Bronze versa 316 s/steel as through hull fittings and seacocks.

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Peter

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Got to change a few plastic through hull fittings and looking at the costs they are cost differences between bronze and 316 s/steel, favouring s/steel, especially when I can get 316 ball valves at cost. So what are the differences in using the 2 different materials, bronze or 316s/stel (not mixed) as below the water through hull fittings?

Peter
 
You can mix SS and real bronze. IE bronze with no zinc in it. But I wouldnt mix them, use all SS or all bronze, then you cant go wrong.
 
Stainless steel has the drawbacks that it can seize up "galling" and stainless is not immune to corrosion. Indeed, unless oxygen is present, stainless rusts like any other steel. Plastic, usually Nylon of one sort or another, is fine - I haven't heard of any problems. It is said that they would be a problem in a fire (!?) but then again maybe a bit of water is a good thing if the temperature rises to that extent? Bronze can de-zincify with electrolytic action but then again, you need to address that to protect your prop, P bracket, propshaft, etc.

My yacht, while GRP, is built fairly traditionally and when my skin fittings need replacement I will stick with bronze as a prospective buyer might be put off if I used plastic. Othewise I'd go for plastic.
 
David, real bronze doesnt have zinc in it, as I'm sure you know, like phosphor bronze et al. The "bronzes" that we see most of the time, in the chandlers, are brass, or DZR brass. ASAP supplies have real bronze fittings, which is where I obtain mine, not cheap though!!
A little smear of copper grease, will stop your stainless nuts, bolts, bottle screws and any other stainless threads from galling, I never assemble stainless without it.

Just to add to this, I will not under any circumstances use anything but real bronze below the waterline on my boat, the shafts, props, p brackets, rudder and shaft, seacocks and shaft logs are all bronze, as are the keel bands and screws.

Stainless is OKish above the waterline, but still seen crevis corrosion and failures in shackles and other fittings, I just dont trust it. I inspect my stainless fittings a lot!!!

I dont trust any kind of stainless and even monel has problems as we use it and stainless in the oilfield a lot and I have seen many failures, but none in bronze, which we also use, as well a berrylium copper.
 
Colin, yes, I agree. Thanks for the tip about copper grease. I'll get a tube; galled stainless can be a real headache.

One other negative against stainless has just come to mind...crevice corrosion. If the skin fitting is pulled hard down then it would be possible for a crack to develop unseen. A friend of mine who has been on a cruise since 1976 (sic) has a 'rogues gallery' of stainless parts that have failed in service due to crevice corrosion. He has collected parts from other yachties and yards. Fascinating to see what can happen. No stainless skin fittings in his collection, I should add but I cannot see why they should be immune.

David
 
would strongly advise against using stainless steel in this application. as is often pointed out it needs oxygen to maintain its protective oxide film and it cant get this in stagnant water

as an illustration of what can happen I removed the 316 bolts holding on the rudder bearings to the skegs on my Prout cat. the bolts were 12mm yet 3 of them came out in two pieces having corroded through with what looked almost like worm holes. not a single one of the other 9 bolts was re-useable.

I had a similar experience with the lifting outboard mechanism on my Hunter. the bolts had corroded away where they went through the grp. theory is that you can protect them by carefully bedding in sealant to avoid any partial water ingress but in real life sealants age, water gets part way in and sits there stagnant.

given the choice with a new boat I would go for the specialised plastic sea cocks such as Marelon. pal has them on his Freedom and after 20 years in use they are still perfect and as tough as old boots.
 
There are aplictations that are safe.Like prop shafts.It is very rare that they corrode.Except when enclosed in long shaft tubes where the seawater stagnates and oxigen levels are low,Screws in wet material will corrode because of absence of oxigen.Through hulls are probably ok as long as the threads are liberaly coated in sealant and water is kept away .
Not long ago I saw a seventies Swan with a stainless P bracket in perfect condition so as I say it can be used underwater,just not everywhere.
I certainly prefer a 316 through hull and seacock to a dodgy bronze alternative.
 
Whats "dodgy" bronze?

Have you seen a shaft made useless by corrosion under the prop, on the taper? Very common problem. I have changed quite a few.
 
I'm sure you have.I've been into boats my whole life and never saw a corroded shaft.Admitedly I saw a few pictures but that's all.How can you know the ones you changed were 316 and not something else?
Dodgy bronze is well,not bronze.There are trade names like manganese bronze for instance for what is only high tensile brass wich contains zinc.You can buy dezincifcation resistant "bronze" wich is a fancy brass with another metal added. can't remember wich (mercury?)to enhance it's corrosion resistance but it's still a brass.There was a report of a boat that sank because a fitting made of Tonval (think that's the name) dezincified and broke up.
 
Yep, dodgy bronze. I know what you mean, was just trying to find out if you knew! True bronze is an alloy of copper and tin, with some additions to make it phosphor bronze, aluminium bronze etc.
I could tell the SS was at least non-magnetic, so not 314, could have been monel, but monel doesnt corrode like that, as it has no iron in it, or very little.
Tonval is brass, I ave always thought that manufacturers should not be allowed to call any alloy that has zinc in it bronze.
By the way, proper manganese bronze, has no zinc in it either, cant remember where, but there is a list of the "bronzes" with what they have in them, I'll see if I can find it.

Found it, classicmarine website. Look for reference data.
 
Manganese bronze is not as far as I know a trade name but it is the common name for a copper zinc alloy (therefore technically a brass) also containing manganese. It is resistant to corrosion in saltwater and is a common material for propellers. Some suppliers now do call it manganese brass.

DZR brass is a high tensile brass containing lead but items made from it are also heat treated. That seems to be an essential part of the dezincification resistance.

Tonval is a leaded brass but is not resistant, it seems to dezincification. I think you are referring to the "Random Harvest" It did not actually sink but came close to it when a Tonval fitting failed. There were to be fair other factors contributing to this incident and it can all be found in the MAIB report on their website.
 
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I could tell the SS was at least non-magnetic, so not 314, could have been monel, but monel doesnt corrode like that, as it has no iron in it, or very little

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304 ( I assume the 314 was a typo) is non magnetic just like 316 unless cold formed sufficiently to have martensite in the structure. this is unlikely with a boat shaft which will have been annealed and then ground or lightly cold drawn and ground and will be fully austenitic.

the magnetic stainless steels are mainly the 400 series
 
We have recently arrived at our winter spot in Turkey and are pretty busy with complicated airline reservations and 101 other rush things. So, I am sorry, but you do not have my full attention.

We would never consider using stainless, even 316, for a thru hull. I am told that there are some very good plastic ones if you are allergic to bronze, but we prefer bronze.

Briefly, here is something I wrote to a friend considering a stainless fuel tank.

“Stainless” needs oxygen to stay stainless. Some kinds are much worse than others. Wet dirt or water below fuel or bedding compound, or paint, or tape, etc can cut off the air and cause ‘crevice corrosion’. This can possibly happen within weeks, but usually takes years. Let your sink stay dirty and it will corrode. Keep it clean and it will last for 50 years. A colleague saw a brand new shiny stainless bolt wrapped tightly with a rubber band that only covered a small percentage of the length of the bolt. It was placed in a solution similar to seawater and in a week the bolt had crevice corroded into two parts under the rubber band. The exposed parts still looked like new. There was no stress at all on the bolt. But, stress makes it much worse. I suspect that the demonstrator had secretly scratched the bolt just before putting on the rubber band, but still…

We had a similar real-world experience. See below.

If you really have a bee in your bonnet about using stainless, get the finished tank electropolished inside and out and mount it so that it stays dry with no trapped water. Electropolishing makes the chrome and nickel layer on the surface much thicker by removing the surface iron and makes the stainless passive. It makes an excellent or even mediocre stainless into what most people thought stainless should do to begin with. But, any scratch will remove the protective layer to expose active stainless. Active stainless and passive stainless are very different on the electro-chemical scale.

See http://www.classicmarine.co.uk/Articles/metals.htm

Add water, especially salt or chlorinated water, and you make a battery out of a single piece of metal. It won’t last long. As shown on the link above, 316 is much less susceptible than 304, but still has risk. A marine metallurgist once told me that the many good marine bronzes were about 800 times less likely to fail in this manner than the best stainless. Brasses are, in general, much less useful in a marine uses than bronzes although there are some good ones.

A much more detailed and authoritative website for copper alloys is

http://www.copper.org/

Have a look at the next link for a list of the USA numbers for the various copper alloys.

http://www.copper.org/alloypdf/alloyall.pdf

We built our boat 1974 to 1976 in Seattle, sailed to the South Pacific ’77 to ’79. We left again in 1981 and have been living aboard continuously ever since. We have had many stainless items fail due to restricted oxygen (crevice corrosion). Nyloc nuts, the self-locking nuts that have a piece of nylon built into the top to lock them, are particularly bad. They split in two. I can email photos.

Our stainless U-bolt used for our safety lines in the cockpit was removed after about 5 years to make it easier to paint the area. It was in 3 pieces. BOTH sides of the U, where it went through the wooden bulkhead had crevice corroded in two. Who knows how many years our safety lines were attached to the boat only by the sealant.

I must get back to chores. I can email photos is desired.

Dave & Janet
 
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Plastic, usually Nylon of one sort or another, is fine - I haven't heard of any problems.

[/ QUOTE ] Most 'plastics' (nylon included) are susceptible to U/V deterioration. I've seen some horrific cracks in four year old waterline fittings, and in an insurance study in USA some 20% of of dockside sinkings were found to be due to 'plastic' skin fittings failing. The same study pointed out that US build standards only recognised Marelon (a specific type of hard plastic) and bronze as suitable for skin fittings. (Sorry, I can't find the link at the moment).

With respect to bronze, it's very important to ensure that the fastening method doesn't rely on incompatible bolts (typically brass!) which then fail . . .
 
Dave, to save you time as you are busy, if you email the photos to me I'll size and host them on photobucket and insert them in a post here. - David
 
That's interesting, Jim. From a quick search it appears that Marelon is glass-reinforced DuPont Zytel. As you say, components made from Marelon have been UL listed. Presumably just using Marelon does not of itself confer a UL listing. You'd want to see the UL stamp on the packaging and buy from a reputable chandler.
 
Yep, make sure you use silicon bronze bolts and nuts! Very good point, some peeps forget, and watch blakes bolts, I know for certain, some are not bronze.

With blakes seacocks (and I have 8 of them) I opulkl a bolt everytime I slip te boat, a different one each time, if I find it "waisted" I replace all in that seacock, wit proper silicon bronze. But still pull one in each seacock everytime, easy to keep note in the log off which bolt as been pulled.
 
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