And how strange that one is not rusting?
That is because the shank is popped inside the aka. The rivet itself is not rusting in any of the shots... it is the shanks that bleed a bit. Not structural.
Hobie is going to resolve? That would be me of course. I have offered (Mariner) to replace your Monel rivets that have carbon in the shanks for straight stainless rivets, Mariner will be contacting you.
We use different rivets for different applications. Over the years Monel has been specified so we also offer them.
Couple of internet quotes about this issue:
Unlike Stainless Steel, Monel is mostly Nickel (67% Ni, 30% Cu, 1.4% Fe). Some Monel alloys (Monel K-500 for example) can be much stronger than 18-8 or 316 stainless steel. And yes, the monel rivets have a carbon steel shank.
Monel won't react with the alum. like the stainless. As to the shank being steel. I have found the ones that are not steel tend to break off before the rivet is fully drawn tight. If you don't like the rusting shank, Neatly dab your 4200 there.