Stainless bolts in aluminum

I need to bolt some GRP framing to aluminum below the waterline. I was planning to use stainless bolts (grade not determined yet). What are thoughts on how to best make the holes watertight as well as minimize the potential for dissimilar metal galvanic corrosion?

 

Plastic washers, to isolate the s/s from the alloy.

 

Use aluminum bolts and nuts.

 

"Nutserts" (riveted threaded inserts) are a common choice. It would be helpful to know a little more about the application.

 

Would having a less noble noble metal such as zinc included in the equation save the aluminium from corrosion ?

 

Thanks all. The application is a GRP hull extension on an aluminum catamaran. It will be through-bolted onto the reinforced transom. These bolts could see fairly high cyclical loads, so I'm not willing to use something other than steel.

I've been leaning toward plastic washers and inserts for isolation, perhaps with a neoprene or similar washer beneath the plastic to help seal from leakage. Should I be using something like 5200 as well?

 

5200, ideal, yes the washers will of course seal the water, 316L grade is suitable for the stainless steel.

 

Plastic or neoprene will permit movement where you really don't want any. So, you have a 'glass extension through bolting an aluminum hull. Use stainless nutserts and stainless machine screws (or bolts, depending on size). 3M5200 wouldn't be a bad idea, really is just a coating/bedding helping keep corrosion away. There's still a dissimilar material issue though. Maybe you should look into some tabs or ears welded to the aluminum, possibly a Triplate transition joint, so you can eliminate the corrosion issue all together.

 

I'm not following the tabs or ears suggestion. Wouldn't they need to be bolted to as well?

 

A dissimilar metals transition joint can be accomplished a few ways. The usual way is to use a bimetallic product, that explosive welded (at the factory), this permits you to weld an aluminum part to an aluminum hull, yet the other side is stainless, so you can bolt or weld to it as well. I picture a set of ears or tabs (with holes) to bolt the 'glass extension to (stainless bolts through stainless ears or tabs). These bimetallic plates are commonly employed and can solve the issue.

 

Got it. Thanks.

 

Had the same problem back in 1982. Bolted deck gear to aluminum boat as well as lots of equipment on the interior. Plastic inserts and washers and bedding compound under the fittings worked fine. No galvanic corrosion problems.

 

But underwater is different, no ?

 

Yes. You should minimize as much as possible dissimilar metals underwater and making sure there are no DC current leaks to the hull or water becomes critical.