Mast corrosion - repair options?

Hi all,

I discovered some pitting corrosion under a stainless steel fitting on my mast. It is right under the spreaders. I think the fitting used to host spreader sockets in the past, which were ground off.

It looks like there is a lot of aluminum left, no cracks, so I'd rather strengthen it instead of getting a new mast. I'm considering the following options:

1) Fabricating a larger and thicker stainless steel fitting, drilling new holes outside of the damaged area, and fixing it with monel rivets.

2) Bending an aluminum plate around the mast (a doubler?), covering the damaged area and 6 inches above and below it. Fixing it with epoxy, as well as rivets.

3) Finding a mast section that would fit around my mast after removing the track, extending 1 foot above and below. Epoxy and many rivets.

What do you think about it?

One person suggested moving the spreaders, the lowers, and the inner stay, below the damaged area to reduce compressive stress on the weakest spot. Good idea?

Tom

 

Welcome to the Forum Santomas.
What type / size vessel is this mast for? It looks like it might be for a large dinghy, but the scale is deceptive.
How old is it?
Has it been in service until fairly recently?
If it has been working well until now, I think I would be inclined to fill the pits with a high build epoxy putty and re-attach the S/S fitting for the forestay and side shrouds. You could use a suitable bedding compound to help to insulate the mast from the fitting.
You definitely do not want to start drilling more holes in the mast.

 

Thanks. It's for a Catalac 9M catamaran, launched in 1972 and I believe the mast might be just as old.

I was told it sailed a couple of years ago, not regularly.

 

Re how it is for a Catalac, ok, the forestay fitting will be for a baby stay then, rather than an inner forestay for a staysail.

If you are not a member of the Catalac group on Facebook, it would be worthwhile signing up - they would have a lot of knowledge and resources on tap there.
https://www.facebook.com/groups/238722366816514/

Edit - just a thought re the wire rigging - do you know how old it is?

 

1972-2021 is a year short of 50 years

If able, start new.

 

My guess is that it is corrosion caused by dissimilar metals in contact, with water seeping between them to create perfect electrolysis conditions. If that corrosion is not excessive, it can almost certainly be ground out, then filled with thickened epoxy, sanded back and primed. Be especially sure to fill the mounting holes, then re-drill them after the epoxy has hardened. The epoxy will be way stronger than the metal
Before replacing the stainless fitting, a "gasket" of electrically neutral material like thin rubber will prevent any further reaction.
The amount of work involved is not great and will save megabuck for a new mast. I doubt the current stainless fitting needs replacing either.
Hope this helps

 

Good advice above. Don't move the spreaders. Your pictures show the end of the compression bar that is suppose to resists the forces on the sides of the mast. If you move your spreaders, you will have to install a new compression bar too. Not necessary.

 

Bajansailor:
Thanks, I asked on the Catalac Facebook group already. The rigging is all new.

Aussiebushman:
Great tips about using thickened epoxy. I'm just a little worried grinding the corroded area can make the aluminum even thinner.

How much corrosion/thickness loss would you call excessive?

 

I think that 'grinding' would be very brutal and unnecessary - I would just give it a light sand, and etch the aluminium with a suitable etching agent prior to applying the epoxy filler.

Like with rust, the corrosion product (ie the white deposit) will appear to be a lot, yet the actual amount of aluminium (or steel) that has corroded is actually much less.
In your photos I just see some slight pitting - if there were obvious holes corroded through in the wall of the mast then that would be a much greater cause for concern.
I would agree with Aussie - if you apply epoxy on it it should be at least as strong as before, if not stronger.

 

Precisely. When I say "grind" I do not mean remove heaps of metal, even if it is corroded. You merely want to get rid of the worst of the corrosion. Personally I would use a "flap disk" NOT a grinding wheel. You do not need to grind out the pitting - the epoxy will fill that. If necessary, use a wire brush to get it clean. All you are really doing is getting a firm substrate for the epoxy because you are creating a mechanical bond - not a chemical one.

As I said before, make sure there is plenty of fill in and around the mounting holes. You might even want to oversize them a little before you bog, then redrill to the correct size after the epoxy has cured. Just be patient about that. Actually, perhaps instead of the rubber "gasket I suggested earlier, you could wrap a single layer of thin fibreglass cloth over the epoxy bog while it is still tacky, them smooth it down with your hand. It is non-conductive and will also strengthen the repair. ​

 

Thanks, Aussiebushman! Let me see if I can find a cloth that is thin enough...
Would you use etch primer or vinegar solution before applying epoxy or is it unnecessary?

 

Sorry about delay in responding - have just changed computers and typically, none of my favorites or passwords wanted to work.

Re your question, white vinegar is used to clean epoxy from tools etc before it kicks - will not work after that. You are best applying the epoxy to clean metal, meaning immediately it has been sanded and dusted off, though it may help to clean the mast areas with it. I do NOT recommend priming, because if you do, you are only bonding to the primer and not the metal.

Another little trick is to slightly thicken the epoxy with some filler to a thin mayonnaise consistency before applying the cloth. Don't overdo the amount of filler - it is merely to ensure the glass cloth sticks firmly because if it is completely liquid, there may be a tendency for the cloth to fall off. In this respect, use just a medium weight cloth - just strong enough to assist the repair. The epoxy will do most of the work.

Hope this helps
Alan