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Old 26-02-2021, 15:41   #1
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Material choices to isolate stainless / aluminum

I would like to hear other opinions on materials to isolate 316 stainless from marine aluminum in above and below waterline applications. Specifically the following. A threaded 316 thru hull flanged adaptor plate from Groco. Im replacing Forespar marelon.
Currently 1/2 thick aluminum backing plates were TIG welded to the hull. They were tapped for bolts prior to welding. The current thru hulls are Marelon and are flush below the waterline. I can add an aluminum or plastic backup plate with new Marelon thru hulls. If I go aluminum, Im thinking a thin isolation layer between the stainless Groco plate and the aluminum hull backup plate. Lots of choices of materials. The sealant will be 5200. Above the waterline, Id like to use 316 mushroom thru hulls. Between the inner side of the mushroom and the hull, Im not sure which plastic to use. Thin, strong and something I can scuff up to grab the 5200. Suggestions?
Thanks to all in advance.
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Old 28-02-2021, 10:38   #2
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Re: Material choices to isolate stainless / aluminum

You could isolate them with 5200 alone if you don't squeeze it too tightly before it sets. Check with an ohm meter between the hull and the fitting.

Or, render three average sized manatees to obtain an adequate supply of waterproof and non-conductive grease.
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Old 28-02-2021, 15:33   #3
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Re: Material choices to isolate stainless / aluminum

The title had a typographical error, and has been fixed.
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Old 02-03-2021, 00:02   #4
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Re: Material choices to isolate stainless / aluminum

Maybe worth looking at if you haven't already?

Duralac is an anti-corrosive jointing compound for use between joints of dissimilar metals. It is in the form of a tough, flexible, yellow paste with low moisture permeability, and contains barium chromate in an inert filler matrix. The barium chromate is only very slightly soluble in water, so it remains in the joint, even in the presence of a considerable flow of water. It has a long useful life because it only becomes active in the presence of conditions which promote corrosion.
Duralac may be used:
Between aluminium alloy plates, extrusions and bolts or rivets used in building applications.
In general engineering applications where dissimilar contact cannot be avoided.
In vehicle building where aluminium alloy contacts steel.
For marine application where corrosion is accelerated by brine concentration.
In close proximity to the sea where a salt laden atmosphere will meet with structures and set up electrolytic cells.
In aerospace where rivets/bolts are secured into aluminium plates.

https://www.ebay.com.au/itm/283499960091?chn=ps&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&itemid=283499960091&targetid=11369912462 20&device=c&mktype=pla&googleloc=1000637&poi=&camp aignid=10101785183&mkgroupid=102311926220&rlsatarg et=pla-1136991246220&abcId=9300368&merchantid=7364522&gcl id=Cj0KCQiAvvKBBhCXARIsACTePW_KAzv7nfkBMNptYZtj7h7 OBTc0HN8tL07q9I1zsapywwdCPGpEcdwaApvXEALw_wcB

How to Prevent Galvanic Corrosion


https://www.amisales.com.au/product-article-2
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Old 02-03-2021, 02:41   #5
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Re: Material choices to isolate stainless / aluminum

For aluminum, a single coat of chrome phosphate pretreatment followed by primer and high-performance paint may effectively prevent galvanic corrosion. Alternatively, a single field-applied coat of heavy-bodied bituminous paint can be employed. Anodic coatings alone are usually insufficient for galvanic separation.8 May 2


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Old 02-03-2021, 04:26   #6
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Re: Material choices to isolate stainless / aluminum

Any thin plastic material will do: there is a self-adhesive heavy-duty UHMW material that is commonly put on carbon spars where blocks and lines would slap. Can't remember what they call it, but the rig shop sells it by the foot, 6" wide. I don't think much will stick to it, though--nor to any other plastic. You could use a very thin G-10 plate; McMaster has a wide selection of those, and they can be sanded to grip 5200.
Me, I'd weld an alloy standpipe everywhere I wanted a through-hull on an alloy boat, bring it above the waterline, then thread it for a Marelon ball valve. No corrosion; no caulking; no worries.
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Old 02-03-2021, 05:25   #7
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Re: Material choices to isolate stainless / aluminum

Just wondering if you could weld the stainless steel to the aluminum hull?
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Old 02-03-2021, 05:40   #8
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Re: Material choices to isolate stainless / aluminum

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Just wondering if you could weld the stainless steel to the aluminum hull?


Not a chance.
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Old 02-03-2021, 06:00   #9
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Re: Material choices to isolate stainless / aluminum

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Not a chance.
Why do you say that?
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Old 02-03-2021, 06:25   #10
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Re: Material choices to isolate stainless / aluminum

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Why do you say that?

For so many reasons it's difficult to pick one.


Think about it this way: Welding is a chemical process that occurs under high heat. The metal in the weld bead is a brand new alloy that gets invented by the guy mixing it all together. If he's good, and the weld was planned out well, the resulting new alloy will have good properties and the weld will be strong.
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Old 02-03-2021, 06:31   #11
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Re: Material choices to isolate stainless / aluminum

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Originally Posted by jordanbettis View Post
For so many reasons it's difficult to pick one.


Think about it this way: Welding is a chemical process that occurs under high heat. The metal in the weld bead is a brand new alloy that gets invented by the guy mixing it all together. If he's good, and the weld was planned out well, the resulting new alloy will have good properties and the weld will be strong.

How stupid is that response? You obviously know nothing about welding
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Old 02-03-2021, 09:06   #12
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Re: Material choices to isolate stainless / aluminum

Greetings all. Im leaning toward very thin G10. We tried 5200 but its hard to get a uniform thickness. Someone suggested making a very thin washer with pre- sanded Mylar, but Mylar is very slippery. Ive got a call into the CeraKote guys .
Rather than trying to put something in between, maybe I should consider coating the stainless .
We can always go G10 and 5200. I just wanted to tap into the collective knowledge here as it is always surprising.
We have used Marelon flush head thru hulls after we weld on a 1/2 backing plate which has been drilled and tapped. We vee out the hull and part of the plate .
Looks beautiful and there is nothing sticking out but its a lot of work.
Standpipes with supports are difficult to TIG in tight spots but do work perfectly.
Again, thanks to all for the suggestions.
Mark and crew.
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Old 02-03-2021, 10:00   #13
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Re: Material choices to isolate stainless / aluminum

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Originally Posted by coopec43 View Post
Why do you say that?


For starters the approximate 1200 degree difference in melting points.

There are methods to join the two metals, non of them are feasible to use in the suggested scenario, nor are they the correct methodology for this type of problem.
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Old 02-03-2021, 12:46   #14
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Re: Material choices to isolate stainless / aluminum

I used GPO-3 - an isolating fiberglass - for these types if things. It's the red cheap fiberglass that's readily available.


PVC sheets bond well to 5200, so that maybe an option too.

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Old 02-03-2021, 15:05   #15
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Re: Material choices to isolate stainless / aluminum

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Originally Posted by Sailmonkey View Post
For starters the approximate 1200 degree difference in melting points.

There are methods to join the two metals, non of them are feasible to use in the suggested scenario, nor are they the correct methodology for this type of problem.

You obviously have no idea at all Geez!


Seriously, thank you Sailmonkey for your patience and your erudite response! Have a look at this.

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