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samgary 12-08-2011 04:28 PM

corrosion in aluminum mast - options
 
I have recently un-stepped my mast in order to make a repair. Upon further inspection i have noticed a small area of galvanic corrosion above the cabin, on the forward side of the mast, where a stainless fitting was mounted. The corrosion has eaten a small hole, less than 1/4" in diameter through the mast The mast is keel stepped. The rest of the mast is in very good condition, with a minimal amount of corrosion.

My plan is to make a 4"x4" aluminum "band-aid" and rivet it on for reinforcement. I also plan on applying something to stop the existing corrosion as well as re-bedding all current fittings with tef-gel.

Any feedback/advice on process and or products would be greatly appreciated.

hellosailor 12-08-2011 04:48 PM

Sam, I would wonder whether the four rivet holes were going to cause more problems than simply cleaning up the 1/4" hole you have and then leaving it be. Perhaps just reinstalling the stainless with proper galvanic protection, as it was.

pdqaltair 12-08-2011 05:10 PM

I would consider ultrasound inspection
 
The trouble is, you don't know how far the thinning extgends. A 1/4-inch hole may not be important at all, or there could be a large thin area. Ultrasound can also tell if the inside is smooth or rough. Of course, I do UT inspections and so this is free for me.

I would also be very surprised if the bandaid carried any important amount of load; compare the amount of missing area to the cross section of the rivets; it would take a LOT of rivets, close tolerance on the holes so that they carried load at the same time the mast began to deflect, and require a lot of drilling. Just a little slop under load, and the mast will begin to tear before patch loads up.

Is it a high load spot, and is it in compression or tension? Most masts fail by buckling up near the spreaders, not at the deck. A small hole at the deck in compression is probably unimportant.

kd3pc 12-08-2011 05:24 PM

If any repair is made, I would opt for a sleeve and welding, by a metals familiar welder. The added rivets and needed drilling in the 4"x4" area(small), may actually weaken the mentioned 1/4" hole area.

Ultrasound of the entire extrusion is the best way to actually know what is going on...anything else is a guess or speculation at best.

If not, you may want to purchase an inexpensive bore camera and see what is going on inside.

INMA 12-08-2011 09:05 PM

I had a similar issue, a rigger replaced the stainless fitting with an aluminium pad and aluminium fitting welded so that the weaker area was straddled and there was less chance of galvanic corrosion.

On another mast I noted a sleave welded over a weakened section.

In both cases the rigger avoided more holes and rivets.

If I was stuck away from a competent yard, then a rivetted patch would be an option but it would be replaced with a welded solution once I reached a suitable yard.

samgary 12-09-2011 02:04 PM

Thanks for all of the feedback!
After thinking about it more, most of the load is compression, as it is only 6 inches above the cabin. If anything, i think welding a patch would be the better solution, or applying some epoxy based paint to prevent further corrosion and keep an eye on it.

Thanks again for the feedback


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