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**Jim Cate** · 15-06-2014, 15:23

Quote:

Originally Posted by S/V Alchemy

My pleasure. I own a steel sailboat and had to get better informed on this topic. Here's a brief primer on the different types and causes of corrosion in stainless: Article: Corrosion mechanisms in stainless steel and here's an old if illustrated document from the oil industry, who have a variety of metals in contact in electrolytic fluids: https://www.slb.com/~/media/Files/re...494/p04_18.pdf

Here's a pretty basic Boat US article, which demostrates that there's a number of places on a boat where stainless and other metals can quietly fail: http://www.cruisersforum.com/images/smilies/popcorn.gif Types of Marine Corrosion - BoatTECH - BoatUS

But the book I return to time and time again (if you REALLY want to learn and remediate) is Nigel Warren's Metal Corrosion in Boats. It sits on the same shelf aboard as do the works of Calder and Gerr.

This will get you well-grounded (pun intended) in the subject, which, for obvious reasons, I find compelling. But I've still never seen "as it happened" photos like the one Noelex's Mermaid got. That's amazing stuff. I hope this has been helpful.

Well, Alchemy, I had a look at those links, and frankly, I found nothing that supported your contention. I'll admit to not reading the whole oil industry treatise, for it was directed at a rather different set of conditions than we are addressing in this case.

But the one from the Stainless industrial source had the following to say about Bi-metallic or Galvanic corrosion:

"Bimetallic (galvanic) corrosion

...

Austenitic stainless steels usually form the cathode in a bimetallic couple and
therefore do not suffer corrosion. Contact between austenitic stainless steels and zinc or aluminium may result in some additional corrosion of the latter two metals. This is unlikely to be significant structurally, but the resulting white/grey powder may be deemed unsightly"

Thus, the galvanizing on the chain and/or the carbon steel chain would not have contributed to the cracking in the stainless swivel.

However, one likely candidate for the cause would be either stress corrosion cracking or crevice corrosion:

"Stress corrosion cracking (SCC)

The development of stress corrosion cracking (SCC) requires the simultaneous presence of tensile stresses and specific environmental factors. It is uncommon in normal building atmospheres. The stresses do not need to be very high in relation to the proof stress of the material and may be due to loading and/or residual effects from manufacturing processes such as welding or bending. Caution should be exercised when stainless steel members containing high residual stresses (e.g. due to cold working) are used in chloride rich environments (e.g. swimming pools enclosures, marine, offshore)."

This scenario seems applicable, especially if the swivel was forged, or if cast, not annealed after machining... ie, with residual stresses in the metal. Once again, no influence from dissimilar metals is noted.

So, Alchemy, perhaps I have missed something in the references, something that suggests that mixing stainless swivels/shackles and galvanized chain would cause such a failure. If you can point me towards a specific citation that supports the idea, I would be happy to read it and be better informed.

Cheers,

Jim

"

SVNeko · 15-06-2014, 16:06

My understanding is that the zinc on galvanized steel and stainless steel are dissimilar enough to cause galvanic action. However, the degree of corrosion is related to surface area and when the nobler piece is much smaller than the anodic piece, corrosion will be minimal, and vice versa. Thus, having a SS shackle in an otherwise galvanized ground tackle setup should not be a proble,. However, you would not want to use a galvanized shackle with a SS rode. Thats how I understand it. I have a very stout SS swivel connected to galvanized pieces and see zero evidence of corrosion after 3 years.

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GMac · 15-06-2014, 21:44

Quote:

Originally Posted by SVNeko

My understanding is that the zinc on galvanized steel and stainless steel are dissimilar enough to cause galvanic action. However, the degree of corrosion is related to surface area and when the nobler piece is much smaller than the anodic piece, corrosion will be minimal, and vice versa. Thus, having a SS shackle in an otherwise galvanized ground tackle setup should not be a proble,. However, you would not want to use a galvanized shackle with a SS rode. Thats how I understand it. I have a very stout SS swivel connected to galvanized pieces and see zero evidence of corrosion after 3 years.

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I'd tend to agree with you on that. I get told so many times if you mix SS and Galv in a anchoring rode it's all doom and gloom. In real life we haven't seen anythinbg to suggest that's correct. In a mooring system then Yes bit not an anchoring.

Sure that swivel didn't fail just because it was made for 5c in a eastern country? And I kid you not about the price, 1/2" ones cost us around US$0.50 each.... thats why we don't use them in anchor rodes.

**Jim Cate** · 15-06-2014, 23:39

Quote:

Originally Posted by SVNeko

My understanding is that the zinc on galvanized steel and stainless steel are dissimilar enough to cause galvanic action. However, the degree of corrosion is related to surface area and when the nobler piece is much smaller than the anodic piece, corrosion will be minimal, and vice versa. Thus, having a SS shackle in an otherwise galvanized ground tackle setup should not be a proble,. However, you would not want to use a galvanized shackle with a SS rode. Thats how I understand it. I have a very stout SS swivel connected to galvanized pieces and see zero evidence of corrosion after 3 years.

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The main thing is not the relative sizes, but that the stainless part is the cathode and the zinc is the anode... and it is the anode that is depleted by the galvanic couple. So, one could possible experience loss of galvo near the stainless portion, but the stainless would not be adversely affected.

Cheers,

Jim

SVNeko · 16-06-2014, 03:07

Yes, of course, zinc is the anode and the SS the cathode. So your point is that galvanic corrosion did not lead to the failure in the picture. I quite agree. I was merely responding to the age old issue of galvo and SS being dissimilar metals and hence should never be mixed in ground tackle.

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poiu · 16-06-2014, 04:31

This is a stress corrosion cracking failure. There is evidence of rust in the image. The crack would have been most of the way through before the anchor was dropped and evidenced by a rusting line. Crevice corrosion is unlikely to happen on this part as it is exposed to air/water most of the time. They should have changed the part earlier or bought one out of galv steel or some non corroding metal or a grade of stainless not susceptible to SCC such as duplex or ferritic.

Great pics.

darylat8750 · 16-06-2014, 06:20

Quote:

Originally Posted by poiu

This is a stress corrosion cracking failure. There is evidence of rust in the image. The crack would have been most of the way through before the anchor was dropped and evidenced by a rusting line. Crevice corrosion is unlikely to happen on this part as it is exposed to air/water most of the time. They should have changed the part earlier or bought one out of galv steel or some non corroding metal or a grade of stainless not susceptible to SCC such as duplex or ferritic.

Great pics.

This is my opinion also. When I first bought my boat I put a pretty SS swivel on my anchor chain. When considering building a boat out of metal I read several books about suitable metal and stress and crevice corrosion is a very issue. There is no stainless steel in my anchor system now. Also noticed the apparent lack of seizing wire on the shackles.

One of the interesting articals: http://www.kastenmarine.com/_pdf/mbqMetRef.pdf

I thought one of the forum rules was that when Mermaids take and post amazing photos that they include a selfie.

S/V Alchemy · 16-06-2014, 09:13

Quote:

Originally Posted by donradcliffe

I doubt the dissimilar metals had anything to do with the failure. I've used SS anchor shackles for decades with no corrosion problems. In addition, the failed part was hooked to SS shackles at both ends, so it didn't see dissimilar materials itself.

My understanding is that if they touch, the "circuit" is completed, and the failure can come at any pre-extant weak spot in that link or link.

You may have a boat with better isolation, or fewer stray millivolts in the water, or just more on top of eyeballing corrosion. There's a lot of variables that could make your setup last as long as all galvanized, and someone else's setup be a recipe for an early failure.

S/V Alchemy · 16-06-2014, 09:28

Quote:

Originally Posted by Jim Cate

So, Alchemy, perhaps I have missed something in the references, something that suggests that mixing stainless swivels/shackles and galvanized chain would cause such a failure. If you can point me towards a specific citation that supports the idea, I would be happy to read it and be better informed.

Jim, there's two potential problems: Firstly, the SS shackle is more noble than the zinc coating of the galvanized chain, and this can be eaten away with corrosion increase on the now-unprotection mild steel links:

Here's a reasonably rational thread on the topic: SailboatOwners.com - Stainless Shackles on Galvanized Anchor Chain

Secondly, there's embrittlement in a low-oxygen environment. Here's a PS review of the embrittlement due to the oxygen starvation to which stainless steel (chain in this case, although shackles and swivels are also mentioned) is prone: Mooring Chain Test - Inside Practical Sailor Blog Article

A linked PS article discusses crevice corrosion in deck-level shackles. I've had this sort of failure (I caught it just before using the shackle in question) even in fresh water. Mild steel deforms first unless it's already eaten away...that why on our club's mooring committee we have a V-slotted hand gauge to determine the percentage of metal wastage in the links before the mooring is "condemned" and replaced...

Stainless, especially if it is getting twisting side loads, just tends to part without a telltale "thinning out".

Anyway, those examples are easier and quicker to read than my first fruits of Googling.

hellosailor · 16-06-2014, 14:46

Gmac-
"Sure that swivel didn't fail just because it was made for 5c in a eastern country? "

You foreigners from outside the US might never have heard this, but along time ago, maybe 30-40 years ago, there was a big kerfluffle in the US National Nooze about radioactive stainless steel dinnerware being sold on the US market.

Apparently some clever scrapyard bought some hospital metal scrap, and some of that metal scrap was a nice stainless steel box containing some high radiation source for medical purposes.

And it all went into the smelter for recycling, to make generic "stainless steel" dinnerware.

Guys, you know, metal casting today is done no more carefully than rocket science, and the same way that NASA occasionally blows O-rings, confuses metric with inch, and manages to land less than 1/3 of all Mars probes on Mars (I know that's not NASA's fault, the Martian air defenses are among the finest in this solar system)...

Who's to say what is in an anonymous piece of metal from an anonymous vendor in an anonymous supply chain where everyone keeps trying to squeeze another nickel out?

How to make money while cruising: set up an x-ray and metallurgy lab across from your galley, and offer rigging inspections on the high seas?

sailorboy1 · 16-06-2014, 15:14

I say it failed because it cracked!!!!!!!!!!!! I believe this to be fact.

I say it became a problem because the boat owner didn't care and couldn't be bothered to inspect in who know how long, and didn't care even when told of the problem. I believe this to be a story to internet battle over.

BTW - I not planning on removing my stainless steel sirvel that connects my galvanized anchor to my galvanized chain. So be sure to stay the heck out of my anchorages.