Corrosion and Its Prevention Notes

Corrosion and Its Prevention

Outcome

  • Describe corrosion in detail, including chemical equations.

  • Describe actions to prevent corrosion.

Keywords

  • Rusting: Corrosion of iron or steel in the presence of water and oxygen.

  • Corrosion: Gradual deterioration of a substance reacting with its environment (e.g., metal oxidation in air).

  • Sacrificial Protection: Using a more reactive metal to corrode preferentially, protecting another metal.

  • Tarnishing: Formation of a thin, discolored layer on a metal surface due to environmental reactions (e.g., oxidation).

  • Electroplating: Using electricity to deposit a thin layer of metal onto a conductive object.

Understanding Corrosion

Metal Oxides

  • Metals react with oxygen to form oxides (oxidation).

  • Group 1 metals oxidize quickly, tarnishing the surface.

  • Most metals oxidize slowly.

Tarnishing

  • Surface phenomenon: metal develops a film due to chemical reaction with air substances.

  • Occurs on metals like silver, copper, and brass.

  • Forms a thin layer of oxide or sulfide.

  • Results in discoloration, not structural damage.

Protective Oxide Layers

  • When aluminum reacts with oxygen, it forms aluminum oxide (Al2O3).

    • Thin.

    • Adheres strongly to the aluminum.

    • Prevents further oxidation (and corrosion).

    • Effectively transparent.

  • Some metals form thin, stable oxide layers, preventing further corrosion; this can make the metal appear dull.

Examples of Metals with Protective Layers

  • Iron → Iron oxide (rust): Porous and flakes off, leading to more corrosion.

  • Aluminum → Aluminum oxide: Used in food containers.

  • Chromium → Chromium oxide: Used in stainless steel.

  • Titanium → Titanium oxide: Used in aerospace.

Rusting

  • Specific type of corrosion affecting iron (and steel).

  • Conditions:

    • Iron.

    • Oxygen (air).

    • Water.

  • Overall reaction: iron + oxygen + water → (hydrated) iron(Ⅲ) oxide

Practice Task A

  • Describe what is meant by the terms ‘corrosion’ and ‘rusting’, giving details on how they are similar and different.

  • Explain how a metal oxide layer can protect the underlying metal from further corrosion. Provide an example of a metal that does this.

  • Explain what oxidation means in the context of rusting. Identify which element is oxidised during the rusting process and describe how you can tell.

  • 4Fe(s) + 3O2(g) + 2H2O(l) \rightarrow 2Fe2O3 \cdot 2H_2O(s)

Feedback Task A

  • Corrosion vs. Rusting:

    • Corrosion: General process of metal degradation.

    • Rusting: Specific corrosion of iron/steel with water and oxygen.

  • Similarities:

    • Both involve deterioration of metals.

    • Both can weaken metal structures.

  • Differences:

    • Corrosion affects various metals; rusting affects only iron and steel.

    • Rusting specifically needs oxygen and water.

  • Protection Mechanism of Metal Oxide Layer:

    • Forms a physical barrier.

    • Prevents oxygen and water from reaching the underlying metal.

  • Example: Aluminium forms aluminium oxide.

    • The oxide layer is thin, adheres strongly, and prevents further oxidation.

    • The thin layer of aluminium oxide is also nearly transparent, so it doesn't significantly alter the appearance of the metal.

  • Oxidation in Rusting:

    • Iron reacts with oxygen to form a new compound.

    • Iron is oxidized during rusting (gains oxygen to form iron oxide).

  • 4Fe(s) + 3O2(g) + 2H2O(l) \rightarrow 2Fe2O3 \cdot 2H_2O(s)

Preventing Corrosion

Desiccants

  • Substances that absorb moisture from the air, keeping sealed environments dry.

  • Examples:

    • Silica gel: small, transparent beads (used in packaging).

    • Calcium chloride: white beads/granules/powder (used in industrial applications).

Physical Barriers

  • Covering the metal to provide a physical barrier.

  • Methods:

    • Greasing.

    • Painting.

    • Plastic coatings.

  • Coatings can be damaged easily and require regular maintenance.

Sacrificial Protection

  • Using a more reactive metal to protect a less reactive one.

  • The more reactive metal corrodes (sacrificed) instead of the protected metal.

  • Example: Zinc blocks (anodes) on a ship’s hull.

Galvanizing

  • Specific type of sacrificial protection: coat iron or steel with a layer of zinc.

  • Most commonly by hot-dipping.

  • Zinc corrodes, preventing rust.

Electroplating

  • Coating a metal object with a thin layer of another metal using electric current.

  • Enhances appearance and improves corrosion resistance.

  • Basic Principles:

    • Metal (Me) to be plated is the cathode.

    • Metal salt solution contains ions of the plating metal.

    • Electric current causes metal to deposit on the object.

  • Benefits:

    • Shiny, decorative finish.

    • Protects the base metal from corrosion.

  • Expensive and requires specialist equipment.

Regular Checks and Maintenance

  • Ensure long-term protection.

  • Key activities:

    • Inspections.

    • Cleaning.

    • Touch-ups.

  • Prolongs lifespan, reduces replacement/repair costs, increases safety and reliability.

Corrosion-Resistant Alloys

  • Alloys are mixtures of two or more elements, with at least one metal.

  • Stainless steel: iron, nickel, and chromium (chromium forms a metal oxide protective layer).

Practice Task B

Method

Description

Example

Advantage

Disadvantage

Desiccants

Absorb moisture from the air, reducing humidity around the metal.

Calcium chloride or silica gel in sealed containers.

Simple and effective in enclosed spaces.

Limited to enclosed or sealed environments.

Physical Barrier

Physically separates the metal from environmental factors.

Painting, plastic coating, greasing.

Inexpensive, easy to apply, provides immediate protection.

Can be damaged easily, requires regular maintenance.

Alloying

Mixing metals with other elements to improve properties.

Stainless steel (iron, nickel, chromium).

High resistance to rust and long-lasting.

Can be more expensive than pure metals.

Sacrificial Protection

More reactive metal corrodes in place of the protected metal.

Zinc anodes on steel ship hulls.

Provides long-term protection.

Requires periodic replacement of sacrificial metal.

Galvanizing

Coating iron or steel with a layer of zinc.

Hot-dip galvanized steel handrail.

Provides durable protection, even if scratched.

Zinc layer can eventually wear away and need recoating.

Electroplating

Coating a metal object with a thin layer of another metal using electric current.

Silver-plated jewellery.

Enhances appearance and corrosion resistance.

Can be expensive, requires specialist equipment.

Summary

  • Corrosion: General process of metal degradation.

  • Rusting: Specific type of corrosion affecting iron in the presence of air and moisture.

  • Metal oxide layer can protect if it forms a physical barrier.

  • Sacrificial protection: reactive coatings with a more reactive metal (e.g., zinc in galvanizing).

  • Electroplating: improves appearance and/or corrosion resistance.

Desiccants Can Reduce Metal Oxidation

  • Rust prevention achieved by eliminating oxygen or water.

  • Iron nail in four test tubes:

    • Boiled water and oil layer.

    • Salt water solution.

    • Open to the air.

    • Sealed container with calcium chloride.

Test Tube Experiment Results

Test tube

Conditions

Result

A

Boiled water and oil layer

No rust. Boiling water removes oxygen, and the oil stops new oxygen entering.

B

Salt water solution

Severe rust. Salt water solution acts as an electrolyte, speeding up rusting.

C

Open to the air

Rust. Air and moisture cause normal rusting.

D

Sealed with calcium chloride

No rust. Calcium chloride dries out the air.