Stainless fasteners into aluminium?

Tim Good

Tim Good

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The quote on the BSS site is "Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion"

However I have some 10mm bolts holding a large plate to my aluminium mast. Behind which is my mast furling gear. In each side it is secured with approx 10 hex countersunk bolts screwed into the mast. However many are loose no as the thread has corroded.

Two questions:

1. Is there some substance I can coat the threads with if screw stainless into aluminium to prevent corrosion?

2. For the tapped holes in the mast where the threads have deteriotated what is a good option to avoid re tapping for a larger bolt?
 
VicS said:
Tefgel, which is teflon based, is probably most suited to close fitting joints

Duralac, which is a barium chromate paste, is probably the better choice for poorly fitting joints. It is messy stuff to use!
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'Academic' question - is Tefgel readily, as readily, available as Duralac?

You cannot underline how messy Duralac can be - but it has worked for decades. Tefgel is much more recent.

Never joint aluminium and stainless without one or other.
 
Tefgel is just an inert grease. Any inert grease will prevent corrosion by excluding water, for as long as the grease takes to wash away or whatever.
Duralac is a chemical 'fix' to the problem, in a compound which sets and won't wash away.
Un-passivated stainless into aluminium threads is a recipe for corrosion, but the outcome is usually that the fasteners won't come undone, rather than the assembly falling apart.
On motorbikes we just use copper grease and it works.
If a thread has gone slack due to corrosion, the options include threaed inserts, rivnuts, larger fasteners.
 
Neeves said:
'Academic' question - is Tefgel readily, as readily, available as Duralac?

You cannot underline how messy Duralac can be - but it has worked for decades. Tefgel is much more recent.

Never joint aluminium and stainless without one or other.
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There are dozens of ptfe loaded greases on the market. Tefgel is just a favourite of a few on this forum.
 
+1 for Rivinuts. I would advise Monel for the purpose of using SS bolts into a aluminium mast section. If you do use SS, by all means passivate the rivinuts before fixing.
 
Neeves said:
You cannot underline how messy Duralac can be - but it has worked for decades. Tefgel is much more recent.
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I find it is much more controllable and less messy using it from a tin rather than a tube and applying it with a small artist's size paintbrush. A small tin is no more £ than a tube and when it separates (as it always does) you can stir it back together which you can't with a tube. Aviation suppliers sell it in tins...

https://www.lasaero.com/products/article/P002OHHP1
 
AngusMcDoon said:
I find it is much more controllable and less messy using it from a tin rather than a tube and applying it with a small artist's size paintbrush. A small tin is no more £ than a tube and when it separates (as it always does) you can stir it back together which you can't with a tube. Aviation suppliers sell it in tins...

https://www.lasaero.com/products/article/P002OHHP1
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I purchased a tin direct from the manufacturer last time I was in the UK

http://www.llewellyn-ryland.co.uk/downloads/duralac.pdf
 
northcave said:
2. For the tapped holes in the mast where the threads have deteriotated what is a good option to avoid re tapping for a larger bolt?
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Thread inserts. (Often generically called 'Helicoils', which is a brand name. Others, such as Recoil, are widely available and use a simpler, less expensive fitting kit.)

Easy to use and I wouldn't be without them on a boat. They're usually stainless, so any corrosion occurs relatively harmlessly between the mother metal an the insert, rather than around the fastener. The 'fix' is invariably stronger than the original thread. Many larger fasteners on better-quality masts have thread inserts as standard for precisely these reasons. Porsche also employ them on critical threads into aluminium.

They're particularly worthwhile on the stupid little grub screws holding a furling drum to the foil, which are otherwise almost guaranteed to seize sold.

Incidentally, they come in a variety of lengths, usually specified as 'l' x diameter of fastener. Rather than stocking several lengths, it's worth buying longer than typically needed: they can easily be shortened, one coil at a time, with side-cutters.
 
AngusMcDoon said:
I find it is much more controllable and less messy using it from a tin rather than a tube and applying it with a small artist's size paintbrush. A small tin is no more £ than a tube and when it separates (as it always does) you can stir it back together which you can't with a tube. Aviation suppliers sell it in tins...

https://www.lasaero.com/products/article/P002OHHP1
Click to expand...

That's good to know for anyone who doesn't already have enough Duralac to keep them going for the next 20 years, in a mangled sticky tube.
 
macd said:
Thread inserts. (Often generically called 'Helicoils', which is a brand name. Others, such as Recoil, are widely available and use a simpler, less expensive fitting kit.)
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Inserts are OK where you have sufficient depth of material, but thin sections, like mast extrusions, they are not so useful, rivinuts or their like are a better idea.
 
Topcat47 said:
Inserts are OK where you have sufficient depth of material, but thin sections, like mast extrusions, they are not so useful, rivinuts or their like are a better idea.
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Perfectly true, and the reason I have both. But I very much doubt the OP's 10mm bolts are into a remotely thin section. In fact it would be rare (and probably unsuitable) for any sort of tapped thread to be into thin aluminium section. Rivnuts, on the other hand, may not be suitable for use in thick sections.
 
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steve66 said:
I use copper grease. Works fine for me
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Copper grease is well known to increase galvanic corrosion in a marine setting. In fact, I have done studies of this. I'm sure it worked for you, but other things work better. Copper is NOT what you want next to aluminum. It is really for high-temp steel and cast iron applications.

I suggests you try some side by side testing with a metals-free antiseize from the same manufacture. Locktite LB 8023 is very good.
 
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