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Discussion Starter · #1 ·
We just replaced this 5/8" shackle that was connecting our bob-stay to the boat, right at the waterline. That pin was responsible for supporting such useful items as: the mast. I can't believe how corroded it turned out to be, it's pretty much hollow inside. There was very little indication of a problem from the outside - when swimming this summer we noticed a little pitting, so put it on the to-do list. Makes you really think about checking your rig.

 

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NOW you know what 'crevice corrosion' looks like. Stainless steel, in the presense of water, and not exposed to the air, plus mechanical galling (stainless against stainless is extremely bad) gets you really accelerated corrosion. This is why stainless is not used below the water line. It is also why stainless keel bolts with just a tiny leak around the keel can literally be gone before you know it!
 

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gary
i thought the whole reason to use stainless was it corroded less than steel. also, why would you get galling with two like metals.
 

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From what I've read, even high quality stainless (ie 316) needs oxygen to be effective. I suspect in dhornsey's example, a lower grade stainless was used. Be interesting to know if those peices are slightly magnetic.

Also that shackle pin is under a fair amount of tension and movement (although slight) through wave action on the bob stay and sail pressures through the forestay. Pin could have developed a slight crack and the water did the rest.
 

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I'm going to hazard a guess that that is not crevice corrosion. I'd think it much more likely to be due to galvanic action with the pin being the less noble metal.
 

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Galling is pretty similar to welding, and is often called cold welding. Two like metals will often gall because they have the same physical properties. It's a lot like rubbing two candles together, rub them together and they'll stick, take them apart and you have bits of each stuck in the other.
Aluminum to aluminum, steel to steel etc will all gall.
Iron to Iron, and bronze to bronze is normally fine though. (iron has a high ratio of carbon which acts as a lube so it doesn't gall under ordinary conditions)
Dissimilar metals won't gall, but then you introduce galvanic corrosion.

Galling is not a problem for parts securely fastened together, only when they are free to move against each other.

Ken.

Meant to quote wchevron
 

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Discussion Starter · #8 ·
There was definitely a lot of movement between the shackle pin and it's mounting plate, as we fasten our anchor line to that shackle at times. I have no idea how old the shackle is, but the plate that is is fastened to shows no signs of corrosion whatsoever. Also, being right at the waterline, they are both constantly dipped in salt water. Seems like the perfect environment for corrosion, whatever type it is. I'll certainly check these things more often.
 

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Galling is not a problem for parts securely fastened together, only when they are free to move against each other.
This is true but you need one more ingredient to assure galling: Pressure

Actually this phenomenon is put to good use in industry, where parts can be permanently welded by just rubbing them together under extreme pressure.

Eric
 

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Without knowing if it is 3 years old or 30 it is a tough call, At 30 it was just left unchecked to long at 3 there is something wrong :D
 

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tommays has it...even good stainless will eventually corrode in the situation this stuff was in.

do put a magnet to it. I'm suprised at how often I get a small amount of stick from a magnet on stainless.
 

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Are there any maker's stamps on the what is left? Or was it generic "Made in China" stainless?

There have been all sorts of improper alloys used over the years, including one lot of stainless from Mexico that was infamous because the foundry used scrap, and a batch of radioactive material got into their product. Which, IIRC, was "stainless steel" dinnerware. That was probably 25-30 years ago, but China is no better.
 

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Discussion Starter · #13 ·
I couldn't find a logo anywhere on it, so it was probably just generic. I'll try the magnet test when I get home today.
 

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Failing shackles

On another thread I previously had posted some pictures of a Lewmar shackle on my main sheet that failed suddenly, with no outward sign of corrosion. Lewmar rather quickly sent me replacements for all of the main sheet shackles and blocks, but I never got a satisfactory explanation of the problem. It looked to me from the failure that it was defective mettalurgy, like the oft-repeated poor quality Chinese forgings, but I thoguht Lewmar was supposed to be better. By the way, this shackle was on the boom, so it did not get much salt water, except spray now and then, and it in its third season.


 

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Discussion Starter · #15 ·
Magnetic Shackle

I just checked the failed shackle with a magnet, and it is extremely magnetic. By comparison, even the cheapest stainless hardware I have on board was only very slightly attracted to a strong magnet. So what does that imply? Do you think it was just made of very poor quality stainless?

Thanks.
 

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I just checked the failed shackle with a magnet, and it is extremely magnetic. By comparison, even the cheapest stainless hardware I have on board was only very slightly attracted to a strong magnet. So what does that imply? Do you think it was just made of very poor quality stainless?

Thanks.
Absolutely!

Crevice corrosion occurs due to a lack of oxygen, a situation your shackle did not experience. One also notes that the threaded portion of the pin shows much less degradation than the rest of the exposed pin. Were oxygen deficiency related to the corrosion you'd expect it to be most severe in the threads, and it's not.

I'd even hazard a guess that you'll find the bow of the shackle somewhat less magnetic than the pin. The bow and the pin are then dissimilar metals from one another and the pin was the less noble. The pin would also be less noble than the padeye on the hull. Regardless of which exactly was the cause, the pin acted like a sacrificial anode and deteriorated due to galvanic action. To the extent that it was once shiny, it was stainless. (g)
 

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"So what does that imply?" Let's see...No name, no brand stamp, extremely magnetic...Wait, that's a special alloy from that premier French marine outfitter, Count Erfeit.

I expect that some vendors turn a blind eye to some suppliers, who in turn are intentionally supplying junk as marine grade stainless.

As our Fearless Leader (and before him, Chairman Mao!) said, "Trust but verify". Especially if it is a "Jesus pin" holding your rigging up.
 

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Probably some variation on 400 series stainless, "Looks like stainless"!!!

Be glad you caught it when you did!
 

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Looks like bad stainless—probably not an austentitic 300-series grade anyways.

Be aware that stainless steel require oxygen for the chemical reaction that protects it to occur. Without sufficient oxygen it rusts like any other steel, just not as quickly—which is why it is STAINLESS not STAINPROOF steel...it stains less.

Marine grade stainless steels are non-magnetic...so check stuff when you buy it...

As for galling, it is very common on stainless steel-to-stainless steel contact situations. Using LanoCote, Tefgel or some other compound to prevent it is generally a really, really good idea.
 
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