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crevice corrosion
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andydavis



Joined: 11 Apr 1999
Posts: 268
Location: Point Isabel

PostPosted: Mon Jul 22, 2013 10:43 am    Post subject: crevice corrosion Reply with quote

You might be interested...rigging up Saturday, pulled a fin out of the back and noticed the bolt had a huge crack in it. I think it is pure crevice corrosion. I rinse off all my gear at the end of the day...except the fins and bolt Sad

Wonder what the condition of the threads on the nut embedded in the fin is?



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spennie



Joined: 13 Oct 1995
Posts: 824
Location: Thousand Oaks, CA

PostPosted: Mon Jul 22, 2013 1:04 pm    Post subject: Reply with quote

Unlikely that was caused by corrosion, that bolt is stainless steel. While many grades of stainless will rust eventually, that one has very little corrosion showing, making it unlikely that that was the problem. I would guess that bolt was bent & straightened at some point, or has a manufacturing flaw in it. Good that you caught it, but I wouldn't worry about it in the future, I think it's a one-in-a-million fluke.

The "nut" in the fin is brass, also not prone to corrsion.

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rigitrite



Joined: 19 Sep 2007
Posts: 280
Location: Kansas City

PostPosted: Mon Jul 22, 2013 2:36 pm    Post subject: Reply with quote

Actually, I believe he’s correct about this being crevice corrosion. The crevice corrosion is the mode of initialization, but the failure mode is classic fatigue failure: even from your low res picture, you can see there’s some kind of defect in thread notch that starts the crack, then beach marks (darkish area) as the crack progresses, and at about ½ way across, brittle failure (dull metallic speckeled area). Crevice corrosion occurs where there’s no way to get oxygen to the metal surface to re-passivate the chrome oxide layer. The only thing that makes stainless steel stainless, is a layer of chrome oxide a couple molecules thick (~130 anstroms): the passivisation layer. Remove this layer and SS will rust just like mild steel. My guess, is the crack formed where there was a nut or something to prevent water from evaporating.

rats....I geeked out again. Sad

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andydavis



Joined: 11 Apr 1999
Posts: 268
Location: Point Isabel

PostPosted: Mon Jul 22, 2013 2:52 pm    Post subject: Reply with quote

spennie wrote:
Unlikely that was caused by corrosion, that bolt is stainless steel. While many grades of stainless will rust eventually, that one has very little corrosion showing, making it unlikely that that was the problem. I would guess that bolt was bent & straightened at some point, or has a manufacturing flaw in it. Good that you caught it, but I wouldn't worry about it in the future, I think it's a one-in-a-million fluke.

The "nut" in the fin is brass, also not prone to corrsion.


I'm sure you're a nice guy, but you really don't know what you're talking about. The corrosion resistance of a stainless steel depends presence of a passive, thin protective oxide film. If there's any kind of break in the film due to a crack, a weld or a thread, there is potential difference between the protected surface and the bottom of the pit. In the presence of an electrolyte (seawater). This kind of corrosion is extremely common in boats, especially, if you have a stainless bolt embedded in a wet, oxygen poor environment.

I'm hoping that the nuts in the fins are bronze or at least a "DZR" brass. Brass is notoriously prone to a selective leaching of the zinc in the presence of seawater...dezincification. Some brasses are alloyed with tin or arsenic to make them DeZincificationResistant. Here's are pictures of a shackle and a bolt, both of which failed due to crevice corrosion. Also, see the image of the brass propeller and the adjacent pit corrosion in the ss shaft.

I'm not saying "be worried" -- just sayin' I changed all my fin bolts yesterday.



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andydavis



Joined: 11 Apr 1999
Posts: 268
Location: Point Isabel

PostPosted: Mon Jul 22, 2013 3:05 pm    Post subject: Reply with quote

rigitrite wrote:
...even from your low res picture, you can see there’s some kind of defect in thread notch that starts the crack, then beach marks (darkish area) as the crack progresses, and at about ½ way across, brittle failure (dull metallic speckeled area)...

rats....I geeked out again. Sad


Yeah, doesn't look like the kind of classic failure I'm used to. I always remove the fins from my boards right after sailing so the boards can ride on the truck. I loosely put the fin bolt back a few threads (just to keep from losing it), then put it in a box in the back. When I withdrew that bolt earlier this week it look fine, but when I looked at it Friday it was busted...I think just from throwing some stuff on top of it.
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spennie



Joined: 13 Oct 1995
Posts: 824
Location: Thousand Oaks, CA

PostPosted: Mon Jul 22, 2013 3:41 pm    Post subject: Reply with quote

Wow, sorry, I didn't know any of that! I always thought since SS is an alloy it would be corrosion-resistant all the way through. Well, you learn something new every day!
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kcorr1



Joined: 15 Feb 1998
Posts: 51
Location: Moss Beach

PostPosted: Mon Jul 22, 2013 7:02 pm    Post subject: Reply with quote

one thought I had was consistent torque/lube in future fin screw installs. Getta torque wench. Very Happy
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isobars



Joined: 12 Dec 1999
Posts: 14181

PostPosted: Tue Jul 23, 2013 8:10 am    Post subject: Reply with quote

I put on a pair of rubber gloves before clicking on this thread.
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inf2003



Joined: 15 Jan 2009
Posts: 137

PostPosted: Tue Jul 23, 2013 9:08 am    Post subject: Reply with quote

Quote:
The only thing that makes stainless steel stainless, is a layer of chrome oxide a couple molecules thick (~130 anstroms): the passivisation layer. Remove this layer and SS will rust just like mild steel.


Rigitrite, Question for you. I also thought stainless was an alloy that was consistent throughout the metal. Are you saying there is a protective layer over the surface? Why is it that I have fabricated boat and other outdoor parts with stainless and have been cut or ground on the surfaces. The surfaces didn't seem to rust at the cuts any more than un ground surfaces. Not arguing but just trying to understand.
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rigitrite



Joined: 19 Sep 2007
Posts: 280
Location: Kansas City

PostPosted: Tue Jul 23, 2013 10:08 am    Post subject: Reply with quote

This is a little bit counter-intuitive. Stainless Steel is an alloy throughout. The way it becomes “stainless” is a chemical reaction with oxygen and Chromium which forms a layer a couple molecules thick of chrome oxide called a passivization layer on the surface and only the surface. This coats the metal and prevents general attack (what most people think of when they talk about corrosion). However….if the chrome is reacted with other elements, like chlorine, carbon, bromine, etc… it essentially gets used up and the passivization layer is removed at that location and general attack can occur. The way to fix that, is to clean off the area so that new chrome is exposed and the passivisation layer is reestablished. You can see this process on your flatware at home, which is typically a 400 series stainless. After a couple years of dishwashing, you start to see circular rust marks, where water has beaded on the flatware, the chemicals in the rinse water have reacted with the chrome and veeeeery slowly started to corrode the metal. If you rub it down with steel wool and remove the rust, it’ll be like new.
A buddy of mine accidentally left the lid off a container of chlorine for his pool and went on vacation. When he came back two weeks later, all the steel tools in the shop where the chemicals were stored were covered in rust, including things that were 316 SS. He was baffled…..and pissed! I had to go through the whole explanation above to keep him from making an angry call to the store he bought stuff at. He thought the metal was defective.

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