State some common barrier methods, including painting, greasing and coating with plastic

Metals – Corrosion of Metals (IGCSE 0620)

Learning Objective

State the common barrier methods used to protect metals from corrosion – painting, greasing and coating with plastic – and recognise other important protection methods and the conditions required for corrosion.

1. What is corrosion?

• Corrosion is the gradual destruction of a metal or alloy by a chemical reaction with its environment.
• For iron the most familiar form is rusting. A useful overall equation is

  4 Fe + 3 O₂ + 6 H₂O → 2 Fe₂O₃·3H₂O

  (hydrated iron(III) oxide). The value of n in Fe₂O₃·nH₂O denotes the number of water molecules attached to the rust.

2. Conditions required for rusting (iron)

All three components must be present simultaneously; removal of any one stops the reaction.

1. Iron (or an iron‑based alloy)
2. Water (or moisture) – provides the electrolyte that allows ion transport.
3. Oxygen – the oxidising agent.

Diagram suggestion: a cross‑section showing a metal surface, a thin film of water, and dissolved O₂, each labelled.

3. Why protect metals from corrosion?

• Maintain structural integrity and safety (bridges, aircraft, pipelines).
• Preserve appearance and functionality (automotive body panels, household appliances).
• Extend service life and reduce maintenance or replacement costs.
• Prevent loss of valuable material (e.g., steel in ships, copper in wiring).

4. Barrier methods – core syllabus requirement

4.1 Painting – a barrier method

• How it works: A dried film of paint is essentially impermeable to both water and oxygen, isolating the metal surface.
• Key point for the exam: Paint blocks the two agents (H₂O and O₂) required for rusting, so the reaction cannot start.
• Typical materials: Oil‑based, water‑based, enamel, epoxy paints.
• Advantages: Low cost, easy to apply, available in many colours, can be re‑coated.
• Disadvantages: Chips or cracks expose the metal; regular inspection and maintenance are needed.

4.2 Greasing – a barrier method

• How it works: Grease forms a thick, hydrophobic film that repels water and limits diffusion of oxygen.
• Key point for the exam: The grease layer prevents the formation of an electrolyte film, so ion transport stops.
• Typical materials: Mineral oil, lithium grease, silicone grease.
• Advantages: Simple to apply; also reduces friction and wear on moving parts.
• Disadvantages: Attracts dust and dirt, can break down at high temperatures, not suitable for permanent outdoor exposure.

4.3 Plastic (polymer) coating – a barrier method

• How it works: Polymers such as epoxy, polyurethane, polyester or PVC create a tough, chemically resistant film that is virtually impermeable to water, oxygen and many chemicals.
• Key point for the exam: The polymer coating isolates the metal from both moisture and oxygen, stopping the corrosion process.
• Typical materials: Epoxy resin, polyurethane, PVC, polyester resin.
• Advantages: Very durable, resistant to chemicals, UV and abrasion; can be applied by spray, dip or brush.
• Disadvantages: Higher initial cost; surface must be clean and dry before application; repair can be difficult.

5. Supplementary – other important protection methods

5.1 Sacrificial protection (galvanising)

• Principle: A more reactive metal (usually zinc) is attached to the iron/steel. The sacrificial metal oxidises preferentially, protecting the underlying metal.
• Key point for the exam: The coating metal acts as a “sacrificial anode”.
• Typical example: Hot‑dip zinc‑galvanised steel used for nails, bolts, car body panels and bridge girders.
• Reaction equations:

  Zn → Zn²⁺ + 2e⁻ (anodic oxidation)

  Fe²⁺ + 2e⁻ → Fe (cathodic reduction)

  The overall effect is that zinc corrodes while the iron remains protected.
• Advantages: Self‑renewing protection; works even if the coating is scratched.
• Disadvantages: Requires a more reactive metal; zinc layer can be consumed over time; may add weight.

5.2 Natural oxide layer on aluminium

• Aluminium rapidly forms a thin, adherent layer of aluminium oxide (Al₂O₃) when exposed to air.
• The oxide film is dense and insoluble, preventing further reaction of the underlying metal with water or oxygen.
• Because of this protective layer, aluminium appears “non‑corrosive” in many everyday situations.

6. Practical investigation (AO3 – investigation)

Objective: Observe the effect of the three barrier methods on the rate of rusting of iron.

1. Prepare four identical iron nails.
2. Treat them as follows:
   • Control: no protection.
   • Painted: apply a thin coat of water‑based paint and allow to dry.
   • Greased: coat with a thin layer of mineral oil.
   • Plastic‑coated: dip in a quick‑dry epoxy resin.
3. Place all nails in a shallow tray containing a thin layer of tap water (to supply moisture) and expose to ambient air for 7 days.
4. Record observations (colour change, amount of rust) and rank the effectiveness of each method.
5. Safety note: wear gloves, work in a well‑ventilated area, and dispose of waste according to school guidelines.

7. Comparison of common barrier methods

8. Real‑world examples (Why corrosion control matters)

• Bridges: Steel girders are regularly painted or hot‑dip galvanised to prevent catastrophic failure.
• Automotive industry: Body panels are typically painted over a zinc‑galvanised steel sheet.
• Marine vessels: Hulls receive epoxy/polyurethane coatings and sacrificial anodes to resist salt‑water corrosion.

9. Key points to remember

1. Rusting of iron requires iron + water + oxygen; any one missing stops the reaction.
2. Barrier methods protect by physically separating the metal from water and oxygen.
3. Painting, greasing and plastic (polymer) coating are the three barrier methods explicitly required by the syllabus.
4. Supplementary methods – galvanising (sacrificial protection) and the natural oxide layer on aluminium – are also important in practice.
5. The choice of protection depends on the environment, cost, durability required and ease of maintenance.