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Old 04-16-2014, 12:45 PM   #1
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mating aluminum with stainless steel

I'm fabricated some custom aluminum motor mounts upon which I intend to rest the full weight of my genset. Trouble is, that the genset drain pan (which is actually the base of the genset) is stainless steel. Do I need to insulate these from one another or can I simply attach both to the bonding system of the boat and hope for the best? How about a zinc? Thanks in advance for any help.
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Old 04-16-2014, 01:23 PM   #2
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Typically you do not have a problem until the two dissimilar metals are in the presence of an electrolyte. Aluminum and stainless are far enough apart in the activity series you should insulate them. I would not bond them together.
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Old 04-16-2014, 01:26 PM   #3
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Aluminum and stainless live quite well together, just look at any sailboat. It certainly would not hurt to put something between the two just to prevent water ingress. Windshield tape comes to mind.
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Old 04-16-2014, 01:56 PM   #4
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Quote:
Originally Posted by Brooksie View Post
Aluminum and stainless live quite well together, just look at any sailboat. It certainly would not hurt to put something between the two just to prevent water ingress. Windshield tape comes to mind.
No they do not! They do not really need water to start interacting with each other particularly when bolted to each other. The underside of a generator can be a funky environment. However, Tef-Gel is an easy solution. Handy stuff.
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Old 04-16-2014, 02:22 PM   #5
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Larry: We've been using Duralac for years with good success.

http://www.llewellyn-ryland.co.uk/downloads/duralac.pdf
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Old 04-16-2014, 10:02 PM   #6
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However, Tef-Gel is an easy solution. Handy stuff.
A pain to clean off, but otherwise amazing. Don't leave home without it if your job includes kicking hydrophones, cameras, and ROVs over the side.
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Old 04-16-2014, 10:08 PM   #7
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+ 2 (or whatever) on Tef-Gel. Particularly if your Al is threaded.

A simple nylon washer would probably suffice if it is thru-bolt.
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Old 04-16-2014, 11:26 PM   #8
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Larry, keep them apart. Tef-Gel, nylon washer, thin plastic, what ever you have that will prevent direct contact. Like NS said, Tef-Gel on any dissimilar threads for sure.
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Old 04-17-2014, 07:20 AM   #9
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Looks like I've got some re-engineering to do. I've made the tolerances so close that squeezing in an additional 1/8th inch or so of insulating material will really be a squeeze. Thanks for the valuable feedback. Here's a shot of the mounts.
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Old 04-17-2014, 08:05 AM   #10
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You can mate the two...I would use some sort of epoxy coating on top of the aluminum and use insulating washers the best I could...

If it's a decent alloy designed for marine environments....you will notice corrosion with it if in a relative high electrolyte situation..but not all that much.

My old boss insisted on using aluminum for everything and some insulating goo for stainless bolts. But he usually tapped/threaded the aluminum which often was a disaster and I gave up and would drill oversize and use bolts with nut. The moral of the story is sure.... lots of fittings had some corrosion around them when exposed to high levels of salt spray...but on the interior of the booats, the corrosion over decades was pretty minor.
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Old 04-17-2014, 10:25 AM   #11
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Like PSN said, it's not a huge issue but it is an issue. I'd probably look for some very thin plastic sheet or just coat the surface with Tef-Gel. Don't go nuts re-engineering it.
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Old 04-17-2014, 11:06 AM   #12
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Just don't use black carbon rubber. It would be worse than the stainless. HDPE, PTFE or Gore 500 tape ($$$) would work fine.

I'd go to the dollar store and buy one of those super flimsy (~1/32") cutting boards or dough rolling sheets, cut it up and layer in there. They are HDPE.

FWIW, I removed copper pipe nipples from my aluminum fuel and water tanks. They had been in place for nearly 30 years. Yes there was corrosion, but it was dry so not a lot of ion exchange. I replaced with SS nipples and used Boeshield, I think.
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Old 04-17-2014, 01:17 PM   #13
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"I'd go to the dollar store and buy one of those super flimsy (~1/32") cutting boards or dough rolling sheets, cut it up and layer in there."

Perfect!
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Old 04-17-2014, 01:56 PM   #14
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I have stainless fittings and stainless rivets on my 30 year old aluminum mast. I don't know if they were originally coated with any barrier coating, but there is only very minor corrosion that has slightly lifted the paint around some fittings.

I've fitted some new fittings and cut a gasket from the plastic of a milk carton as suggested by an experienced friend. Something like that may work for your application, Larry.
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Old 04-17-2014, 03:46 PM   #15
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GREAT input! GREAT ideas. Heck, I think I might have a half dozen of those cutting board sheets laying around here. I completely forgot how durable and thin they are. But below, you'll see why I'm so concerned about it.

The first shot is of the mounts just pushed up onto the underside of the genset drain pan....the space for something as thin as those cutting board sheets is there, but you can see that the mounts are tight enough to hang onto the pan with no retainers.

The second shot shows where and how the mounts fit over the stringers. See what I mean? Just below is the PSS shaft seal which, according to mfg. testimony, does mist a bit. I suppose I could put a plastic hood over the seal area, but that kind of misting would provide a potentially disastrous electrolytic source, no?
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Old 04-17-2014, 04:06 PM   #16
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You probably want the marine version however.

I'd recommend the thin, durable cut-to-fit galvanic isolators now on sale at Hopkins Carter Marine Supply & Fishing Tackle for $24.99 a sheet.
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Old 04-17-2014, 04:12 PM   #17
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Greetings,
Mr. H. Wow, talk about a "honeymoon fit". I would suspect the salt water "mist" would indeed inoculate the dissimilar bits quite nicely. How about one of the cutting sheets left long to hang forward over the stuffing box? Being loose it would still allow inspection of the box but stop any upward movement of the mist.
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Old 04-17-2014, 04:16 PM   #18
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Definitely want a spray deflector on that shaft seal if gennie is in the same box. Gennies don't like breathing salt mist. I cut up mixing pots and zip tie to gland housing. Not sure if that works on a PSS (is that what you're using??).
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Old 04-17-2014, 04:50 PM   #19
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Way over thinking and over engineering this IMO as far as the dissimilar metal part goes. Tef Gel or the stuff Larry M linked to will take good care of that. And AusCan, these products are perfect for your application, very simple and very effective.

Now as far as having the genset right over a "misting" shaft log, mmmm... not so sure about that. Guess I'd want to know how much mist they are talking about... do you see any signs of this now? It's the electrical end I'd be more concerned about, doesn't seem to have effected your engine has it (assuming a good air filter)?
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Old 04-17-2014, 05:00 PM   #20
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It's the electrical end that hates mist. It's air cooled with iron core pieces. Exterior of the machine gets corroded too with salt mist.

Salted gennies paid for most of the construction of my boat, so I have some experience with this!!!

Anything you can do to keep it dry and salt free is worthwhile. Spray deflectors convert spray into drips, and the drips go down to the bilge and cause little harm.
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