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Metals and Non-metals - Corrosion and its prevention

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Corrosion is the process in which metals are eaten up gradually by the action of air, moisture, or a chemical (such as an acid) on their surface.

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Rusting of Iron: Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust. Both air (O2O_2) and moisture (H2OH_2O) are essential for rusting.

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Silver Corrosion: Silver articles become black after some time when exposed to air. This is because it reacts with sulfur in the air to form a coating of silver sulfide (Ag2SAg_2S).

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Copper Corrosion: Copper reacts with moist carbon dioxide (CO2CO_2) in the air and slowly loses its shiny brown surface and gains a green coat. This green substance is basic copper carbonate (CuCO3⋅Cu(OH)2CuCO_3 \cdot Cu(OH)_2).

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Prevention - Galvanization: It is a method of protecting steel and iron from rusting by coating them with a thin layer of zinc (ZnZn). The protective layer works even if the coating is broken because zinc is more reactive than iron.

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Prevention - Alloying: An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. For example, Stainless Steel (Iron mixed with NiNi and CrCr) does not rust.

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Other Prevention Methods: Painting, oiling, greasing, chrome plating, or anodising are common ways to prevent the surface of the metal from coming into contact with air and moisture.

📐Formulae

4Fe+3O2+2xH2O→2Fe2O3⋅xH2O (Hydrated Ferric Oxide/Rust)4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O \text{ (Hydrated Ferric Oxide/Rust)}

2Ag+H2S→Ag2S+H2 (Silver Sulfide - Black aspect)2Ag + H_2S \rightarrow Ag_2S + H_2 \text{ (Silver Sulfide - Black aspect)}

2Cu+H2O+CO2+O2→CuCO3⋅Cu(OH)2 (Basic Copper Carbonate - Green aspect)2Cu + H_2O + CO_2 + O_2 \rightarrow CuCO_3 \cdot Cu(OH)_2 \text{ (Basic Copper Carbonate - Green aspect)}

💡Examples

Problem 1:

An iron nail is kept in three different test tubes: (A) containing dry air and calcium chloride, (B) containing boiled distilled water with a layer of oil, and (C) containing ordinary tap water. In which test tube will the nail rust?

Solution:

The nail will rust only in test tube (C).

Explanation:

In test tube (A), there is no moisture (CaCl2CaCl_2 absorbs it). In test tube (B), the boiled water has no dissolved oxygen and the oil prevents air from entering. In test tube (C), both air (oxygen) and water are present, which are the two necessary conditions for the formation of Fe2O3⋅xH2OFe_2O_3 \cdot xH_2O.

Problem 2:

Why does a galvanized iron article not rust even if the zinc layer is broken or scratched?

Solution:

This is due to sacrificial protection. Zinc (ZnZn) is more reactive than Iron (FeFe).

Explanation:

Even if the coating is scratched, the zinc continues to corrode preferentially (acts as an anode), protecting the underlying iron from oxidation.

Problem 3:

What is the composition of Stainless Steel and why is it used for making surgical instruments?

Solution:

Stainless steel is an alloy of Iron (FeFe), Nickel (NiNi), and Chromium (CrCr).

Explanation:

The addition of NiNi and CrCr makes the iron hard and resistant to rusting. Since surgical instruments require high durability and must remain sterile/rust-free, stainless steel is the ideal material.