7.5.2 Prevention of Corrosion

Corrosion is a widespread issue that affects industries, infrastructures, and everyday objects. Therefore, understanding corrosion and implementing preventive measures are crucial.

Prevention of corrosion is an important way of conserving our natural resources.

Following methods have been devised to protect metals from corrosion:

1. Coating with Paint

Corrosion can be prevented by applying a protective coating such as paint or epoxy, creating a barrier between the metal surface and its environment.

• Paint is cheap and can be applied easily.
• Provides visual appeal.

Uses: - Protects many everyday steel objects such as cars, trucks, trains, bikes, bridges, etc.

Simple explanation: Paint keeps oxygen and water away from the metal.

2. Alloying

The tendency of iron to oxidize can be greatly reduced by alloying it with other metals.

Example: Adding chromium to iron forms stainless steel, which is highly resistant to corrosion.

Simple explanation: Mixing iron with other metals makes it harder for oxygen and water to attack it.

3. Coating with a Thin Layer of Another Metal

Metals that readily corrode can be protected by coating with a thin layer of another metal that resists corrosion.

This can be done by:

• In the process of tin plating, a clean iron sheet is dipped in a bath of molten tin.
• It is then passed through a hot pair of rollers.
• Tin protects iron effectively since it is very stable.

• The process of galvanizing consists of dipping a clean iron sheet in a hot zinc chloride bath and heating.
• After this, the sheet is rolled into a zinc bath and cooled.

(c) Electroplating: - In electroplating, an electrolytic process is used to deposit one metal on another metal.

Simple explanation: Covering iron with a more stable metal (like tin or zinc) protects it from rusting.

4. Cathodic Protection (Sacrificial Protection)

• The metal to be protected from corrosion is made the cathode.
• It is connected to an active metal such as magnesium or aluminium.
• These metals are more active than iron, so they act as the anode.
• The more active metal oxidizes itself and saves iron from corrosion.

• Prevent iron and steel structures such as pipes, tanks, oil rigs, etc., in moist underground and marine environments.
• Large bars of magnesium are used to help protect ships against rusting.

Simple explanation: A more reactive metal "sacrifices" itself to protect iron. The magnesium or aluminium corrodes instead of the iron.

Intermediate Level

The Electrochemistry of Corrosion

Corrosion is an electrochemical process:

At the anode (oxidation): Fe → Fe²⁺ + 2e^-

At the cathode (reduction): O₂ + 2H₂O + 4e^- → 4OH^-

Formation of rust: Fe²⁺ + O₂ + H₂O → Fe₂O₃ · xH₂O

Simple explanation: Iron gives up electrons at one spot (anode) and oxygen takes them at another (cathode). The iron ions then react with oxygen to form rust.

Why Oxygen and Water Are Necessary

• Oxygen is needed for the cathodic reaction (reduction of O₂ to OH⁻).
• Water acts as the electrolyte — it allows ions to move between the anode and cathode.
• Without either, the electrochemical cell cannot work, and rusting does not occur.

Methods of Prevention — Detailed

• Cheap and easy to apply.
• Creates a physical barrier.
• Used on cars, bridges, buildings.

• Mixing iron with chromium → stainless steel.
• Stainless steel is highly resistant to corrosion.

• Tinning: dipping iron in molten tin.
• Galvanizing: coating iron with zinc (by dipping in molten zinc).
• Electroplating: using electrolysis to deposit a metal layer.

• Connect iron to a more active metal (Mg, Al).
• The more active metal acts as the anode and corrodes instead of iron.
• Used for ships, pipelines, underground tanks.

Comparison of Prevention Methods

Important Points

• - Corrosion: natural electrochemical process where a metal reacts with its environment.
• - Rusting of iron: most familiar example of corrosion.
• - Oxygen and water are necessary for iron to rust.
• - Corrosion is a redox reaction.
• - Anode: iron is oxidized (Fe → Fe²⁺ + 2e⁻).
• - Cathode: oxygen is reduced (O₂ + 2H₂O + 4e⁻ → 4OH⁻).
• - Rust: Fe₂O₃·xH₂O (hydrated iron(III) oxide).
• - Prevention methods:
• 1. Coating with paint — barrier.
• 2. Alloying — stainless steel.
• 3. Coating with another metal — tinning, galvanizing, electroplating.
• 4. Cathodic protection — sacrificial metal (Mg, Al).
• - Cathodic protection: iron is made the cathode; more active metal (Mg, Al) acts as the anode and corrodes instead.

Remember

• - Corrosion = natural electrochemical breakdown of metals.
• - Rusting of iron: requires oxygen and water.
• - Anode: Fe → Fe²⁺ + 2e⁻ (oxidation).
• - Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduction).
• - Rust: Fe₂O₃·xH₂O.
• - Prevention:
• - Paint → barrier.
• - Alloying → stainless steel.
• - Tinning → coat with tin.
• - Galvanizing → coat with zinc.
• - Electroplating → electrolytic deposition.
• - Cathodic protection → sacrificial metal (Mg, Al).

Common Mistakes

• 1. Thinking corrosion is only rusting of iron. Corrosion happens to many metals (e.g., aluminium, copper).
• 2. Thinking only oxygen is needed for rusting. Both oxygen and water are needed.
• 3. Confusing tinning and galvanizing. Tinning = coating with tin; galvanizing = coating with zinc.
• 4. Thinking cathodic protection works by coating. It works by connecting iron to a more active metal that corrodes instead.
• 5. Forgetting that corrosion is a redox reaction. Oxidation occurs at the anode; reduction occurs at the cathode.
• 6. Thinking paint is the only prevention method. There are several methods: alloying, tinning, galvanizing, electroplating, cathodic protection.

Quick Revision

• Corrosion: a natural electrochemical process that occurs when a metal reacts with its environment; the metal reacts with oxygen and moisture; converts refined metals to more stable metal oxides.
• Rusting of iron: most familiar example; oxygen and water are necessary; corrosion is a redox reaction.
• Anodic region: Fe → Fe²⁺ + 2e⁻ (oxidation).
• Cathodic region: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduction).
• Rust: Fe₂O₃·xH₂O.
• Prevention methods:
• Cathodic protection is used for pipes, tanks, oil rigs, and ships.

60-Second Revision

• Corrosion = natural electrochemical process.
• Rusting requires oxygen + water.
• Anode: Fe → Fe²⁺ + 2e⁻.
• Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻.
• Prevention: paint, alloying, tinning, galvanizing, electroplating, cathodic protection.
• Warning: oxygen and water are both necessary for iron to rust.