What the heck is rust?
Oxidation.
Rust is a colour, not a condition.
The correct term for corrosion would be oxidation, if the metal would. Nor that the goofs care, they hasrd to explain what a chassis is for their audience.
As a side note, I have to wonder if the rover life span will be ocsd enough to matter in case it did experience corrosion, the rovers of the moon were not painted/ coated and they worked just fine.