Would it be possible to move the ISS to an orbit of the sun, say 120 million miles away from the sun, putting it halfway between Earth and Mars? If it was purged of oxygen there would be little or no oxidation. Heating and cooling might be tricky! Perhaps it could be a staging/emergency post for Mars?
Putting it half-way between Earth and Mars doesn't make as much sense as you think. It's something akin to putting a ship half-way between two other ships. It's a moving target, and doesn't really make sense as a result. The closest is something like the Deep Space Gateway, which orbits the Moon, although that is of questionable value.
There is some discussion of moving the ISS to be a deep space space station. At best, a small part of the station could be used. Simply put, the radiation, thermal, and power systems are all designed to be in low Earth orbit, and wouldn't function well at anything else.
A purposely build mission to store fuel could quickly and easily be developed, and would be far easier than trying to bring the entire Space Station somewhere. It would also probably do the job better.