That's our right-hand sink drain. The side of the sink that the tell-tale for the refrigeration pump discharges into, discharges salt water into that is. And that is not just staining... it is rust.
Simplistically, stainless steel is an alloy of iron, chromium, nickel and carbon. There are over 150 alloys called "stainless steel", each optimized for a particular set of properties. The alloys typically found on a boat are SAE 304 (AKA "18-8", 18-20% chromium and 8-10.5% nickel), and 316 (AKA "surgical stainless", 16-18% chromium, 10-14% nickel and 2-3% molybdenum). Between these two, 304 is the stronger and 316 is the more corrosion resistant.
But when you are buying plumbing fittings, you are not typically provided any choice of alloy (or even, usually, an alloy description). The single exception to this is the sink, where sometimes an alloy description may be given. All stainless alloys are somewhat attracted to a magnet, but 316 is attracted a lot less than 304. But you would really have to have samples of both to be able to make a valid comparison. To confuse matters more, the amount of cold work that the piece of metal has received will affect its magnetic properties - a lot. And finally, we don't have any loose magnets aboard Eolian, for obvious reasons, so magnetic testing of the new drain fittings is not going to happen.
It is clear that whatever the alloys of the sink and the drain fittings, the drain fittings have the least corrosion resistance. Still, I installed those drain fittings over 15 years ago, when I replaced the original severely corroded chrome-plated brass ones.
So I guess I am good for another 15 years now.