Aluminum screws instead of stainless - Page 2 - SailNet Community

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  #11  
Old 04-08-2013
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Re: Aluminum screws instead of stainless

the aluminum will suffer more. the ti will not corrode but the aluminum will do all the corroding. much cheaper to replace bolts then an alum mast
How is it easier to isolate? the bolts are the exact same dimensions and function exactly the same
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Re: Aluminum screws instead of stainless

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Originally Posted by overbored View Post
the aluminum will suffer more. the ti will not corrode but the aluminum will do all the corroding. much cheaper to replace bolts then an alum mast
How is it easier to isolate? the bolts are the exact same dimensions and function exactly the same
They don't tend to work as well as an electrode as steel does. And as for same size: Why would you use the same size? On second thought, don't answer that, because:

In any case, that wasn't the point. The point was that surgical screws, nuts and bolts aren't exactly needed in a marine setting.
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Re: Aluminum screws instead of stainless

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They don't tend to work as well as an electrode as steel does.

In any case, that wasn't the point. The point was that surgical screws, nuts and bolts aren't exactly needed in a marine setting.
but that is the point of the galvanic series, they make a better electrode then stainless with regards to aluminum. they will eat away at the aluminum better then the stainless. and are not needed in the marine environment unless you have a carbon parts on your boat
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Re: Aluminum screws instead of stainless

Overboard,

Titanium is higher on the galvanic chart, but it has the advantage that when exposed to air or water titanium instantly creates a titanium oxide coating that is an insulater. So that a titanium-aluminium joint has a lot of galvanic potential, but very little current. Simply because the resistance is so high.

I DO NOT recommend installing titanium directly onto other metals without insulation, but I can point you to a few studies that found that a titanium-stainless joint actually corrodes slower than a stainless-stainless joint. Because of the insulating properties of titanium.

And as for cost... Somewhere between the cost of 316L and siliconized bronze fasteners typically. Specialty fasteners can vary widely though.
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Re: Aluminum screws instead of stainless

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Originally Posted by overbored View Post
but that is the point of the galvanic series, they make a better electrode then stainless with regards to aluminum. they will eat away at the aluminum better then the stainless. and are not needed in the marine environment unless you have a carbon parts on your boat
Carbon is even more prone to corroding everything.

However, even if it's higher on the galvanic scale (for pure titanium), there are different titanium alloys, and it still doesn't work as an electrode to the same degree as a stainless piece of metal. I think it's to do with it being a piss-poor conductor, even if caked with salt and therefore moisture.
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Re: Aluminum screws instead of stainless

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I DO NOT recommend installing titanium directly onto other metals without insulation, but I can point you to a few studies that found that a titanium-stainless joint actually corrodes slower than a stainless-stainless joint. Because of the insulating properties of titanium.

Hmm, yeah, as I said ...

Edit: Sorry, I thought i responded to Overbored.
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Re: Aluminum screws instead of stainless

Also, aluminum has dreadful fatigue and work hardening properties. A threaded aluminum fastener would develop stress risers and fail very, very quickly. Even the threads themselves are prone to work hardening under cyclical loads, then stripping catastrophically. Had an air compressor head blow right across the shop that way. FWIW, I'm not keen on tapping any fasteners into aluminum spars for that reason, not if they see tensile loads. Thru bolting makes me happier.
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Re: Aluminum screws instead of stainless

How about Monel rivets?
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Re: Aluminum screws instead of stainless

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How about Monel rivets?
Monel sits just below titanium on the galvanic chart (so it is slightly less anodic depending in general) but close enough that you have to look at exacally what alloy. For aluminium applications Monel is worse than stainless at causing galvanic corrosion.

It is also more expensive ( often more expensive than titanium)
Very heavy
And hard to machine. Monel rivets would be a pain to instal without power tools.
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Re: Aluminum screws instead of stainless

I went ahead and pulled the article I referenced above. This is a pear reviewed study from NIH investigation surgical implants which live in a more corrosive environment than even our boats. It's warm, wet, and salty all the time. But there is enough free oxygen to allow for self healing metals (like titanium and stainless) to operate.

Is galvanic corrosion between titanium alloy... [Spine J. 2004 Jul-Aug] - PubMed - NCBI

And some other experts in the field:
http://www.corrosionist.com/Galvanic..._Titanium.html
http://www.sciencedirect.com/science...11916401004350
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