|Cosmic Rust redirects here. For the episode in which it first appears, see Cosmic Rust (episode).|
Cosmic Rust is a microorganism that devours Cybertronian metals. An infected Transformer starts growing a rust-like substance all over their body. Eventually, the corrosion causes the Transformer to waste away to nothing. Perceptor describes it as the Cybertronian equivalent of the Bubonic plague, the "Black Death".
The Transformers cartoon
Cosmic Rust was first discovered in the ruins of an ancient Autobot colony in deep space, on the planet Antilla. The Decepticons who visited the planet were accidentally infected by the rust courtesy of a stray meteor. The radiation provided from a weapon found on the planet (known as The Lightning Bug) caused the virus to accelerate substantially.
A new and improved 'Cosmic Rust' disease once again surfaced in the year 2003, thanks to Swindle's tampering. Here, the Decepticons flood the Earth with it, destroying the original bodies of most of the Autobot garrison stationed there - before apparently backfiring on the Decepticons once again. The bulk of both forces however was unaffected as the bulk of the war's fighting has shifted to the Cybertronian-Theater. The Rust itself is never cured, but thanks to a fusion of Cybertronian technology and human car manufacturers, new temporary-bodies were built for the damaged Autobots. While the Decepticons had to resort to Mind Control in order to trick the human engineers into building them bodies of their own.
Nightbeat, head of the Autobot Secret Police, suffers from a chronic case of Cosmic Rust due to a failed assassination attempt. Though usually kept in check by an unspecified treatment, it can flare up and cause him to malfunction and become contagious if not monitored continually.
The Decepticon chemist, Oil Slick often made use of cosmic rust against Autobots, it is one of his favourite weapons. After attacking an Autobot base that held a Space bridge, only Rodimus Prime stood in their path; Oil Slick dispatched him by throwing a vial/grenade of cosmic rust on him. Making him crumble away.
Dark of the Moon film
Sentinel Prime carries a Cosmic Rust cannon that causes any metal it fires upon to rust and crumple away. He used it to murder Ironhide. It is a powerful weapon and doesn't just infect him with cosmic rust, but causes horrific exterior damage to Ironhide, killing him in just three shots and the effect rusts the Autobot veteran away with his body falling apart. His action shows that he revealed his true colors — Sentinel had made a deal with Megatron. Although this weapon is never used to its full potential.
As for the weapon in general, not many Autobots like it, as they thought it was something the Decepticons would likely use, and wasn't up to their honorable standards.