Toredo worm eats stainless steel at least thats what it looks like.

[William_H]

I was given somme "stainless steel" chain about 10 years ago for my swing mooring. It came out of a ceement works kiln. Links 18mm and about 10cm diameter round links. Becuae the buoy I ma obliged to use has stainless steel attachment and built in swivel I fitted 316 ss shackles and this chain down to an iron mass (railway wheel). This cahin worked beautifully for 10 years or more with no signs of wear.
Now the government who control the moorings have mandated a 2 yearly inspectionn of the moorings by a licenced inspector. So though I have cared for my mooring for 30 years I had to pay someone to check and report to the authority. 2 years ago the inspector reuired a new railway wheel mass. I thought the old mass was OK. Anyway inspection time again this different guy reported SS chain rotten. I had some more spare so he changed it. Now I have the rotten bit at home and indeed it is scary. Just one link the top which attached to aa 316ss shackle then the ss loop of the buoy/swivel.
The inside of the 18mm link has been hollowed out for about 90mm the openening and surface damage on the outside of the link. Just as if a worm had got in and eaten the SS. So the rubbing surface where wear occurs is perfect as is the 316 shackle it attached to. All the other links are perfect.
The only thing is that this top part of the chain always had a lot of growth on the bottom part being very shiny from contact with the bottom. It is a 2.5 to 3.5 metre deep mooring. Not much tide.
So leaves me wondering if this is a case of the SS that fails due to lack of oxygen? Or if there was some sort of inclusionj of foreign metal in the link. While I believe the metal is stainless steel is is probably more designed for heat resistance than 316 or similar.
Anyway I am going to stick with the present arrangement. It certainly lasts longer than GI. Especially where it attaches to the SS buoy.
Any metalurgy experts here? sorry gave up on uploading photo. How do you delete existing uploaded files in attachment manager?olewill

 

[vyv_cox]

I have seen stainless steel chain previously with holes in it, but in this case anchor chain so definitely not marine organism connected. There was a photo of it in YM quite a few years ago, the owner had sent a photo of him poking a length of wire into the hole. It had been supplied new with a HR that he had bought. I was not consulted (not that I would have known the cause) but Nigel Calder suggested it was due to poor manufacture, in particular low on alloying elements. I could not agree or disagree with that, it needs a proper investigation to be certain.

 

[PuffTheMagicDragon]

One possibility might be that the 'adhesive' that keeps barnacles firmly attached to a surface will also, very effectively, cut off any exposure to oxygen.
Just a personal thought that I cannot back with any proof, although I have seen severe corrosion on a SS carabiner clip right under where there was a rope tied tightly to it; the rest of the clip (exposed to sea water) was still shiny as if it were new.

 

[VicS]

sorry gave up on uploading photo. How do you delete existing uploaded files in attachment manager?olewill

AFAIK cant be done from attachment manager
therefore :
Go to your "setttings".
Scroll to the bottom of the L.hand menu
Select "attachments"
Tick the boxes on the R. hand side for the pictures you want to delete
Scroll down and hit the "delete selected" button.

But better to forget the attachment system and use Photobucket, or another photo-hosting site of your choice.

 

[RichardS]

AFAIK cant be done from attachment manager
therefore :
Go to your "setttings".
Scroll to the bottom of the L.hand menu
Select "attachments"
Tick the boxes on the R. hand side for the pictures you want to delete
Scroll down and hit the "delete selected" button.

But better to forget the attachment system and use Photobucket, or another photo-hosting site of your choice.

But you might find that your attachment allocation space is being taken up by photos which do not appear in the list provided by VicS instructions.

There appears to be nothing you can do about this. Hence VicS final suggestion!

Richard

 

[Iliade]

I'd look at crevice-crack corrosion. Also if there there is an impressed current somewhere, or a prior chemical attack, the oxide surface could have been compromised, allowing CC corrosion an entry point.

I have a feeling that it may be possible to 'passivate,' or whatever the correct term is*, the chain to reinvigorate the oxide** layer.

* And to think that in a dim and distant past I ran a materials lab...
** Stainless works by the formation of an atomically thin oxide layer. e.g., With certain stainless steels, spilt battery acid will terminally ;-) damage the surface structure and red rust will result.

 

[William_H]

Thanks VIc Viv Richard and SAPurdie. Yes it really did look like a cave inside the SS link. Funny all the rest of the links including those adjacent to the bad one so covered in growth are still perfect. I guess it will remain a mystery. olewill

 

[vyv_cox]

I'd look at crevice-crack corrosion. Also if there there is an impressed current somewhere, or a prior chemical attack, the oxide surface could have been compromised, allowing CC corrosion an entry point.

I have a feeling that it may be possible to 'passivate,' or whatever the correct term is*, the chain to reinvigorate the oxide** layer.

* And to think that in a dim and distant past I ran a materials lab...
** Stainless works by the formation of an atomically thin oxide layer. e.g., With certain stainless steels, spilt battery acid will terminally ;-) damage the surface structure and red rust will result.

Pitting in stainless steel occurs by exactly the same mechanism as crevice corrosion, see my website for more info. However, I have never seen pits that remain narrow enough to form a deep worm-hole, they usually widen into more of a crater.

 

[yachtorion]

The surveyor who checked my boat recently reckoned you shouldn't use stainless underwater, because of the unpredictable crevice corrosion issues I think. Certainly I've personally seen a year old stainless bolt succumb to it.

 

[vyv_cox]

The surveyor who checked my boat recently reckoned you shouldn't use stainless underwater, because of the unpredictable crevice corrosion issues I think. Certainly I've personally seen a year old stainless bolt succumb to it.

The problem is not the stainless steel, it's the crevices! The vast majority of us have stainless steel drive shafts, with which corrosion issues are rare. There are a few on my website. My Blakes seacocks and P-bracket are secured using stainless steel bolts and remain in perfect condition after 30 years. Provided the crevices are sealed there is no problem, although I would agree that exposed threads are difficult.

 

[KellysEye]

It does sound like crevice corrosion, I remember a boat anchored in Chaguaramas, Trinidad that had stainless chain which broke due to crevice corrosion. I don't think it's worth the risk of using it.