Medieval steel and rust... Medieval smiths were quite capable of making good steel... some of it was even crucible-steel imported from the east, but even local smiths could make good steel if they had the right materials... Steel made in and around Toledo Spain, f'rinstance, had a bit of nickel and chrome in the ore... so they could make really good alloys. The thing is, good steel was expensive and it could only be made in small quantities. That's why you don't see steel crossbows before @ 1350... because the steel 'industry' was mostly very small shops that couldn't make steel in big billets. Typically the best medieval swords (and other tools) had good, hard steel for cutting edges, but the rest of the blade was made of lesser stuff.
It's likely that most triggers, lockplates, bow-irons, etc. were made from wrought iron or very low-carbon steel. But good quality arrow points, the edges of tailors' scissors (not shears, real scissors) good quality knives, etc. were actually made of very good steel.
Outside oil-quenching and blueing, most armor-steel was painted. just because we see it in the white (bare) in a museum, doesn't mean it was used that way. (The famous/infamous English White Company of mercenaries were apparently so-named because they kept their armor bright) A fair number of steel crossbows... like the Fels-Colonna bow in the Wallace Collection in London, had their steel prods covered with painted parchment, perhaps partially for decoration, but also for protection.
As for protecting run of the mill stuff in the white... your guess is good as mine... probably they treated it with something like linseed oil, that dries to leave a slightly gummy residue. Otherwise, I suspect you just work your steel over once a week with oil and sand. Geezer.