Bronze/Aluminum Windlass... Bolts?

[bstreep]

OK, we've removed our Simpson / Nillson V1000 Windlass to rebed it.

The above deck assembly is bronze, and the drive unit is in an aluminum casting. The old bolts are bronze, and need to be replaced. The bronze above deck looks good. The aluminum casting... Not so much. But still serviceable.

So, my question is, do I need to replace these bolts with bronze, or can I go with stainless. I mean, the bronze was pretty rough on the aluminum, will stainless be worse?

[roverhi]

If the bolts are anywhere near the aluminum go with stainless. Bronze is way away from aluminum on the galvanic charts. Stainless is not right next door but close enough that it's the standard fastener for anything aluminum. There shouldn't any problem with stainless fasteners and the bronze.

[Stumble]

Rover,

I think you have it backwards.

When you look at a galvanic compatability chart, I have Galvanic Corrosion Chart* bookmarked, but there are a umber of them. You want to use materials as close as possible to each other.

In order for aluminium this would be
Aluminium bronze
Siliconized bronze
304
316
Titanium

I skipped a bunch of metals for other reasons (availability, cost or rusting). For stainless alloys, ignore the white boxes. They are indicators for below the water applications.

[roverhi]

No I don't, Bronze is further away and thus more reactive with aluminum. On masts, you can get by, though not reccomended, putting SS fittings directly against the aluminum. Bronze needs an insulating layer. Remember one winch with a bronze base that pretty much eaten up the aluminum that it was mounted because it wasn't insulated. Pretty much left a large, perfectly round hole.

The chart is a little confusing because there are two states for SS. One in an acid rich environment and one where it's been passivated. Passivated SS is better than Bronze by a little bit on the chart but by a whole lot in practice. Surprized that Titanium is way down the list from Aluminum as well. Maybe the galvanic charts don't tell the whole story cause there sure is a lot of mixing titanium and aluminum in aircraft. Anyway, the point is use stainless around aluminum if possible. Definitely don't put a bronze bolt into aluminum.
http://us.mg4.mail.yahoo.com/neo/lau...D%7D&box=Inbox

[xymotic]

Use a lot of Tef-Gel or Mare-lube.

[bstreep]

Hmm. So, folks, which is it?

[Charlie]

Go with SS and install plastic washers and bushings between any contact point. Also email Nillson and ask them.

[Stumble]

Rover,

The unshaded area is for still water, and high acid conditions (inside crevices), not the interface between a plate and a bolt/fastner.

[noelex 77]

Looking at the galvanic chart is only part of the answer. SS forms a passive film that is less conducting. Bronze does not. Since electrical contact is nessessary the film on SS reduces the galvanic action compared to what would be expected from the galvanic tale alone.

So in general SS is slightly better than bronze. The overall design however is poor. SS can produce acceptable results if the aluminium part is massive compared to the SS, say in case of a mast with a SS bolt, but that is not the case here.

Use lots of Duralac, or Tef-Gel (depending on your religion) and if you can minimise the contact with plastic bushings etc. A thin plastic washer und the SS one helps. Keep the area as dry as possible it needs to be wet for a problem to occur.

BTW Stumble titanium also forms a very good less conducting coating so Titanium produces less galvanic corrosion in adjacent materials than its placement on the galvanic table would indicate. It's an important selling point as many people are horrified at using titanium fasinings because of its placement on the galvanic table. Titanium can still produce a severe galvanic reaction because the coating is less, not non, conducting.

If you could devise a totally insulating coating for titanium say a sort of Inert anodising we would have the perfect marine fastening. All I ask is 0.01c from the sale of every bolt and shakle with the "Noelex" coating to keep me in my crusing lifestyle.

[Stumble]

Lol,

We already offer an extra thick anodizing, but no one except for satellites seems to be interested in paying for it. We also have made gold, and platinum plated stuff before, but only for special use projects.