Ingredient X

There is no ingredient X to be found—anywhere. Anywhere.

If ingredient X is a previously-unknown element, it certainly would have a very high atomic number and weight, and would most likely be radioactive, with a very short half-life. This may account for its rarity.

Nothing is known about the physical or chemical properties of ingredient X, except that it has something to do with corrostop.

Fiction

The Transformers cartoon

Corrostop could not be produced without ingredient X. The reason is not clear. Perceptor referred to it as a key alloy element—which would imply it is a metal, and is used up in forming corrostop. Optimus Prime, however, called it a catalyst, without being contradicted by his friendly neighbourhood science-bot. This would imply it was NOT used up in forming corrostop, but was lost in the reaction in some other way. One thing is certain: Cosmos and Blaster could not find any more of it, anywhere...leaving Earth the richest source of the substance in the universe. Cosmic Rust

Animated cartoon

Ingredient X was a component of the cure to cosmic rust, called Corrostop by Ratchet. One part ingredient X is mixed with ten parts razon gas and then processed. The AllSpark Almanac II