IGCSE Chemistry (0620) Study Notes: Corrosion of Metals (Topic 9.5)

Welcome to the notes on the Corrosion of Metals! This topic is all about why metals wear away and how we can stop it from happening. Understanding corrosion is crucial because it helps engineers and scientists save billions of dollars every year by protecting structures like bridges, cars, and pipelines. Let's get started!

What is Corrosion?

In simple terms, corrosion is the destructive chemical attack on a metal by substances in its environment. It is usually a redox reaction where the metal loses electrons (oxidation).

The most common and destructive type of corrosion is the rusting of iron and steel.

The Chemistry of Rusting (Iron Corrosion)

Rusting is the specific term used for the corrosion of iron or steel (which is an alloy primarily made of iron).

Don't worry if this seems tricky at first—just remember the two essential ingredients!

The syllabus requires you to know the two conditions necessary for rusting:

  1. Oxygen (O₂)
  2. Water (H₂O)

Analogy: Think of it like cooking a recipe for rust—you need both water and air (oxygen) to complete the dish. If you remove either ingredient, the rusting stops!

The product formed when iron rusts is hydrated iron(III) oxide.

The general equation for the formation of rust is complex, but the product you must remember is:

\( \text{Iron} + \text{Oxygen} + \text{Water} \to \text{Hydrated iron(III) oxide} \)

This product—rust—is flaky and porous, meaning it falls off easily and allows the oxygen and water to reach the fresh iron underneath, causing the corrosion to continue until the iron object is completely destroyed.

Quick Review: Conditions for Rusting

  • Metal involved: Iron or Steel
  • Conditions: Must have BOTH Water and Oxygen.
  • Product: Hydrated iron(III) oxide (Rust).

Preventing Corrosion: Barrier Methods (Core)

Since we know iron needs both oxygen and water to rust, the simplest way to prevent corrosion is to apply a barrier method. These methods create a physical layer that separates the metal surface from the air and moisture.

Here are three common barrier methods you need to know:

1. Painting

  • Used for large structures, such as car bodies, bridges, and outdoor railings.

  • The paint layer is impermeable, excluding both oxygen and water.

2. Greasing or Oiling

  • Used mainly for moving parts of machinery, such as engine components, chains, and industrial tools.

  • The oil/grease repels water and prevents oxygen contact.

3. Coating with Plastic (Polymer)

  • Used for objects that need durability and a smooth finish, such as refrigerator shelves, garden furniture, or wire fences.

  • The plastic completely seals the metal, excluding oxygen and water.

Key Takeaway for Barrier Methods: If the barrier gets scratched or chipped, the underlying iron is exposed to oxygen and water, and rusting will begin immediately at that point.

Advanced Protection Methods (Extended/Supplement)

For situations where we need long-lasting protection, or where the metal is likely to get scratched, we need more advanced techniques.

1. Galvanising (Barrier and Sacrificial Protection)

Galvanising is the process of coating iron or steel objects with a thin layer of zinc.

This method is highly effective because it provides two types of protection at once:

a) Barrier Protection

Initially, the zinc coating acts just like paint or grease—it provides a physical barrier, preventing water and oxygen from reaching the iron surface.

b) Sacrificial Protection (The Clever Part!)

This protection method relies on the Reactivity Series. Remember that zinc is more reactive than iron.

In the reactivity series:

K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au

When the zinc layer is scratched, exposing the iron underneath, the zinc still protects the iron. How?

The more reactive metal (zinc) 'sacrifices' itself to protect the less reactive metal (iron).

Step-by-Step Explanation (in terms of electron loss):

  1. Rusting is an oxidation process where iron loses electrons:
    \( \text{Fe} \to \text{Fe}^{2+} + 2e^- \) (We want to stop this!)
  2. Since zinc is more reactive, it has a stronger tendency to lose electrons (it is a better reducing agent).
  3. When water and oxygen hit the scratch, the zinc is oxidised first, losing its electrons:
    \( \text{Zn} \to \text{Zn}^{2+} + 2e^- \)
  4. These electrons flow to the iron, preventing the iron from losing its own electrons. As long as there is zinc present nearby, the iron stays intact.

Analogy: Imagine Zinc is the metal bodyguard. When danger (corrosive agents) comes, the bodyguard (Zinc) gets attacked and destroyed first, ensuring the VIP (Iron) remains safe.

2. Sacrificial Anodes (Extended)

Sacrificial protection isn't just used in galvanising; it's also used to protect large structures like underground pipelines or ship hulls, often by attaching large blocks of a more reactive metal (like magnesium or zinc) directly to the structure.

  • These large blocks are called sacrificial anodes.
  • They are intentionally allowed to corrode (oxidise) while the expensive iron/steel structure is protected.
  • The anodes must be replaced periodically once they are used up.

Quick Review: Sacrificial Protection

  • Method: Coating iron with a metal higher in the reactivity series (e.g., Zinc).
  • Mechanism: The more reactive metal loses electrons (oxidises) instead of the iron.
  • Key Term: Protection is due to the difference in the metal's tendency to lose electrons.

Did You Know? The Paradox of Aluminium (A Useful Connection)

Aluminium is very high up in the reactivity series (above zinc and iron), meaning it should corrode extremely easily. Yet, we use aluminium for things like aircraft and window frames because it seems unreactive!

The syllabus (9.4 Supplement 5) notes that aluminium's apparent unreactivity is due to its oxide layer.

  • When aluminium reacts with air, it quickly forms a very thin, tough, non-porous layer of aluminium oxide (\( \text{Al}_2\text{O}_3 \)).
  • Unlike rust, this oxide layer sticks firmly to the surface and acts as a superb, natural barrier, preventing any further corrosion.

You have now mastered the conditions for corrosion, the basics of barrier protection, and the chemistry behind sacrificial protection—excellent work!