Before I get the drill and break the screws off in the mast, what should I use to break down corroded salt? WD-40? Hot water? Vinegar? Tequila?
Aluminum mast and fuse 3 M6’s. I really only care about salvaging the fuse and not having to redrill the inserts 
cheers!
I’d try a come-along, (with cardboard on the ground/floor under the seized setup), apply good tension, then hot water (maybe a few times), then tapping. Repeat and gently increase come-along tension. Make sure to use webbing slings and minimize leverage on the fuse over it’s length.
Before resulting to an extractor, I would try a manual impact screwdriver.
Sorry–I missed the fact that the screws are seized in there. if soaked with penetrant, sometimes an impact driver (which I would normally never use) can break the screw loose if a lot of pressure is applied downward into the screw. If not, it’ll just break the screw or strip the head
apply Kano Kroil or PB Blaster penetrating oil. WD40 doesn’t work nearly as well but you could try it if you have it.
Then use a heat-gun on the aluminum mast, hot to touch. It will expand a little bit and break free some of the siezed condition and the oil will penetrate better.
Then carefully un-torque the fastener with a fairly new sharp tool not a rounded off rusted tool. If you munge up the fastener head you’re on to step two, so please be patient before you mess this up.
Step 2 once you mess up the head. Use a dremel cut-off wheel to cut a slot in the heat. Then use a big wide slotted screwdriver to untorque the bolt.
Step 3 once that doesn’t work. Get a drill bit and start drilling. Start with a reasonably small drill bit and step up until you get to just under the diameter of the fastener. The head will pop off allowing you to disassemble. Then use a vice-grip on the exposed part of the bolt to get it out.
Try penetrating lubricant, heat the fuse and then tap directly onto the head of the bolt with a hammer a bunch of times. Once this fails use an Easy out screw extractor (m6 size and a sharp 3mm drill bit)
I’ve done this 4 different times on fused fuse to mast bolts. It always works and leave the thread intact with out damaging anything.
Old mechanics trick, get the bolts hot, be smart and don’t burn off the anodizing kind of thing but the bolts need to be hot. rub a candle on them, the heat melts the wax and pulls it into the threads. it get’s in there better than penetrator does, it’s an old school method and it works really well, both auto and marine mechanics have been doing that for years. personally I’d soak in penetrator overnight or for a few hours at least then do this. I’d Use a manual / hand impact to on the bolts and more importantly take your time. extractor bit’s don’t stand a chance against titanium or stainless hardware, I’ve removed broken bolts from masts before, it’s not fun. a manual impact and light hammer taps will make short abrupt motions, ideally cracking the threads and unseize them without breaking the bolts or stripping them or putting excess tension on the mast. be patient with it, work slow until they’re all the way out, it may take a while but most of the time they come out.
Its worth noting that the manual impact wrench will do two things, the first is the hammer tap will help break/crack the bond. The second, it turns a small amount while under load..
The above advice is good. Penetrating oil, and cycled heat. Don’t be afraid to heat it up and let it cool a few times. The fastener will expand/contract differently than the aluminum parts will. Cycling the heat will further free things up. The wax trick works well, but I doubt it’s necessary.
If you don’t have a manual impact, a large center punch and a hammer can help break the bond, just hit directly in center of each screw head with a firm hammer strike. Then an impact driver(screw driver style, not 3/8” wrench) should be enough to get it out.
I had an Axis mast fuse together for over a year that seemed frozen, but it eventually came apart.
I just bought these because I realized I’m out of the oil and my impact wrench got loaned away years ago. these are things that belong in the toolbox before you need them or you’ll try to get creative so you can go foiling tomorrow morning.
https://www.amazon.com/dp/B00DR7F4AO
https://www.amazon.com/dp/B08LYSHNCW