Stainless Steel Through hull fittings?

Norman_E

Norman_E

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After all the information on possible failure of brass through hulls, I have decided to replace all of mine at next lift out (which may not be until October 2012), even though I think that all of mine are actually bronze or decent DZR brass. The ball valves are more suspect and all will also be replaced.

I have found stainless steel through hull fittings in Marmaris, to go with stainless ball valves. As none of my through hulls are electrically connected to anything else they seem like a good idea, especially as you cannot readily tell if ones sold as "bronze" actually are bronze or DZR brass, or even common brass.

Are stainless through hulls actually a good idea? They should be mechanically stronger than the yellow metal ones, but will they corrode? Has anyone tried them?
 
You will have to wire them them to an anode system as stainless steel has the unique ability to corrode itself without being in contact with any other metal. Personally I'd stick with what you've got as you could be making matters far worse.
 
PCUK said:
You will have to wire them them to an anode system as stainless steel has the unique ability to corrode itself without being in contact with any other metal. Personally I'd stick with what you've got as you could be making matters far worse.
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But it is usually crevice corrosion which is easily avoided and anodes wont make the slightest bit of difference.

If you do go that route, make sure they are 316 and the valves and tails are all the same. Vetus list the full range - but be prepared for a really big bill - roughly twice the price of DZR and comparable with Bronze.
 
spent most of my working life in the steel industry including making stainless steel - and I would not use it for under water fittings. A much better choice would be the re-inforced plastic ones - Marelon. You would never need to worry again.
 
VicS said:
And do any of the links that Googling produces tell us that a sacrificial anode will prevent crevice corrosion?

Like Tranona I dont think it will.
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Correct, it doesn't. Nor pitting, which is effectively the same thing. Bonding of seacocks and other underwater fittings to anodes is very questionable anyway and should not be done without very good advice.

Stainless steel underwater, without crevices, is perfectly OK, e.g. propshafts. Problems may occur at the flange-to-hull connection and at the skin fitting-to-seacock connection, where crevices and, in the second case, galvanic corrosion, may be difficult to eliminate.

You can tell bronze from DZR and brass by colour. Compare with plumbing compression fittings, brass and much yellower in colour than bronze.
 
Bosun Higgs said:
spent most of my working life in the steel industry including making stainless steel - and I would not use it for under water fittings. A much better choice would be the re-inforced plastic ones - Marelon. You would never need to worry again.
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How strong are the Marelon ones? I have heard of the smaller sizes breaking off.
 
Very strong. Jump on them strong. But then how much hammer does a skin fitting have to take anyway?

A pal has them fitted to a 39 ft freedom schooner and like everything on that boat, they are real heavy duty. The actual cocks are a delight with nice big handles, easy movement and freedom from maintenance.

But make sure they are Marelon and not just the typical white plastic ones. Incidentally, unless you have an unusual boat you already have re-inforced plastic skin fittings for your log and depth sounder. I do. And on my previous boat, these went through grit blasting no problem.
 
Those who think that stainless steel seacocks and related fittings are no good in sea water may like to explain why they are almost universally used on modern high quality large sail and power boats and smaller aluminium boats with no issues at all.

And the poster who "authoratively" stated that they all have to be bonded if used is also talking absolute rubbish, I have no idea where these people get such ideas from. Obviously not from any sort of professional experience with them.

Regarding Marelon, as another poster has said it is almost indestructable. There is also a wide range of suitable fibre reinforced plastic ball valves used in industry that are more ruggedly constructed and designed for pressures a couple of hundred times the pressure faced by a seacock. Due to economies of scale these valves can be very cheap to purchase, and be very reliable and maintenance free (we used them for many years and never had to replace one, that included after 16 years of service on one boat with no maintenance of them whatsoever).
 
CelebrityScandel said:
Those who think that stainless steel seacocks and related fittings are no good in sea water may like to explain why they are almost universally used on modern high quality large sail and power boats and smaller aluminium boats with no issues at all.
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Perhaps because such vessels either have professional crew on board who check them regularly, or are so petrified of corrosion issues that they do likewise? :)

Are these fittings 316 or duplex?

Around 25 years ago many seawater valves and pumps on offshore platforms were changed to 316 stainless, at the time far cheaper than the bronze that was in use. It wasn't very long before corrosion problems began to occur, usually crevice and other anaerobic problems. Duplex was substituted but that turned out not to be the universal material that was hoped. Now bronze is back in favour again, despite the cost.
 
CelebrityScandel said:
Those who think that stainless steel seacocks and related fittings are no good in sea water may like to explain why they are almost universally used on modern high quality large sail and power boats and smaller aluminium boats with no issues at all.

And the poster who "authoratively" stated that they all have to be bonded if used is also talking absolute rubbish, I have no idea where these people get such ideas from. Obviously not from any sort of professional experience with them.

Regarding Marelon, as another poster has said it is almost indestructable. There is also a wide range of suitable fibre reinforced plastic ball valves used in industry that are more ruggedly constructed and designed for pressures a couple of hundred times the pressure faced by a seacock. Due to economies of scale these valves can be very cheap to purchase, and be very reliable and maintenance free (we used them for many years and never had to replace one, that included after 16 years of service on one boat with no maintenance of them whatsoever).
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Wouldnt like to use stainless with aluminium, corrodes together wonderfully and they are not almost universally used on high quality motor/sail boats.
 
Interesting replies. Stainless is certainly getting more popular here, but Marelon looks interesting. The only problem I can see with Marelon is the bulk of the seacock, which appears from the adverts to be about three times the diameter of the through hull. That would rule it out for some of mine, though a combination of Marelon through hull and stainless ball valve might work. I am concerned that the pipework can put a bending load on the through hull, which the big diameter valve screwed flush to the hull is designed to take, and which would be a risk with a standard valve.
 
vyv_cox said:
Perhaps because such vessels either have professional crew on board who check them regularly, or are so petrified of corrosion issues that they do likewise? :)

Are these fittings 316 or duplex?
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Generally one finds that the operators of bigger aluminium vessels whether pleasure or commercial are not as paranoid about corrosion as the small boat forums would have one believe one should be. Just a simple example; I have never known a larger aluminium vessel to paranoidically hang extra anodes on wires over the side or worry about berthing next to a steel vessel :). We certainly don't worry.

Of course, they also generally do not have electrical and protective systems built by or "improved" by (sic) amateurs :). Generally, corrosion issues to worry about are in the likes of the lowest points in plated in fuel tanks between bilge floors or in the undrained lowest point in the bilge behind watertight bulkheads where undrained water, often which has become acidic, collects.

I am not sure what grade is most commonly used Vyv but certainly 316 (or its cast equivalent) is widely used. Of course, on aluminium vessels such as our own the ss is electrically isolated from the hull (or piping in the case of strainers). Not realised by many is that many bronze ball type seacocks have 316 ss balls on plastic wipers in them (Groco, for example) and they too seem to be well regarded.

Another place where ss valves, usually butterfly, are used is on close to waterline wet exhaust discharges both for main engine(s) and generators for emergency shut off, say in the case of engine room fire, and one would assume that corrosion is a far greater issue there.

Personally, for aluminium vessels without going to a much more expensive material such as titanium I am not sure what material would galvanically be a better choice (putting aside the possibility of reinforced plastics) else we would have the bronze fanatics telling us we should use bronze fastenings on aluminium spars, for instance :).

I think a possible service condition that improves the performance of ss on larger vessels is that they are in use all the time (well mostly), so there is little chance of stagnation. That said I have a close relationship with a number of smaller (14-15m) aluminium and frp boats too that have ss valves; these are high quality custom builds but I have seen some production boats advertised with ss sea cocks. We used reinforced plastic on our own smaller boats and we ran all systems weekly whenever that was possible while the boat was laid up between cruises.
 
The most galvanically compatible material for water inlets on an aluminum hull is aluminum of course. Either pipe welded on and a valve inside or a standpipe with several pipes drawn off of it. Little sense bolting on when you can weld it. Same would go for most items of deck gear.

Marelon seacocks have to meet the same standards as bronze or any other material for ABYC approval. 500 lbs sideways force for a period of I believe 30 seconds with no damage.

Bonding of seacocks is not thought to be a good idea by experts nowadays. Made some sense before marinas and shorepower.

Crevice corrosion in stainless happens when there is moisture present in the absence of oxygen. This occure when chainplates are buried in the deck and a leak allows water in for example. Seacocks would be prone to this which is why they are generally not recommended, although common on some steel boats
 
CelebrityScandel said:
Of course, on aluminium vessels such as our own the ss is electrically isolated from the hull (or piping in the case of strainers). Not realised by many is that many bronze ball type seacocks have 316 ss balls on plastic wipers in them (Groco, for example) and they too seem to be well regarded.
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Electrical isolation is definitely the answer, but difficult to achieve in most cases. How is this done? And having achieved it with, for example, plastic inserts, then a good sealant is required to avoid crevices. My feeling is that ss underwater equipment is fine if the installation is carried out to a high standard. Unfortunately in the leisure yachting business this is often not the case. :)
 
mitiempo said:
Crevice corrosion in stainless happens when there is moisture present in the absence of oxygen. This occure when chainplates are buried in the deck and a leak allows water in for example.
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This is the confusion shared by many on the forums. The (relative) absence of oxygen is created by the tip of the crevice. There is no requirement for the general area to be low in oxygen. Crevice corrosion can happen anywhere that the water becomes stagnant, for example bolted joints in seawater pumps. There is dissolved oxygen throughout the environment of the pump, except in an extremely small local crevice.

The same phenomenon is responsible for the tarnishing of shroud wires, where minute crevices are created by drawing the wire through the dies. The oxygen in this case is plentiful and the wires are not even submerged in water.

Some more information at http://coxengineering.co.uk/Crevice.aspx
 
I'll give you an illustration of why I wont trust stainless steel for such fittings.

I once owned a Prout cat which had factory made 316 stainless steel bottom bearings between skegs and rudders, held onto the skegs with four 12mm 316 stainless steel bolts passing through the solid grp of the skeg from one side to the other. I should say that Prout were a very good boatbuilder who did not cut corners - for example fitting nothing but Blakes seacocks, and if what follows can happen to Prout, how much greater the risk with the others?

I was concerned about the wear/ corrosion that was occurring where the stainless shoe on the rudder pivoted on the stainless plate of the skeg fitting so I decided to remove the skeg fittings to have a better look. Of the 8 bolts I removed, only one of them came away as a complete bolt. All the other 7 had such bad corrosion mid bolt that they sheered rather than undid. The corrosion looked for all the world like worm holes. Mind you, the heads and the nuts were fine and shiny!

The rubbing bearing surfaces had suffered from the same sort of corrosion as the rubbing had removed the protective oxide coating and allowed corrosion to take place to a depth of maybe 2mm in places after 10 years from commissioning..

The culprit for the bolt corrosion was stagnant water round the bolt either because the sealant round the bolts had not been installed correctly by Prout or had been installed but had failed in use. But what was alarming was that this corrosion was invisible from the outside as indeed it would be with stainless skin fittings if sealant failure allowed water in between the flange and the hull.

Your hull is re-inforced plastic. Why not use re-inforced plastic for the skin fittings?
 
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