Duralac paste needed with monel rivets?

eddystone said:
When I needed to remove the boom end cap (aluminium to aluminium) a couple of months ago, the guys at Aladdin's Cave where I bought the rivets told me you didn't need chromate paste with monel rivets. Now I'm not so sure - should I drill them out and do it again with paste this time?
Click to expand...

No need IMO. Thats how fittings were put into masts before Duralac became available in the yacht market. Mind you, if I were putting new rivets in, I would use duralac - nothing to lose by doing so. But thats belt and braces.
 
Surprised nobody has mentioned Tefgel as the more effective, less messy (but much more expensive) substitute for Duralac.
Nobody using it?
 
Plevier said:
Surprised nobody has mentioned Tefgel as the more effective, less messy (but much more expensive) substitute for Duralac.
Nobody using it?
Click to expand...

Tef-Gel is a good alternative to Duralac but it does not have the corrosion inhibiting properties of Duralac. It is PTFE based and works by preventing the ingress of water into the joint between dissimilar metals. It is, therefore, a good choice for a close fitting joint although Duralac is possibly a better choice for a badly fitting joint with gaps to fill.

http://www.tefgel.com/contain.php?param=tefgel_infor
 
Vic as a chemist what do you think the mechanics of the " corrosion inhibiting properties " of Duralac are when used between stainless steel and aluminium and even through the position of stainless and monel on the galvanic scale are close there is significantly more corrosion reaction with stainless than monel in the junction without Duralac
 
Rogershaw said:
Vic as a chemist what do you think the mechanics of the " corrosion inhibiting properties " of Duralac are when used between stainless steel and aluminium and even through the position of stainless and monel on the galvanic scale are close there is significantly more corrosion reaction with stainless than monel in the junction without Duralac
Click to expand...

I dont know. I have not managed to find an explanation. I suspect it's due to the oxidising properties of the chromate maintaining a protective oxide film on the aluminium.

Equally I dont understand why corrosion of aluminum appears to be worse in contact with stainless steel than with Monel,. I wonder if that simply due to the surface areas involved. Usually we are talking about stainless fittings, which have a fairly large area in contact with the aluminium, whereas with Monel it's usually only the rivets.
It would be interesting to compare the corrosion occurring in physically identical and controlled situations.
 
vyv_cox said:
Why do you think that stainless steel on aluminium, which everybody agrees should have a Duralac treatment, is different from monel on aluminium? Monel and 300 series stainless steels occupy almost exactly the same position in the galvanic series table, with a potential difference of between 0.7 and 0.9 volts. Aluminium masts and spars corrode badly around stainless steel fittings and fasteners that are unprotected, so why should monel be any different?
Click to expand...

This implies that there is only minimum liklihood of galvanic corrosion between s/s and monel, but both should be insulated from aluminium.
 
Spirit of Glenans said:
This implies that there is only minimum liklihood of galvanic corrosion between s/s and monel, but both should be insulated from aluminium.
Click to expand...

Probably true. Monel has better resistance to corrosion than stainless steel, particularly crevice and pitting corrosion. Copper-nickel alloys are superb in seawater but they can form galvanic couples with other metals as you would expect.

I just noticed that there is an advert for a rivet company in Shanghai at the masthead of this page. Does everyone see that or just me? Clever stuff, whatever.
 
Having worked for Sailspar, Z Spar and Fox' Rigging (Selden) Monel rivets do not need any paste under them, as has been mentioned any stainless fitting must be insulated from alloy, the one problem with Monel rivets is they are expensive! Drilling out the rivet is difficult but all you need to do is drill of the head and knock the rivet inside the mast/boom. If when making a mast all the rivets had paste on the cost would go sky high! Some spar companies (Sailspar) fit a plastic plug to seal of the rivet, this looks nice but again not needed.
 
vyv_cox;5013959 I just noticed that there is an advert for a rivet company in Shanghai at the masthead of this page. Does everyone see that or just me? Clever stuff said:
I've got adverts for domestic boilers at the top of the page. I presume PBO must get some kick back from the advertisers to help with the running costs of this excellent forum.
Click to expand...
 
Last edited:
I guess you are into 3 two way cells and the stainless / aluminium one is the most active one almost always with the biggest contact area? Either way, the practical experience seems to be that the ally under the stainless suffers more.
 
Not wishing to move off-topic, but what metal is my mast (and boom) probably made from? The mast is about 23ft long, most likely 40 years old, and weighs around 11kg I think.

Being a racing class, a fair amount of mast-bend seems to be encouraged, but all this talk of vulnerable metals makes me wonder what metal or combination of metals is used.
 
vyv_cox said:
Why do you think that stainless steel on aluminium, which everybody agrees should have a Duralac treatment, is different from monel on aluminium? Monel and 300 series stainless steels occupy almost exactly the same position in the galvanic series table, with a potential difference of between 0.7 and 0.9 volts. Aluminium masts and spars corrode badly around stainless steel fittings and fasteners that are unprotected, so why should monel be any different?
Click to expand...

Always assuming the rivets are 400 monel. It could be argued that everything but zinc, magnesium and Beryllium are lower than aluminium alloys - http://rickthompson.com/blog/galvanic-series/

Totally true if both are in seawater and thus part of a cell. I had my boom/gas-strut connection part with similar results to the holes in the photos - on that occasion it was wasting of the holes by the stainless fitting and finally mechanical wear. I replaced the fitting with an interleaf of PTFE and with monel rivets, no sign of corrosion 8 years and 15K Med miles later.
This last season the lower gas-strut fitting parted company due to the heads of the aluminium rivets shearing - they've been replaced with monel, admittedly with Duralac, mainly because it was handy.
You'll note that all strut manufacturers use aluminium rivets, frequently not up to the job and riggers seem to use monel without Duralac.
However there is a story of a monel-plated boat, which corroded away its steel frame in a 6-month period and so had to be scrapped.
Most of the limited specialist literature on monel, stresses it's corrosive effect with nearly any metal, so I'd support Vyv's contention, that monel needs Duralac whenever it's used where salt water might get.
The actual mechanism might be slightly more complex than suggested - but monel and aluminium are as reactive as stainless and aluminium.
 
Last edited:
" but monel and aluminium are as reactive as stainless and aluminium "

I agree in theory but on my mast the Monel rivets fitted with no Duralac show no sign of damage to the Aluminium after 40 years or more but the stainless that was fitted without Duralac 6 years ago when I rebuilt the mast shows extensive corrosion such that screws cannot be removed.

I would say that the actual mechanism is not what we expect hence my question to VicS the chemist of this parish.
 
Whereas monel is typically used only for rivets stainless is usually for larger pieces where greater thermal movement may increase seawater penetration? Or similarly the greater size increases gaps again allowing water to get in more readily.

A possible explanation?
 
You must log in or register to reply here.

Share this page

Top