Metals - Corrosion: Barrier Methods to Prevent Rusting

Rusting is a form of corrosion that specifically affects iron and its alloys. It's an electrochemical process that requires the presence of oxygen and water. Preventing rusting often involves creating a barrier between the metal and the corrosive elements. This lesson will explore various barrier methods and how they work.

How Barrier Methods Prevent Rusting

Barrier methods work by physically preventing oxygen and water from reaching the metal surface. This interrupts the electrochemical process that leads to rusting. Different methods achieve this in different ways.

Common Barrier Methods

• Painting: Applying a layer of paint creates a physical barrier that prevents water and oxygen from contacting the metal.
• Greasing: A layer of grease or oil forms a hydrophobic barrier, repelling water and thus preventing corrosion.
• Plastic Coating: Coating the metal with plastic (e.g., PVC) provides an excellent barrier against moisture and oxygen.
• Galvanisation: Coating iron or steel with a layer of zinc.
• Electroplating: Coating a metal with a thin layer of another metal (e.g., chromium plating on steel).

Detailed Explanation of Galvanisation

Galvanisation is a particularly effective barrier method. It involves coating iron or steel with a layer of zinc. Zinc is more reactive than iron, so it corrodes preferentially, protecting the underlying iron. This is because zinc is more readily oxidized than iron. The zinc coating acts as a sacrificial anode.

Property	Zinc	Iron
Reactivity	More reactive	Less reactive
Oxidation Potential	More negative	Less negative
Role in Galvanisation	Sacrificial Anode	Protected Metal

When the zinc coating is scratched or damaged, the zinc will corrode instead of the iron, thus protecting the iron from rusting. This is a key advantage of galvanisation.

Electroplating

Electroplating involves coating a metal object with a thin layer of another metal using electrolysis. For example, steel can be plated with chromium to give it a shiny, corrosion-resistant surface. The plating process creates a barrier that prevents the underlying metal from reacting with its environment.

Summary Table of Barrier Methods

Method	Principle	Advantages	Disadvantages
Painting	Physical barrier	Relatively inexpensive, wide range of colours	Can chip or crack, requiring re-painting
Greasing	Hydrophobic barrier	Simple, effective in some applications	Not suitable for all environments, can be messy
Plastic Coating	Excellent barrier against moisture	Durable, resistant to chemicals	Can be expensive, may not be suitable for all shapes
Galvanisation	Sacrificial Anode (Zinc)	Highly effective, protects even if coating is damaged	Can be expensive, appearance may not be desired
Electroplating	Physical barrier with another metal	Good corrosion resistance, can improve appearance	Can be expensive, requires specialized equipment

Understanding these barrier methods is crucial for preventing corrosion in various applications, from protecting bridges and vehicles to preserving metal tools and equipment.