SST and aluminum
 

Yup, many of us have broken and had to drill SST bolts out of aluminum. it's particularly a problem in moist or a salty environment. In the 70s on jet engine design we used wet zinc chromate primer on SST fasteners threaded into aluminum. Often this wasn't foolproof, so tried to design for enough material around the fastener that it could be drilled out and a helicoil or different type of internally threaded insert could be used to repair the hole. I often use an anti-seize compound with SST threaded into aluminum.

Brass is closer to aluminum on the galvanic series than any of the SSTs and therefore it's a good choice in aluminum. The 400 series SST are closer to aluminum than the 300 series. But I considered the 400 series as the rustier SST , those that would flunk my magnet test and I would try to avoid for boat hardware.

I remember when I built my RV 6 experiment airplane the builders that were smarter than I were very Anal about making sure thet neither came into contact and yes using methods u mentioned. Good news you won’t have to land a boat

Great discussion. I think I’m going go with brass and will inspect when I pull inspection covers once a year

Most marine tanks are made form 5052 (5000 series) aluminum. It is one of the most corrosion resistant grades of aluminum and yields higher strength than say 3000 series which a lot of AL stuff is made of. 5052 has more magnesium in it to aid in its properties. If you can find marine grade fittings that are 5052 grade or 5000 series you will have better corrosion resistance. A lot of fittings are 6061 aluminum and have good resistance to corrosion but the 5052 grade will be a better match if you could find them. I wouldn't be afraid of brass if that's what you had tho.

Aluminum fitting on a tank
 

I should have mentioned my tank fitting which I drilled to remove the anti-siphon ball is aluminum