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.
On Cybertron, there is an entire sea of unstable planetary matter called the Rust Sea.
Fiction
Marvel The Transformers comics
Under certain malign conditions, Transformers may develop a rust rash. The Technobot Lightspeed is a known victim of this affliction. Lightspeed's Universe bio
As a physician, Ratchet was quite concerned about rust. When he observed humans using thin strips of wood to traverse moist, powdery surfaces, he regarded it as foolhardy — surely it would cause them to rust! Repeat Performance!
Later, when Skids brought the human Donny Finkleberg into Ratchet's medical bay to speak with Optimus Prime, Ratchet was most displeased. The medical bay was a sterile environment, and while humans don't rust, they sweat, which could have gotten moisture into Optimus Prime's wound, thereby starting a rust infection. Ratchet demanded that Skids not return until he had dehumidified the human, but he was overruled by Prime. Heavy Traffic!
"Rustbucket" is a fairly common insult among the Transformers. The Seacons once insultingly referred to Blaster as "Autobot rust-droppings". Club Con!
After a mission to retrieve a power cell from Dreadwing at the bottom of the sea, Getaway complained when he found a spot of rust on his wrist. Dreadwing Down!
The Transformers 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. Cosmic Rust
Beast Wars cartoon
When Cheetor wondered aloud the cause of Depth Charge's attitude problem, Rattrap guessed that it was probably rust. Deep Metal
Live-action film series
Transformers film
Ironhide complained his foot would rust after Sam Witwicky's dog urinated on it. Transformers
Revenge of the Fallen film
Jetfire, ancient and energon-deprived as he was, had rust all over him. He complained aloud: "Itchy, wretched rust in my aft!" According to Jetfire, deprivation of energon would cause a Transformer to rust and eventually fall apart. Revenge of the Fallen
Dark of the Moon film
Ironhide met his fate by a blast from Sentinel Prime's Cosmic Rust cannon. Dark of the Moon
IDW Generation 1 continuity
When Nominus Prime died, the Senate reported that he had succumbed to a long-standing rust infection, though his physician Ratchet knew that to be a lie. Post Hoc
