That is not rust, lol.
If you set your phone down on varying surfaces without a case, you stand the chance of getting marks on the back of it...you'd be really surprised what tiny particles can do when squished against a hard surface (table, counter, etc) by your phone...it has no where else to go, so damage will be had.
Corrosion is not going to easily (or quickly) happen to the metal pieces of the iPhone...the case on yours is essentially the same material as Apple's macbook line is made out of, and trust me, if the stuff was prone to ANY kind of corrosion issue, we'd have heard about it by now (since they've used the same stuff for quite some time now).
You have surface scratches which I'm guessing are from either not using a case or putting it into a tight fitting case uncleaned and little particles caused this while it moved around in the case itself.