Rust Question

I hope this is the right section. I'm in school and the section i am on now is called 'lithium bromide absorption systems'.

Now what has me baffled is in this section i read it says in a brand new system there will be a film/layer of rust on the inside of the 'vessels' as they put it.

Now what i can not understand for any reason whatsoever is why would there be rust there? I mean if it's new/straight from the factory/manufacturer why would there be rust on it? I have to do a lot more studying to get a better understanding on how these systems work but i read that and it just makes no sense to me. Thanks in advance for any information.