Re: ACMO rigging - unusual failure


Brent Cameron
 

What a wonderful forum. The expertise and experience here is astonishing. I do take issue with the comment “stainless steel doesn’t rust” although I suspect something may have got lost in the Franglish translation.  I suspect that over the years we’ve all left a knife or something not well built in a wet stainless steel sink for a few days and have come back to see rust stains on the sink as much as the knife. Luckily a bit of a polishing seems to resolve the issue and keep our marriages intact. 

Stainless Steel depends on OXYGEN not to rust. When it gets exposed to oxygen, the chromium and nickel form an oxide coating film that occur to cover up  even minor scratches preventing rusting. It’s that shiny oxide layer that gives it its “rust free “ appearance.  

Stainless steel certainly can rust when deprived of oxygen and when exposed to “halide” agents that can break down the oxide layer such as fluorine, iodine and CLORINE. Guess what salt is made of...  NaCl (Sodium Chloride).   So left in grease, in a salt water environment deep into the swage where there isn’t a lot of oxygen... it’ll rust... eventually.   Scary how you didn’t really have much warning of the impending failure.  Delos cut one open in one of their Thailand videos a few years back and most of the damage was deep down in the swage on that as well.   

Brent

On Apr 13, 2021, 10:44 PM -0400, Bill Kinney <cruisingconsulting@...>, wrote:
Craig,

Your description of the amount of tension on your backstays sounds perfect...  so much for my theory...  The construction of the swage certainly looks good mechanically.  The photo is not detailed enough, but the broken ends of the wire, either in  the swage, or at the end of the cable... they look quite rusty and brown.  A sudden catastrophic failure from work hardening would leave the ends of the wire that came apart bright and shiny, but grainy looking.  What I THINK I see is evidence of slow stress corrosion cracking of the inner strands of the wire.  We had a failure of that type on one of our jib cars.  The photos there might give you an idea what I am trying to describe...  

https://fetchinketch.net/2019/07/26/failure-analysis/

I'm a bit surprised at the comment that stainless steel "can not rust."  That certainly isn't my experience either in industrial or nautical situations.  On our old ACMO rig we would polish off the surface rust off the swages and lower foot or two of wire every six months or so, and it would come back. More slowly if we waxed it, but still inevitably. I doubt the swages were still oozing assembly grease a decade after they were made.

Bill Kinney
SM160, Harmonie
Hollywood, FL, USA

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Brent Cameron

Future Amel Owner & Amel Owner Registry Moderator

Oro-Medonte, Ontario, Canada

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