Cosmic Rust was an infection similar to standard rust, discovered by the Decepticons. Its effects were identical to that of regular rust, except that it was capable of infecting the Transformers, who are apparently immune to normal rust. The only cure ever found for it was Corrostop (spelling obtained from its purpose: to stop corrosion), created by Perceptor in an unrelated project.
During an exploration mission, Astrotrain discovered an ancient Autobot colony far from Earth, on the planet Antilla. On the planet, they found an object of great power which they called the Lightning Bug. Testing the Lightning Bug, they destroyed an asteroid, causing a piece of it to lodge itself into Megatron's chest. Starscream removed the rock, but it had already infected Megatron with Cosmic Rust.
The infection spread quickly, and the Decepticons kidnapped Perceptor in an attempt to force him to cure them. Perceptor agreed, and tried out a new liquid he had recently invented as a corrosion remedy, and used on the Statue of Liberty. The cure worked, but in the process, he used up all the Corrostop he had created. Only with a sample of Corrostop could Perceptor replicate more.
Seeing a perfect opportunity to destroy the Autobots, Megatron tied up Perceptor and left him with explosives for his comrades to rescue him. As planned, the Autobots who rescued him were infected with Cosmic Rust, and spread it to the others. Perceptor made it known that they needed to obtain a sample of Corrostop, and the only place they could do so would be at the Statue of Liberty.
The Decepticons had, of course, forseen this, and were hidden at the statue in ambush. The ambush was a failure, the Lightning Bug was destroyed, and the Autobots obtained their much-needed Corrostop sample. Cosmic Rust was erradicated, and has not reappeared since then.