Stainless Steel Through hull fittings?

Here's a picture of corroded keel bolts. The corrosion was only visible after the keel was dropped. The visible parts of the keel bolts in the sump looked fine.
 
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vyv_cox said:
Electrical isolation is definitely the answer, but difficult to achieve in most cases. How is this done? And having achieved it with, for example, plastic inserts, then a good sealant is required to avoid crevices.
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Sorry about the delay in answering but I have been detained by scandals :).

When I said isolation I really meant isolation of the bulk of the mass of the ss from direct contact with the aluminium on the wet side rather than being completely isolated. So in a similar way to ss fittings on spars, etc where the mass of the ss is isolated fom the spar by a gasket but the ss fastenings into the aluminium but hidden from the environment are not (well, apart from a little bit of compound).

In our own case (and some others I've looked at) the ss valve is flanged with a neoprene or similar gasket to the alumium. The aluminium plating (say on the stand pipe or the top of a sea chest as in our case) is thickened by a tapping plate, drilled to match the flange PCD and ss Helicoil or similar inserts inserted for ss machine screw fasteners. In the case of the valve being flanged direct to the top plating of a sea chest, for example, the ss insert, the bottom end of the machine screw and aluminium are in contact with the sea on their wet side but this seems to work fine and is widely used. In our case ss piping is used on the boat side of the valve for seawater.

Fire services have been mentioned and again that is mostly 316 ss too as far as I am aware with no problems (we use ss butterfly valves) but note that these are not wet sprinkler or otherwise still systems but just supply to hoses which are also used for hose down, etc so stagnation is not an issue.
 
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