Describe how barrier methods prevent rusting by excluding oxygen or water

Metals – Corrosion of Metals (Cambridge IGCSE 0620)

1. Where Corrosion Fits in the Whole Syllabus

The corrosion topic belongs to Section 9 – Metals of the Cambridge IGCSE Chemistry syllabus (0620). To use the notes effectively, students should already be familiar with the following prerequisite ideas (AO 1):

• States of matter, atomic structure and the periodic table (Topics 1‑8) – symbols for solid (s), liquid (l), gas (g) and aqueous (aq) species.
• General properties of metals, the reactivity series and extraction of metals (Topic 9) – especially the position of iron, zinc and aluminium in the series, because the reactivity series explains why zinc can act as a sacrificial metal.
• Environmental chemistry (Topic 10) – the role of moisture, salt (chloride ions) and acidity in accelerating corrosion.
• Organic chemistry (Topic 11) – a reminder that many barrier coatings (paints, polymers, waxes) are organic materials.
• Experimental techniques (Topic 12) – a simple practical investigation is included at the end of the notes to satisfy AO 3 requirements.

2. Why Does Iron Rust?

Rusting is an electro‑chemical (redox) process that needs both oxygen (O₂) and water (H₂O). When either reactant is removed, the reaction stops.

2.1 Overall (simplified) equation

2.2 Half‑reactions (with state symbols)

• Oxidation (anodic) – iron:
  $$\text{Fe(s)} \;\rightarrow\; \text{Fe}^{2+}\text{(aq)} + 2e^{-}$$
• Reduction (cathodic) – oxygen dissolved in water:
  $$\text{O}_{2(g)} + 2\;\text{H}_{2}\text{O(l)} + 4e^{-} \;\rightarrow\; 4\;\text{OH}^{-}\text{(aq)}$$

Electrons flow from the anodic (oxidising) iron to the cathodic sites where oxygen is reduced. The resulting hydroxide ions combine with Fe²⁺ to form Fe(OH)₃.

2.3 Conditions Required for Rusting (Cambridge Syllabus 9.5)

• Presence of iron (or another susceptible metal).
• Presence of oxygen (O₂).
• Presence of water (H₂O) – provides the electrolyte and the medium for the reduction reaction.
• Formation of anodic and cathodic sites on the metal surface (electro‑chemical cells).

3. Barrier Methods – General Principle

Barrier methods protect metal by creating a continuous, crack‑free layer that isolates the metal surface from the surrounding atmosphere. By doing so they either:

1. Prevent oxygen from reaching the metal (physical seal),
2. Prevent water from forming a conductive electrolyte (hydrophobic layer), or
3. Provide a more reactive metal that corrodes first (sacrificial protection).

4. Common Barrier Methods

4.1 How Each Method Excludes Oxygen or Water

1. Physical seal – Paint (1), Enamel (3), Plastic/Polymer (5). The coating is continuous and impermeable, so neither O₂ nor H₂O can reach the metal.
2. Hydrophobic layer – Oil/Grease (2), Wax/Bitumen (6). The water‑repellent coating prevents the formation of a thin electrolyte film, thereby limiting the supply of H₂O (and the dissolved O₂ it carries).
3. Sacrificial protection – Galvanising (4). Zinc oxidises according to
   $$\text{Zn(s)} \;\rightarrow\; \text{Zn}^{2+}\text{(aq)} + 2e^{-}$$ The electrons released protect the underlying iron, while the zinc consumes the available O₂ and H₂O.

5. Practical Investigation (AO 3)

Objective: Compare the rate of rusting of untreated iron with that of iron protected by a barrier method.

1. Record the initial mass of each nail (to 0.01 g).
2. Apply the designated coating (paint or oil) and allow it to dry.
3. Place each nail in a separate container, add a small amount of water to create a humid atmosphere, and seal the lids.
4. Leave the set‑up for 7 days at room temperature.
5. After 7 days, gently wipe off any loose rust, re‑weigh each nail and record the final masses.
6. Calculate the mass gain (Δm) for each nail – the larger the gain, the more rust formed.

Expected outcome: The painted nail (physical seal) should show the smallest mass increase, the oil‑coated nail a moderate increase, and the untreated nail the largest increase. The results illustrate how excluding oxygen and/or water slows the electro‑chemical corrosion process.

6. Key Points to Remember (AO 1 + AO 2)

• Rusting requires both oxygen and water; removing either stops the reaction.
• Barrier methods must be continuous and free of cracks to be effective.
• The three mechanisms required by the syllabus are:
  1. Physical seal (excludes O₂ and H₂O).
  2. Hydrophobic layer (repels water, limits electrolyte formation).
  3. Sacrificial protection (more reactive metal oxidises first).
• The reactivity series explains sacrificial protection: a metal higher in the series (e.g., Zn) will oxidise before iron.
• Environmental factors such as chloride ions (salt) and acidic water accelerate rusting; barrier methods are especially valuable in marine or industrial settings.
• Regular inspection and maintenance of the coating are essential for long‑term protection.
• Choice of barrier method depends on cost, durability, appearance, and the specific service environment.