A fun project that just came across my desk

[dLj]

A friend of mine has an old converted Chesapeake work boat - it's really lovely. Old wooden hull really nicely redone for pleasure cruising. Wish I had photos.

He recently gave me a shackle that broke in his steering system. The steering system is run through two cables that attach to the rudder through two shackles. These shackles actually hang just below the water line where they attach to the rudder. So they are actually submerged. He wasn't sure how long they have been on th boat, he bought the boat just a year or two ago so it could have been changed shortly before he bought the boat or they could be original to the boat, making them somewhere in the 50 year old range.

Of course the question is - what should he use as a replacement. Have fun...

I'm attaching some images.

 

[Ralph Johnstone]

Do we presume the original shackle to be SS ?

Interesting differences in corrosion forms between the two fractures. And that's further presuming that neither fracture face has been cleaned up.

 

[dLj]

Do we presume the original shackle to be SS ?

Interesting differences in corrosion forms between the two fractures. And that's further presuming that neither fracture face has been cleaned up.

Indeed, the assumption of the boat owner was that the original shackle was SS.

It was looking at those two fracture surfaces that made me think something really wacko is happening here. Want the answer?

dj

 

[shemandr]

Not yet! What fun would that be?
Being underwater most of the time, and presumably deprived of oxygen, I would consider the deterioration of the oxide layer. There are other areas of pitted corrosion. There could be galvanic deterioration of the SS too. But dLj showed us the threads for a reason. They look perfect to me. What about cavitation?
I'm not expert in metals but I have casted metal and sometimes porosity happens. In my field we discarded those. I doubt a porous casting would make it to market. It's pretty obvious because you can't polish it.

 

[dLj]

Not yet! What fun would that be?
Being underwater most of the time, and presumably deprived of oxygen, I would consider the deterioration of the oxide layer. There are other areas of pitted corrosion. There could be galvanic deterioration of the SS too. But dLj showed us the threads for a reason. They look perfect to me. What about cavitation?
I'm not expert in metals but I have casted metal and sometimes porosity happens. In my field we discarded those. I doubt a porous casting would make it to market. It's pretty obvious because you can't polish it.

I love the attitude! I'm current away from my computer - it will be a few hours before I'm back at my computer.

Cavitation is an interesting one - it's not, but I'll see if I can get images of cavitation to show you. Cavitation looks kind of like the surface of the moon with roundish depressions, like fields of them. More later.

I did indeed show those threads for a reason - also the flange of the other side showing nothing. Also all the pitting along the top of the shackle where that threaded side of the pin was located.

dj

 

[Captain Larry-DH]

Crevice corrosion or electrolyses. I‘ve seen pitting like that on the flat surface of the stainless disk in my PSS shaft seal. It was continuously submerged and mated against a flat carbon disk. It took several years to develop, and the maker suggested I flip the disk to extend its life.

 

[dLj]

Crevice corrosion or electrolyses. I‘ve seen pitting like that on the flat surface of the stainless disk in my PSS shaft seal. It was continuously submerged and mated against a flat carbon disk. It took several years to develop, and the maker suggested I flip the disk to extend its life.

Crevice corrosion it's not. Electrolysis is a misnomer for several corrosion processes. So I'm not sure exactly what you are referring to. It's often used to mean stray current corrosion. It's not stray current corrosion.

dj

 

[dlochner]

Are you certain that is it SS? What's the weight of the shackle compared to known SS shackles? Clearly the pin and the shackle body are not the same metal or the corrosion would be similar on both parts. Perhaps the shackle is chrome plated mystery metal. Any way to get it tested to see what it is composed of?

How was the loop on the steering cable made? A proper splice or compression sleeves of copper? The copper near the mystery metal could cause galvanic corrosion.

 

[dLj]

Are you certain that is it SS? What's the weight of the shackle compared to known SS shackles? Clearly the pin and the shackle body are not the same metal or the corrosion would be similar on both parts. Perhaps the shackle is chrome plated mystery metal. Any way to get it tested to see what it is composed of?

How was the loop on the steering cable made? A proper splice or compression sleeves of copper? The copper near the mystery metal could cause galvanic corrosion.

Exactly - when I was handed the part I had no indications that the shackle and pin were not the same material. Visually they looked the same. When I got home and started looking at the details it was making no sense at all. The corrosion patterns were wrong, the fracture surfaces did not look anything like stainless. In fact they aren't fracture surfaces - they are corrosion planes. So I looked around to see if I could find a metal recycling yard with a handheld XRF gun to tell me what the pin and the shackle were made from. The owner was convinced it was a 316 shackle.

It turns out, the shackle is an aluminum alloy and the pin is likely 304 stainless. So the pin is being protected by the shackle body, hence it is practically pristine and you see that corrosion pattern all around the top of the shackle. The break surfaces are all contaminated with residual debris from the water due to being simply active corrosion sites. It's Chesapeake water.

I was so stunned when the guy told me aluminum, when I got home I checked the density of the shackle and pin and sure enough. The shackle is aluminum! I have no idea where that shackle was purchased. But what an eye opener! I've never seen aluminum look so much like stainless steel. Even after knowing this, hold the piece and looking at it, it looks like SS... I can usually spot aluminum from far away...

dj

 

[High Current]

Are you certain that is it SS? What's the weight of the shackle compared to known SS shackles? Clearly the pin and the shackle body are not the same metal or the corrosion would be similar on both parts. Perhaps the shackle is chrome plated mystery metal. Any way to get it tested to see what it is composed of?

How was the loop on the steering cable made? A proper splice or compression sleeves of copper? The copper near the mystery metal could cause galvanic corrosion.

I noticed that too: the pin looks pristine which makes the other part suspect. The pitting pattern reminds me of a raw casting. I would guess a defect in the casting process such that the alloy was not pure/homogenous: some bits lacked proper chromium and therefore corroded away. Or maybe weren’t even steel to begin with (sand from the mold?). Though I don’t know enough about metallurgy to know if either is actually plausible…

 

[High Current]

Aaand never mind I see you posted the answer while I was typing.

Crazy!

 

[dLj]

Aaand never mind I see you posted the answer while I was typing.

Crazy!

Hahaha - No worries. By the way, the questions you had above are actually all possible. They are extremely rare in any modern casting process but are found frequently in ancient casting technologies. In fact some of the patterns found can help date artifacts. Archeometallurgy is a really interesting field.

dj