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1. To become thoroughly corroded by rust, especially as to cause something not to work or be able to move correctly. The nuts and bolts have all rusted up, so getting these brackets off is going to be a pain in the butt. You see here? The engine block is totally rusted up, meaning we'll have the replace the whole thing.
2. To cause something to become thoroughly corroded by rust. In this usage, a noun or pronoun can be used between "rust" and "up." You should have kept your bike in a shed or something, because the rain has rusted the chain up completely. The sea air in this town rusts up machines really badly, so we constantly have to have our things repaired or replaced.
1. To become thoroughly corroded: The walls of the old ship had rusted up.
2. To become immobile or stuck due to corrosion: The bolts have rusted up; I can't remove them.
3. To cause something to be thoroughly corroded: Don't use these chemicals; they will rust up the tank. Exposure to salt rusted the fender up.