Ingredient X

Ingredient X is an uncreatively named substance in the Generation 1 continuity family.

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

Generation 1 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