Sentient iron

Sentient iron filings were intelligent creatures which 'breathed' metal and 'exhaled' rust.

In the 1970s, three groups of filings came to Earth in a crashed meteorite. It was examined by the Third Doctor and Liz Shaw and taken to a museum, but en route the filings escaped and began corroding metal structures, including a bridge, a cafe and a chemical plant, causing them to collapse. The Doctor used an anti-corrosive fluid to neutralise the two groups of filings, then traced the third to the crash site, which had attached itself to a plane about to take off with school children aboard. The Doctor arrived in time and used the fluid to end the threat. (TVC: The Metal Eaters)