Eli5 How come some metals rust and others don’t?



Eli5 How come some metals rust and others don’t?

In: Chemistry

Corrosion happens when a metal reacts with something. The most reactive “something” that most metals encounter is oxygen in the air, so most corrosion, including rust, is reaction with oxygen.

A metal corrodes if oxygen is able to rip electrons off it. Oxygen is good at that. It can corrode copper, iron, nickel, cobalt, zinc, aluminum, magnesium, calcium, strontium…a whole list. But not everybody.

Some metals have a strong enough grip on their electrons that oxygen can’t pull them off. You can find those metals on a ‘redox table’ – they include gold, iridium and platinum. It’s not a coincidence that many of them are metals we make jewelry out of – we do that because these metals stay shiny and bright.

Here’s a redox table. You can see at 0.40 volts, that’s the reaction that corrodes most metals, when they get exposed to oxygen and moisture at the same time. Every metal lower than that will corrode just from being outdoors; metals above that are much harder to corrode and will often be found as bright, shiny nuggets, even if they’ve been in the ground for thousands or millions of years.

Rust is specifically Iron oxide, therefore non-iron based metals won’t rust, but they will oxidize.

Iron happens to be a very common metal so we see rusted things quite often, but copper and brass will oxidize to give you the green layer on the Statue of Liberty. Aluminum quickly oxidizes and you only ever see oxidized aluminum but since its still fairly shiny you don’t notice.

Some iron based metals like stainless steel will have other metals mixed in such as chromium that stay shinier and reduce the formation of rust, but they can still oxidize and pick up a duller finish, its just not flaky rust

Most, if not all metals will form an oxide layer where the bare metal contacts the atmosphere and reacts with oxygen. On most metals though, the oxide layer stays well attached to the base metal. One ferrous metals (Iron and steel), the oxide layer doesn’t adhere as well to the metal and can flake off, exposing fresh metal that can form another oxide layer and flake off again. This is why steel and iron typically need some protective coating or maintenance to prevent rusting. Stainless steels try to get around this by adding nickle and chromium, which can form well attached oxide layers and protect the iron from oxygen. Iron oxide in specific is what we call rust. Other metals can corrode under different circumstances, but that’s not chemically rust.

There are other factors that can corrode metals beyond just oxygen though. Contact between dissimilar metals can result in a galvanic reaction where one metal causes the other to oxidize. This can also be used to protect metals though. It’s not uncommon for zinc to be used to protect steel, since the galvanic reaction will cause the zinc to corrode before the steel.

Everything they said. Chemical reaction where a material wants to return to its chemically most stable state. Certain metals like gold are fairly chemically stable and don’t mind existing in nature. Iron however has an atomic weight of 26 with only 2 electrons in its outermost shell giving it a +2 ionic charge, oxygen has a -2 ionic charge, so wherever possible, iron and oxygen are best buds and wanna be together in their natural state. This is rust