I have devised an alien species for a universe I am working on. They originate from the same planet as my high-pressure creatures (What would happen to a high-pressure creature in a low-pressure atmosphere (50atm)?), but can survive comfortably in a much broader range of environments due to their robust nature (they have survived nearly every mass extinction that the planet has to throw at them and can even survive for extended periods of time submerged in boiling water, freezing water or exposed to the vacuum of space. Think of them as being a little like tardigrades).
Although they did evolve on this particular planet they have since been transported around the universe and are currently taking up residence inside of ships and space stations that are made of metal.
The idea behind them is that they are essentially a pest species - not dangerous to any organic life form, but making their living eating away at the hulls of such ships and stations, often causing massive damage and even hull bursts.
They possess a tough exoskeleton made of aragonite (like a chiton) and bear a vague resemblance to some kind of arachnid or crustacean. Is such a creature possible? Can any biological organism (carbon-based or otherwise) survive by chewing up metal, and metal alone?