Ruthenium

Ruthenium, a hard silvery-white metal provides outstanding corrosion resistance because it does not tarnish at room temperature, nor is it attacked by hot or cold acids.

Ruthenium's excellent resistance is the reason for its major use as a hardener in conjunction with various other metals. Alloys of ruthenium and platinum or palladium are used for wear-resistant electrical contacts. Titanium's resistance to corrosion is increased by the addition of ruthenium.

Ruthenium has no biological role. However, it will severely stain skin. When ingested, ruthenium is retained in the bones making it highly toxic and carcinogenic.