore wind turbine. The turbine's support structure will be made of steel and submerged in seawater. Seawater contains dissolved salts, which accelerate the corrosion of steel.

(a) Explain why the presence of dissolved salts in seawater increases the rate of corrosion of steel. [3]

(b) The company is considering two methods to protect the steel support structure from corrosion: * Painting the steel with a special anti-corrosion paint. * Attaching zinc blocks to the steel structure.

Compare and contrast these two methods, discussing the advantages and disadvantages of each. [7]

Worked Solution:

(a)

1. Dissolved salts in seawater act as an electrolyte. This identifies the role of dissolved salts.
2. This electrolyte increases the conductivity of the water, facilitating the transfer of electrons during the corrosion process. This explains how the electrolyte increases the rate of corrosion.
3. This speeds up the oxidation of iron to form iron ions and hydrated iron(III) oxide (rust). This links the electron transfer to the formation of rust.

How to earn full marks:

• You must mention that dissolved salts act as an electrolyte.
• You must explain that the electrolyte increases the conductivity of the water.
• You must explain how this increased conductivity speeds up the corrosion process.

(b)

1. Painting: Advantage - Provides a barrier to exclude oxygen and water, relatively inexpensive to apply initially. This identifies an advantage of painting.
2. Painting: Disadvantage - The paint can be scratched or damaged, exposing the steel to corrosion. Requires regular maintenance (re-painting). This identifies a disadvantage of painting.
3. Zinc blocks: Advantage - Provides sacrificial protection, even if the steel is scratched or damaged. This identifies an advantage of using zinc blocks.
4. Zinc blocks: Disadvantage - The zinc blocks corrode over time and need to be replaced periodically, which can be costly and difficult to do underwater. This identifies a disadvantage of using zinc blocks.
5. Comparison: Painting is a barrier method, preventing contact with corrosive substances, while zinc blocks provide sacrificial protection by corroding in place of the steel. This compares the two methods.
6. Comparison: Painting requires more frequent maintenance but is cheaper initially, while zinc blocks require less frequent maintenance but are more expensive. This compares the cost and maintenance requirements.
7. Conclusion: The best method depends on the specific circumstances, including the cost, the ease of maintenance, and the expected lifespan of the wind turbine. A conclusion is drawn.

How to earn full marks:

• You must give at least two distinct advantages and two distinct disadvantages.
• You must compare the two methods, highlighting their differences.
• Your comparison should consider both the advantages and disadvantages of each method.
• A conclusion or summary statement is expected.

Common Pitfall: When comparing methods of corrosion prevention, students often only focus on the advantages of one method without considering its disadvantages, or vice versa. Make sure you discuss both the pros and cons of each method, and then directly compare them. Also, remember that the presence of salt accelerates corrosion by acting as an electrolyte, increasing conductivity.