alloy with stainless fixings

tom52

tom52

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Wotcha all,

I am fixing alloy cleats (using a suitably chunky marine ply backing pad) with stainless M8 nuts and bolts.
Do I really need to worry about the interaction between s/s and the alloy?
If yes whats the best solution ?
Thanks in advance for your advice.

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Yes!!

Yes, you must protect against corrosion due to the interface of the dissimilar metals. The stuff most people use is Duralac - it's a sticky yellow paste which you apply liberally to the mating surfaces before assembling. Seems to work quite well. Also well worthwhile taking care to seal the underside of the cleats against moisture, by using a bedding compound.

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As pvb says, use Duralac, I have had no corrosion problems when used on stainless bolts threaded directly into alloy. Use plenty and wipe off the excess. Duralac is used by the aerospace industry for just this purpose.

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Where can you buy Duralac ? cannot recall seeing it advertised anywhere,

thanks mike

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Any Chandler should stock it, I got my tube from Aladins Cave, about a fiver but should last forever.

Martin

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I fully appreciate the theory behind this but thought I'd just mention that there are plenty of stainless / ally interfaces on my boat that seem fine!

I have loads of stainless self-tappers going into the boom, stainless bolts holding cast aluminium cleats to GRP, stainless bolts holding the cast aluminium mast foot to the deck, stainless bolts holding the cast aluminium gooseneck to the extruded aluminium mast and stainless bolts holding a fabricated aluminium autohelm bracket to a plywood bulkhead. Some of these joints are over 30 years old and seem absolutely fine! I've never dared use anything like that below the waterline but I'm starting to come to the conclusion that as long as the oxide layer on the aluminium is good, there may not be that much to worry about!

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only really need to worry with stainless male thread into female ally tapping. in the case you mention both threads are stainless, no problem.

never bothered about stainless bolts through plain holes & have had no trouble.

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thats good to hear, put some stainless bolts through the boom last year,
think I will have a look at them though,

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My experience too. My aluminium alloy cleats have been fixed to my deck with stainless steel bolts for almost 20 years without any evidence of either chromate paste or corrosion. Having said that, the aluminium is anodised, which passivates it considerably. Most fittings you are likely to buy would be treated in the same way, so no big worry. It also helps that the anode (aluminium) is much bigger than the cathode (the bolts) The other way around might give more problems, but unless constantly immersed I wouldn't worry.

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My alloy stanchion sockets are bolted to the toerail using stainless bolts. The stanchion itself I'm fairly sure is stainless steel too. I had to replace two of these sockets due to them splitting and visibly everything looked fine and I thought this would be quite easy. However, whilst trying to separate them I quickly found that the whole stanchion/socket/bolts seemed to be fused into a single block! It took me nearly 2 days of careful drilling and chiselling to remove the stanchions without damaging them so I have reassembled using copious quantities of the sticky yellow paste!

Last week I found that another two bases have split - not a job I'm looking forward to!

<hr width=100% size=1>Gavin
 
Learning the hard way...

A few years ago I had to replace the alloy bow cleats on my boat because corrosion where the stainless bolts passed through the cleats had got so bad that the cleats were literally bursting apart. It was a messy and expensive job, and I assembled the new cleats with a good coating of Duralac.

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Thanks everyone.

I will also be fitting fairleads which do involve stainless bolts into tapped alloy so it looks like a trip to the sticky yellow paste shop unless.......

Would a physical barrier be any good ? I was thinking of something like PTFE tape around the threads?

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Physical barriers...

On balance, I think the paste is a better bet. I wondered about using heat-shrink tubing, but I think it would get damaged when tightening up the bolts.

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