[C38] crevice corrosion?

[S Orton]

All,  It looks to me like there lots of crevices where the stranded wire is roller swaged to a fitting.  I believe that most marine stainless used is 316.  Some SS (304) are "free machining", which have sulfur added to the alloy.  This alloy could present problems in salt environment.  I will keep reading.

Steve O     
 


Date: Sat, 10 Jul 2010 15:27:43 -0700
From: drstrong at ucdavis.edu
To: listserve at catalina38.org
Subject: [C38] crevice corrosion?

Steve: 
My friend the marine engineer told me that what we worry about in tiny protected areas of stainless steel, and with low grades of stainless that have impurities, is called crevice corrosion. The Wikipedia has a post that begins, 

"The corrosion resistance of a stainless steel is dependent on the presence of a protective oxide layer on its surface, but it is possible under certain conditions for this oxide layer to break down, for example in reducing acids, or in some types of combustion where the atmosphere is reducing. Areas where the oxide layer can break down can also sometimes be the result of the way components are designed, for example under gaskets, in sharp re-entrant corners or associated with incomplete weld penetration or overlapping surfaces. These can all form crevices which can promote corrosion. To function as a corrosion site, a crevice has to be of sufficient width to permit entry of the corrodent, but sufficiently narrow to ensure that the corrodent remains stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not found in grooves or slots in which circulation of the corrodent is possible."...

http://en.wikipedia.org/wiki/Crevice_corrosion
  Don


S Orton wrote: 


I was going to replace my life lines as recommended by the CC, but on detailed inspection, I declined.  My reasoning:
1.  Where there was rust at the swaged end fitting, I cut away the plastic, cleaned up the wire with a "fine" wire brush rotary tool, bent the wire back and forth, and observed no failed strands.
2.  As far as know Catalina still uses coated wire and West Marine still sells the stuff.
3.  I don't understand this oxygen deprived corrosion.  When you think about the roller swaging of the end fittings on all our rigging, there isn't much oxygen moving into the fittings.  It has to be rather stagnate inside the fittings.
4.  My surveyor told me this was a "recommended" not a "mandatory" requirement and it didn't affect my insurance.
5.  I'm not a metallurgist by training, but in my previous life, I worked as a ME in aerospace for 40 years, working with corrosion resistant materials and never coming across the term "oxygen deprived corrosion" in SS.  If some body has some factual engineering info on the subject I would love to read it.
With that said I inspected every inch of the life lines, looking for rust marks, cutting away the plastic, and found no broken stands of wire (fish hooks).  Now I did replace my rigging (25 yrs old) at a material cost of $1700.
 
Cheers, Steve O (Santa Susanna- #304)  
> From: 1derful at comcast.net
> To: Listserve at catalina38.org
> Date: Thu, 8 Jul 2010 19:49:45 -0700
> Subject: [C38] Life-lines
> 
> Someone suggested I replace my life-lines because they are coated in plastic 
> so I can tell if they are fraying. They advised stainless steel. Seems 
> like an expensive retro-fit.. I can't remember every really using the life 
> line to prevent falling overboard.
> 
> Also does anyone know the height of our Catalina 38's? I've been going 
> under some bridges lately and from the cockpit it looks as though I am gonna 
> lose my mast. Hasn't happened yet.
> 
> Patrick
> Blue eyes
> S F Bay 
> 
> 
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