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-   -   Corroded Shackle (http://www.sailnet.com/forums/gear-maintenance/50210-corroded-shackle.html)

dhornsey 01-04-2009 05:27 PM

Corroded Shackle
 
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.

http://lh5.ggpht.com/_0-Jj9jQnB6A/SW...boat%20007.JPG

fullkeel7 01-04-2009 06:34 PM

WOW! Good catch. Maybe some of that counterfeit stainless? At any rate, looks like you dodged a real bullet there d.

GaryHLucas 01-04-2009 06:59 PM

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!

wchevron 01-04-2009 07:05 PM

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.

fullkeel7 01-04-2009 08:13 PM

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.

sailaway21 01-04-2009 08:16 PM

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.

merc2dogs 01-04-2009 08:42 PM

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

dhornsey 01-04-2009 10:32 PM

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.

AllThumbs 01-05-2009 09:14 AM

Quote:

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

tommays 01-05-2009 10:22 AM

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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