Why do some metals rust faster than others?

Val wanted to know why some metals rust faster than others, so Chris Smith put the question to materials physicist Jess Wade...

Jess - The technical term I think is corrode. Rust we only think about iron so iron rusts and goes that typical kind of rusty, bronze colour so that’s right. And corrode is when metals interact with oxygen, usually in the presence of water, and it’s because oxygen is super reactive; the oxygen that’s in the air. It’s a really hungry little gas. It needs to fill two electron spaces so it attacks these metals, which are kind of seas of electrons, and tries to pull them off that to react with it.

In some metals this seems to happen very quickly, so in things like iron. The interesting ones are the things where it doesn’t happen and aluminium actually super quickly, aluminum that we use in cans and in an awful lot of products, covers itself with a very very strong layer of aluminium oxide so that makes it look like its not rusted, but that layer is super tough and sturdy and no more oxygen can get through it. So we can do that.

We can do other kinds of clever things with metals where we blend them with other metals to stop them from corroding. I think the ones that are really interesting are things like gold and platinum, which have this sea of electrons. You know gold is really good at conducting electricity, which is why we use it so often and it’s beautiful if you buy expensive jewellry. But that doesn’t seem to rust, so the kind of interesting questions are why is that not happening? And I think that’s much much more complicated than we think. It’s a kind of quantum chemistry question why it doesn’t happen in gold.

But certainly all metals do seem to rust. They want to get back to their kind of state where we found them in these mountains and when we started refining them in the first place. So it’s a rate: you can look at a table to see which will rust faster.

Chris - You can make gold dissolve, but you need a mixture of all of the three sort of king acids, don’t you? Nitric, sulphuric and hydrochloric acid. Aquarega I think isn’t it to dissolve gold and it’s pretty horrible stuff?

Jess - Yeah. I don’t advise doing that at home.