Cathodic protection prevents iron from corroding by attaching it to a more reactive metal which corrodes instead of the iron.
Understanding the Principle of Cathodic Protection
Corrosion of iron (rusting) is an electrochemical process. To prevent iron from corroding, this process needs to be interrupted. One effective method is cathodic protection.
Based on the reference, cathodic protection involves a specific technique:
- The iron object that needs to be protected is attached to another metal.
- Common examples of metals used for attachment are zinc or magnesium. These metals are chosen because they are more electrochemically active than iron.
Why This Method Works: The Role of the Sacrificial Metal
The key to how this works lies in the different reactivities of the metals involved:
- Metals like zinc or magnesium give up electrons to oxygen more readily than iron does.
- When attached to iron in the presence of an electrolyte (like moisture), these more reactive metals act as the anode in a galvanic cell.
- The reference states, "The iron to be protected is attached to another metal such as zinc or magnesium, which give up electrons to oxygen more readily than does iron."
- Because the attached metal gives up electrons preferentially, it becomes the site of oxidation (corrosion).
- The iron, in this setup, becomes the cathode, where reduction occurs (typically oxygen reduction in rusting).
- Crucially, the reference notes, "The so- called sacrificial cathode will then corrode and the iron will not." (Note: While electrochemically the corroding metal is the anode, the reference terms it the "sacrificial cathode" because it is consumed to protect the actual cathode, the iron). This means the more reactive metal sacrifices itself to protect the iron.
Practical Applications and Examples
This principle is widely applied:
- Galvanization: Coating steel or iron with a layer of zinc. The zinc layer acts as a barrier, but if scratched, the surrounding zinc still corrodes preferentially, protecting the underlying iron cathodically.
- Protecting Pipelines and Structures: Sacrificial anodes (blocks of magnesium or zinc) are buried near steel pipelines or attached to ship hulls. These anodes corrode over time and must be replaced.
- Water Heaters: Magnesium rods are often installed in water heaters to protect the steel tank from corrosion.
In essence, cathodic protection using a sacrificial metal ensures that any electrochemical activity that would normally lead to iron corrosion is directed towards the more reactive attached metal instead, leaving the iron intact.
