Iridium

The fact that gold and platinum are exceptionally resistant to corrosion is one of the reasons they have been prized for so long. While copper and iron will eventually lose their luster, gold and platinum can do so for an extended period of time. It will take some effort, but you can destroy those metals. You can watch it at work by pouring some boiling nitric acid [HNO3] and hydrochloric acid [HCl] on either one. What happens if you treat iridium the same way? Nothing at all.

Even though iridium is one of the world's rarest elements, it has one of the highest corrosion resistance rates. Almost nothing can penetrate it. Iridium is unaffected by things that would turn other metals into a puddle of bubbling goo. This is partly because of its extraordinary density. It is the densest element, second only to osmium. But it's quite rare because we can only make a few tons of it every year. It also has the advantage of being implicated in the meteor that killed out the dinosaurs, according to evidence.