Galvanised chain : reply from Chich.hbr

[andyball]

There was a thread recently where I trotted out Chichester Harbour Conservancy's advice NOT to use galvanised chain.

As ponted out tehn, this goes against most advice, so I asked 'em why....reply follows.

"It is our experience that galvanised chain used underwater as part of a mooring system adds an element of uncertainty to the integrity and life expectancy of a mooring. It appears to us that as the galvanised layer starts to wear a galvanic reaction is set up between the steel and its galvanised coating, causing the links to pit and weaken. This tends to happen to a lesser extent with a self-coloured chain and the rate of wear and therefore its integrety is easier to predict. This phenomina is not displayed equally in all mooring situations; in some drying moorings in soft mud it may be minimal or non-existent, wheras on another deep-water mooring it may be quite marked.

It is often assumed that galvanised chain is better and our article in the harbour news aimed to redress this expectation in the light of our experience. We would accept that in many circumstances galvanised chain will perform adequately underwater, but as our advice covers the whole gambit of mooring scenario's we recommend a self-colour chain as a safer general bet; although of course the coating of the chain is only one aspect, the material it is made of is of more importance.

We have found that a mixture of self-colour chain and galvanised components i.e. shackles or chain, is particularly bad and the accelarated errosion can be very marked! "


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[vyv_cox]

Thanks a lot for going to the trouble of obtaining this. Unfortunately I would suggest that it is technically incorrect. If it were true, then zinc anodes would not work. Of course a galvanic action is set up between the steel and zinc. The result is that the more electropositive metal (the steel) is protected at the expense of the zinc, which corrodes. It doesn't matter that the zinc has holes in it or even that large areas of the steel are exposed, so long as both metals are in electrical contact and not too far apart when immersed in seawater.

I do agree that black, forged links have good corrosion resistance by comparison with bright steel ones. But the addition of zinc will always improve the situation.

On the last paragraph, to my knowledge almost every mooring on Menai Strait is constructed in this way, without markedly high corrosion rates.

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[Evadne]

I don't have the experience of laying my own mooring chains, but I have used various objects moored in deep water. i.e. totally immersed.
We always use greased, ungalvanised shackles and galvanised chains for deep water moorings and there is never any galvanic corrosion. This is true for "mixed" seawater and anoxic conditions, so it backs up the Chichester Harbour Conservancy's findings quite convincingly that ungalvanised steel is a safe option, although I personally am among those who are surprised that galvanising can be a liability on yacht moorings. It goes to show that these things are almost inherently unpredictable, so experience is the defining factor.

I'ts not been brought up, and any knowledgable readers can stop here, but for anyone who isn't familiar with the problem, stainless steel should never ever be used. It self-corrodes in anoxic conditions, even on the thread of a shackle in otherwise well-mixed water, with frightening speed. You might get away with it, but it's not worth the risk.
Dave


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[longjohnsilver]

All I know is that the galvanised chain I put down as the top half of the riser on my mooring 2 years ago is still going strong, whereas the previous self coloured chain I replaced after 1 year.

Think I'll stick with the galvanised!

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[vyv_cox]

Just for fun and to fill the last ten minutes before going home for our last long weekend until Xmas, I punched "Galvanized mooring chain" into Google. Found quite a few USA "standards" and yacht club rules on the subject, all insisting that chain, shackles and even anchors be galvanised.

All part of life's rich tapestry, I suppose.

One rule that made me blink, from a YC in New York "The weight of the anchor shall be ten times the length of the boat". Que?

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[Evadne]

<<ten times the length of the boat>>

.... would that be in hurricane country?

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Dave_White on 06/06/2003 16:09 (server time).</FONT></P>

 

[oldestgit]

What about the Chain.... I suppose 3/8th is out???

Then again what about the Windlass.

Then again what about the........... Yep its just our Yank'ie friends.......

Everything has to be bigger over there.... including the Waistlines !!!!

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[Mirelle]

Stainless

Curiously, I was advised that the local practice here (R Deben, Suffolk) is to use stainless swivels on the riser - with substantial anodes on the black iron chain, of course! Arguably, no part of the swivel is in anoxic conditions.

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[Swivelhead]

Can't elp but think that empirical always beats the boffins.
First Timer ready for the hits, but not always as hard as the rock.
Sincerely.
Swiv


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[philip_stevens]

This is one article that ought to be copied to the next (Sept or Oct - to late for Aug) issue of PBO.

<hr width=100% size=1>regards,
Philip

 

[Birdseye]

I suspect that the observations of the Harbour conservancy are correct but that the conclusion may be wrong.

As someone has already said, the zinc coating is sacrificial and is eroded at a rate that depends on the electrical connection and proximity of the bare steel - as well as a lot of other variables including leakage currents from the boat. Thus the lifespan of a galvanised chain, particularly with bare steel components, will vary considerably. However, I cannot imagine any circumstances in which it will be less than that of bare steel. How could it be? What is the technical mechanism involved?

One other issue, of course, is the standard of galvanising involved. Most shackles and chain are imported from places where the lack of environmental controls make galvanising cheap, and the coatings are not to BS1461. the mooring shackles i have been sold are particularly bad in this respect

Stainless is a bit different. If there is oxygen in the water (ie if fish live in it) then the oxide coating is renewed and stainless, particularly 316, has lasted well on my moorings (at the surface, not at the sea bed). However, if there is no oxygen, then corrosion can become horrific. I removed 8 10mm stainless bolts from the skegs of my cat where they had been bedded, badly, in sealant for 8 years, and found not one was re-useable, and two broke in half during removal. The corrosion was frightening.

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[classworks]

There is a plastic anchor on auction this week on ebay-- would it be suitable for use with plastic chain?

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[oldestgit]

What a futuristic world we now live in.....

50 years ago you would have said I was mad if I said you would be sailing in a plastic boat !!!.... (some say we still are)

And navigating from the stars without a sextant.......

Well..... Shiver me timbers

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[oldharry]

I used black chain for ground chains on my moorings in N Wales - they were covered when a Sand bank shifted. 8 years later the bank shifted back, and the chains re-appeared good as new. The galvanised riser had to be replaced every 3 years due to wear (mud and sand grinding paste!) so the long term life of the galvanising was never proved. But it kept the decks cleaner!

Now, my new mooring in Chi harbour has black chains lying in silt which seem to be as good as when they were laid ?3 years ago, with no significant corrosion. The galvanised shackle at the top of the riser had almost burned through due to having been seized with Stainless wire, and all the galvanised elements of the mooring were badly corroded, and needed replacement when I took the mooring over this season.

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[Birdseye]

contacted the technical dept of the galvanisers association - the trade organisation for hot dip galvanising in the uk. when i worked in engineering, they were always a well respected source of tech info.

to paraphrase their comments, they cannot see any situation in which a galvanised chain would not outlast an identical self coloured one in the same location. indeed, as a rule of thumb they quote the normal rate of erosion of the galv coating as 10 microns per annum for chain fully immersed in sea water, with a bs coating of the order of 85 micron thick. so that means that it is 5 years or so before it starts corroding in the same way as self coloured. at the surface ie splashed by sea water, the coating life would reduce by maybe half.

yes, ok - we know its not as simple as that since some bits will last longer, some less well, hit by boats etc etc. but we're generalising here just as chichester harbour have done. (it would be interesting if they would post the scientific study on which they have based their comments).

so after 5 years or so you get it re-galvanised. not that expensive actually.

self coloured chains will last very well buried in anaerobic mud - no oxygen for them to rust with.

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[VicS]

Its good to have expert opinion even if relayed but I think 5 years life for galvanising in Chichester is very optimistic. Based on my own experience 2 would be maximum. By 5 years I would say there is considerable redution in the thickness of the chain. Thats based on a muddy drying area and in any case constant wear between the links erodes the zinc very quickly, and the steel after that, locally. The more steel that is exposed the faster the remaining zinc will go.

I wonder if the chain we buy really does have 85 micron of zinc all over.

Remember a chain is only as strong as its weakest link and if you carefully inspect a length after a few years service, and I mean link by link, you are very likeky to get a shock.

My own opinion is that galvanising may extend the life of a chain by only 1 year

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