Mysterious "growth" under my mast step - HELP!

[bluedragon]

Mysterious \"growth\" under my mast step - HELP!

My newly acquired Halcyon 27 has a die cast Al alloy mast step mounted on a large block of teak sitting on top of the coach roof. I'm just in the process of replacing the wood due to aging and bowing. Today I removed the casting for possibly the first time in >20 years and was intrigued and a bit conerned by what I saw underneath. I can only describe it as an extensive growth of a hard mineral-like deposit, v.light brown in colour. This had "eaten" depressions into the teak to a depth of perhaps 5mm in places and to some extent into the underside of the Al casting. This deposit was wet and powdery in places, and quite stone-like in others. I was able to remove it by scraping. Does anybody have any idea what this is, what caused it, and how to avoid when I re-assemble everything with a new Iroko block?? The casting was attached to the block with SS wood screws. These did not show any signicant corrosion much to my surprise, neither did the casting around the holes. It seems to be a reaction between the Al casting and the teak brought about by a moist / salty environment over the years.

 

[VicS]

Re: Mysterious \"growth\" under my mast step - HELP!

Almost certainly just corrosion of the aluminium. The volume of the aluminium oxide produced always seems out of all proportion to the to the amount of metal lost. The fact that in places the deposit was wet suggests the, not surprising, presence of aluminium trichloride as well as just oxide.
The stainless steel screws will not have helped.

I suggest you clean the aluminium of all the corrosion products and, if you can, get it sand blasted with a soft grit to get right in to all the pits. Paint the underside with a couple of coats of zinc chromate and use some zinc chromate paste on the ss screws (or use more zinc chromate paint).

Maybe setting it on some sealant will help but it will need to be a pretty hard one to withstand the loads.

If it lasts another 20 yrs I guess you'll be happy.

 

[alahol2]

Re: Mysterious \"growth\" under my mast step - HELP!

I wonder if your 'growth' is the remains of somebody placing a coin beneath the mast foot? I believe it's a long held tradition.

 

[bluedragon]

Re: Mysterious \"growth\" under my mast step - HELP!

Well, I wouldn't have thought of that possibility!! Presumably it's some kind of good luck thing?

 

[30boat]

Re: Mysterious \"growth\" under my mast step - HELP!

Thats good advice.I would also sheath the Iroko heavily with roving and epoxy and paint with two pack poliurethane paint.The class sheathing will resist compression very well providing it is thick enough and won't absorb water .

 

[Colvic Watson]

Re: Mysterious \"growth\" under my mast step - HELP!

Very common good luck 'charm'. Usually a penny. Copper and aluminum, what a lovely combination!

 

[AndrewB]

Re: Mysterious \"growth\" under my mast step - HELP!

I've had the exactly the same happen where aluminium and teak have been held together for many years under cockpit winch mountings. As others say, its aluminium oxide. Possible made worse in your case by a coin placed under the mast, though if so I would have expected it to still be there.

When I reassembled my winches, I followed VicS and 30boat's suggestion to paint. Like you, I replaced teak with iroko. Polyurethane varnish was used on the wood, and epoxy primer/polyurethane topcoat paint on the aluminium winch base, which I'd also recommend for your casting. (Your casting will also probably need to be faired with epoxy filler). Sikaflex was used between the winch mounting and the block to help prevent seawater getting trapped between them in future.

 

[dulcibella]

Re: Mysterious \"growth\" under my mast step - HELP!

My old Halmatic 30 had alloy plates embedded in the lay-up supporting shrouds, mainsheet fixings etc. Inevitably some salt water reached them through the bolt holes in time, leading to bulging and cracking of the overlying GRP. Amazing that such a great boat got that one so wrong.