According to Steve Barker :
Real stainless has no iron in it and does not rust. If it rusts, then it's
not pure stainless steel.

Stainless steel is a variety of alloys of iron with other metals.
There's no such thing as "pure stainless steel", because there isn't
just one "stainless steel".

There are different alloys/grades of stainless, which have different
levels of protection against corrosion.

The stainless steel you see in cutlery is just as pure as any other,
but expose it to, say, high temperature caustic solutions, it rusts.

Most "consumer grade" stainless won't stand up to continuous exposure
to salt and moisture for decades on end. Or at least, not at 30 gauge.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.