Galvanic Corrosion

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Bill Edmundson
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Galvanic Corrosion

Postby Bill Edmundson » Wed Feb 04, 2015 6:53 am

I'm working on rub rails on the bartender right now. I'm thinking about using 1/8 or 3/16 aluminum (6061) bar on it instead of SS.

I guess, from a corrosion stand point, which fasteners to use, SS or S-bronze?

Bill

BTW:
1/8 x 1 Al. is $1.23/ft.
3/16 x 1 Al. is $1.74/ft.
1" 316 SS hollow back is $5.49/ft.
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
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mrintense
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Re: Galvanic Corrosion

Postby mrintense » Wed Feb 04, 2015 7:20 am

It might be a bit more work but if you opt for the SS screws, you can isolate them from the aluminum by placing a dab of sealant in each countersink before adding the screw. Be sure to wipe the part down afterwards and you won't notice any sealant. When I was working aircraft, it was always standard practice to isolate dissimilar metals either through paint or sealant.
Carl
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gdcarpenter
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Re: Galvanic Corrosion

Postby gdcarpenter » Wed Feb 04, 2015 9:37 am

There's a product called Tef Gel that is apparently the perfect 'isolator' for SS fasteners in aluminum. I will be trying it when I reassemble my old outboard engine with some new fasteners. At least with SS the screws will be less noticeable.
This is my first, last and only boat build.

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kens
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Re: Galvanic Corrosion

Postby kens » Wed Feb 04, 2015 4:47 pm

I pondered the same thing Bill,
I went with 3/4 x 1" quarter-sawn Sapele; scarfed to make a single 26' rail.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Kevin Morin
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Re: Galvanic Corrosion

Postby Kevin Morin » Wed Feb 04, 2015 6:14 pm

Bill, I'd like to offer some experience in aluminum with SS fasteners and give some proposed install details.

If the rub rail is a flat bar, that is not a profile or extrusion shape like half pipe then I'll make the assumption mounting screws will be counter sunk- and I'd recommend against that if there is a possibility of another faster hear pattern? Flush head/stove head screws with the conic seating area will usually squeeze out any boundary material, like TefGel which can be useful, and that allows water to stand in the crevice between the two metals. This is a guaranteed corrosion site because almost all boundary layers that provide real isolation end up needing to be rigid.

Next is the barrel of bore of the remaining flat bar hole, if this can be over-bored to fit a small pieces of inert tubing that holds the two metals away from one another it is the correct means of isolation. TefGel can help but will wash out in a season or two and corrosion follows.

Last but not least, if you can use a cap screw style head where there is no cone to bury the two metals' interface and use GALVANIZED fasteners you will have moved to the very least galvanic difference on the chart/scale so you'd have minimized the galvanic potential.

Of course I realize you have plans and aesthetic considerations that may prevent bolts' head lining your sheer! So I'd like to suggest a little more time consuming method of installing a flat rail on a wood boat. If you're able to put up the rub railing, drill it and screw it in place without the plastic tube liner inserts (??) THEN go back and loosen all the screws in a 1;3 pattern. The rail stays where it is.

When a screw is backed off, put a circle of tape around the entire hole, and the fill the cone with 5200 and redrive the screw ONLY till the head is just outside the bar's face. Wait til the 520 hundred kicks and drive the screw home. This puts a layer of fairly impermeable reasonably stiff sealant around the screw's conic head buried in the aluminum bar. So you 'cast' a formed conic plastic barrier between the two metals. It is also a water tight joint (usually) and if the excess sealant is wiped clean with acetone just after the initial re-drive, the outer edge is clean, crisp and not smeared, then pull the tape circle.

Now you've installed an isolated fastener between the two metals and can expect decent service life. If you don't then it will begin to chalk in one season.

My bigger concern is how will you be bedding the aluminum to the wooden sheer clamp?

http://s689.photobucket.com/user/3f8/sl ... %20keeping reference recently posted on a metal boat site? These hole were sealed with TefGel and used SS bolts.

Also, please investigate passivating SS http://www.mmsonline.com/articles/how-t ... teel-parts this reduces surface reactivity.
Kevin Morin

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Bill Edmundson
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Re: Galvanic Corrosion

Postby Bill Edmundson » Wed Feb 04, 2015 7:38 pm

Thanks All,

Lots to think about here. This is why I asked.

Kevin,
I might just spend the money and go SS on SS. I'm in salt/brackish water.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Kevin Morin
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Re: Galvanic Corrosion

Postby Kevin Morin » Wed Feb 04, 2015 8:10 pm

Bill what about exploring the D shapes in various metals or plastics? There are plenty of thermoplastics that are super tough, could be found or sawn to a D out of round stock and would take fasteners and may well be available in lots of colors?

https://www.interstateplastics.com/Abs- ... -ABSNE.php vendor like this one have more shapes and materials than you can shake a stick at... might not be the lowest cost but sure would stop corrosion.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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kens
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Re: Galvanic Corrosion

Postby kens » Fri Feb 06, 2015 6:15 pm

Bill Edmundson wrote:I'm working on rub rails on the bartender right now. I'm thinking about using 1/8 or 3/16 aluminum (6061) bar on it instead of SS.

I guess, from a corrosion stand point, which fasteners to use, SS or S-bronze?

Bill

BTW:
1/8 x 1 Al. is $1.23/ft.
3/16 x 1 Al. is $1.74/ft.
1" 316 SS hollow back is $5.49/ft.


Bill,
I have a mixed bag of hardware in my build.
I use it primarily in salt water.
Aluminum sometimes corrodes by itself without fastners around of any kind,
Bronze just turns a snotty green, but seems to stop at that point,
Stainless sometimes corrodes, not knowing the base alloy or, alloy of fastners,
I have many fishing hooks, lures, bait knives, scissors, etc., all of stainless that rust like common steel, it is only a slower process.
I have no wood that is corroded.
At this point, 7 years in salt water, of fishing offshore, fish scales, shark blood, et al;
The 2 least corrosive things on the boat is wood and bronze.

For 'least corrosive' rub rails my vote goes out to a wood rub rail fastened with bronze screws.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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chugalug
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Re: Galvanic Corrosion

Postby chugalug » Fri Feb 06, 2015 6:25 pm

:D Sounds like TREX run through saws and routers for you too.You can also bend it by taping cardboard foundation tubes together and use an ipco? heater to warm it up. :roll:
Working on regular-sized Bo-Jest


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steveh41
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Re: Galvanic Corrosion

Postby steveh41 » Fri Feb 06, 2015 10:47 pm

Bill,

I don't yet have a lot of direct experience with galvanic corrosion, but from what I've read the combination of SS fasteners with aluminum hardware should be acceptable from a corrosion standpoint if trapped moisture can be avoided. I would steer clear of the bronze fastener/ aluminum combination.

A reference article:

http://www.bssa.org.uk/topics.php?article=89

I've been bedding all exposed fasteners and hardware in 5200 with the hope of reducing or eliminating trapped moisture and galvanic activity. Complete electrical insulation would be ideal when materials are dissimilar but hard to achieve with countersunk fasteners.

Regards,

Steve
The longest journey begins with a single step… then repeat as necessary!


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