Corrosion, Its Prevention, and Alloys

Iron rusts, silver tarnishes, copper turns green — corrosion eats away at a metal’s surface over time, but it needs specific conditions, and it can be prevented or even turned into a useful new material altogether.

In this lesson you will

Define corrosion and name the specific product for iron (rust), silver (tarnish) and copper (green coat).
State, from experimental evidence, that rusting needs both air and water — neither alone is enough.
List methods of preventing corrosion: painting, oiling, greasing, galvanising, chrome plating, anodising, alloying.
Define an alloy and explain why alloying changes a metal’s properties, with steel, stainless steel, brass, bronze, solder and amalgams as examples.

What corrosion looks like

Left exposed, many metals slowly react with substances around them and their surface deteriorates — this is corrosion. Silver articles turn black because silver reacts with sulphur compounds in the air to form silver sulphide. Copper loses its shiny brown surface and gains a green coat of basic copper carbonate, formed by reaction with moist carbon dioxide. Iron exposed to moist air for long enough develops a brown, flaky coating called rust.

What does iron actually need, to rust?

A clean iron nail is set up three different ways: sitting in ordinary water with air above it; sitting in air-free boiled water sealed under oil so no air can dissolve back in; and surrounded only by air kept bone-dry with a drying agent. After several days, only the nail with BOTH air and water present has rusted. The other two — water without air, and air without water — stay rust-free. Rusting is not caused by air alone or water alone; it needs both together.

Preventing corrosion

Corrosion can be slowed or stopped by keeping air and moisture away from the metal surface: painting, oiling, greasing, chrome plating, and two specific techniques worth knowing by name —

Galvanisation: coating steel or iron with a thin layer of zinc. The article stays protected from rusting even if the zinc coating gets scratched.
Anodising: thickening the natural, protective oxide layer on aluminium by electrolysis, making it more resistant to corrosion (and easy to dye for a finished look).

Alloying — changing a metal on purpose

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal, made by melting the main metal and dissolving the others into it in a fixed proportion, then cooling. Pure iron is soft and stretches easily when hot — almost never used on its own. Mixed with about 0.05% carbon, it becomes hard, strong steel. Mixed with nickel and chromium, it becomes stainless steel: hard and resistant to rust.

Alloying changes properties predictably: brass (copper + zinc) and bronze (copper + tin) both conduct electricity far worse than pure copper; solder (lead + tin) has an unusually low melting point, which is exactly why it is used to weld electrical wires together without damaging them. When one of the metals in an alloy is mercury, the alloy is called an amalgam. Pure (24-carat) gold is too soft for jewellery, so it is alloyed with silver or copper — 22-carat gold is 22 parts pure gold to 2 parts of that other metal.

Key terms

Corrosion: The gradual eating away of a metal’s surface by reaction with substances in its surroundings, such as moisture, acids or gases in the air.
Galvanisation: Coating iron or steel with a thin protective layer of zinc to prevent rusting — the article stays protected even if the coating is scratched.
Alloy: A homogeneous mixture of two or more metals, or a metal and a non-metal, made by melting the primary metal and dissolving the others into it in a fixed proportion before cooling.
Amalgam: An alloy in which one of the components is mercury.

Key relationships

Silver tarnishes black by reacting with sulphur in the air to form silver sulphide.
Copper reacts with moist carbon dioxide to form a green coat of basic copper carbonate.
Iron exposed to moist air for a long time forms a brown flaky coating called rust.
Rusting needs BOTH air and water present together — either alone is not enough.
Alloying almost always changes a metal’s properties: pure iron is soft, but iron with ~0.05% carbon is hard steel; iron with nickel and chromium is stainless steel.
Most alloys conduct electricity and heat less well than the pure metal they are made from.

Activities

Activity 3.14 — The three-test-tube rusting investigation

What you do: Place a clean iron nail in each of three test tubes: tube A has ordinary water, corked, with air above it; tube B has boiled, air-free water sealed under a layer of oil (no dissolved air can reach it); tube C has only dry air, kept dry by anhydrous calcium chloride. Leave all three for several days.

What you see: The nail in tube A rusts. The nails in tubes B and C do not rust at all.

What it shows: Rusting needs both air and water together — water alone (tube B, air excluded) and air alone (tube C, moisture excluded) are each insufficient on their own.

What you should be able to do

Recognise

Which of the three test tubes in Activity 3.14 rusts, and why the other two do not.
A named prevention method (galvanising, anodising, alloying, etc.) from a description of how it works.

Explain

Why a galvanised article stays protected even after the zinc coating is scratched.
Why pure iron is rarely used on its own, and what carbon/nickel/chromium do to it.
Why copper, not an iron alloy, is used for hot-water tanks.

State

At least four ways to prevent corrosion.
The definition of an alloy, with two named examples and what each is made of.

NCERT alignment

This topic corresponds to NCERT Class 10 Science, Chapter “Metals and Non-metals”, sections 3.5, 3.5.1; activities 3.14. Explanations, visuals and worked material here are original.

Quick revision

Corrosion: silver → black silver sulphide; copper → green basic copper carbonate; iron → rust.
Rusting needs BOTH air and water — proven by the three-test-tube experiment (Activity 3.14).
Prevention: painting, oiling, greasing, galvanising (zinc coating, protects even if scratched), chrome plating, anodising, alloying.
Alloy = homogeneous mixture of metals (or metal + non-metal); changes properties — steel, stainless steel, brass, bronze, solder, amalgam.
Alloys usually conduct electricity/heat less well than the pure metal.