What is corrosion?
Corrosion is the process by which materials — particularly metals — are degraded through chemical or electrochemical reactions with their environment. The most familiar example is rusting of steel and iron when exposed to moisture and oxygen. Corrosion is one of the greatest threats to the lifespan of building structures.
How does corrosion occur?
In steel and iron, the process works as follows:
- Moisture and oxygen reach the steel surface
- Electrochemical reaction — the iron oxidises, forming iron oxide (rust)
- Volume increase — rust occupies a greater volume than the original steel
- Flaking layer — the rust layer is not protective, so the process continues
Applications
Corrosion protection is essential in construction:
- Reinforcement in concrete — the concrete cover protects the reinforcing bars from moisture and CO₂
- Steel structures — protected by paint systems, galvanising or hot-dip zinc coating
- Cladding — aluminium and stainless steel are more corrosion-resistant than untreated steel
- Pipework — copper and plastic pipes are less susceptible than steel pipes
Corrosion in concrete
When CO₂ penetrates concrete (carbonation), the pH drops. Below about pH 9, the reinforcement loses its protective passive layer and corrosion can begin — causing concrete spalling as the corroding bars expand.
Related terms
- Reinforcement
- Steel
- Concrete
- Galvanising
- Concrete deterioration
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