corrosion, bolts etc

[bob the builder]

hi all.

i'll be wanting to attach 15 ft of stainless steel traveller track to an aluminium truss beam.


what would different people here do?



thanks,
mal.

[wardd]

be careful of dissimilar metal contact

[bob the builder]

would you put a strip of wood / epoxy fiberglass between them?

[wardd]

when i worked on aircraft we used a tape to isolate them and a zink chromate paint the fasteners were dipped in. otherwise corrosion would develop

that was many years ago, there may be better ways now

its something youll have to look into

some of the boat builders here could be a better source of info on current methods

[bob the builder]

?!

from
http://www.scorpiosail.com/web/mast_2.html



http://www.scorpiosail.com/photos/repairs/boomcorrosion_fix/images/2008.11.180.web.jpg


i don't ever want to play that game


can you install a sacrificial anode on the mast?

what about using aluminium rivets?
surely this is better than coupling more dissimilar electrolytic metals?


plastic washers & sleeves

plastic bolts
Nylon Bolts

keep it dry

epoxy with ceramic powder ie fumed silica to decrease the osmotic diffusion rate to the lowest you can




plastic bolt manufacturers
http://www.craftechind.com/materials.php

Yield strength of reinforced plastic is comparable to aluminum

[mark775]

An aluminum rivet is of a different alloy and thus dissimilar.

[CDK]

Make or have an intermediate part made from epoxy and glass or carbon fiber.
Two half circles clamping around the mast would avoid all direct contact between dissimilar metals.

[Ad Hoc]

woahhhh....looks like it could be cheaper (inferior) 304 SS...which doesnt help!

[Jimbo1490]

Sensible anti-corrosion measures:


Use Monel rivets (http://www.inox.ie/acatalog/Rivets.html) rather than stainless rivets. Monel (http://en.wikipedia.org/wiki/Monel) is closer to the Al alloys on the Galvanic Scale (http://en.wikipedia.org/wiki/Galvanic_scale), so less conducive to dissimilar metal corrosion.


As Ward mentioned, an isolation tape or other coating can help. A good coating of a primer with hexavalent chromium will also work here. The common 'Mil-Spec' primer is such a coating, containing strontium chromate. You can brush this on; it does not have to be sprayed. The beam (I assume) will already be anodized. This complicates getting a primer to adhere properly, but it certainly can be done. Tapes, OTOH, always have their best adhesion to anodized aluminum, so no prep would be needed there.

One caveat:

Since tapes are not mated nearly as intimately with the substrate as a primer, they may harbor moisture in the adhesive layer, which could potentially defeat the purpose of the tape.


An isolation/anti-sieze coating such a Mastinox (http://buyat.ppg.com/REP_aerospace_files/Sealants/mastinox.pdf) can be applied to the fasteners as well, delaying the onset of corrosion for years or decades. While pricey (http://www.skygeek.com/ppg-mastinox-6856k-yellow-160-ml-catridge.html), in the grand scheme of things, this is a small fraction of the cost of your project.


Jimbo

[alan white]

Formica laminate will be cheap and effective as an isolation material. Low compression and easily cut, free from kitchen counter fab shop scrap pile.

[Jimbo1490]

Formica laminate will be cheap and effective as an isolation material. Low compression and easily cut, free from kitchen counter fab shop scrap pile.

Great Idea!

[Jimbo1490]

woahhhh....looks like it could be cheaper (inferior) 304 SS...which doesnt help!

Just as with we sometimes do with aluminum, we lump a lot of different alloys together with the term "stainless steel", when in reality this is a diverse range of alloys with varying properties.

Some have no place on a boat just as some are not at all suitable for food contact.

Jimbo

[wardd]

lay some fiberglass tape down on a flat surface and make your own isolator

[owene]

To be honest, I'd forget it. Get back to basics. The basic issue is that mixing SS & ally ain't a runner so why not purchase or fabricate the track in ally. End of problem.

[Astute Boats]

A very good product for this application is DURALAC. I use this for all fastenings an alloy tracks when fitting out and maintaining composite race yachts.