Fuel system question

So, in the process of undersealing "buttercup" before re-fitting fuel tank (I hate undersealing, real pain of a job laying on your back under the car) however question is....what is the black plastic box above the near side of rear axle, has rubber pipe from top of tank plus plastic pipe into metal pipe disappearing under the heatshield towards the engine bay, is it part of the breather system?
reason for asking is that the metal section in the pipe (runs across the car along with the rear brake pipes) is rusty, I wondered if I could replace this rusty section with rubber hose?
the rusty section is only about 3 ft long but looks horrible.
If it merely a breather pipe then can I simply wire brush it and paint it?
Thoughts please

It's a Charcoal filter canister, to soak up fuel in case you roll the car. The pipes run forward to vent the pressure/fumes from the petrol tank to a solenoid valve on the nearside of the engine bay bulkhead. he solenoid then gets a signal from the ECU to periodically open to vent the pressure into the inlet manifold.

The metal pipe you refer to is therefore carrying fumes.

You could replace it with some sae j R9 petrol hose & proper petrol hose clamp style clips not jubilees, however ensure that there are no sharp bends that could kink the the hose .

It would be better to replace the metal pipe with an equivalent diameter piece of copper piping assuming that it is available in that size.

Many thanks for that info DOH, super helpful, think I will look at the copper pipe option, it looks to be near enough the same as the brake pipe alongside it anyway, just makes sense to tidy up what I can while the tank is out of the way.
Cheers