Problem 53 - Chemistry and Chemical Reactivity

Question

Use \(E^{\circ}\) values to predict which of the following metals, if coated on iron, will provide cathodic protection against corrosion to iron. (a) Cu (b) \(\mathrm{Mg}\) (c) Ni (d) Sn

Step-by-Step Solution

Answer

Mg can provide cathodic protection to iron.

Step 1: Understanding Cathodic Protection

Cathodic protection occurs when a metal with a more negative standard electrode potential (\(E^{\circ}\)) is used as a sacrificial anode to protect a metal with a less negative \(E^{\circ}\). The metal with a more negative \(E^{\circ}\) will corrode preferentially.

Step 2: List Standard Electrode Potentials

Find the standard electrode potentials (\(E^{\circ}\)) for iron and the given metals: - \(E^{\circ}(\text{Fe}^{2+}/\text{Fe}) = -0.44\,\text{V}\) - \(E^{\circ}(\text{Cu}^{2+}/\text{Cu}) = +0.34\,\text{V}\) - \(E^{\circ}(\text{Mg}^{2+}/\text{Mg}) = -2.37\,\text{V}\) - \(E^{\circ}(\text{Ni}^{2+}/\text{Ni}) = -0.25\,\text{V}\) - \(E^{\circ}(\text{Sn}^{2+}/\text{Sn}) = -0.14\,\text{V}\)

Step 3: Compare Electrode Potentials

Compare each metal's \(E^{\circ}\) to that of iron (\(-0.44\,\text{V}\)). A metal with a more negative \(E^{\circ}\) can act as a sacrificial anode for iron: - \(E^{\circ}(\text{Mg}) = -2.37\,\text{V}\) is more negative than \(-0.44\,\text{V}\). - \(E^{\circ}(\text{Cu}) = +0.34\,\text{V}\) is less negative. - \(E^{\circ}(\text{Ni}) = -0.25\,\text{V}\) is less negative. - \(E^{\circ}(\text{Sn}) = -0.14\,\text{V}\) is less negative.

Step 4: Determine the Suitable Metal for Protection

Since magnesium (\(\text{Mg}\)) has the most negative \(E^{\circ}\), it can provide cathodic protection to iron by corroding first, thus acting as a sacrificial anode while protecting the iron.

Key Concepts

Standard Electrode Potential

In the world of electrochemistry, the standard electrode potential ( E^{ ext{o}} ) is crucial. It helps us predict how different metals will react in an electrochemical cell. E^{ ext{o}} is measured in volts and represents a metal's tendency to gain electrons, also known as reduction potential. The more positive the E^{ ext{o}} value, the greater the metal's ability to undergo reduction.
This value can be compared to assess which metal will donate electrons (oxidized) and which one will accept them (reduced) when paired together.

• Standard conditions include a 1 M concentration for ions and a temperature of 25°C (298 K).
• The standard electrode potential of hydrogen is used as a reference and is assigned a value of 0 volts.

Knowing this, for cathodic protection, we pick a metal with a more negative E^{ ext{o}} to ensure it loses electrons more readily than the protected metal. This ensures that the metal with a less negative E^{ ext{o}} is preserved.

Sacrificial Anode

A sacrificial anode is a key player in the cathodic protection system. This metal is deliberately allowed to corrode to save another metal from corrosion, such as iron. This anode has a more negative standard electrode potential ( E^{ ext{o}} ), making it more likely to oxidize first.
By doing so, the sacrificial anode protects the main metal from corrosion.

Here's how it works:

• A sacrificial anode becomes the anode of an electrochemical cell and corrodes instead of iron.
• This process requires that the anode has a more negative E^{ ext{o}} compared to the metal it protects.
• Metals like magnesium and zinc are often used since they have highly negative E^{ ext{o}} values.

Magnesium and zinc are common choices for sacrificial anodes. In this exercise, magnesium provides cathodic protection to iron because it has a more negative E^{ ext{o}} value, thus sacrificing itself and preventing iron from rusting.

Corrosion of Iron

Corrosion is the gradual destruction of a metal due to chemical reactions with its environment. Iron, when exposed to oxygen and moisture, forms rust, an iron oxide compound. This process is a concern where durability and safety of structures are important.
Corrosion of iron typically involves an electrochemical reaction where iron loses electrons and reacts with oxygen:\[ \text{4Fe + 3O}_2 + 6\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3 \]
Strategies to prevent corrosion involve disrupting these reactions. Cathodic protection is one such method.

To safeguard structures like pipelines or marine vessels, a sacrificial anode can effectively protect iron by serving as an alternative site for oxidation. Thus, corrosion is prevented or significantly reduced on the iron itself, extending its lifespan. In this context, choosing the correct sacrificial anode based on its E^{ ext{o}} value can make a huge difference in corrosion prevention.