Iron is coated with zinc, a process known as galvanization, precisely because zinc is more reactive than iron. This higher reactivity allows zinc to act as a sacrificial layer, protecting the iron from corrosion.
The Science Behind Sacrificial Protection
When iron coated with zinc is exposed to the environment (moisture and oxygen), an electrochemical cell is formed. In this cell, the more reactive metal, zinc, acts as the anode, while the less reactive metal, iron, acts as the cathode.
As stated in the reference, "whenever the metal comes in contact with moisture and water then there is formation of electrochemical cell and oxidation of metal takes place, zinc is having lower reduction potential and acts as anode and oxidation of zinc take place rather than iron". This means that oxidation, the process of losing electrons (which leads to corrosion), preferentially happens to the zinc.
- Zinc (Zn) has a lower reduction potential (meaning it is more easily oxidized) than iron.
- Therefore, zinc readily loses electrons and corrodes.
- Iron (Fe), having a higher reduction potential, is protected as long as the zinc is present.
Essentially, the zinc "sacrifices" itself by corroding first, leaving the iron intact.
How Sacrificial Protection Works
This method of protection is highly effective, offering significant advantages over simply painting or coating with a less reactive material:
- Protection Even When Scratched: Unlike a paint layer which, if scratched, exposes the iron to rust, the zinc layer continues to protect the iron even if the coating is damaged or scratched, exposing the underlying iron. The electrochemical cell mechanism ensures that the zinc surrounding the scratch will corrode instead of the exposed iron.
- Complete Coverage: The sacrificial action provides protection across the entire surface, including edges and any minor imperfections.
- Extended Lifespan: This process significantly extends the lifespan of iron and steel structures by preventing rust formation.
Comparing Zinc and Iron Reactivity
We can look at the standard reduction potentials to understand why zinc is more reactive (more easily oxidized) than iron:
| Metal | Standard Reduction Potential (V) | Reactivity |
|---|---|---|
| Zinc | -0.76 | More |
| Iron | -0.44 | Less |
A lower reduction potential indicates a greater tendency to be oxidized (corrode). Zinc's lower value (-0.76V) compared to iron's (-0.44V) confirms its higher reactivity and its role as the sacrificial anode in galvanization.
In conclusion, although zinc is more reactive, this property is utilized to provide superior and long-lasting protection to iron by ensuring that the zinc corrodes preferentially, saving the iron from rusting.
