What metal for screws attaching brass under the waterline?

[Greenheart]

I benefited from many useful replies about replacing my keel band back in the winter...and I succeeded in doing the job quite satisfactorily...

...but now, after a only a few hours in the briny, I've discovered some of the screws are turning to rust. The unaffected ones will be the originals that I reused; the others are some tat I added without thinking.

The keel-band is brass. What's the best metal I can use for the screws, to prevent corrosion of either screws or keel band? I was careful to use plenty of Duralac to isolate the screws, but it hasn't worked...or perhaps the dud screws I used were just totally vulnerable to salt water. Any suggestions?

 

[pmagowan]

Brass ?

 

[Tranona]

Brass or bronze (if you won the lottery tomorrow). Brass will eventually dezincify, but if you bed the keel band in Sika it won't come off and the screws will probably outlast the band.

 

[VicS]

Brass screws were supplied for the brass keel band when i built my Mirror but they likely to be difficult into screw into GRP.

Silicon bronze might be be a better choice.

But if you are screwing into GRP I think stainless steel self tapping screws would be the best bet. 316 grade of course and put in with a dollop of sealant to seal the thread into the hole.

 

[Greenheart]

Thanks gents, it hadn't occurred to me that strong-enough screws were available in brass.

Are bronze screws prohibitively expensive? I'll probably only need half a dozen, and they're tiny.

The screws which had held the keel band previously look like stainless. They're a bit dull now but not corroded. How much does stainless suffer, in contact with brass?

 

[Tranona]

It won't. The potential problem is crevice corrosion in the threads, but plenty of sealant will avoid that. The key point is that if you use an adhesive sealant, the screws are rather irrelevant - only there to locate the band, not hold it on.

I have just removed some lacing hooks which have been on for 20 years. The threads of the brass screws crumbled away, but the sealant held them on.

 

[Woodlouse]

Thanks gents, it hadn't occurred to me that strong-enough screws were available in brass.

Are bronze screws prohibitively expensive? I'll probably only need half a dozen, and they're tiny.

The screws which had held the keel band previously look like stainless. They're a bit dull now but not corroded. How much does stainless suffer, in contact with brass?

A small number of small bronze screws won't break the bank and are the best choice for the application you want them for. Yes they'll cost more than brass or stainless but if you can't afford a couple of quid then the screws in your keel band are probably not your biggest concern. Bronze only really gets expensive when you start buying large fastenings in large numbers.

 

[Greenheart]

Thanks again.

I used that Two Tonne epoxy when I attached the keelband - so I'm reasonably confident that it's secure, and I also replaced the screws which had previously held the band in place.

I wouldn't be concerned except that certain of the screws which I sourced hurriedly, are just crumbling, despite minimum time immersed. I'll go to Screwfix.

 

[David2452]

Probably not relevant to the OP as he is likely using existing holes but with new stuff when using brass screws I like to use a steel one first then remove it and replace with brass as they can be very soft and sheer or the heads can mar.

 

[rob2]

+1 for cutting the thread with a steel screw and then fitting brass. A drop of epoxy or mastic in the hole as you change over and the seal should hold for years. Actually, brass screws do seem to crumble faster than the band they're holding, so phosphor bronze would be worth the money if you can find them.

Rob.

 

[VicS]

+1 for cutting the thread with a steel screw and then fitting brass. A drop of epoxy or mastic in the hole as you change over and the seal should hold for years. Actually, brass screws do seem to crumble faster than the band they're holding, so phosphor bronze would be worth the money if you can find them.

Rob.

Why phosphor bronze ? Why not silicon bronze? I though that was more usual for bolts and screws etc

 

[rob2]

Oops, my mistake - typed phosphor whilst still half asleep! I was more worried about the spelling than getting the correct term! Yes, silicon bronze - mea culpa.

Rob.

Damn, this hair shirt is itchy...

 

[Greenheart]

Thank you, I'll try to find what I need in silicon bronze.

I wonder why the generous application of Duralac hasn't prevented immediate corrosion? Is it likely that the cheap screws would have rotted regardless of the superior metal adjacent?

 

[Tranona]

Thank you, I'll try to find what I need in silicon bronze.

I wonder why the generous application of Duralac hasn't prevented immediate corrosion? Is it likely that the cheap screws would have rotted regardless of the superior metal adjacent?

Duralec isolates stainless from aluminium. You don't say what your cheap screws were. If they are steel or plated steel they will rust away in water, irrespective of the metal they are connected to!

You are making a mountain out of a molehill. Whack some stainless or brass in with plenty of sealant and waste the time saved on dreaming about gaff rigged schooners. Forget about wasting hours trying to find somebody who will sell you 6 tiny bronze screws - and having a bad turn when you see the price (plus postage and packing, sir!)