Corrosion and Its Prevention – Long Answer Questions (CBSE Class 10 Science)

Medium Level (Application & Explanation)

Q1. Define corrosion and explain how it affects metals in daily life. Give suitable examples and write the relevant chemical equations where applicable.

Answer: Corrosion is the gradual destruction of metals due to reactions with air, moisture, acids, or gases in the environment. It reduces strength, causes economic loss, and spoils appearance. For example, iron rusts by reacting with oxygen and water to form hydrated iron(III) oxide, which is flaky and weak: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. Silver tarnishes by reacting with H₂S to form black Ag₂S: 2Ag + H₂S → Ag₂S + H₂. Copper develops a green coating of basic copper carbonate due to moisture, CO₂, and O₂: 2Cu + H₂O + CO₂ + O₂ → Cu(OH)₂ + CuCO₃. On the other hand, aluminium resists further corrosion due to a thin Al₂O₃ film that protects it. Thus, corrosion can weaken bridges, pipelines, and tools, while also increasing maintenance costs.

Q2. Describe the rusting of iron. State the conditions required, the chemical changes involved, and how a simple activity proves these conditions.

Answer: Rusting of iron is a specific type of corrosion where iron forms a brown flaky rust (hydrated iron(III) oxide). The reaction is: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. The two essential conditions are the presence of oxygen (air) and water (moisture). This is proven by the classic three test tube activity: in Test Tube A (iron nail with water and air), rusting occurs; in Test Tube B (boiled water with oil layer preventing air), no rust; in Test Tube C (anhydrous calcium chloride absorbing moisture), no rust. This shows both air and water are necessary. Rusting causes loss of strength and durability, making iron structures brittle. It is often seen on gates, bridges, railings, tools, and nails left in damp air. Preventive steps include painting, oiling, greasing, galvanization, and alloying.

Q3. Aluminium appears to resist corrosion better than iron. Explain why, and describe how anodization further improves its resistance with examples.

Answer: Aluminium seems corrosion-proof because it quickly forms a thin, hard, and adherent oxide film (Al₂O₃) when exposed to air. This passive layer seals the surface and prevents further reaction with oxygen or moisture, unlike iron, whose rust is porous and flakes off, allowing continuous corrosion. Anodization enhances this natural protection by passing electric current through aluminium in an acidic bath, building a thick, uniform Al₂O₃ coating. This layer can also be dyed for aesthetic use and is more scratch-resistant. Applications include aluminium window frames, aircraft bodies, and colored water bottles. While iron needs frequent painting or galvanization, aluminium largely remains shiny and intact for years due to its self-protecting oxide film and improved performance through anodization.

Q4. What is galvanization? Explain how it prevents rusting of iron and steel, and give practical applications from daily life.

Answer: Galvanization is the process of coating iron or steel with a thin layer of zinc. It protects in two ways. First, zinc acts as a physical barrier, keeping air and moisture away from the iron. Second, zinc offers sacrificial protection because it is more reactive than iron; even if the coating is scratched, zinc corrodes in preference to iron, forming zinc oxide or basic zinc carbonate, which further seals the surface. This dual protection makes galvanization very effective for outdoor and wet environments. Everyday uses include galvanized water pipes, buckets, fences, wire mesh, and sheet roofing. The silvery look of these items is due to the zinc layer. Galvanization extends the life of iron objects, reduces maintenance, and prevents leaks and failures in pipelines and structures.

Q5. Compare painting, oiling/greasing, and electroplating as methods of preventing corrosion. Include their advantages, limits, and common uses.

Answer:

Painting: Forms a protective film that blocks air and moisture. It is easy and colorful, suitable for gates, railings, lamp posts, cars. However, scratches or peeling expose metal, needing regular maintenance.
Oiling/Greasing: Creates a water-repellent barrier. Ideal for moving parts like bicycle chains and tools in storage. Needs reapplication and is messy; not good for high-temperature or decorative surfaces.
Electroplating: Deposits a thin layer of a less reactive metal (e.g., nickel, chromium, silver) for both protection and shine. Used for bike handles (chrome), kitchen tongs (nickel), cutlery (silver plate). It requires equipment and may wear off over time. All three methods prevent contact with oxygen and water, slowing corrosion. Choice depends on cost, appearance, durability, and environmental exposure.

High Complexity (Analytical & Scenario-Based)

Q6. A steel bridge near the sea shows frequent rusting. Explain why corrosion is faster in coastal regions and propose a layered protection plan.

Answer: Coastal areas have high humidity, salt-laden air, and frequent spray, which accelerate corrosion. The presence of electrolytes (salt, mainly NaCl) increases the conductivity of water films on metal, speeding up rust formation: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. A layered plan should include:

Surface preparation: sandblasting to remove old rust and contaminants.
Primary barrier: zinc-rich primer (acts like partial galvanization for sacrificial protection).
Secondary barrier: epoxy paint for strong adhesion and moisture resistance.
Topcoat: UV-resistant polyurethane to withstand sunlight and weathering.
Critical zones: seal joints and crevices; use grease in moving parts.
Maintenance: scheduled inspections and touch-ups after monsoons. Combining a sacrificial layer (zinc) with durable coatings and regular maintenance offers long-term protection in marine environments.

Q7. A city must choose materials for an underground water pipeline. Compare galvanized iron pipes with stainless steel and explain how cathodic (sacrificial) protection could extend pipe life.

Answer: Galvanized iron (GI) pipes are economical and protected by a zinc coating that offers barrier and sacrificial action. However, in soil with moisture and salts, coatings can get damaged over time. Stainless steel (Fe with Cr and Ni) offers better corrosion resistance due to a passive Cr₂O₃ layer, but costs more. For budget-sensitive projects, GI pipes with added cathodic protection are practical. In sacrificial protection, more reactive metals like magnesium or zinc blocks are attached to the pipeline; these corrode preferentially, protecting iron. Additionally, apply bituminous or epoxy coatings and ensure proper backfilling to prevent damage. Regular potential checks and replacement of sacrificial anodes are necessary. Thus, GI plus coatings and sacrificial protection balances cost and durability for underground water systems.

Q8. A museum reports that silver trophies blacken while an outdoor copper statue turns green. Explain the chemistry of both and suggest safe cleaning and prevention strategies.

Answer: Silver darkens due to tarnish—formation of silver sulphide (Ag₂S) by reacting with H₂S in air: 2Ag + H₂S → Ag₂S + H₂. The copper statue develops a green patina of CuCO₃ and Cu(OH)₂ formed by CO₂, O₂, and moisture: 2Cu + H₂O + CO₂ + O₂ → Cu(OH)₂ + CuCO₃. For silver, use gentle polishing with a mild cleaner to remove Ag₂S; store with desiccants (silica gel) and airtight cases to reduce moisture and gases. For copper statues, the patina can be protective; harsh cleaning may damage the surface. If cleaning is needed, use mild aqueous methods and then apply a clear protective wax or lacquer. For both, improve ventilation, minimize exposure to pollutants, and use display cases with moisture control to slow further corrosion.

Q9. Design an investigation to compare the rate of rusting of iron nails in different conditions (dry air, distilled water, salt water, and boiled water with oil). State variables, steps, observations, and conclusion.

Answer:

Variables:
- Independent: Environment (dry air with desiccant, distilled water, salt water, boiled water with oil layer).
- Dependent: Extent of rusting (color change, mass change).
- Controlled: Same nail size, exposure time, temperature.
Steps:
- Place clean nails in four labeled test tubes: A (dry with calcium chloride), B (distilled water), C (salt solution), D (boiled distilled water with oil to block air).
- Leave for 3–5 days; observe daily.
Observations:
- A: No rust (no moisture).
- B: Rust forms (air + water present).
- C: Faster rusting (salts increase conductivity).
- D: No rust (air excluded).
Conclusion:
- Air (O₂) and water are essential; salts accelerate rusting. Blocking either air or moisture prevents rusting.

Q10. Propose a comprehensive anti-corrosion plan for a factory storing iron tools, operating vehicles, and metal pipelines. Use methods like barrier coatings, alloying, desiccants, and sacrificial protection with justification.

Answer: Adopt a multi-layer strategy tailored to different assets:

Stored tools: Apply oil/grease films; store with desiccants (silica gel) in sealed cabinets to reduce moisture. Periodically clean and reapply coatings.
Operating vehicles and machinery: Use paint or powder coatings on expos...