Mixing aluminum and stainless

[buzzz]

I have an aluminum swim platform support I need to refasten under the waterline. (The boat is in salt water) Can I use a stainlees steel bolt? What is the rule on dissimilar metals?

 

[tommays]

Re: Mixing aluminum and stainless

You really do not have any choice BUT SS bolts

Adding more Anodes may be nessary to protect everything


Tommays

 

[External Combustion]

Re: Mixing aluminum and stainless

Fastenal offers aluminum bolts and cap screws from size 10 through 1/2" along with washers and nuts.

If you use stainless steel use the 304 family as the galvanic potential is less.

Google Fastenal for the store nearest you. They are good people to know.

.fastenal.com

 

[Texasmark]

Re: Mixing aluminum and stainless

From the "hosses mouth".

MIL-STD 454 defines what is permissible for US defense department airborne environments, and it doesn't differentiate between seaplanes operating in salt water, or U2 atmospheric testers.

The spec says that if you attach metals in a particular group to one-another, and sometimes to metals in an adjacent group, you will have minimim galvanic activity.....corrosion.

Group II includes: Aluminum, Aluminum Alloys, Zinc, Cadmium, Tin, Stainless Steel, Tin-Lead (Solder).

If it's ok for Uncle Sam, surely it's ok for you .

Go for it.

Mark

 

[Jdubb16]

Re: Mixing aluminum and stainless

I would not trust that info. I design steel, stainless steel and aluminum and I always isolate them. I am actually required to do so on gov't projects, so I am not sure they follow their own standards (what's new).

Stainless steel will begin to corrode the aluminum, I would find a peel and stick type of dielctric isolator to stick around the throat of the bolt (the portion that may come in contact with the aluminum) as well as using neoprene or high density platstic washer where the nut and bolt head might touch the aluminum. I would be a little conservative as I assume this is a connection that will be hidden and you will only be able to monitior it when you actual look under the swim platform. You would not want to find out the aluminum is corroding when the swim platform breaks off.

Just my opinion.

J-dubb

 

[Frank Acampora]

Re: Mixing aluminum and stainless

Ahhh! sorry to disagree, but no matter what Uncle Sam says, in a salt water environment, any aluminum in contact with stainless is going to corrode. Look at the engine mountings where the transom bolts go through! Matter of fact---on one of my engines, corrosion split two mounting clamps where the bolts went through. But again, you don't have much choice if you want the strength of steel fasteners. My suggestion would be to drill the holes slightly oversize and use nylon flanged collars around the screws to isolate them electricaly from the aluminum.

 

[F_R]

Re: Mixing aluminum and stainless

I fully realize that the smart people all say that isolating the dissimilar metals will prevent corrosion. But down here in the real world it is obvious that they corrode anyway. I wonder how it's possible to isolate them when a conductor (salt water) is joining them together? Sounds like witchcraft to me.

 

[Willyclay]

Re: Mixing aluminum and stainless

My experience is with a 1965 Lonestar 16.5ft painted aluminum hull. The transom had SS bolts through it holding an aluminum angle that ran side-to-side. The head of each SS bolt had a nylon washer isolating it from the transom skin. There was ZERO corrosion under or around these bolts. The rest of the transom looked like Swiss cheese. Good luck.

 

[Boatist]

Re: Mixing aluminum and stainless

My 1980 21 foot aluminum Crestliner has had stainless steel bennett trim installed with Stainless steel bolts and washers inside and out since 1981 and see no sign of any corrosion at all. I did put sealer behind every bolt and washer to prevent leaks. Also every bolt on the boat is stainless steel from the factory. Again I see no sign of any problem.

I boat 50 percent of the time in the Pacific Ocean but it is a trailer boat so pull it and flush it at the end of the day.

 

[John_S]

Re: Mixing aluminum and stainless

Mark,
I might have missed it, tell me how you really feel.


I am not a metals expert, but from a recent work project, we had 16ga 304 stainless steel cap, rivited (structural steel rivits w/zinc coating) to a 6063T6 aluminum extrusion, and it passed salt fog testing at D.T. Brown labs on LI. This was an accellerated corrosion test, and the product is not designed to be in contact with salt-water.

 

[tashasdaddy]

Re: Mixing aluminum and stainless

closed for review

 

[Don S]

Re: Mixing aluminum and stainless

jheron and Texasmark. That is enough from both of you.

DO NOT post on this thread again either of you.

I have deleted several of the posts.

 

[over indulgence]

Re: Mixing aluminum and stainless

from what i do know is if you use the same grade stainless bolt and nut (i.e 304L bolt and 304L nut) then you have to use an anti-sieze when installing nut to bolt. otherwise you run a good chance of galling the stainless and bolt will break before you get it tight. just a word from experiance might save you some headache.