Metals: Surface treatments and finishes

Introduction: Why Finish Metals?

Imagine you have just finished making a beautiful steel coat hook. It looks great now, but if you leave it as "raw" metal, two things will likely happen: it might start to corrode (rust) because of moisture in the air, and it might look a bit dull.

In this chapter, we look at surface treatments and finishes. These are processes applied to the outside of a metal product for two main reasons: Aesthetics (to make it look better) and Function (to protect it from the environment and make it last longer).

1. Painting

This is one of the most common ways to finish metal. It involves applying a liquid pigment to the surface.

• How it works: The metal is usually cleaned, then a primer is applied to help the paint stick, followed by the top coats of paint.
• Why use it? It provides a huge range of colours and finishes (gloss, matt, satin). It acts as a barrier, stopping water and oxygen from reaching the metal and causing rust.
• Applications: Car bodies, metal gates, and toy models.

2. Dip Coating

Think of this as giving your metal product a "plastic jacket." It is often called polymer dip coating.

• How it works: The metal part is heated up and then dipped into a tank of fine polymer powder (like polyethylene). The heat melts the powder onto the metal, forming a smooth, thick plastic layer.
• Why use it? It provides a comfortable grip, adds electrical insulation, and protects the metal from chemicals and weather.
• Applications: Tool handles (like pliers), dishwasher racks, and coat hangers.

Quick Review: Why do we finish metal?

1. Protection: To stop corrosion (rusting).
2. Appearance: To make it look shiny, colourful, or professional.

3. Electroplating

This is a clever process that uses electricity to coat a cheap metal with a thin layer of a more expensive or attractive metal.

• How it works: The product is placed in a chemical bath, and an electric current is passed through it. This "pulls" atoms of a plating metal (like silver, gold, or chrome) onto the surface of the product.
• Why use it? It makes cheap metals look expensive, increases hardness, and provides excellent corrosion resistance.
• Applications: Silver-plated jewellery, "chrome" taps, and gold-plated electronic connectors.

4. Anodising

This process is specifically used for Aluminium. It’s like "super-charging" the metal's natural protection.

• How it works: The aluminium is placed in an acid bath, and an electric current is passed through it. This makes the natural oxide layer on the surface much thicker and harder.
• Why use it? It makes the surface very hard-wearing and scratch-resistant. It also allows the metal to be dyed vibrant colours.
• Applications: iPhone casings, high-end bicycle parts, and torches.

Did you know? Anodised aluminium can be dyed almost any colour, and because the dye is "trapped" in the surface layer, it won't flake off like paint!

5. Galvanising

This is a heavy-duty protection method, specifically for Steel or Iron.

• How it works: The steel is dipped into a vat of molten zinc. This creates a thick, dull grey coating.
• Why use it? Zinc is a "sacrificial" metal. Even if the coating gets scratched, the zinc will corrode instead of the steel underneath. It is extremely durable.
• Applications: Street lamp posts, industrial buckets, and motorway crash barriers.

6. Powder Coating

This is like painting, but much tougher. It uses static electricity to get the job done.

• How it works: A dry powder is sprayed onto the metal. The powder is given an electric charge, which makes it stick to the grounded metal part like a magnet. The part is then "baked" in an oven to melt the powder into a hard, even skin.
• Why use it? It is much harder and more resistant to chipping than liquid paint. It is also more environmentally friendly as it doesn't use solvents.
• Applications: Washing machines, metal office furniture, and car wheels.

7. Lacquering

If you want to protect a metal but still see its natural beauty, you use lacquer.

• How it works: A clear, quick-drying resin is sprayed or brushed onto the surface.
• Why use it? It provides a transparent barrier that stops the metal from tarnishing (turning dull/green) when it reacts with air.
• Applications: Brass musical instruments (like trumpets) and copper ornaments.

8. Polishing

Sometimes the best finish is just the metal itself, rubbed until it shines.

• How it works: The surface is rubbed with very fine abrasives or a "polishing mop" and a special wax-like compound.
• Why use it? It creates a highly reflective, mirrored surface and removes small scratches. It is purely for aesthetics.
• Applications: Stainless steel cutlery, high-end car trims, and silverware.

Common Mistake to Avoid

Don't confuse Galvanising with Anodising!
Remember: Galvanising uses Zinc on Steel. Anodising is for Aluminium.

Summary: Comparison Table

When choosing a finish, you must decide what is most important for your product:

Key Takeaways

• Protection: Finishes stop corrosion by acting as a barrier against moisture and air.
• Aesthetics: Finishes improve the look through colour, shine, or texture.
• Specifics: Use Galvanising for heavy-duty outdoor steel; use Anodising for colourful, hard aluminium; use Dip Coating for handles and grips.