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Old 04-03-2014, 23:01   #16
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Re: Aluminum bronze vs silicon bronze keel bolts

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Originally Posted by Andrew Troup View Post
Under free exposure conditions in fresh waters or sea water, aluminium bronzes free from gamma 2 phase do not show selective phase corrosion but, under crevice conditions, beneath deposits or marine growths or under the influence of galvanic corrosion or of electrical leakage corrosion, selective phase attack can occur. In the alphabeta alloys this takes the form of slightly preferential attack on the beta phase.
A lovely writeup Andrew and informative. The crevice corrosion is interesting as is galvanic. Since its galvanic corrosion that to me is the main issue.

I would love to see a study of aluminum bronze and sea water effects on it over time (1-4 years). If Aluminum bronze props didn't require zincs, I would be less concerned with its corrosion resistance. Perhaps its only a minimum loss of material so no real loss of strength.

I find that dealuminification being mentioned at all is interesting as well as
<<It can be very largely prevented under most conditions>> . Since zinc in brass/bronze has been around for well over 200 years, I would expect it to show up more. Aluminum Bronze was not so common.

But in truth, I am not that familiar with the aluminum bronze combinations. But am well aware of aluminum's weaknesses.
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Old 04-03-2014, 23:04   #17
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Re: Aluminum bronze vs silicon bronze keel bolts

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Originally Posted by Andrew Troup View Post
In general, and by any yardstick the Aluminium Bronzes are seriously tough materials, and yet very strong.
The strongest bronze as far as I know is Superston, widely used in ships' propellers. It is an Al Bronze with Mn, Fe, and Ni, about 7 - 8% Al.

I believe it's C95700.
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Old 05-03-2014, 16:24   #18
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Re: Aluminum bronze vs silicon bronze keel bolts

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Thanks everyone for these insights. The foundry I am using in Auckland is casting new chainplates and hanging knees in Aluminum Bronze. It has been used for many of the classic restorations in NZ. I restored the 1904 B Rater Ngatira and put new keel bolts of Silicon Bronze which I obtained from Jamestown Distributors. I contacted them about supplying again with no response. The foundry in NZ say they can cast rod in Si Bronze if I want and then have it machined to dimension. Similarly the nuts are very expensive and it appears it is more economic to cast them and machine than to buy a length of hex bar.
overtheseas:

If you can source it, continuously cast (or better still, wrought) bar would have better and more reliable properties than sticks of sand cast bronze.

(Which is not to say that the latter would not be satisfactory, especially if the keel leverage vs bolt disposition, proportions & quantity are favourable -- as they tended to be in earlier times)

Here's a possible source of (presumably) wrought bar, in silicon bronze of alloy 655
Atlas Metal Sales

These guys even list silicon bronze square tubing !!! (alloy 651)
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Old 05-03-2014, 20:32   #19
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Atlas is used by lots of folks in the wooden boat world and comes recommended.
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