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Old 21-03-2010, 09:41   #1
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SS and Aluminium

Hi all,

Ive made a anchor roller platform out of aluminium which will be bolted through the deck. I was plannin on using SS bolts and am wondering about the possible consequences of corrosion. I could use nylon washers to prevent contact between the dissimilar metals. is it a bad idea to go this route? how back would the cossion be? i've seen it in the past as a light white dust, nothing major. but being on the bow and being exposed to seawater, I am thinking that it will be worse. The backing plate is also alumium.
hank
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Old 21-03-2010, 09:51   #2
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Use Tef-gel between the ss bolts and the aluminum. Fantastic product.
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Old 21-03-2010, 12:22   #3
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Since Stainless is more noble than Aluminum, a small stainless bolt in a large aluminum structure, is the right proportion of metals to limit the galvanic corrosion. A dielectric grease, like Tef-Gel, will further limit any potential corrosion.
The major problem is galling (which prevents easy removal of the fastener), occuring with stainless fasteners threaded into aluminum hardware. Through-bolted stainless fasteners are ok on aluminum.
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Old 21-03-2010, 18:35   #4
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Well, the alloy will be the loser, so ... huge dia alloy fasteners can be used.

Or else - I-S-O-L-A-T-E.

Tef-gel. Lanocote, washers. And a lot of them.


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Old 21-03-2010, 18:47   #5
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I have used both nylon washers and Tef-gel doing essentially the same thing. No problems, even years later. In my experience, Lanocoat is not as good as Tef-gel.
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Old 21-03-2010, 18:50   #6
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G'Day All,

A real world observation:

Maxwell windlasses, cast aluminum body, have s/s bolts securing them to the deck. The OEM provides plastic sleeves with integral washers to isolate the s/s from the powder coated aluminum. I just assisted a friend in the removal, overhaul and replacement of a 2200HWC which had been on deck for 15 years. There was noticeable corrosion around all four bolts... enough to breach the powder coating, but there was no serious erosion of material, and I believe no perceptable weakening of the structure.

So, in the case of the bow roller structure, the use of nylon washers under the heads of the attaching cap screws, plus the use of reasonable clearance holes in the alloy with the voids filled with sealant of some sort should give good service. Inspection during use should show up any corrosion developing, and renewing the sealant might be required now and then. Or, one could perhaps buy the appropriate sleeve/washer isolators from Maxwell (or other sources).

Good luck with it all.

Cheers,

Jim and Ann s/v Insatiable II lying Church Point, NSW, Oz
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Old 21-03-2010, 18:53   #7
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FYI - Backing plate could be steel. Generally use aluminum for strength, and steel for stiffness ... kilo for kilo ... until carbon came along ...
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Old 22-03-2010, 01:33   #8
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Yes, TefGel is much better than Lanocote. I use Lanocote for SS to SS to prevent galling and lubrication. The reason is that it is cheaper (I'm Dutch :-) because I think TefGel would be better for that too ;-)

cheers,
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Old 23-03-2010, 19:39   #9
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lanocote vs. tef-gel

Quote:
Originally Posted by s/v Jedi View Post
Yes, TefGel is much better than Lanocote. I use Lanocote for SS to SS to prevent galling and lubrication. The reason is that it is cheaper (I'm Dutch :-) because I think TefGel would be better for that too ;-)

cheers,
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I use -
:lanocote - where there is movement, (e.g. joints)
:tef-gel - where there is no movement (e.g. ss crews into alloy) (esp. if painting over)
plastic washers - where I cannot use tef-gel or lano

When using plastic washers I will still supplement them with one of the goos.

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Old 23-03-2010, 20:25   #10
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and lanocote smells like sheep...........kinda soothing in a way.
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Old 05-04-2010, 19:27   #11
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If you want to keep things simple you can use hot dipped galvanized steel nuts and bolts. Mild steel is very close to aluminum on the galvanic scale being slightly more noble. Zinc (galvanized) is just below aluminum bieng slightly less noble. Stainless should not be in contact with aluminum as it is way, way away on the galvanic scale.

Galvanic corrosion is not the only type you should be worried about with aluminum. Anywhere moisture can be trapped or held can present a serious problem. If certain alloys are allowed to be primarily in the presence of oxygen aluminum can be very corrosion resistant. If in a salt environment use 5000 series. In any case, if the aluminum will be bolted down to something make sure you paint any surface (imediately after sanding it to remove the oxide) that may trap moisture. A good bedding will help also.
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Old 06-04-2010, 06:06   #12
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Hank

Since you will not be removing the anchor roller platform anytime soon, here is a procedure that will work in the long term: do not use delrin washers as they distort under pressure and also absorb moisture over the years. Use ss bolts that are truss head (they have a slightly larger head diameter). Each bolthole has to be cleaned out well of any grease, cutting fluid etc.

Clean each bolt and backing fender washer in acetone and let dry without touching with bare hands.

Now apply a good amount of Lifeseal (from Boatlife) to each bolt and the inside of the hole. Insert bolt, apply lifeseal to washer side that will go against the aluminum and tighten the nut. Lifeseal on the thread is OK as it will prevent the nuts from loosening.

Cleanup is with mineral spirits. Make sure you clean all traces of bedding because it is almost impossible to take off after it's dry. A roll of paper towels will work wonders.

The lifeseal bedded under the truss heads and washers will work as a capping barrier, sealing moisture away from the contact point of aluminum and ss. The bond is also permanent. Tuff gel works well also but not for as long.

This procedure is an old trick riggers use on alum masts where screws are tapped in aluminum where there is a potential for galvanic corrosion.
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Old 06-04-2010, 06:11   #13
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[QUOTE=ConradG;431295]If you want to keep things simple you can use hot dipped galvanized steel nuts and bolts. Mild steel is very close to aluminum on the galvanic scale being slightly more noble. Zinc (galvanized) is just below aluminum bieng slightly less noble. Stainless should not be in contact with aluminum as it is way, way away on the galvanic scale.

<><><><><><><><>><><><><><><><><><><><><><><><>< >

The only problem with zinc dipped is if one scratches the zinc coating during tightening on the nuts, the fastener will start becoming active in that spot. A small scratch is enough on an anchor roller as this sees a lot of salt spray....

Coating will help but it is not easy to paint alum properly
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Old 06-04-2010, 07:58   #14
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Thank you all for your insights/ advice.

Hank
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Old 06-04-2010, 16:33   #15
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Quote:
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and lanocote smells like sheep...........kinda soothing in a way.
Only if you are from the land down under. ;-))))

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