SS crevice corrosion..what else to use?

[Finnerty]

Removed the stainless steel screws to replace the stainless steel trim tabs on my boat (which is kept in saltwater) and discovered the shaft of the screws were almost entirely eaten away. Apparently this is not electrolysis but is crevice corrosion that occurs in pockets of stagnant (no oxygen) water around the fitting. So what would be the best solution - silicon bronze? That would mean dissimilar metals (with SS trim tabs). Should I use bronze screws? Or, should I use bronze bolts so that I can run a bonding wire between them inside the hull?

 

[minuteman62-64]

Re: SS crevice corrosion..what else to use?

Are the screws going into metal or into glass/ply transom?

I've had good results using Napa Aviation Gasket sealer on ss threads going into metal. Maybe some 5200 or 4200 on threads going into glass/ply?

My understanding (from reading - I'm not an expert) is that you need to "seal" the ss screw threads to prevent/reduce the crevice corrosion.

 

[Finnerty]

Re: SS crevice corrosion..what else to use?

Are the screws going into metal or into glass/ply transom?

I've had good results using Napa Aviation Gasket sealer on ss threads going into metal. Maybe some 5200 or 4200 on threads going into glass/ply?

My understanding (from reading - I'm not an expert) is that you need to "seal" the ss screw threads to prevent/reduce the crevice corrosion.

The screws are going into solid fiberglass. The problem I believe is that the screws pass through two layers of stainless steel, being the hinge plate of the trim tab and the mounting plate. This has the potential to trap seawater between the plates. I suppose if I used lots of 4200 it would seal them off. But if there's a better solution, like using bronze bolts, I'll go that route.

 

[Finnerty]

Re: SS crevice corrosion..what else to use?

Are the screws going into metal or into glass/ply transom?

I've had good results using Napa Aviation Gasket sealer on ss threads going into metal. Maybe some 5200 or 4200 on threads going into glass/ply?

My understanding (from reading - I'm not an expert) is that you need to "seal" the ss screw threads to prevent/reduce the crevice corrosion.

I decided to stay with SS screws and seal it well with 4200. Other advice I received was that bronze fasteners will be too soft in this application and may shear off with the forces exerted by the trim tabs. I don't know how long the corroded SS screws were in there that I removed, but it could have been decades (!)...moral of the story, nothing lasts forever....