Explain, using appropriate equations, illustrations, reduction voltages, and description, how the buried iron pile can be protected (via anodic sacrificial protection) from corrosion. Also explain why Mg as a sacrificial anode is better suited for protecting iron, compared to Ag or Cu. What other metal can be used in place of Mg to protect iron from corroding? Explain your answer.
The Correct Answer and Explanation is:
Protection of Buried Iron Pile via Anodic Sacrificial Protection
Anodic sacrificial protection is a method to prevent corrosion by using a more easily corroded metal (sacrificial anode) that sacrifices itself instead of the iron. In this case, the buried iron pile is protected by placing a sacrificial anode, typically magnesium (Mg), adjacent to the iron. The process is explained by the following electrochemical reactions.
Electrochemical Reactions:
- Iron (Fe) Corrosion: Fe→Fe2++2e−(oxidation at the iron surface)Fe \rightarrow Fe^{2+} + 2e^- \quad \text{(oxidation at the iron surface)}Fe→Fe2++2e−(oxidation at the iron surface) The iron undergoes oxidation, releasing electrons that would normally cause it to corrode.
- Magnesium as the Sacrificial Anode: Mg→Mg2++2e−(oxidation at the magnesium surface)Mg \rightarrow Mg^{2+} + 2e^- \quad \text{(oxidation at the magnesium surface)}Mg→Mg2++2e−(oxidation at the magnesium surface) The magnesium metal undergoes oxidation, releasing electrons. Since magnesium has a more negative reduction potential, it becomes the anode in this system, sacrificing itself to protect the iron.
Potential Difference (Reduction Voltages):
Magnesium has a standard reduction potential of -2.37 V, while iron has a standard reduction potential of -0.44 V. This means that magnesium is more easily oxidized than iron, making it a better sacrificial anode. The electrochemical potential difference ensures that magnesium corrodes instead of the iron, protecting the iron from rusting.
- Iron’s Reduction Reaction (cathodic reaction): O2+2H2O+4e−→4OH−O_2 + 2H_2O + 4e^- \rightarrow 4OH^-O2+2H2O+4e−→4OH− The oxygen reduction takes place at the cathode, typically on the surface of the iron.
Why Magnesium is Better Than Silver (Ag) or Copper (Cu):
- Magnesium is the most suitable sacrificial anode because it has the lowest reduction potential among commonly used metals. This ensures that magnesium will corrode preferentially, providing long-term protection for the iron.
- Silver (Ag) and Copper (Cu) have much higher reduction potentials (+0.80 V for Ag and +0.34 V for Cu), meaning they are less likely to corrode than iron. As a result, they wouldn’t effectively protect the iron from corrosion because they don’t offer the same driving force for sacrificial oxidation.
Other Metals That Can Be Used:
Zinc (Zn) is another metal that can be used in place of magnesium. Zinc has a standard reduction potential of -0.76 V, which is more negative than iron’s reduction potential, making it a good alternative to magnesium. It corrodes preferentially, offering protection to the iron in similar environments.
In summary, anodic sacrificial protection works by using a more easily corroded metal to “sacrifice” itself. Magnesium is the best choice due to its highly negative reduction potential, but zinc can also be used effectively.
