Not strictly true! Aluminium doesn't go the brown colour of rust, as that is the distinctive colour of Iron Oxide (rust) but Aluminium will corrode in salt water and produce a white powdery material Aluminium Oxide which actuslly can prevent further corrosion taking place, but will prevent the tripod working properly.
An extract from a site on metal corrosion:
Anything ferrous exposed to saltwater or salt atmosphere will deteriorate very quickly. Saltwater will also have a deleterious effect on non-ferrous metals like aluminum. They are subject to corrosion and eventually become brittle. Aluminum cans thrown into saltwater will completely dissolve in a very short time. Zinc-coated (galvanized) surfaces will eventually yield to corrosive elements if not properly maintained.
So if you want a non-corroding (or rusting) tripod you need it made from plastic or carbon.