# Aluminum screws instead of stainless



## Frogwatch (Jan 22, 2011)

Considering that stainless screws into aluminum will corrode in spite of using Tefzel gel (the gel will eventually wash out). Why not use aluminum screws?


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## norahs arc (Jan 23, 2013)

Aluminum screws are not very strong. They may be alright for fastening trim but for anything else the are likely to fail.


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## DonScribner (Jan 9, 2011)

Aluminum screws should actually work fine. 6061-T6 has a min yield of 35ksi compared to 316 stainless steel with a yield of 45ksi. That means that a 3/8" machine screw of 6061-T6 will handle 3864 pounds before deforming permanently where the stainless can go to 4968 pounds. Yes, there is a difference, but is the aluminum screw already overkill? That's something that you'd have to decide. Is the decrease in strength worth the decrease corrosion? But . . . have you pulled any two aluminum components apart lately? I pulled my gooseneck out of the boom and found gobs of corrosion inside. Quite frankly, regardless of the numbers, stainless is still probably the best answer. New technologies aside, sometimes the old ways are the best ways, tried and true.


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## Alex W (Nov 1, 2012)

You are correct that the yield strengths are in the same ballpark, but the tensile strength of 316 is 90kpsi, compared to 42kpsi for 6061-t6.


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## Frogwatch (Jan 22, 2011)

Looking over the galvanic series, Aluminum bronze looks good.........until you try to find any fasteners made from it. There are a couple of companies in India selling them. There must be something better than SS.


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## svHyLyte (Nov 13, 2008)

Frogwatch said:


> Considering that stainless screws into aluminum will corrode in spite of using Tefzel gel (the gel will eventually wash out). Why not use aluminum screws?


Screws function in either tension or shear--i.e. "pull" along the center axis of the screw or, in shear, pull-push across with width of the screw. On a mast, almost all fasteners are loaded primarily in shear although mast mounted winches subject their lower fasteners to both shear and tension. Aluminum is relatively poor in tension, compared with a similar sized ss screw, and much poorer in shear. That said, one can certainly size aluminum screws for the loads they are likely to be subjected to although the fasteners will be proportionally larger and, being used less frequently, more difficult to find and costly. Being treated with TefGel, or Lanacote for that matter, and properly bedded to prevent water infiltration into the threads will prevent or limit the corrosion problem.


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## HUGOSALT (Jun 15, 2004)

An aside...thought I had an inside track on some very light, very strong screws when
my son was home sorting out some Titanium screws used in spine surgery...until he 
explained each screw sold for many hundreds of dollars!


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## One (Mar 20, 2013)

HUGOSALT said:


> An aside...thought I had an inside track on some very light, very strong screws when
> my son was home sorting out some Titanium screws used in spine surgery...until he
> explained each screw sold for many hundreds of dollars!


Well, you can get non-surgical titanium bolts, screws and nuts for much less - (track) motorcycle tweakers use them a lot.

Hell, you can even get carbon fiber bolts and nuts (yes, really).


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## overbored (Oct 8, 2010)

except that titanium is worst then stainless on the galvanic series


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## One (Mar 20, 2013)

overbored said:


> except that titanium is worst then stainless on the galvanic series


Yes, depending on which ti alloy is used. However, it is easier to isolate electrically than stainless, and it doesn't suffer from crevice corrosion.


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## overbored (Oct 8, 2010)

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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## One (Mar 20, 2013)

overbored said:


> 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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## overbored (Oct 8, 2010)

One said:


> 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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## Stumble (Feb 2, 2012)

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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## One (Mar 20, 2013)

overbored said:


> 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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## One (Mar 20, 2013)

Stumble said:


> 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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## bobmcgov (Jul 19, 2007)

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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## MarkSF (Feb 21, 2011)

How about Monel rivets?


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## Stumble (Feb 2, 2012)

MarkSF said:


> 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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## Stumble (Feb 2, 2012)

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_Corrosion_Titanium.html
http://www.sciencedirect.com/science/article/pii/S0011916401004350


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## overbored (Oct 8, 2010)

MarkSF said:


> How about Monel rivets?


Monel grade 50 cherry rivets ( blind or pop rivets ) are use in aircraft some times on aluminum because they are lower on the Galvanic chart then the SS grade 51 cherry rivets but they are not a strong as the SS grade 51's in both shear and tensile strength


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## tommays (Sep 9, 2008)

And outboard motor goes through hell and something as simple as a coating of PERFECT SEAL GASKET SEALER is more than enough to make a saltwater lower unit pretty easy to take apart on the next service 


If you look at BIG OUTBOARDS were there is plenty of money one of the solutions used by the OEMs who know a bit is chrome plated SS bolts


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