Question

Several blocks of magnesium are fixed to the bottom of a ship to: (a) Keep away the sharks (b) Prevent action of water and salt (c) Make the ship lighter (d) Prevent puncturing by under sea rocks

Short answer

(b) Prevent action of water and salt.

Step-by-step solution

Understand the Role of Magnesium Blocks

Magnesium blocks are used as sacrificial anodes to protect other metal parts of the ship, typically made of iron or steel. These blocks corrode first due to their higher electrochemical reactivity compared to iron or steel.

Identify the Protective Function

The correct answer refers to a process called cathodic protection. Magnesium prevents the metal parts of the ship from corrosion caused by the electrochemical action in saltwater, protecting the ship's structure from degrading over time.

Eliminate Incorrect Options

Evaluate each option considering the actual purpose of magnesium blocks: they do not repel sharks, float the ship, or protect from physical impacts, such as puncturing from rocks.

Select the Correct Answer

Having examined the role of magnesium as outlined in Step 2, the correct option to the question is prevention of corrosion due to water and salt exposure, which corresponds to option (b).

Key concepts

Sacrificial Anode

In the realm of corrosion prevention, a sacrificial anode plays a crucial role. It is a component made from a metal that is more reactive than the metal it protects. When in contact with an electrolyte like saltwater, the more reactive metal will corrode first. This deliberate corrosion is what saves the other metal.
Although different metals can be used, magnesium is a common choice for sacrificial anodes because of its high reactivity. When magnesium is fixed to the bottom of a ship, it will naturally corrode and generate a protective effect. This is because magnesium gives up its electrons more willingly than the ship’s hull material, which is typically steel.

• The sacrificial anode corrodes first, hence the name 'sacrificial'.
• It is a practical solution in maritime environments.
• Maintains the integrity of important structural materials.

By sacrificing the magnesium, the integrity of the ship's hull is preserved, extending its lifespan.

Corrosion Prevention

Corrosion is a natural process that deteriorates metal materials due to reactions with environmental elements like air and water. Preventing corrosion is essential, especially in marine environments where saltwater acts as a catalyst.
The technique known as cathodic protection, in which sacrificial anodes like magnesium are used, is a common solution. This method effectively shifts the corrosion process to the less critical sacrificial anode, leaving the primary structure intact.

• Corrosion prevention is about safeguarding valuable assets.
• It involves both chemical and physical processes.
• Requires periodic maintenance to replace corroded anodes.

By continuously replacing these anodes, ships and other metal structures in harsh environments can be protected over time.

Electrochemical Reactivity

Electrochemical reactivity is a key concept when it comes to understanding how sacrificial anodes work. Reactivity refers to an element's tendency to lose electrons and undergo a chemical reaction. In the context of cathodic protection, metals with high electrochemical reactivity are used because they lose electrons readily.
The more reactive a metal, the more likely it is to donate electrons, thereby corroding faster. This is why magnesium is often selected as a sacrificial anode. It has a higher potential to lose electrons compared to steel, making it a good choice for protecting steel structures.

• High reactivity equals greater protection for less reactive metals.
• Used extensively in marine and coastal infrastructure.
• Reliability depends on correct placement and management of anodes.

It's about using the natural properties of metals to control where and how corrosion occurs, ensuring only the sacrificial anode is affected.