Repairing an aluminum spar

[Sonosailor]

My mast has a small crack. It is the result of strain on an eyestrap that holds a block to reroute the mainsheet. The eyestrap is actually recessed into the mast but riveted to the outside shell, and the crack is between the lower fastening rivet and the hole through which the eyestrap exits the mast.

I have a sheet of 316 stainless steel to cover the area, and am intending to re-install the rivets through the steel to the eyestrap, and then rivet around the area. This is the repair suggested by Z-Spar. My question is whether I should use any material like silicone between the steel and the aluminum to control crevice corrosion. Any need? Any other suggestions?

[GordMay]

Yes, you should place an insulating barrier between the Stainless Steel and the Aluminum.
Silicone may not be the best choice.
Try instead:
"No-Corrode"
"No-Alox" (Ideal)
"Penetrox" (Burndy)
"Ox-Guard" (GB)
#1602/1603 Sealers (3M)
et al
Regards,
Gord

[delmarrey]

As a Toolmaker Machinist, I would agree with the Z-Spar patch and with GORD on a better insulator. Welding is not an option. It will change the properties of the metal (softer) and make matters worse. Since the crack has run into a hole that should be the end of it. But it may try to go the other way. Keep an eye on it. If it starts to go the other way, drill another 3/16" hole about 1/4" ahead of it.

[Sonosailor]

Gord:

I gave your list of products to Binnacle in Halifax, and they came back with an antiseize product by STARBRITE called ANTISEIZE. Are they on the right track, or should I keep looking?

[delmarrey]

That is an anti lock, corrosion resistant oil based lubricant. Not used as a sealant. But used on screws and bolts to keep them from seizing up in time or stress. On a hot day that stuff would be running down your mast.

I think I woud just stick with the 3M sealers.
Put your (contoured) plate on the mast and transfer the rivet holes, remove, then put on the sealer and the plate using the un-crimped rivets to hold it in place for around 6 hours, then crimp down the rivets. That will create a seal and a barrier from corrosion without squeezing out all the sealer.

Del

[Sonosailor]

Of course. No thinking at this end on that one. I'll try to get the 3M product, and attempt a prefit and sealant application before riveting.

[GordMay]

SONOSAILOR & DEL:
Thanks to DEL for pointing out the potential for "liquifaction" on some of these products.

Upon revisting my earlier posting, I understand why "Binacle" offered up "Starbright Anti-Sieze". It's closely related to the 'No-Corrode', 'No-Alox', 'Penetrox', and 'Ox-Guard' products I recommended; all of which could be subject to thinning out at higher temperatures.

As DEL suggested, try the 3M Sealers, or perhaps Epoxy Resin.

My apologies for my earlier ill concieved advice - and thanks to DEL for thinking it through.

Regards,
Gord