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Old 04-03-2014, 03:56   #1
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Aluminum bronze vs silicon bronze keel bolts

I would like to ask everyone in our world of wooden boat restoration a question that continually challenges. I was always taught that the correct material to use for keel bolts on a lead keel is silicon bronze. The reason being that a copper alloy like one of the bronzes would be closer to lead in the galvanic series than a ferrous alloy, basically a steel like stainless (so called). Nowadays it is very difficult to source silicon bronze round bar or nuts at a reasonable cost. Most foundries prefer to cast in aluminum bronze with the number C95400. This amalgam is easy to machine, is good in seawater and has a good corrosion resistance however it is relatively low in relation to lead. Has anyone any thoughts about a keel bolt in ali bronze through lead in saltwater?
BTW there is no emoticon about metal fizzing!
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Old 04-03-2014, 04:19   #2
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Re: Aluminum bronze vs silicon bronze keel bolts

Have you thought about shopping around on the internets?

Kiwi Shipping are very affordable, and you can use them as a domestic shipping address for US suppliers... I think they also operate to Oz, if that's your current location...

I personally wouldn't risk Ali bronze in that combination, if you are aiming at an enduring restoration...
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Old 04-03-2014, 13:03   #3
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Re: Aluminum bronze vs silicon bronze keel bolts

When I was looking to source material for new chain plates ran into the near total unattainability of silicon bronze in anything but round stock. The 950 Aluminum Bronze was the only stuff out there in a variety of shapes. Couldn't get feedback on its suitability for marine use, good or bad. Specifications all looked good but no one would admit to having used it on a boat.

The reason to steer clear of 300 series stainless is crevice corrosion not its location on the galvanic scale. Bronze doesn't suffer from this issue and is much better material for use where the metal may be in an oxygen starved environment. Suspect that bronze was the metal of choice on older boats because of availability. A port town of any decent size and many large boatyards had a foundry capable of casting bronze pieces. If you needed a part, you had it made. SS wasn't widely available or as cheap as bronze and SS fabrication was more difficult especially welding until inert gas systems became widely available. Unfortunately times have changed.

Ask around and see if there are any metallurgists out there who can shed some light on Aluminum Bronze. The spec's all seemed good and maybe there are some out there who've actually had experience with it in a marine environment.
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Old 04-03-2014, 13:38   #4
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Re: Aluminum bronze vs silicon bronze keel bolts

I believe a lot propellers are Al Bronze, but it still remains the question of galvanic corrosion properties with lead..
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Old 04-03-2014, 14:04   #5
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Quote:
Originally Posted by TeddyDiver View Post
I believe a lot propellers are Al Bronze, but it still remains the question of galvanic corrosion properties with lead..
Afaik lots of props are nibral, nickel bronze aluminium
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Old 04-03-2014, 14:30   #6
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Re: Aluminum bronze vs silicon bronze keel bolts

It's still Aluminium Bronze With 10% Al, 5% Ni. Naming it Nibral is cheating... IMHO
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Old 04-03-2014, 16:44   #7
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Re: Aluminum bronze vs silicon bronze keel bolts

Roverhi

Aluminium bronze is a fantastic marine material; my sole reservation about the OP's proposed use was the adverse effect it might have on his keel, particularly at the interface of the bolts with the lead.

The alu bronze would if anything be less affected than silicon bronze bolts, it's the lead which would suffer.

I did some engineering for a friend who built a wooden Lyle Hess cutter to very high (trad) standards (like me, he was a mech eng) and he used aluminium bronze extensively, among other things for mast fittings, bowsprit fittings, chainplates, and block straps.

The boat is now twenty years in the water, and those items could pass for new if repolished.

I'm as sure as I can be that he used silicon bronze, though, for the keel bolts. I remember making a special cutter for counterboring the lead, but I didn't make the keel bolts.

- - - - - -

The only way aluminium bronze can be improved is by adding nickel.

Nickel Aluminium Bronzes, AB2, 955 , (the term Nibral seems to be largely restricted to prop marketers- maybe they invented it?) are legendary in the practical engineering community, being reputed as a material that tends to perform better than the raw properties data would suggest ... and the data are impressive enough, already. Virtually no downsides, from a metallurgical standpoint; corrosion resistance, for instance is better than most. There are inevitable tradeoffs but they are nuances rather than vices, for instance AB2 corrosion resistance is enhanced by annealing, at some decrease in hardness and tensile strength, but improved toughness.

In general, and by any yardstick the Aluminium Bronzes are seriously tough materials, and yet very strong.
- That apparent redundancy might perplex someone not familiar with the exact technical significance of those terms.

Ordinary glass and brittle toffee are very strong, and yet not tough in the slightest.
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Old 04-03-2014, 17:22   #8
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Re: Aluminum bronze vs silicon bronze keel bolts

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.
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Old 04-03-2014, 18:40   #9
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Re: Aluminum bronze vs silicon bronze keel bolts

Hum, Aluminum is just up from zinc on the galvanic scale. Its a very strong material and its getting very common in brass/ bronze alloys now a days. But I would have reservations on using it where it is in contact with salt water and without the benefit of zinc anodes to protect the aluminum in the Al/bronze from micro galvanic corrosion.

In my mind, aluminum bronze is only slightly better then zinc/ brass. Aluminum will still sacrifice its self when mixed with copper/tin/nickle. Just at a slightly slower pace then zinc would.

Where zinc anodes are installed, such as at prop shafts, then its a satisfactory material underwater. Most folks don't have zinc's on their keel bolts, and that would create a wee bit of a problem. I would not recommend aluminum bronze for keel boats for that reason.
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Old 04-03-2014, 21:20   #10
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Re: Aluminum bronze vs silicon bronze keel bolts

Hey-

I've used exactly that alloy (C954) in many marine applications, but not admittedly in a keel bolt yet.

I replaced all the chainplates, deck stancions, rudder pintles and fabricated many custom fittings from it for my previous (traditional wood) boat. It is fabulously tough material - difficult to work, but easily weldable with the right filler rods, and develops a lovely patina after a few years.

I've also designed and had made many bearings/bushings, wear plates, mould press cavities, plastic injection mould release pins/press parts and related industrial components from this alloy. I am a mechanical engineer, not metallurgical, but I do have a metallurgical eng working for me and he loves this stuff too - the mechanical properties are so superior to most other easily available bronzes. ... although he is not a yachtie and I have not pressed him on the galvanic issues when immersed full time in seawater.

my experience so far with it in immersed locations (~8 years) says no problem. I've seen no evidence of any decay or change in properties on close visual inspection, nor noticed any change in the rate of decay of zincs or plating/erosion anywhere else after using the material.

...but as always, ymmv... Good Luck!
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Old 04-03-2014, 21:29   #11
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Re: Aluminum bronze vs silicon bronze keel bolts

Hmm, sailorchic34

Your instincts would be correct if all alloys were just a mixture of discrete elemental metals, dissolved in each other, but retaining their separate properties at the macro scale.

For various reasons, this does not usually happen. One simple example is the formation of 'intermetallic' compounds.

This is where the atoms of different metals share electrons, the most commonly known being the various compounds of Fe and C, like austenite, martensite, pearlite, cementite, etc. - which give different grades of steel their peculiar properties.

Some of these compounds of iron and carbon have properties (strength, resistance to attack) which are unrecognisably different from either constituent, and from each other.

Whereas, in cast iron, which has roughly an order of magnitude more carbon than ordinary "carbon steel" (which we usually just call steel), most of the carbon is present as clumps of graphite. The graphite is not chemically compounded with the Fe of iron.

And zinc, in brass, is (to a crude approximation) like graphite in iron: it retains its separate identity.

Because of this, zinc exhibits much the same vulnerability to attack in seawater, when present as an alloying constituent (perhaps more accurately as a solid in solution) in brass, as it would if you studded the brass with little zinc rivets.

Aluminium bronze, far from being unsuitable for seawater service, is one of the most suitable bronzes of all.
Nickel aluminium bronze is even better, and is widely used when money is no object, or when small amounts are involved


To make sure you were not onto something I didn't know about, I checked with one of the "bibles" on the topic, "Corrosion Metal Environment Reactions", Vol I of which runs to over 1400 pages.

Here's an excerpt which is relevant to this discussion, particularly the first paragraph:

<< Many of the alloys of copper are more resistant to corrosion than is copper
itself, owing to the incorporation either of relatively corrosion-resistant
metals such as nickel or tin, or of metals such as aluminium or beryllium that
would be expected to assist in the formation of protective oxide films.

Several of the copper alloys are liable to undergo a selective type of corrosion in certain circumstances, the most notable example being the dezincification of
brasses.>>

I noted a couple of instance of 'dealuminisation' in the book, but they were not in the body text; they appeared in the title line of a couple of obscure fifty-year old scientific papers. Dealuminification did not appear.
In comparison, the word "dezincification"appeared roughly a hundred times.

But this pricked my interest, as I have never encountered selective dealloying (aka 'parting') of aluminium from bronze, either in the real world or in the literature, so I searched further.

And came across this, in

"Aluminium Bronze Alloys - Corrosion Resistance Guide"

when I hunted for "dealuminification", I found this:

<<It can be very largely prevented under most conditions of service by ensuring that the alloy used is free from gamma 2 phase.>>

Gamma 2 phase is a particular structure of an intermetallic compound of copper and aluminium, which forms only on cooling, and only in certain conditions.

Which probably explains why it is not a well-known problem: the metallurgists who work for bronze founders would surely know all about it, though; it's their job to make sure nobody else needs to know about it.


For anyone interested in the detail, and the implications for Nickel Alu Bronze:


<<The danger of selective phase attack occurring on the gamma 2 phase in aluminium bronzes has already been discussed in Section 1, where it was also explained that the formation of this phase can be avoided by suitable control of composition and/or cooling rate.

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.

In the nickel aluminium bronzes, selective phase attack may affect small amounts of residual beta phase if any is present, but is more likely to affect the narrow band of alpha phase immediately adjoining the lamellar kappa and to spread from that into the kappa phase itself. This selective phase attack in aluminium bronzes is not usually of great significance and occurs only when they are subjected to particular severe service conditions. For such conditions of service it can be beneficial to apply to nickel aluminium bronze castings the heat treatment required in DGS Specification 348 (six hours at 675C + 15C followed by cooling in still air). This is, however, only necessary if the rate of cooling of the casting from about 900C has been too rapid for formation of the normal alpha-plus-kappa structure.>>

In short: the problem is negligible for aluminium bronzes, and minuscule for nickel aluminium bronzes. Which helps explain the popularity of the latter for propellors, where the amount of metal is not large, and even shallow corrosion is potentially very consequential.

The problems they mention for alu bronzes, even in the worst cases, tend to be self limiting. They are not in any way comparable with the crevice corrosion vulnerability of, say, 316 stainless.

Stainless is almost unique in having the capacity to attack itself under certain (regrettably, commonly encountered) conditions.
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Old 04-03-2014, 21:38   #12
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Re: Aluminum bronze vs silicon bronze keel bolts

Monel - Wikipedia, the free encyclopedia

Should be closer to lead on the galvanic index than the silicon bronze. I like the bit about it being used for anchor cables on minesweepers because of its complete lack of magnetism.

It's been a while but I sourced some Monel staples for a cold-molding project once. You can leave them in the lay-up, saves time pulling staples.
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Old 04-03-2014, 22:19   #13
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Re: Aluminum bronze vs silicon bronze keel bolts

Monel V nice, even tougher than Nickel Alu Bronze, but (for anything bigger than rivets, staples etc), expen$$$$ive.
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Old 04-03-2014, 22:22   #14
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Re: Aluminum bronze vs silicon bronze keel bolts

BTW the galvanic tables do not tell the whole story.

Some combos work a lot better than the table predicts, others a lot worse.
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Old 04-03-2014, 22:40   #15
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Re: Aluminum bronze vs silicon bronze keel bolts

Interesting topic

I just checked my copy of Nigel Warrens 'Metal Corrosion in Boats' and he mentions that "de-aluminification" of submerged Alu Bronze "has been known" under certain unspecified circumstances, so I was wrong to say I'd never heard/read of it, just forgotten! My bad ...

He doesn't go into detail, unlike the more detailed source I quoted above, but does mention that the addition of at least 4% Nickel makes the problem go away. (Which, of course, does not mean that other things do not)

He lists the following viable options for keelbolts in lead keels:

Silicon Bronze, Gunmetal (!) Monel, Alu Bronze or better, Nickel Alu Bronze; and specifies that "good electrical insulation should be provided for any of these alloys"
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