Corrosion and Its Prevention – Long Answer Questions

Medium Level (Application & Explanation)

Answer:

Corrosion is the slow destruction of metals due to chemical reactions with substances in the environment, like air or water. A common example is the rusting of iron, where iron reacts with oxygen and moisture to form a brownish powder called rust. The chemical equation is:
4Fe(s) + 3O₂(g) + xH₂O(l) → 2Fe₂O₃·xH₂O(s).
Another example is the tarnishing of silver. Silver reacts with hydrogen sulphide in the air to form silver sulphide, turning the metal black:
2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g).
Both these reactions show how metals deteriorate and lose their shine or strength.

Answer:

Painting creates a physical barrier on iron surfaces. This prevents air and water moisture from reaching the metal, which are essential for rusting. As long as the paint layer is intact, iron remains safe from corrosion.
Greasing or oiling works similarly. The layer of oil or grease covers the surface and keeps out water and air. This is very helpful for moving machine parts, like bicycle chains, where exposure to weather is common.
Both methods are simple and cost-effective but require regular maintenance because scratches or cracks can allow corrosion to start.

Answer:

Galvanization is the process of coating iron or steel objects with a thin layer of zinc. Zinc is used because it is more reactive than iron. When exposed to air, zinc forms a protective zinc oxide layer that stops moisture and air from reaching the iron.
Even if the zinc coating is scratched, zinc still protects iron by corroding itself first (called “sacrificial protection”). This is why galvanized iron pipes or buckets last longer in moist environments.
Thus, galvanization is a highly effective way to prevent rusting of iron.

Answer:

Alloying means mixing a metal with other metals or non-metals to improve its properties. Alloys are usually more resistant to corrosion than pure metals.
For example, stainless steel is made by mixing iron with chromium and nickel. Chromium forms a thin oxide layer on the surface, preventing rust.
Brass (copper + zinc) and bronze (copper + tin) also resist corrosion better than copper alone. Alloying changes the metal’s properties, making it tougher and less likely to react with water or air.
That is why we use alloys for tools, utensils, coins, and many other items.

Answer:

Anodization is mainly used for aluminium. Electricity is passed through aluminium in presence of acid, forming a thick, stable oxide layer. This layer acts as a shield and makes the metal more resistant to corrosion. You often see anodized aluminium in window frames or kitchen utensils.
Electroplating involves depositing a thin layer of a less reactive metal such as chromium, nickel, or silver on another metal using electrical current. This improves the look and also protects against corrosion. Common examples include chromium-plated bicycle handles and silver-plated utensils.
Both methods increase the life and appearance of metal objects.

Answer:

Without paint, the iron bridge is continuously exposed to air and moisture, which speeds up rusting. Over time, the iron weakens as rust flakes off, making parts of the bridge less strong. Weak, corroded parts may break, causing accidents.
Repairs become more frequent and expensive, increasing the financial burden on authorities. If ignored, the bridge could even collapse, risking lives. Thus, corrosion also creates safety hazards and economic loss due to frequent repairs or total replacement.
Timely prevention like painting helps avoid these serious consequences.

Answer:

Sea water has high salt content and moisture, which speeds up the corrosion of iron in ships. The process is more rapid than in normal air because salts make water a better conductor, helping corrosion reactions happen faster.
To protect ships, owners often use cathodic protection (attaching magnesium or zinc blocks) so these sacrifice themselves and corrode instead of the iron hull. Ships are also painted with special anti-rust paints.
Using corrosion-resistant alloys and regular maintenance also help extend the life of ships in salty water.

Answer:

Aluminium is quite reactive, but when it reacts with oxygen in the air, it forms a thin, tough layer of aluminium oxide (Al₂O₃) on its surface. This oxide coating is stable and protects the underlying metal from further reaction with air or water.
So, although the metal is reactive, it doesn’t keep corroding. This property makes aluminium useful for making window frames, kitchen utensils, and aircraft bodies. Unlike iron, aluminium does not keep losing its mass and strength because the oxide layer sticks firmly and doesn’t flake off.

Answer:

Pure iron has a crystal structure that lets water and oxygen easily react with it, causing rust. When chromium and nickel are added, they fit into the gaps in the iron structure and change how the atoms are arranged.
Chromium forms a thin, invisible layer of chromium oxide on the surface. This layer is stable and doesn’t allow water or oxygen to reach the iron beneath.
Nickel further strengthens the structure and also resists corrosion. This combination produces stainless steel, which is shiny, does not rust easily, and is used for cutlery, utensils, and surgical tools.
Thus, alloying changes both the composition and physical arrangement of atoms, making the metal much less reactive.

Answer:

For underground pipelines, the risk of corrosion is high due to moisture and soil chemicals. I would recommend using galvanized iron (iron coated with zinc) or stainless steel pipes, as both are highly resistant to corrosion.
Additionally, cathodic protection can be used—magnesium or zinc blocks are attached to the pipeline, so they corrode in place of the iron. Protective coatings such as tar or plastic can also be applied to the pipes.
These steps together ensure the pipelines last longer, reduce the risk of leaks, and avoid costly repairs. Therefore, material choice and protection methods are both essential when designing pipelines.