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:
To protect a buried iron pile from corrosion, anodic sacrificial protection is employed. In this method, a more easily corroded metal is connected to the iron pile. The sacrificial metal undergoes oxidation (corrodes) instead of the iron, protecting the iron structure. This is based on Galvanic corrosion, which depends on the difference in electrode potentials of metals.
Basic Principle:
The general reaction for corrosion of iron (anode reaction) is:Fe(s)→Fe2+(aq)+2e−Fe (s) \rightarrow Fe^{2+} (aq) + 2e^-Fe(s)→Fe2+(aq)+2e−
For the sacrificial anode (magnesium, for example), the oxidation reaction is:Mg(s)→Mg2+(aq)+2e−Mg (s) \rightarrow Mg^{2+} (aq) + 2e^-Mg(s)→Mg2+(aq)+2e−
How Anodic Sacrificial Protection Works:
- Connection to Iron: The sacrificial metal (e.g., magnesium) is electrically connected to the buried iron pile.
- Electron Flow: When connected, magnesium will lose electrons more readily than iron because of its lower reduction potential.
- Corrosion Prevention: Magnesium corrodes (oxidizes) by giving up electrons to the iron, which keeps the iron in a reduced state and prevents it from oxidizing and corroding.
Voltage Comparison:
The reduction potentials of metals are:
- Magnesium: -2.37 V
- Iron: -0.44 V
- Copper: +0.34 V
- Silver: +0.80 V
Since magnesium has the most negative reduction potential, it will act as the anode and corrode first, while the iron remains protected.
Why Magnesium (Mg) is Better:
- Magnesium’s low reduction potential makes it highly reactive, ensuring it corroding before the iron.
- Copper and Silver have very positive reduction potentials, meaning they are not easily oxidized and cannot act as sacrificial anodes. Using them would lead to ineffective protection of iron, as they would not corrode in the same way as magnesium.
Alternative Metals:
- Zinc (Zn) is another metal that can be used as a sacrificial anode to protect iron. Zinc has a reduction potential of -0.76 V, which is still more negative than iron’s, meaning it will corrode instead of the iron.
