Corroding galvanized shackle - anchor

[skunther]

I’ve gone through 2 shackles in as many years. Something seems wrong as I see plenty of other anchor rode without corroding shackles. Anyone have any input on this? I’d just like to get a strong shackle I can trust that won’t corrode and need such regular replacement.

I have G4 chain, a galvanized Bruce, and the shackle in question is a Crosby 3/8" G-209A Alloy Screw Pin Anchor Shackle - 2 Ton WLL - #1017450

I got this response from Crosby regarding the shackle:
“The Anchor is most likely a carbon steel casting. The chain is most likely a carbon steel chain. The Shackle is alloy. These dissimilar metals will cause a galvanic action between the parts. The shackle has a Zinc coating. This Zinc actually becomes the sacrificial anode in this system. The galvanization for a boat anchor is a different chemical mixture than the galvanization of a shackle or chain.”

 

[JoeWhite]

There are stainless steel shackle you can try.

 

[Ralph Johnstone]

There's got to be a reason why the shackle is the only thing corroding and corroding so quickly. Look closely and you'll see :

What makes the galvanic corrosion so bad is that surface area of the SS side wall (cathode) is large while the surface area of the shackle (anode) is small. A recipe for fast corrosion.

A SS shackle would be a good solution because you would then have a small surface area for the shackle (cathode), no interference from the SS side wall and the anchor (anode) has a large surface area . The chain (anode) I'm not too sure of. If it corrodes over many links (anodes) you're safe. If the chain only includes one link (anode) then you may have accelerated corrosion on the one link in contact with the SS shackle.

OR

Convert all your tackle to SS.

OR

Install a 1/4" piece of Starboard on the sidewall where the shackle currently contacts the sidewall. Maybe that would be the easiest. Nicely bevelled forward edge to avoid getting damaged by the shackle as it passes by. Maybe bevel all around with a 45° router bit for a real professional look. You could make a real masterpiece out of it.

 

[jviss]

@Ralph Johnstone that's a really good catch and fascinating analysis. Some way of insulation the shackle from that stainless would do the trick, if you are right.

I am not a fan of stainless ground tackle as it won't stretch under excessive loads as carbon steel will, it will break. And, if also is subject to crevice corrosion, which weakens it. But is sure does look good!

 

[skunther]

Very interesting @Ralph Johnstone! I would never have thought of or caught that. I will have to confirm that is the (only) point of contact and isolate the shackle if it is touching.

 

[NYSail]

Or simply get a pool noodle, cut a 6 inch peice, slice it open and wrap it on this section when you pull the anchor in. Isolation.

greg

 

[Ralph Johnstone]

Or simply get a pool noodle,

But where's the craftsmanship, the artistry, the flair in applying something like that ?

I could waste at least a day or more cutting and installing something like that. And that's not even mentioning the cut fingers, the dropped drill bits into the water along with a few other expensive tools.

 

[thinwater]

... I am not a fan of stainless ground tackle as it won't stretch under excessive loads as carbon steel will, it will break. And, if also is subject to crevice corrosion, which weakens it. But is sure does look good!
[/QUOTE]

Interesting.

316 SS has 35-40% elongation to break. Carbon steel used in grade 30 chain is about 30% elongation to break. Take a piece of 316 bar and flex it back and forth, then do the same with carbon steel. You will learn that 316 is reasonably tough. That's not the problem. If it was, we could not use SS standing rigging, which is highly loaded. This is obvious, right?

But I agree with you re. corrosion and cracking. That's the real story. The problem with 316 is not the sudden failure, it is that people will continue to use it for many years, beyond it's fatigue life, because it does not get rusty and show its age. If people retired SS on the same [U]time basis[/U] as galv. we would not hear of this. I understand why they don't. It's a lot of money to replace an SS rode every 5 years or so.



[U][/U]

 

[jviss]

@thinwater perhaps my knowledge is based on folklore. But, I've seen sudden stainless steel failures due to crevice corrosion, something that I don't think occurs with tough carbon steel.

 

[dLj]

It is an interesting observation by @Ralph Johnstone but I don't think that's the problem. I looked up your shackle and some sources say it's hot dipped galvanized, but the appearance does not look that way to me. It looks more like a mechanical galvanize, a more modern zinc deposition process used instead of hot-dipping (that the marketing people like to call equivalent to hot dipped - it's not). Your chain and anchor do look like hot dipped galvanized from the little I can see in your pictures. It would also make sense that Crosby would use the mechanical galvanize method as they are using high strength alloy for the shackle - hot dipping oten lowers the tensile strength of these alloys.

Your shackle in your photo does not look bad to me. It's just got surface rust on it. If it were me, I'd simply clean it up and spray it with cold galvanizing spray paint, as many layers as I had the patience to put on, and go sail. If that rust bothered me, I'd swap out to a stainless shackle.

dj

 

[dLj]

In your photo, note the more visible rust at the contact points at the chain and anchor. That is actually indicative not of galvanic corrosion as suggested to you, but rather mechanical abrasion, removing the galvanized layer in these locations. If the corrosion was occurring due to the contact with the stainless guide, the majority of the corrosion would be on that side. This is additional indications to me that the shackle is not hot dipped galvanized and the galvanizing in this region is simply being abraded away through contact with the hot dipped galvanizing of the chain and anchor.

dj

 

[dLj]

@thinwater perhaps my knowledge is based on folklore. But, I've seen sudden stainless steel failures due to crevice corrosion, something that I don't think occurs with tough carbon steel.

You are mixing up two different things. In your original post, you said stainless was not tough and steels were. Then you mixed in crevice corrosion. Those are two different things.

Stainless steel, as @thinwater pointed out, is quite tough. Often more so than a number of the steels. But nothing really of notable difference in a general sense.

Crevice corrosion is a degradation mechanism that austenitic stainless steels are susceptible to - as are some of the carbon steel alloys - we just don't typically see those alloys on our boats.

The "sudden" failures of stainless components are really not sudden. The corrosion process will have been going on a long time but no one noticed (or even looked)... Then, one day, enough degradation has occurred and the component breaks, seemingly "suddenly"...

dj

 

[jviss]

Crevice corrosion is a degradation mechanism that austenitic stainless steels are susceptible to - as are some of the carbon steel alloys - we just don't typically see those alloys on our boats.

I don't believe that's the case. From Swagelock's website:

"
Crevice Corrosion

Similar to pitting corrosion, crevice corrosion is initiated with the breakdown of stainless steel’s protective oxide film and continues with the formation of shallow pits. However, rather than occurring in plain sight, crevice corrosion—as its name implies—occurs in crevices."

Pitting Corrosion vs. Crevice Corrosion

Crevice corrosion certainly is a problem with stainless alloys used on boats.

 

[dlochner]

In your photo, note the more visible rust at the contact points at the chain and anchor. That is actually indicative not of galvanic corrosion as suggested to you, but rather mechanical abrasion, removing the galvanized layer in these locations. If the corrosion was occurring due to the contact with the stainless guide, the majority of the corrosion would be on that side. This is additional indications to me that the shackle is not hot dipped galvanized and the galvanizing in this region is simply being abraded away through contact with the hot dipped galvanizing of the chain and anchor.

dj

View attachment 214896

Interesting observation. I have a similar wear issue on my anchor. When raising or lowering a short section of the shank often scrapes against the bottom of the furling drum, which has worn away the galvanizing. The plan is to clean it this spring and spray some galvanizing paint on it.

 

[skunther]

It is an interesting observation by @Ralph Johnstone but I don't think that's the problem. I looked up your shackle and some sources say it's hot dipped galvanized, but the appearance does not look that way to me. It looks more like a mechanical galvanize, a more modern zinc deposition process used instead of hot-dipping (that the marketing people like to call equivalent to hot dipped - it's not). Your chain and anchor do look like hot dipped galvanized from the little I can see in your pictures. It would also make sense that Crosby would use the mechanical galvanize method as they are using high strength alloy for the shackle - hot dipping oten lowers the tensile strength of these alloys.

Your shackle in your photo does not look bad to me. It's just got surface rust on it. If it were me, I'd simply clean it up and spray it with cold galvanizing spray paint, as many layers as I had the patience to put on, and go sail. If that rust bothered me, I'd swap out to a stainless shackle.

dj

If the shackle I got wasn’t actually hot dipped as advertised (Crosby 3/4" G-209A Alloy Screw Pin Anchor Shackle - 7 Ton WLL - #1017538), then what shackle would you recommend? For <$20 I’d rather just have a shackle that doesn’t corrode than to mess with cleaning and painting it every year.

For reference, here’s what the previous shackle (same Crosby shackle before replacement) looked like after 2 mos in the weather (PNW) and 7 months in the roller

 

[jviss]

@dLj , also, found this:

"In Short What Is The Difference Between Stainless Steel and Carbon Steel?

• Stainless steel has a high chromium content which acts as a protective layer against corrosion and rust.
• Carbon steel is high in carbon that when exposed to moisture can corrode and rust quickly.
• Stainless steel is is more appealing to the eye and can be use for decorative products.
• Carbon Steel is stronger and more durable then stainless steel."

What’s the Difference Between Stainless Steel and Carbon Steel?

 

[danooxnard]

Looking at the pictures, I think dLj explanation is the correct one. It looks like surface rust to me. Dano

 

[dLj]

I don't believe that's the case. From Swagelock's website:

I'm not understanding what you are saying is "not the case"?

dj

 

[dLj]

If the shackle I got wasn’t actually hot dipped as advertised (Crosby 3/4" G-209A Alloy Screw Pin Anchor Shackle - 7 Ton WLL - #1017538), then what shackle would you recommend? For <$20 I’d rather just have a shackle that doesn’t corrode than to mess with cleaning and painting it every year.

For reference, here’s what the previous shackle (same Crosby shackle before replacement) looked like after 2 mos in the weather (PNW) and 7 months in the roller

I actually don't think anyone is making hot dipped galvanized shackles today. I'd suggest you go to a stainless steel shackle. Get a high strength one - not easy to find - but ask for 17-4 PH stainless steel shackle.

dj

 

[dLj]

@dLj , also, found this:

"In Short What Is The Difference Between Stainless Steel and Carbon Steel?

• Stainless steel has a high chromium content which acts as a protective layer against corrosion and rust.
• Carbon steel is high in carbon that when exposed to moisture can corrode and rust quickly.
• Stainless steel is is more appealing to the eye and can be use for decorative products.
• Carbon Steel is stronger and more durable then stainless steel."

What’s the Difference Between Stainless Steel and Carbon Steel?

OK, what are you trying to explain? So for starters, the third and fourth lines are at best misleading.

3rd - Stainless steel as well as carbon steel can be used for decoratie products, it depends upon the environment and if you want to use coating etc...

4th - Carbon steel is stronger and more durable that stainless steel is simply wrong. As a blanket statement it's frankly ridiculous.

There are five families of stainless steels, within each of these there are many alloys:

1) Austenitic
2) Martensitic
3) Ferritic
4) Duplex
5) Precipitation hardening

Of those five families, in the marine industry, we typically see the austenitics (304, 316 lots of others); the duplex (not so common but coming more into vogue) and the precipitation hardening stainless steels.

I'm not even going to get into how many types of carbon steels there are, literally hundreds...

So if you have something you'd like to clarify or ask about, let me know. I'm not following your posts.

dj