Eli5 : How does stainless steel stay stainless?

Question

Eli5 : How does stainless steel stay stainless?

In: Chemistry

Answer 1

So all metals, other than gold really, corrode. That is they react with oxygen to form some sort of metal oxide, which is basically saying the burn very, very slowly. A metal oxide is sort of like a rock or a salt. That’s often what the ore we mine it as already is, or similar enough.

Let’s look at iron. It rusts (corrodes, but specific to iron), which turns into flaky red/black stuff, that falls off and exposes more iron to rust further.

Let’s look at copper. It doesn’t corrode as fast as iron. And it turns into this green coating (look at the statue of liberty), which doesn’t flake quite as bad as iron.

Let’s look at aluminum, it doesn’t rust/corrode, right? Wrong. Aluminum corrodes very fast and any aluminum you have ever seen is “rusted”. Except the “rust” forms very fast and sticks to the aluminum very well. This provides a very thin and very protective layer that prevents the rest of the aluminum from corroding further. So the key to being a corrosion resistant metal (gold aside) is to have a thin layer of “rust” that sticks to it well and prevents further inwards metal from “rusting”.

So what is stainless steel? It’s iron with at least 10% chromium, and some other things. 316L stainless steel (there’s many different types of stainless steel, but this is a common one) is about 16% chromium, 10% nickel, 2% molybdenum. Even regular old rusting steel is still an alloy, it has more than just pure iron, like carbon in carbon steel obviously. The chromium does the same thing aluminum does, forms a thin and hard layer that protects the chromium and iron from further corrosion. What if I scratch the layer? Shouldn’t it rust then? No, because the chromium is throughout the stainless steel. Scratching it will just form a new protective layer anywhere. This is unlike galvanized steel, where iron is just coated with zinc. Scratch the zinc off and the iron will rust, the zinc is only on the outside.

But this doesn’t mean stainless steel is corrosion proof. Given time, water mixed with some other chemicals will corrode it. Some strong acids will make it rust pretty easily. Stainless steel is only stainless under certain conditions.