Beyond the Shine - Knowledge about Aluminum

When I was working on a robotics project, I noticed something strange about the aluminum parts we were using. Unlike steel, they didn't rust or get crusty over time. Even after weeks of exposure, they stayed shiny and clean. But when we tried to weld aluminum for a custom part, the welds didn't behave like steel. That made me curious: why doesn't aluminum rust, and why is welding so much harder?

The Science Behind It

Aluminum is actually very reactive. If you cut into a fresh piece, it reacts with oxygen in the air almost instantly. But unlike iron, which forms red, flaky rust (iron oxide), aluminum forms a super thin layer of aluminum oxide (Al₂O₃). This layer is invisible but incredibly protective. It creates a barrier between the aluminum and the air and stops further corrosion.

Oxidation Reaction:

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

Al = aluminum metal

O₂ = oxygen gas

Al₂O₃ = aluminum oxide (protective layer)

This layer is also self-healing. If you scratch it, the aluminum underneath reacts with air again and quickly forms a new barrier. That's why aluminum doesn't need paint or coating to avoid rusting; It protects itself.

Why Is Welding Aluminum So Hard?

The same oxide layer that protects aluminum also makes welding more difficult.

Different Melting Points:

The aluminum oxide layer melts at around 2,000°C, which is much higher than the aluminum underneath, which melts at about 660°C. So when you apply heat, the metal melts before the oxide does. That creates a problem: the oxide acts like a crust that blocks the melted metal from flowing together properly. If you don't remove it, you end up with weak or messy welds.

Cleaning the Oxide:

Before welding, you have to remove this layer using a wire brush or a chemical cleaner. Some welding techniques also use an electric current (like in TIG welding with AC current) to help break up the oxide during the welding process.

Aluminum Conducts Heat Fast:

Aluminum spreads heat quickly, so the area around the weld cools down faster than with steel. That means you need to manage your heat input carefully, or you might get cracks or an incomplete fusion.

What I Learned

Aluminum might look simple as a lightweight, shiny, and common metal, but it behaves very differently from other metals. Its ability to form an invisible shield makes it nearly rust-proof, but the same trait also makes welding it much more challenging.