Very Short Question and Answers - Corrosion and Its Prevention

Corrosion is the slow and gradual destruction of metals by chemical reactions with substances in their surroundings, such as air, moisture, acids, etc.

Rusting of iron is an example of corrosion. Iron reacts with oxygen and water from the air to form a brown flaky substance called rust (Fe₂O₃·xH₂O).

4Fe(s) + 3O₂(g) + xH₂O(l) → 2Fe₂O₃·xH₂O(s)

The black coating is silver sulphide (Ag₂S). It forms when silver reacts with hydrogen sulphide gas (H₂S) present in air: 2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g).

The green coating is basic copper carbonate (CuCO₃·Cu(OH)₂), formed when copper reacts with moist carbon dioxide and oxygen from air.

Aluminium quickly forms a thin, stable, and protective layer of aluminium oxide (Al₂O₃) that prevents further corrosion.

Corrosion weakens structures, leading to possible collapse, and causes economic loss due to repair, replacement, and loss of materials.

Painting forms a protective barrier over metal surfaces, preventing air and moisture from contacting the metal and thus avoiding corrosion.

Galvanization is coating iron or steel objects with a thin layer of zinc. Zinc reacts with air to form zinc oxide, which prevents air and moisture from reaching the iron.

Oiling and greasing coat the metal surface, blocking out moisture and air, thereby preventing corrosion.

Alloys are mixtures that often form protective oxide layers or are less reactive. For example, stainless steel (iron, chromium, nickel) resists corrosion because chromium forms a protective oxide layer.

Anodization is a process of forming a thicker oxide layer on aluminium, making it more resistant to corrosion.

Electroplating deposits a less reactive metal (like chromium or nickel) onto another metal, protecting it from corrosion. For example, chromium plating on bike handles.

2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g)

Oiling is used on bicycle chains, while greasing is common for machinery parts to prevent corrosion.

Sacrificial protection involves attaching a more reactive metal (like magnesium or zinc) to iron. The more reactive metal corrodes instead of the iron, protecting it. Example: magnesium blocks attached to ship hulls.

Desiccants that absorb moisture from the air can be used while packaging metals to prevent corrosion.

Galvanized iron pipes and galvanized buckets.

Alloys like bronze (copper + tin) and brass (copper + zinc) are less reactive and form protective layers on their surfaces, making them less likely to corrode than pure copper.

Cathodic (sacrificial) protection is used, by attaching a block of a more reactive metal such as magnesium, which corrodes in place of the iron pipe, preventing its corrosion.