Life of s/s anchor chain

A very good question. In fact probably one of the best posed on this forum for a long time. No simple answer I am afraid. SS definitely work hardens and is subject to fatigue failure. Micro-cracking of the work hardened surface can precipitate catastrophic failure without warning certainly in theory. Having said that, calculating the practical chance of failure in normal use on a small yacht is problematic, if not impossible. There are simply too many variables. Personally I never use stainless chain and wouldn't want to. Neither would I use stainless shackles or fittings in an anchor chain setup as I don't see any reason to take any risks at all. If galvanised chain shows any signs of corrosion it is cheap enough to bin it and replace with new. IMHO a much better plan than stainless...
 
There is a great deal of mythology about stainless steel, which is completely unjustified. Austenitic stainless steel grades very similar to those from which chain is made is specified for very many duties for which low fatigue life and unpredictable failure would be totally unacceptable, for example every component of an LNG train is made of it, compressors, flanges, pipework, vessels, etc., all operating at 50 bar and -160 C. Every compressor section in every jet engine in the world is made from ferritic stainless steel. All hydraulic tubing on wellheads, control valves, instrumentation on offshore platforms is made of austenitic stainless steel.

In fact, the fatigue limit of austenitic stainless steel is very similar to that of low carbon steel. It does work harden rather more but only when plastic deformation has been involved, a most unlikely scenario in an anchor chain.

The biggest problem with stainless chain is that some of it is made very badly. Poor weld current control creates crevices and porosity, which lead to premature corrosion failure. A visual inspection is usually sufficent to establish whether the chain is good or bad.

IMG_0015.jpg


This one is not very good. The weld is poorly shaped, probably porous, quite likely welded at too low a temperature, and IMHO to be avoided. Chain with a good, smooth weld without these defects will be perfectly OK and will give you the bonus that it stacks in your chain locker far better than the galvanised stuff.
 
Crevice corrosion is the problem. Have a look at this article.....

http://www.corrosionist.com/Pitting_Crevice_Corrosion.htm

Most stainless in inherently weaker than ordinary steel so you might need a bigger calibre. There is a grade of stainless that is as strong but it is many, many times more expensive than ordinary steel. You should talk to a specialist manufacturer if you are really interested but I would certainly consider buying it.
 
Vyv, I use Plastimo ss shackles as part of my anchoring setup. I remember in an earlier thread you recommended Wichard 17/4 shackles, the price of which is a bit scary. I have three options - 1. stay as I am, 2. invest in the 17/4 stuff and don't tell Mrs. Mick, or 3. buy the Wichard 316L shackles. Are all of these viable options?
 
Thank you for the link, it at least gives me some idea of the questions to ask!

So, pardon my ignorance, I have inspected our chain, we already have s/s, it came with the boat. The welds look good, most of the links still have a mirrored like surface(which appears to be a good thing). However there is a reddish discolouration on the length that usually remains in the locker, any suggestions what this could be, and is it a problem? It's not an obvious rust more a dull bloom on the chain itself.
No obvious pitting or faultlines. I understand its Lloyd's certified chain but have no documentation for it, apart from knowing that it cost a small fortune!

The chain is now approx 12 years old, we have end to ended it. We lie at anchor probably 2/3 or the time, last 4 years in the Caribbean(high water temperatures) which may cause a problem according to the article.

So, I am loathe to replace something that shows no obvious faults and yet.....I want to sleep easy at night!

Any more ideas?

www.gerryantics.blogspot.com
 
Use of stainless for LNG is because it tolerates the low temps. Normal steel (mild) would be brittle at such low temps. But it is NOT the only material used. There are other composite materials ...

I was on Shell Brunei LNG ... so my comments based on work.

The statements about stainless failing without warning are valid. Various frames, connections were subject to frequent replacement / repair due to the fact.
 
It's probably the chain rusting in an area where there is no Oxygen. That's the section you want to check carefully link by link every so often. Corrosion (rusting) doesn't have to mean 'crevice corrosion' -- stainless will rust like mild steel in salt water with no (or low) Oxygen which you might get right at the bottom of the anchor locker with a pile of chain on top.
 
No, it's probably superficial. Check it carefully in good light, link by link, and any crevice corrosion should hopefully be seen -- it sounds just like surface corrosion, and very slight at that. As Vic (or Vyv?) said earlier, pay special attention to the welds.
 
Lemain,

I think you are a little confused about crevice corrosion, as I know you have posted this before. We are all familiar with the results of crevice corrosion, as it is the cause of the brown stains that occur under nuts and washers, many stainless fittings, bottle-screws, etc. Unless left for a very long time permanently immersed in water, crevice corrosion is not a huge issue for us.

In well-made chain it simply is not a problem, as there is no crevice. At contact points between adjacent links there is, but this is very rarely a permanent situation, so a lttle brown staining is the worst that is likely. Only in the case of poorly made chain, where there is weld porosity and hence a crevice, is a failure likely to result. One such as this was shown in YM around a year ago but this was the only one i am aware of.

Stainless steel is not 'inherently weaker than ordinary steel'. It is not hardenable by heat treatment, but then neither is a carbon steel with carbon content below 0.23%, which would constitute most normal chain grades. Its UTS is comparable with that of mild steel, as is its fatigue limit.

When I destructively tested 'ordinary' and stainless grades of 8 mm chain for YM, the two galvanised ones failed at 4.93 and 4.4 tonnes, and the stainless steel one failed at 4.25 tonnes. All are well above the DIN 766 requirement. I only tested one pull on each, so these cannot be described as definitive results but they most definitely do not show a marked weakness of the stainless.

Vyv
 
[ QUOTE ]
We are all familiar with the results of crevice corrosion, as it is the cause of the brown stains that occur under nuts and washers,...

[/ QUOTE ]

That is not strictly true. Brown stains are the result of corrosion It need not be 'crevice corrosion' it can be surface corrosion. Crevice corrosion happens where the small size of the crevice allows water to wick deep into the crevice but without enough area for adequate oxygen to get into the water.

[ QUOTE ]
Unless left for a very long time permanently immersed in water, crevice corrosion is not a huge issue for us.

[/ QUOTE ]

Crevice corrosion is a massive issue in stainless parts. Crevice corrosion evolves into stress fracture and the parts give way without warning. Water remains inside joins and threads long after the outside appears to be dry. If you look at examples of failed components you will often find that the crevice corrosion has taken place in a thread, which suddenly gives way. Brown liquid coming out of a threaded part needs to be checked.

Whenever you see brown stains coming from stainless parts you know that corrosion is taking place. Where corrosion is taking place, there is a risk of crevice corrosion commencing. As I said in my post about the chain, I think that the brown stain is probably superficial but the links should be checked particularly at the welds, for any signs of crevice corrosion.

Failures due to crevice corrosion tend to be in places where water can collect and not dry for long periods. Through-bulkhead or hull fixings, tender lifting eyes, etc. The bottom of a chain locker can also spend much of its time wet which is probably why the last section on that chain is showing signs of corrosion.

As for the strength of stainless, when I said that ss is inherently weaker than ordinary steel, I meant 316 stainless which is about 1/3 of the strength of ordinary steel. If you want a really strong stainless for marine use you would choose something like 2205 stainless and that would then be as strong as ordinary steel. 2205 is VERY expensive. Not many leisure and cruising yachties would wish to afford 2205 stainless.

A friend of mine has a little museum of failed stainless parts that he has been collecting since 1976. It really is most interesting and scary. He posted on here a few months ago and he was going to let me have some pics to upload. I'll see what happened to that.
 
The thread is concerned with the life of stainless steel chain. The chains I tested were 316 and galvanised carbon steel, proving your statement on relative strengths to be incorrect. Crevice corrosion does not occur in well-made chain. Surface corrosion on undisturbed chain in the bottom of a locker is tarnishing and is in no way to be considered as threatening the life of the chain.

So far as the rest of your post is concerned, see my PM.
 
Whoa!

probably more info than my brain can absorb, or my intellect comprehend!

So....I am thinking from this thread that unless I am seeing significant 'pitting' on the actual weld that I should not be too worried about discolouration in a small part of the chain?

I understand that we have a German manufactured s/s chain that is approved by Lloyds for anchoring purposes, and that the previous owners wife said she could have had a significant diamond necklace for what it cost!!!

Any idea who the manufacturer could have been or am I asking too much??

www.gerryantics.blogspot.com
 
There is one advantage of stainless chain - because of reduced friction, it does not "cone" in the anchor locker in the same way as galvanized chain - useful when you have a small locker and a lot of chain. If I could afford it I would have stainless chain. "Generally" perhaps the people who argue against stainless chain are similar to those who argue against the new generation of anchors - Wasi, Rocna, Manson etc.
 
Yes, that's about right. Look at some examples of galvanised chain for examples, as this is easier to manufacture. The weld will be smooth, either not raised above the level of the wire or only very slightly. Good stainless steel chain should look as good, I have seen numerous examples.

This photo is a scan of an example given in YM in March 2008
FailedSSchain.jpg

Note the poor weld in the lowest picture, there is clearly a cavity there that seawater may penetrate. The top picture seems to show a cavity in the wire itself, very obviously not a good thing to have in a chain. Anything that looks like these is good reason to reject the chain.

Can't help with the German chain, sorry. Stainless is generally a good deal more expensive than galvanised, but not diamond necklace value in my experience.
 
Please see my PM to you.

Tarnishing is corrosion. As I told the OP, that is not of itself any reason to reject the chain. The pictures you have posted to show points on welds where crevice corrosion is a possibility are very useful. As I said to the OP, he should inspect the chain link by link every so often if there is any sign of corrosion (or 'tarnishing', if you prefer). If in doubt, get out the magnifying glass and look in good light.

Inspecting a stainless chain is very easy because it is not hard on the hands and flows freely, unlike galvanised.
 
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