monel rivets

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TimStewart

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why is it reccomended to use monel rivets to attach stainless steel fittings to aluminium masts, as opposed to stainless steel rivets. Monel and 300 series stainless steel are right next to each other on the galvanic scale so it shouldn't make any difference regarding the corrosion factor. And stainless steel rivets are a lot cheaper than monel rivets.
 
Although Stainless Steel and Aluminiun are next to each other on the galvanic scale, you will find that SS is very much harder than Aluminium and would possibly wear into the aluminium.

Monel is a lot softer comprising of an alloy of nickel and copper (~70:~30) and a very small amounts of other metals.

Like SS, it is highy resistant to corrosion.
 
Hi David
Thanks for replying.
I think you will find that 304/316 stainless steel is at one end of the galvanic scale and aluminium is at the opposite end of the scale. Also monel is a very hard material, which is right beside 300 series stainless steel on the galvanic scale.
 
an excellent question. I've been hunting the internet for information, as metallurgy is well outside my knowledge, and others seem to ask the same question. Especially in the aeronautics field where aluminium riveting is key to keeping the plane in the air

I found all sorts of sites and comments, but to a layman, this set out some of the key issues.
http://www.zenithair.com/kit-data/ht-87-1.html

Is monel one of those word of mouth recommendations that people perpetuate without ever considering the real implications, and that there might be better materials?
 
Although Monels and passivated Stainless Steels are close together on the galvanic (electrochemical)series the chemistry of corrosion is different for the two alloys.

Fixing a stainless steel item to an aluminium mast with aluminium rivets will result in comparitively rapid failure of the aluminium rivet. In addition the aluminium rivets are comparitively weak compared with Stainless or Monel.

As for the choice between Monel rivets and Stainless rivets, the monel rivets are actually stronger than stainless steel but easier to form and do not suffer from "crevice corrosion", and there lies another good reason to use Monel in place of stainless steel rivets.

Monel rivets are also easier to drill out if the need arises - stainless steel rivets can be a real PITA.
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"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
You have hit the nail (or should that be rivet?) on the head Cliff. Monel is not only good for rivets but propshafts and other things susceptible to corrosion. It resists crevice corrosion in situations where other types of SS rivets would fail. Crevice corrosion will occur in 304 or 316 stainless in any situation where oxygen is excluded and oxide films prevented from forming as they would in free atmosphere. A blind (pop) rivet is particularly susceptible as it is not only in contact with the materials it joins but is only thin wall. Corrosion is therefore a big problem. Zinc chromate helps to prevent corrosion and both components and the rivet itself should be liberally coated before assembly ideally if possible even if monel rivets are used. It is not just the rivet that corrodes, it is the metal around it, especially aluminium.
 
I didnt know the crevice corrosion advantage. I knew only that a) the aluminium rivets were out, and b) monel rivets can be put in with typical amateur rivetter but s/s need a LOT more muscle.

In the Selden Mast literature for example, they say use s/s or monel - up to you, the user. From their point of view they are equally desireable (but presumably they dont know about crevice corrosion!). /forums/images/graemlins/tongue.gif
 
[ QUOTE ]
Zinc chromate helps to prevent corrosion

[/ QUOTE ] Just for interest really.

I was surprised to find that Duralac, the compound normally suggested and available from chandleries, is actually based on barium chromate.

See http://www.llewellyn-ryland.co.uk/downloads/duralac.pdf

Annoying because I went to the trouble of making myself a supply of zinc chromate (that I use by making a paste with linseed oil) a few years ago but disposed of a jar of barium chromate!
 
I think it's another nanny health and safety issue Vic. I always use Zinc Chromate but there are always warnings on the jar. (Don't eat it, shove it up your nose, wipe your hands before scratching yer ass....that kind of thing) Barium chromate probably works as well but ZC is still used in shipyards..... /forums/images/graemlins/smile.gif
 
Zinc Chromate got a vey bad name in the paint industry I believe, largely due to poor working practices. It is a carginogen and I think that applies to all chromates.

Barium chromate may be just a bit safer as it may be a bit less soluble, but barium salts, except barium sulphate which is very insoluble, are toxic and I thought barium = toxic, chromate = carcinogenic, we don't use it so lets dispose of it.

That's just a theory, I have not checked any facts.

I was simply surprised to find Duralac was barium chromate as the usual sort of thing one finds on here is that you should use zinc chromate which is available as Duralac.
 
Re: Monel rivets

Speaking as a metallurgist, I would have to disagree with Selden Mast's opinion as to the suitability of SS rivets in the long term or their equivalence to Monel rivets mind you perhaps their metallurgist (if they actually have one) has not spent as a lot of time in forensic metallurgy or failure analysis. /forums/images/graemlins/tongue.gif

I would not use SS rivets on my boat except for riveting SS to SS but that is my choice based on my experience and knowledge of metallurgy. I would leave it to the sailing club/armchair/bar room ex-spurts to make up their own minds.
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hammer.thumb.gif
"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
Thanks very much for everyone's input, especially Cliff and boatmike. Your two answers were exactly what I was looking for. Spot on. Cheers.
 
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