Quick answer: Anodising is a process of forming a thick protective oxide layer on aluminium. A clean aluminium article is made the anode and electrolysed with dilute sulphuric acid. The oxygen released at the anode reacts with the aluminium to form a thicker layer of aluminium oxide (). This layer protects the metal from corrosion and can be dyed easily to give an attractive finish.
Aluminium is quite high in the reactivity series, yet aluminium articles do not corrode the way iron rusts. The reason is a thin layer of aluminium oxide that forms on the surface as soon as aluminium is exposed to air. This layer protects the metal underneath.
The natural layer is very thin. Anodising makes it much thicker and tougher, so the aluminium is better protected and can also be coloured.
In CBSE Class 10 Science, Chapter 3 (Metals and Non-metals), what anodising is and how it is done are frequently asked board exam questions.
What You Will Learn
- Why aluminium resists corrosion even though it is a reactive metal
- What anodising is
- The set-up: anode, cathode and electrolyte
- What happens at each electrode
- Why anodised aluminium can be dyed
- Uses of anodised aluminium
1. Why Aluminium Resists Corrosion
When aluminium is exposed to air, it reacts with oxygen to form a thin layer of aluminium oxide:
| Feature | Rust on iron | Oxide layer on aluminium |
|---|---|---|
| Nature of the layer | Flaky and porous | Thin, tough and tightly attached |
| Does it protect the metal? | No; it falls off and exposes fresh iron | Yes; it seals the metal from air and moisture |
Key point: The oxide layer on aluminium prevents further corrosion. Anodising makes this layer thicker.
2. What Is Anodising?
Definition: Anodising is a process of forming a thick oxide layer on aluminium, by making the aluminium article the anode in the electrolysis of dilute sulphuric acid.
| Part | What is used |
|---|---|
| Anode (positive electrode) | The clean aluminium article |
| Cathode (negative electrode) | An inert electrode, such as lead or graphite |
| Electrolyte | Dilute sulphuric acid |
The process is called anodising because the aluminium article is made the anode.
3. How Anodising Works
Step 1: Clean the article. The aluminium article is cleaned to remove grease and dirt.
Step 2: Set up the cell. The article is made the anode, and an inert electrode is the cathode, both dipped in dilute sulphuric acid.
Step 3: Pass an electric current. Water in the electrolyte is electrolysed:
- Oxygen is released at the anode (the aluminium article).
- Hydrogen is released at the cathode.
Step 4: A thicker oxide layer forms. The oxygen released at the anode reacts with the aluminium surface, building up a thicker layer of aluminium oxide.
4. Why Anodised Aluminium Can Be Dyed
The oxide layer formed by anodising is slightly porous just after it forms. Dyes can soak into these tiny pores. The pores are then sealed, for example by treating the article with hot water, which locks the colour in. This is how coloured aluminium articles are made.
NCERT point: The oxide layer formed by anodising can be dyed easily to give aluminium articles an attractive finish.
5. Uses of Anodised Aluminium
- Cookware, such as pressure cookers and pans, which need a tough, protective surface
- Window frames and door fittings, which must resist weathering
- Decorative and coloured articles, such as some phone and laptop casings
6. Summary Table
| Feature | Details |
|---|---|
| Purpose | To form a thick, protective oxide layer on aluminium |
| Anode | Aluminium article |
| Cathode | Inert electrode (e.g. lead or graphite) |
| Electrolyte | Dilute sulphuric acid |
| At the anode | Oxygen is released and forms with the aluminium |
| At the cathode | Hydrogen is released |
| Extra benefit | The oxide layer can be dyed |
7. Exam Tips and Common Mistakes
Exam Tip: When asked to describe anodising, mention all three parts: the aluminium article as the anode, dilute sulphuric acid as the electrolyte, and oxygen at the anode forming a thicker oxide layer.
Exam Tip: If asked why it is called anodising, say it is because the aluminium article is made the anode.
Common Mistake: Making the aluminium article the cathode. Hydrogen, not oxygen, is released at the cathode, so no oxide layer would form.
Common Mistake: Saying aluminium does not react with air. It does react, but the oxide layer formed protects it from further corrosion.
Frequently Asked Questions
What is anodising?
Anodising is a process of forming a thick oxide layer on aluminium by making it the anode in the electrolysis of dilute sulphuric acid.
Why is it called anodising?
Because the aluminium article is made the anode of the electrolytic cell.
Which electrolyte is used in anodising?
Dilute sulphuric acid.
Why does aluminium not corrode easily?
A thin, tough layer of aluminium oxide forms on its surface and protects the metal underneath from further corrosion.
Why can anodised aluminium be dyed?
The oxide layer formed by anodising can absorb dyes before it is sealed, giving the article an attractive coloured finish.