Shackle pins

[Rum_Pirate]

OK, there is a Guardian anchor in the above pic with a stainless steel chain and shackles.

If you look closely you can just make out (trust me they are there) two stainless steel shackles at the connection to the rope and stainless steel thimble.

One shackle had the pin wear for the most part then corrode away completely in the middle.

Why would the pin corrode and not the bow shackle?

 

[Lady Campanula]

Vyv Cox is yer man for this.

I understand that the protective oxide film which forms on the surface of Austenitic stainless steels in the presence of oxygen - or oxygenated water - is damaged by the threads of pins being screwed into their threaded counterparts in the body of the shackle.

Corrosion commences and develops there much faster than where the protective film is undisturbed.....

That's why shackle pins should NOT be wrenched up mechanically as hard as a long lever can manage.

 

[vyv_cox]

The most likely cause seems to be that the shackle and pin are both stainless steel but not the same grade, or possibly the thimble was different (you didn't say which of the shackles was affected.

On the failure diagnosis course that I present we have a very nice workshop sample. It's a piece of stainless steel mesh used in seawater service. All the warps have fragmented whereas the wefts are intact. The solution is that the warps were 304 whereas the wefts were 316.

In your case I'm surprised that the shackles spent sufficient time in seawater for this to happen, but maybe they were constantly immersed somewhere?

A photo would be nice.

 

[Rum_Pirate]

The most likely cause seems to be that the shackle and pin are both stainless steel but not the same grade, or possibly the thimble was different (you didn't say which of the shackles was affected.

On the failure diagnosis course that I present we have a very nice workshop sample. It's a piece of stainless steel mesh used in seawater service. All the warps have fragmented whereas the wefts are intact. The solution is that the warps were 304 whereas the wefts were 316.

In your case I'm surprised that the shackles spent sufficient time in seawater for this to happen, but maybe they were constantly immersed somewhere?

A photo would be nice.

I tend to use the anchor to moor.

I will get some pics.

Before anyone says anything, I moor a 22'0" Mako centre console with a T-top using two Guardian (Fortress) anchors and also tie on to my 2'0" dia mushroom anchor (1/2"chain with a 1" nylon rope) in salt water.

 

[westernman]

Why when you only want the pin do you have to buy a whole shackle?

I only ever drop the pins overboard, never the shackle itself.

 

[Rum_Pirate]

Why when you only want the pin do you have to buy a whole shackle?

I only ever drop the pins overboard, never the shackle itself.

You too?

 

[Bajansailor]

Doug, I think you need to think composite, and avoid all these corrosion issues with shackles, different metals et al.
How about getting one of these very funky psychedelic pink plastic Cooper anchors for your boat?
http://www.cooperanchors.com.au/

You could use 'soft' dyneema shackles (http://www.colligomarine.com/vmchk/Colligo-Synthetic-Systems/Softies.htm) instead of stainless to join the galvanised (or S/S?) chain to the plastic anchor