It's probably not the 304 stainless that's the problem, but almost certainly an ordinary steel pin or bearing inside the hinge. They look like ball bearing hinges, and the chances are that the bearings aren't stainless.
304 isn't as good as 316 in a marine environment, but it isn't that bad, either. I had some 304 bits on my old yacht (because we had 304 at work and a handy workshop to make things...) and all that happened was that they would get a bit of surface staining, that would wipe off OK.
The chances are that if you could find 316 door hinges they may well still have an ordinary steel pin or bearings, so you may well not be any better off.
I'm pretty sure I have one, maybe two, heavy duty stainless door hinges left over from fitting the doors in the new house, as we fitted heavy oak doors that needed three hinges with bearings, and the hinges came in packs of two. If I remember tomorrow I'll try and dig out the left over one I'm sure I have around and see if I can dismantle it to see how it's put together. I know the hinges we used look very similar to the one in your photo, so there seems a reasonable chance it may be the same internally,
It might be possible to find a way to replace any non-stainless internals with stainless ones. My guess is that they don't use stainless bearings/pins on door hinges because stainless is a bit too soft to make decent bearings, but in your application the bearings aren't really taking any load as they move, so this shouldn't matter.
With luck, I'll post back here tomorrow after I've done an autopsy on a spare hinge to see what's inside.
The problem is that regardless of the stainless grade 304 or 316, or the material of the internal pin -
Fire-door hinges used to be sold including one of the above stainless grades; now they are graded with numbers like 9, 11 and 13. Further to that, Paulie, you have just complicated the matter by referring to A2 and A4....
And in case you wanted another complication, I've just been down to Screwfix and bought a pair of their very finest fire-eating hinges on the grounds that even if one of the originals is a bit dickie, it has taken five or six years to get there. Now that sounds alright doesn't it, except that I didn't notice an extra screw hole has been added and I get the impression that the others have been moved to allow for it, so the next thing is to take it and some WD 40 down to Cobnor and ask Polly Wee what she thinks of the hole positions, or whether she could make do with a hoosh of pressurized fish juice. Before I go back to Screwfix and try again.
I have to visit both locations anyway, as I took the Yaris down to Cobnor over the weekend to offer up the latest tent and once there, discovered that I had left the boom-up crutches in the Transit. The tent is not going to fit anyway - for other reasons. Just like today when I was so excited about new stainless hinges, I forgot to get some hooks for the coat-rack shelf I have just created in the hall
Fire door hinge grades are, I believe, representative of their fire resistance & load bearing capacity, so they may happen to be stainless to achieve this, but it's not an indication of their corrosion resistance. One would use a 60 minute door (a pretty solid thing) & grade 13 hinges to carry it all to keep the fire the other side of the door for one hour or whatever.
A2 & A4 is generally applied to fasteners, A2 will stain in sea water, A4 is generally OK. Most of the staining I get with A4 is where epoxy overlaps & stops the protective layer forming on the surface of the fastener, but nothing I've been concerned about. Given how much exposure to salt water our craft generally get, I often use A2 if I haven't got something the right size in A4.
I'm reasonably sure A2 is the same as 304 & A4 is the same as 316, they are certainly equivalent.
Hold out for 316 hinges - you may have to pay more than Screwfix prices though..... [brace yourself]....
Oddly enough the subject of fire doors came up in conversation today with the bloke who is taking a mould off Snarleyow to create new Bursledon Gigs (we were laboriously removing all the filler he put on the hull to create the exact shape he wants). The other day he was replacing a fire exit door in a nursing home and found he needed new hinges. Unfortunately no members of staff to be seen and the place was full of old dears with dementia who high-tail it for freedom at the least opportunity. He had to stand guard for hours until the owner turned up and he could go to Screwfix.
Sorry for the delayed reply, it took me some time to take one of the spare stainless hinged apart - they are not designed to be taken apart...
As far as I can tell, neither the bearing nor the pin are stainless steel, only the parts on show are. I would imagine that the same may well be the case for any normal heavy duty door hinge, as they aren't usually required to withstand direct exposure to water.
Whilst I was poking around trying to find the spare hinge I did come across a big brass hinge, which doesn't have internal bearings, but does have a brass pin. I've had it years, and can't remember where it came from, but suspect it lay well be one that I bought around 35 years ago, when rebuilding my old gaff cutter, as that had hinged mahogany locker lids in the cockpit, with brass hinges and lockable brass hasps.
I've been working on using stainless every-stressful-where on Polly Wee
But brass is a possibility. The real issue is that unless I use the same style of hinges, the holes in the (locally reinforced) deck will be in the wrong place and similarly those in the rowlock flap as well. Thanks for looking into the problem, gentlemen.
Drifting slightly, well you do as age comes upon you - Ruthie's Dear Old Dad was similarly incarcerated in his dotage; it took him no time at all to work out the code for the door lock and he used to escape repeatedly. I take more after my mother whose approach was much more direct; Colditz nothing, at the age of 85 or so, she had on occasions to be peeled off the heights of an eight foot retaining fence.
Anyway, I've bought another five or six year's worth of stainless