Should you isolate stainless steel ?

[welder]

Dave , keep the Q's coming , we need to know. :D

[DaveP]

It was suggested to me to use roofing tar.. Alum welder who builds boats told me they use it on saltwater fishing boats adn years later, bolts thread out with no problem..
Is he on track ?
BTW..I'll post pics this weekend as boat is finally in water..

[peterbo3]

I cannot see a problem with tar. It is an oil based product. Might be a tad messy though.
I use a product called Lanotec on my SS bolts & nuts. It is lanoline (grease from sheep wool) based. I use NO SS screws to fasten anything to alloy. Every fitting on Drumbeat has been designed to be bolted on to welded hat sections or mounting plates although a few small items are attached by all alloy rivets. I normally use Nyloc nuts on the bolts up to 5/16". Spring washers above this size.
I have noticed no corrosion at all around any fastener but I have an unpainted hull & there is no nyalac on it. Plus I trailer the boat & always wash down with fresh water.
I did get a small amount of corrosion around fasteners on the previous alloy CC. This boat was painted & I feel that seawater gets under the paint around boltholes. It fails to dry & the familiar corrosion bubbles soon appear under the paint because the alloy cannot oxidize properly & seal itself. That is my theory & I am betting a heap of $$$$$ (boat cost) on it being correct.

[85MonarkVC]

I'm gonna try using nylon bolts and nuts to replace a lot of the stainless ones that are being used to keep stuff in place. Ace Hardware has a nice selection of nylon bolts. I wouldn't attach my motor with them but I think they would be fine for many things.

[welder]

A new Internet freind and surfer of AAB has Emailed me with some GREAT info. for you guys . I have put it into a post to maybe answer a few questions.

I REALLY like good info like this.
JettyW & 85MonarkVC

As Jay has mentioned above, both aluminum and high nickel steel, aka stainless steel, resist most corrosion by a transparent thin film of the parent metal combined with oxygen called and oxide. This stuff is hard but fragile like a male thread tap- hard metal to cut steel threads but fragile in that they snap, way to easily, if bent sideways.

If the layer is disturbed by mechanically scratching or abrasion it will reform in aluminum in a few seconds and in SS a little longer but both films can be interrupted while forming by a solution conducting electricity competing for the oxygen and parent metal bonds- and winning the competition. So both metals can/will corrode if they can't get oxygen to renew the oxide film once this film is disturbed; IF THE OXIDE needs to be renewed.

SS is often "passivated" to make sure the oxide film is uniform and has no gaps or voids that would offer sites for corrosion, sometimes called crevice corrosion because it begins in small gaps or crevices in the surface and 'digs' or reacts down into the parent metal. http://www.mmsonline.com/articles/100304.html

http://www.brewingtechniques.com/librar ... spaw2.html

http://www.pfonline.com/articles/119806.html

These links just take you to some reading about the process and some how and why's, not very marine related but metal is metal.

The electricity that might flow between two metals is a result of the 'galvanic differential' or 'galvanic potential' and just means that with some acid, or other conductive liquid like salt water, the two metals could make a battery because they have different 'charges' resulting from their molecular and atomic structure. If both the aluminum and SS are kept apart by a non-conductive film = no battery. If they can be held submerged in saltwater without surface oxides = battery and therefore corrosion.

So, if you have etched and washed aluminum allowed to air dry it forms an oxide which protects the underlying parent metal, in fact this sole property is what allows aluminum to be the "miracle metal" for boat building- it is almost self-healing. Now if the SS were passivated it too would have a protective film unless it were removed. So if you bolt one to the other properly prepared you should be OK? right? well now for the rest of the story. Vibration, agitation and thermal expansion differences. The two metal don't retain their original oxides and if that joint is wet it will begin to deteriorate the aluminum first.

That is why Jay said to put some inert plastic film between the two metals where SS parts are mounted to aluminum boat hull/house/decks, and this is also why the thread joint compounds exist- to coat and separate the metals even in a threaded socket.

JettyWolf,

Most engine manufacturers who build with cast aluminum use passivated fasteners, even if their sales staff doesn't know what it means. So pulling a leg down to coat all the fasteners isn't necessary off the show room floor. What is important to realize is that long term vibration can agitate the two metals' joining surfaces and if water is allowed into a threaded socket in the cast aluminum that has a SS fastener in it- well over time these sites can get corroded.

I've found you can avoid lots of these corrosion sites by making sure you're using passivated fasteners- lots more expensive- or passivate the SS yourself- potentially dangerous goop handling- or/and use inert dielectric goop. Tef-Gel is a suspension of particles of Teflon which is a soft flexible non-conductive plastic that has great lubricity (its slick but dry) and some silicone grease which won't harden or set up - but will wash away leaving the plastic particles to act as a boundary layer- helping the oxide to stay put. Tar has been mentioned and 5200 or a Locktite compound all do the same "thing".

"Thing" -Each of the thread treatments acts to reduce surface friction as you make up the fastener allowing less torque to be used on the fastener (especially important in SS which elongates easily) and still get the full compression of the thread fastener hold the two pieces of equipment together. Also lubed threads in aluminum and SS don't gall or 'weld' as easily and that helps make the joints more reliable and simpler to assemble. Last the goops provide a compressed boundary layer to keep the two oxide films 'separate' but still holding one another mechanically together- which is after all what we need to keep one item bolted to the other.

There is a product line of thread fasteners which have a built in O-ring embedded under the lip of the head of the bolts/machine screws that seals the thread socket or drilled hole from water. These type bolts combined with some thread goop would go along way to keeping the SS to itself leaving the aluminum alone even if the two were threaded together.

85MonarkVC, if there is any way to rethink your idea to replace metal with plastic thread fasteners? I'd like to have you consider the nature of plastic. plastics are notch sensitive so their strength as a rod is reduced very greatly by threads. Yes the are non-conductive or dielectric but at what expense?

If you have the time and energy to remove and change out the metal fasteners with plastic why not consider an isolation kit for each bolt instead? A plastic washer on both sides of the two metals, one under the head and one under the nut will insulate the two gripping faces to one another. A plastic sheet like a gasket, as suggested by Jay in his remarks above Delrin for example will insulate and isolate the aluminum hull/house/decks from the metal equipment base and all that is left is buy a few feet of different size plastic tubing. This will be cut to fit around all the boat's bolts to make a sleeve around the bolt's shaft. These will make the isolation kit complete. You will have to drill out the holes to take the plastic tube sleeve around the bolts, because of the added diameter of the new 'plastic coated' bolts. But if you're willing to gear up with a 'kit' of the right washers and tubing this work isn't all that time consuming and will insure the two metals can't create a battery even if the joint is wetted regularly.

[Ironwoodtuna]

I also think that every two to three seasons you should remove each screw and re-tuffgel them. This seams to be a necessary evil to keep the protection of isolation current... (ooppss! I mean non-current flowing). Marty

[85MonarkVC]

Great info! Thank you for posting it. I like the sleeve idea on the stainless bolts. I've also heard that 316 is the kind of stainless that you want. Most of the bolts that I see for sale are 18-8 and they are not recommended for salt water according to the link below.

Here is another very good link about stainless & corrosion, this link also has other helpful links:

http://www.estainlesssteel.com/corrosion.shtml

John