Esper Refit 34 - Bedding deck fittings with butyl tape, not Sikaflex

demonboy

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After doing my research and taking a punt this week we bedded down our stay-sail track with butyl tape and not polyurethane. I didn't take any photos of this little project but there's some extensive video footage in this week's clip. I haven't tested it yet so I've no idea if I did the right thing, but I followed the general principle and am fairly confident it's the right way to go. I have some white butyl tape on order to continue the bedding.

I did find a bit of corrosion in my aluminium hatch frames. Perhaps Vyv can confirm whether this is also crevice corrosion, I'm not sure if it works in the same way as it does s/s. I filled the corrosion with epoxy and micro-balloons and sanded it back. Not sure if this was the best thing to do but I'm hoping with the butyl tape this should be ok for the next ten years.

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Mey, our Burmese cleaner, was back this week. We set her up with my cheap angle-grinder attached to a block of wood. A couple of polishing disks later and our grotty stanchions were looking like new once more.

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We're also making teak beds for our cleats and fairleads. This adds a little strength to these fittings since we've lost 15mm of teak, and we're fibreglassing them over and will spray them Snow White to match the deck. Here's one of our new fairleads sitting on a not-quite finished bed.

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There's a few other pics on the blog post but do check out the video clip for the bedding of the stay-sail track using the butyl tape. If anyone's used this before for the same type of fitting I'd be interested in hearing from you. I'm waiting for a consignment of white tape from the States as it's less obtrusive than the black stuff on a white deck.

Video clip here: http://followtheboat.com/2014/10/02/esper-refit-week-34/
 
Crevice corrosion affects alloys that depend upon a surface oxide film for their corrosion resistance. This is then the passive condition, with the active being the nascent metal inside the crevice. Some, maybe all, aluminium alloys do depend upon a surface oxide layer, so I suspect that crevice corrosion is the mechanism.
 
Crevice corrosion affects alloys that depend upon a surface oxide film for their corrosion resistance. This is then the passive condition, with the active being the nascent metal inside the crevice. Some, maybe all, aluminium alloys do depend upon a surface oxide layer, so I suspect that crevice corrosion is the mechanism.

Thanks, Vyv. Can you just clear up the nomenclature for me? You describe metals as having a protective 'film' or layer. Using the word 'film' makes it sound like a layer is added manually by someone in a factory. I'm guessing stainless is stainless throughout its structure, so is this layer simply the exposed surface that is being oxidised by the air around it that completes the structure of stainless (which is then lost when covered, ergo crevice corrosion)? My terminology on the subject is confused to say the least.
 
On stainless steels the film is chromium oxide only a few microns thick. It forms very rapidly on a cut or machined surface, due to the desire of metals to return to their lowest energy state. Chromium occurs in nature as the oxide and it takes a great deal of energy to convert it to chromium. Crevice corrosion occurs because chromium oxide (described as passive in the galvanic series tables) has a different potential from the active metal that exists within the crevice. The exchange of electrons between the two is what causes corrosion.

I am nowhere near as educated in aluminium as I am in stainless steels but the mechanism is pretty much identical. Again, aluminium occurs naturally as the oxide and this is the composition of the protective film. The complication is that an oxide film can be generated artificially by anodising, which provides it in a thicker layer to give better protection. Non-anodised aluminium has some corrosion resistance but not as much as is necessary for complete protection.
 
Hi mate. A tip re butyl under fittings: it will continue slowly to extrude out for quite a while, probably several weeks. So there's no point trimming it off too soon, since you'll only have it to do again.

+1. I used it to bed a big wheelhouse roof hatch, did the job brilliantly and cleanly but I was still trimming it six months later.
 
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