marine brass?

[Ships_Cat]

<<<Hope you agree!>>>

I think we have probably been in agreement all along Mike /forums/images/icons/smile.gif.

Is probably important though that it is kept in mind that manganese bronze (being the "bronze" probably most commonly used) is actually a brass and comes in both dezincification resistant and non resistant forms (manganese bronze is not alone in having resistant and non resistant forms as aluminium bronze, for example, does too).

Simply purchasing a "bronze" is not safe, one should always buy from a reputable marine source.

There have been alot of threads in the past mentioning props and seacocks that have suffered from dezincification, and it may very well be that those props are manganese bronze (or in the case of seacocks, supposedly, but not a quality resistant brass such as the heat treated DZR that Blakes use). Perhaps not realizing that "bronzes", such as manganese bronze, can dezincify if not made as resistant and purchasing them from sources not of high reputation is one of the problems there.

Regards

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[boatmike]

Fine John,
As a matter of curiosity, having said I am no metallurgist, why do you keep wanting to designate magnesium bronze as a brass? I accept that all bronzes are in some sense a "brass" in that they are copper zinc alloys but I have never heard magnesium bronze called brass before and am puzzled. What specifically do you believe makes mag-bronze brass and ally-bronze and phos-bronze (or gunmetal bronze for that matter) bronze? Or do you consider all bronze is brass?
As far as source is concerned the average punter just goes in a chandler and buys what is there. It is my belief that, props aside, which usually come from specialist suppliers like Bruntons, skin fittings unless branded, are usually of indeterminate origin. Most bronze ones sold here I believe are from UK suppliers like Blakes and there is a degree of tracability which leads me to trust not only their corrosion resistance but also their tensile strength but a lot of the "brass" ones are of dubious third world origin and gawd knows what they are made of ! DZB tells me nothing so I would rather avoid them completely. This is in fact the core of my argument and I think reading your reply you agree.
None of this actually confirms what "Tonvil" is. Do you know? Is it as I suspect, Naval Brass (Tobin Bronze)? If it is do you know if it is in fact as resistant to de-zincification as suppliers claim? Frankly I don't know....


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[Ships_Cat]

Hi Mike

Brass = alloys of copper and zinc (plus minor constituents)

Bronze = alloys of copper and tin (plus minor constituents), usually, but other metals are sometimes used eg aluminium bronze which is copper and aluminium.

Manganese bronze is an alloy of copper and zinc and therefore is a misnamed brass, not a bronze except in name only. I have had a check of stockists and it is often declared as a brass by them, being listed under the likes of "High Strength Yellow Brasses". Manganese is added to improve its mechanical properties and others may be added (eg arsenic) to improve its resistance to dezincification. Now I am not a metallurgist either but alloys exist in various phases such as alpha, beta, gamma, etc depending on their constituents and their heat treatment (is a molecular structure thing). As far as I am aware, dezincification resistant manganese bronze (and other dezincification resistant brasses, and also some non tin based corrosion resistant bronzes such as aluminium bronze) have to be alpha phase, the other phases not being resistant.

For the DZR brasses, which are what Blakes claim to use and I think (but do not know) are casting brasses they are heat treated, I assume to ensure that they are alpha phase. Interestingly I just had a double check on the Blakes site and while the seacocks themselves are DZR the bolts are phospher bronze - I assume that is because, as I understand it, DZR's are subject to stress related failures when used as fastenings (others may know more about that).

Maybe a metallurgist (eg Cliff) could tidy the above up but I believe that it is pretty much down the right track.

I do not know what "Tonvil" is and have never come across it. If it is used for propellors, perhaps it is similar to the widely used Nibral which is (as far as I know) another brass with nickel and aluminium (spells NiBrAl). This is a respected material which is again it seems, if my understanding of its composition is correct (and I stand to be corrected on that), a brass which is dezincification resistant.

As you say, the details of all the above is not of much interest to most, but it is, however, important that people realise that just because something is called a brass does not mean that it is not suitable underwater (most small boat propellors are technically a brass, but if of correct material do not suffer corrosion). Similarly, just because something is called a bronze does not mean that it is suitable for underwater use, as technically it may be a brass and a non zincification (or other alloying metal) resistant one at that.

I suspect that you may be the same as me, for boats that we have been professionally involved with the assurance is had by dealing only with name respected suppliers for things like propellors and other underwater fittings (eg Blakes, or for here in NZ Henleys for propellors), rather than sort out the complexities of the metallurgy. What I am sure leads to trouble with pleasure boats are instances such as people buying from cheap suppliers or second hand (in fact there is another thread at the moment looking for a second hand propellor).

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[ccscott49]

Just as a matter of interest, I will not use any kind of stainless below the waterline, even my shafts are bronze. (maybe) I have seen stainless screws in a non oxygen enviroment, rust away faster than galvanised steel.

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[macd]

Much impressed by the erudite stuff above, especially from BoatMike and Ships_Cat, and wouldn't presume to be in the same ball-park as their technical knowledge. But, for the record, this is what the MAIB also had to say about Tonval (not Tonvil):

MAIB purchased components similar to the failed Random Harvest seacock from the three different well-known suppliers. These claimed respectively to be 'Tonval ATD', 'bronze approved material' and 'gunmetal'. On analysis "all were found to be made from a 60/40 leaded brass, meeting the composition of alloy CW619N...[which is] “not accepted for use in underwater through-hull fittings”.

MAIB further concluded that the rapid dezincification of the 16 month-old seacock was probably accelerated by stray currents and poor cathodic protection, which seems to concur with AndrewB's observations.

Incidentally, I've tried repeatedly to pin down exactly what Tonval is, without much luck, except that it's basically a 60:40 brass, with 2% lead to aid machining, and probably trace amounts of phosporous. But surely that difficulty of pinning down exactly what it is -- with efforts far more diligent than your average chandlery shopper's -- goes to the heart of the matter. As the MAIB suggestes, it "conceals rather than conveys the nature of the material".

If you buy a packet of frozen peas, the law requires that the packaging inform you precisely what's inside. Why should we put up with anything less? Or does some poor sod have to drown first?

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[Ships_Cat]

Thanks for the clarification on the spelling - so have now found out what it is, a brass with lead as you say so is clearly not a marine material. Have never heard of it here in NZ.

Also have now had a look at the Random Harvest accident report you referred to. Well worth a read and the recommendations on not bonding the thru hulls and sea valves to the anodes (was a frp boat) is well worth a read by the many forumites who seem to believe in putting as much zinc on their boats as possible and then tying everything to it.

Thanks

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[boatmike]

John, I am now embarrassed. What a pratt I am! Of course you are right. The basic difference between "proper" bronze and mag-bronze is that the former is a copper tin alloy and the latter copper zinc which reinforces my preferred choice of gunmetal bronze as it has no zinc in it to de-zinc in the first place ( doh!!) The situation is of course confused by the addition of tin in some brasses and zinc in some bronzes but your basic contention is right on the money and just proves that I am getting old...

However, having got over that piece of Altziemers related memory loss and got back to my basic schoolboy text books I can probably add a little to the "Nibral" thingy.

Manganese bronze (or more correctly brass I suppose) is normally Copper 55-60% Zinc 38-42% tin max 1.5% manganese max 3.5% aluminium max 1.5% iron max 2% lead max 0.4%
Physicals: Tensile 65,000 pounds per sq in. elongation 25% (sorry about the units I cant be bothered to convert)

HIGH TENSILE Manganese bronze is normally Copper 60-68%, Iron2-4%, aluminium 3-6%, manganese 2.5 to 5%, tin max 0.5%, lead max 0.2%, and nickel max 0.5% zinc remainder.
Physicals: Tensile 90,000 to 110,000 psi, elongation 12-20%

Higher aluminium and the addition of trace elements of nickel appear to increase the tensile strength of the alloy and therefore your "Nibral" is possibly still a form of high tensile Manganese bronze at a guess...

The Tonval (correct spelling) thing is not however clear. The more I read the more suspicious I become. I still think the best choice for underwater skin fittings and valves (the primary barriers against seawater ingress) is gunmetal if you can find it.

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[boatmike]

Good man! No problem there then. You might like to file away for later that an excellent substitute for bronze in shafts is Monel. Also highly resistant to crevice corrosion.... Just a detail though. Like you I avoid 316 stainless for propshafts if possible although there are many of them about.

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[wagenaar]

Ships_Cat,
You are basically right that brass is an alloy of copper with mainly zinc and bronze is an alloy of copper with mainly tin. You may notice that I change your definition somewhat, because besides copper and zinc or copper and tin other constituents are present. Some of these constituents are due to the base materials, ores, scrap, used for the manufacturing and sometimes are called "Impurities", while others are added to obtain a specific property or properties of the alloy. In the case of brass manganese is sometimes added to increase the tensile strength and corrosion resistance. Caling this material a manganesebronze is absolutely incorrect and in my opinion should be forbidden, because it confuses things.
Regarding the names of alpha, beta etc.: Most combinations of metals in the liquid state are completely mixed (a well-known exception is lead and iron). In the solid state this is normally not the case. Most metals are to a certain percentage soludable into eachother, but also depending on the temperature. Normally the alloy of the base material with the main constituent in solid solution is called the alpha-phase. With increasing alloy-content other phases may be formed. In the case of copper and zinc the first one is the beta-phase, which consists of approximately 50 at% copper and 50 at% zinc, but during solidification it starts to form at 32,5 % of zinc and 900 C and exists next to the alpha-phase. Above appr. 39 % of zinc its just the beta-phase. Its X-tal structure differs from that of the alpha phase. With decreasing temperature the solubility of zinc in the copper increases to appr. 39 % at 450 C, while the mixed alpha-beta structure exists from 39 to 45.5 % During hot-working of the metal the beta-fase will disove itself, helped by the deformation. In case of a cast structure a heat-treatment is necessary to obtain a homogeneous structure. N.B. With more than 39% of zinc there will always be beta-phase present. The two phases do not always have the same potential and this can cause corrosion.
As regarding the color, it seems evident that with increasing zinc-content the red copper-color changes to yellow. The change of color does not occur at a specific percentage, but it changes gradually.

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[boatmike]

Not that "erudite" really Mac. Just a lot of years of doubtful experience tinged with significant memory loss (on my count anyway!), Ships cat is younger than me and still running around trying to make a living out of building boats (poor sod) where I have retired, so his brain is more awake than mine sometimes.
What you have drawn attention to regarding Tonvil (sorry Tonval) is what I suspected. Everyone thinks it is great stuff because it is a different colour and the trade has presented it as "Kosher". I am reminded of the old story of the Emporors New Clothes...... I think I will start a new thread on this and try to promote a reaction from the trade or elsewhere.
Ships cat is also correct regarding sourceing. Caveat Emptor... Buyer beware.....
Just because it says something on the package does not make it so. I have spent some of my time involved in the building of warships etc and generally the defence industry or in fact a classification society, where there is any doubt will require tracability of the material and mill certification of the material used. This is of course available to commercial builders too but it's expensive so just not often done unless LLoyds DNV or whoever demand it. I have actually asked my friendly local inspector for one of these class societies (a drinking buddy) what Tonvil/Tonval is. His "erudite" answer was "****ed if I know"! His only interest would be to see a material cert which his head office would either accept or not so ?????? Fit gunmetal bronze and be sure.... but how to be sure it is in fact GM? Difficult.....

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[Ships_Cat]

Thanks very much for that Wagenaar, clarifies alot on the phase matter - I had tried to find something on the internet but hit a dead end.

And Mike, I am rapidly catching up quick in the age stakes so it has been known for me to have already practised the alzhiemer's bit too /forums/images/icons/frown.gif. The Tonval thread is interesting, but as I have said I have not heard of it out here - maybe it is a UK name, or else I just haven't come across it 'cos bronze/brass is not common on aluminium vessels which most bigger ones are these days.

Regards

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[boatmike]

"Most bigger ones are these days"
Only because of the cost of mould tools..... All that welding.... nasty stress cracks.... corrosion.....electrolysis...... YUK!!
Still it keeps you out of mischief down under I suppose.... Now it's back to me bucket and roller....
Thought for the day.... If organic composite material was not that good God would not have used it for trees....
New Zealand leg of lamb for dinner tomorrow. Yummy.......


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