Question 32 — Shipwrecks, Corrosion and Conservation (a) Outline the limitations of using paint to protect ships that are in constant use - HSC - SSCE Chemistry - Question 32 - 2015 - Paper 1

A sacrificial anode is a metal that is electrically connected to a target metal to prevent corrosion. The anode is typically made of a more reactive metal, such as zinc or magnesium.

Chemical processes include:

1. Oxidation at Anode: The sacrificial anode undergoes oxidation, releasing electrons.

   Example:

   ext{Zn} ightarrow ext{Zn}^{2+} + 2 ext{e}^-

2. Reduction at Cathode: The electrons prevent the oxidation of the protected metal, reducing ferrous ions back to iron.

   Example:

   ext{Fe}^{2+} + 2 ext{e}^- ightarrow ext{Fe}

This process protects the vessel from corrosion by ensuring it is less likely to oxidize than the anode.

To compare the rates of corrosion, we can conduct an investigation using iron and stainless steel. Steps include:

1. Materials: Gather iron samples and stainless steel samples of the same size and shape.
2. Environment: Place each sample in separate containers filled with an electrolyte solution (e.g., saltwater) to promote corrosion.
3. Measurements: Record the weight of each sample prior to immersion, then allow them to corrode for a specified period (e.g., 2 weeks).
4. Observation: After the exposure period, remove and clean the samples, then weigh them again to determine the mass loss from corrosion.
5. Results: Compare the mass loss of iron and stainless steel to analyze the corrosion rates.

The properties of steel are significantly influenced by its percentage composition of carbon and other alloying elements.

1. Mild Steel: Typically contains around 0.05% to 0.25% carbon. It is soft and malleable, making it easy to work with for various applications such as construction. Its lower carbon content makes it less brittle but also limits its strength.

2. High-Carbon Steel (Structural Steel): Contains approximately 0.6% to 0.9% carbon, leading to higher tensile strength and hardness. However, it is more brittle and less ductile compared to mild steel. This makes it suitable for applications that require strength, such as reinforcing bars in concrete.

The labelled diagram should include the following components:

• A container holding the electrolyte solution (magnesium sulfate)
• Two electrodes: the anode made of magnesium and the cathode (the metal spoon)
• A directional arrow showing electron flow from the anode to the cathode
• Labels indicating the anode (+) and cathode (-) polarities, along with the electrolyte

Davy's experiments with electrolysis provided insights into electron transfer processes, demonstrating that chemical reactions could be driven by electricity. His work showed:

1. Electrolysis Principles: Davy established that substances could be decomposed into their elements through electrolysis, indicating that reactions involve electron movement.
2. Practical Applications: His findings laid the groundwork for the production of alkaline metals, improving our understanding of how electricity could reduce metal ions in solution.

The rusting processes for ships in shallow (60 m) and deep (4000 m) water vary due to environmental factors:

1. Shallow Water: Factors include:

   • Greater oxygen availability enhances rusting.
   • More biological activity may introduce microbes that can accelerate corrosion.

   Reaction:

   ext{4Fe} + 3 ext{O}_2 + 6 ext{H}_2 ext{O} ightarrow 4 ext{Fe(OH)}_3 ightarrow ext{rust}

2. Deep Water: Factors include:

   • Lower temperature and pressure, contributing to slower corrosion rates.
   • Potentially less oxygen, which slows down the rusting process.

   Reaction:** Similar chemical reactions occur, but at a reduced rate due to the conditions.