Surface Treatments for Metals: Anodising, Electroplating, Powder Coating, Dip Coating and Galvanising

Metals are vulnerable to corrosion (rusting in ferrous metals, tarnishing in others) and wear. Surface treatments protect metals and can also improve their appearance. This lesson covers the key metal surface treatments in AQA GCSE Design and Technology (8552), Section 3.2.9.

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Why Do Metals Need Surface Treatments?

Problem	Explanation
Corrosion	Iron and steel rust when exposed to oxygen and moisture, weakening the structure
Tarnishing	Copper, brass and silver develop a dull surface layer over time
Wear	Repeated contact can scratch and damage unprotected metal surfaces
Aesthetics	Raw metal may not have the colour or finish the designer wants

AQA Exam Tip: When explaining why a surface treatment is needed, always link to both protection AND aesthetics. The best answers cover both aspects.

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Anodising

What Is Anodising?

Anodising is an electrolytic process that thickens the natural oxide layer on aluminium, making it harder, more corrosion-resistant and capable of accepting coloured dyes.

How It Works

1. The aluminium part is cleaned and degreased.
2. It is placed in a bath of sulfuric acid (the electrolyte).
3. The aluminium is connected as the anode (positive electrode) — hence the name "anodising."
4. An electric current is passed through the electrolyte.
5. Oxygen is released at the anode and reacts with the aluminium surface, forming a thick layer of aluminium oxide.
6. The porous oxide layer can be dyed by immersing it in a coloured dye solution.
7. The part is sealed in boiling water or a nickel acetate solution to close the pores and lock in the colour.

Properties of Anodised Aluminium

Property	Detail
Hardness	The oxide layer is extremely hard (harder than the base aluminium)
Corrosion resistance	Excellent — the oxide layer acts as a barrier
Colour	Can be dyed any colour; the colour is within the surface, not on top
Electrical insulation	The oxide layer is a poor conductor of electricity

Real-World Examples

• Apple MacBook and iPad housings — anodised aluminium in space grey, silver and gold.
• Bike components — coloured anodised bottle cage bolts, seat post clamps.
• Carabiners and climbing equipment — anodised for identification and corrosion resistance.
• Architectural panels — building facades in major cities.

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Electroplating

What Is Electroplating?

Electroplating uses electrolysis to deposit a thin layer of one metal onto the surface of another. It improves appearance, corrosion resistance and sometimes wear resistance.

How It Works

1. The object to be plated is cleaned thoroughly.
2. It is placed in an electrolyte solution containing dissolved ions of the plating metal.
3. The object acts as the cathode (negative electrode).
4. A piece of the plating metal acts as the anode (positive electrode).
5. When current flows, metal ions from the anode dissolve into the solution and are deposited onto the cathode, forming a thin, even coating.

Common Electroplating Metals

Plating Metal	Purpose	Example Product
Chromium	Hard, shiny, corrosion-resistant	Taps, car bumpers, bicycle handlebars
Nickel	Corrosion resistance, base layer for chrome	Under-layer for chrome plating
Silver	Decorative, conductive	Jewellery, cutlery, electrical contacts
Gold	Decorative, corrosion-proof, conductive	Jewellery, electronic connectors
Zinc	Corrosion protection	Nuts, bolts, brackets
Tin	Corrosion-resistant, food-safe	Food cans (tin-plated steel)

AQA Exam Tip: Electroplating and anodising both use electrolysis but are different processes. In anodising, the workpiece is the ANODE and oxide forms on its surface. In electroplating, the workpiece is the CATHODE and metal is deposited onto it. Know this distinction.

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Powder Coating

What Is Powder Coating?

Powder coating applies a dry, coloured powder (usually polyester or epoxy) to a metal surface using an electrostatic charge, then bakes it in an oven to form a tough, even coating.

How It Works

1. The metal part is cleaned and degreased (often by shot blasting).
2. The part is hung on a rack and given an electrical earth.
3. The powder is sprayed from a powder coating gun that gives each particle a positive electrostatic charge.
4. The charged powder particles are attracted to the earthed metal surface and cling to it evenly.
5. The coated part is placed in an oven at approximately 180–200 °C for 10–20 minutes.
6. The powder melts, flows together and cures to form a continuous, hard coating.

Advantages and Disadvantages

Advantages	Disadvantages
Very durable — resistant to chipping, scratching and fading	Requires an oven — not suitable for heat-sensitive materials
Wide range of colours and textures (matt, satin, gloss, textured)	Minimum film thickness is greater than wet paint
No solvents — environmentally friendly (no VOCs)	Difficult to achieve very thin coatings
Overspray can be collected and reused (minimal waste)	Initial equipment cost is high

Real-World Examples

• Garden furniture — metal chairs and tables are powder coated for outdoor durability.
• Radiators — central heating radiators are powder coated white.
• Bicycle frames — high-quality frames are powder coated for a durable, attractive finish.
• Kitchen appliances — washing machine and dishwasher panels.

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Dip Coating

What Is Dip Coating?

Dip coating (also called plastisol dipping or fluidised bed coating) involves dipping a heated metal part into a bath of polymer powder or liquid polymer to coat it with a thick layer of plastic.

How It Works (Fluidised Bed Method)

1. The metal part is heated in an oven (typically 300–400 °C).
2. The heated part is dipped into a fluidised bed — a tank of fine polymer powder through which air is blown to make the powder behave like a liquid.
3. The polymer powder melts on contact with the hot metal and forms a thick, even coating.
4. The part is removed and allowed to cool and cure.

Applications

Product	Why Dip Coated
Tool handles (pliers, wire cutters)	Comfortable grip, electrical insulation, corrosion protection
Dishwasher racks	Protects wire from rust, provides a smooth surface for dishes
Playground equipment	Soft, durable coating that is safe for children and weather-resistant
Wire fencing	Corrosion protection for outdoor use

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Galvanising

What Is Galvanising?

Galvanising is the process of applying a protective layer of zinc to iron or steel to prevent rusting. It is one of the most important industrial corrosion-protection methods.

How It Works (Hot-Dip Galvanising)

1. The steel or iron part is cleaned by pickling in acid to remove rust and scale.
2. It is dipped into a flux solution (ammonium chloride) to prepare the surface.
3. The part is immersed in a bath of molten zinc at approximately 450 °C.
4. The zinc bonds metallurgically with the steel surface, forming layers of zinc-iron alloy.
5. The part is removed and cooled, leaving a shiny silver-grey zinc coating.

Why Galvanising Works — Sacrificial Protection

Zinc is more reactive than iron (higher in the reactivity series). If the zinc coating is scratched and the steel is exposed, the zinc corrodes preferentially instead of the steel. This is called sacrificial protection.

Real-World Examples

• Street lamp posts and crash barriers — galvanised steel for outdoor durability.
• Galvanised steel buckets — the classic silver-coloured garden bucket.
• Steel lintels and structural steelwork in buildings.
• Corrugated roofing sheets — galvanised to resist rain and weather.

AQA Exam Tip: Galvanising provides BOTH barrier protection (the zinc layer physically blocks moisture and oxygen) AND sacrificial protection (the zinc corrodes instead of the iron). Mentioning both mechanisms in your answer can earn extra marks.

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