Rust

Rust is the result of water reacting with metals such as iron. It is a thoroughly unpleasant situation for any Transformer. Just ask Steeljaw.

Strangely, the fleshy bipeds appear not to suffer from rust. Maybe they're not as weak as everyone first thought.

Generation 1 cartoon
A disease known as Cosmic Rust was responsible for wiping out the Thirteenth Legion of Autobots on Antilla. The Decepticons on Earth later contracted Cosmic Rust, even though they were rust proof. Fortunately for the Decepticons, Perceptor had developed a new solution known as corrostop, which would prevent rust and corrosion, curing the disease. Unfortunately for the Autobots, Megatron infected Perceptor with Cosmic Rust, resulting in the Autobots becoming infected when Optimus Prime rescued Perceptor. Fortunately, the Autobots were able to create more corrostop.