Why is monel recommended for rivets in an aluminium mast?

[ianabc]

Cathodic: Graphite
Monel
Stainless Steel
Bronze
Brass
Copper
Tin
Mild Steel
Aluminum
Zinc

Anodic: Magnesium

As monel is 30 percent copper, I am confused!

 

[oldsaltoz]

Primarily because they are much stronger than standard aluminium rivet but still less corrosion than other types of similar strength.

You will still need to apply a dab of insulating compound tho.

Hope this helps

Avagoodweekend......

 

[pvb]

Strength primarily - monel rivets have a much higher shear strength than aluminium rivets. Although monel and aluminium are quite separated in the galvanic table, bear in mind that a monel rivet is the cathode and the aluminium mast is the anode. Where the surface area of the anode is large relative to the cathode, corrosion will be minimal. In contrast, an aluminium rivet in a monel mast would suffer rapid corrosion and failure.

 

[Ships_Cat]

Aluminium, contrary to popular opinion, does corrode in the presence of monel. Monel on the electrochemical scale is similar'ish to stainless steel and as Salty Oz points out the aluminium should be protected (as one would for a ss fastener).

Three things come to mind leading to usual monel recommendation - aluminium rivets often too weak, is difficult to do ss rivets and in the case where the mass of the fastener is much less than that of a less noble into which it goes corrosion is limited in the less noble metal (ie don't put an aluminium rivet into monel - haven't tried it but suspect would be a problem - but monel rivet into aluminium is less of a problem).

John

 

[savageseadog]

Monel is very corrosion resistant in its own right plus it's malleable/ductile enough to use as a rivet material. I believe that monel can be used submerged in the sea, I've come across it used as control cabling very similar to stainless rigging wire rope.

 

[FullCircle]

Agree with the sages above, and would recommend Duralac as the compound of choice for the job.
Aircraft have flown around for decades with this combination!
Dont forget to coat the mating surface of any Stainless brackets you are rivetting to the mast either!

 

[Norman_E]

All the replies to this confirm the suitability of monel. One other point to consider is that electrolytic corrosion of the mast can only take place when the circuit between the monel rivet and the aluminium mast is completed by the presence of an electrolyte (i.e. sea water). The use of duralac or similar will reduce metal to metal contact between rivet and mast, but given the force involved in closing a rivet really tight, there will always be some contact so there will always be a potential electrolytic circuit IF THERE IS AN ELECTROLYTE PRESENT. If the mast was immersed it might be a problem in time, but spray hitting the mast is not there all the time, so the corrosion effect is limited, especially if you hose off salt deposits from dried spray whenever you clean the boat.

 

[ianabc]

Super answers

....Much appreciated here on the west coast where sunlight is just touching the Comox Glacier above the bay

...Thanks from Ian

 

[Birdseye]

used to flog monel wire to the aircraft rivet makers so you can be sure its OK for a mast. just use duralac on the rivet when installing.