Re: [BCC Forum Post] Ron Thompson: Bobstay fitting rust stain

BCC Forum
1

I pulled Itchen’s bobstay fitting a few years ago because I was
worried by rust stains and what I thought might be the beginnings of a
crevice corrosion crack but which turned out to only be a knife
scratch. I had a local machine shop polish it out and refresh the weld
while he was at it, and then I reinstalled it with extra attention to
getting a good seal where it penetrated the hull, to minimize the
likelihood of oxygen-starved crevice corrosion. Bottom line was that
in fact it was perfectly sound – but I now am confident that it is,
and it is good not to have that worry in the back of my mind. Was not
too big a job once I had all the stuff cleared out of the forepeak. I
used a Fein Multimaster to accurately and quickly cut away the
original fiberglass bond to the hull. In my opinion, rust stains in
themselves are not much of an indication on way or the other of
significant damage to “stainless steel” - crevice cracking and
inter-granular corrosion are the real danger, and there is lot’s of
literature available online regarding the particulars.
Scott
sv/Itchen - BCC #73

On Sun, Nov 11, 2012 at 7:51 PM, BCC Forums bccforums@samlmorse.com wrote:

Author: Ron Thompson
Username: Ron Thompson
Subject: Bobstay fitting rust stain
Forum: BCC Forum
Link: http://www.samlmorse.com/forum/read.php?5,11816,11816#msg-11816

For awhile now I have noticed a rust stain bleeding from the bottom of the bobstay fitting. That indicates to me there is some internal corrosion going on under the fiberglass. It can not be inspected from the inside of the boat as it is completely sealed with fiberglass. My question is how concerned should I be about this. Has anybody had a similar problem. I remember Ben on Elizabeth replaced that fitting. I am tempted to ignore it as a minor issue but not sure if something much more serious is lurking within that fitting.
Thanks,
Ron Thompson
Ho’okahiko 97

2

Ahoy Scott , interesting that our hull numbers are one after another , mine # 72 and your’s # 73 !

I am glad to hear that your lower bobstay SS C/P was easily repaired , and re-installed .

During my studies of SS in the marine environment , I discovered a few things, that surprised me .

I had a swim ladder fabricated out of 316 SS tubing with welded joints. Later in use , rust and subsequent pitting corrosion developed on the exposed welds and the weld boundries. My investigations pointed to the fact that welding SS depleats the welded area of Chrome and Nickle, as the molton weld puddle , boils off Cr and Ni , and leaves that weld with a depleated Cr and Ni content , thus not 316 SS anymore .

Of course cleaning the weld , polishing and electro polishing passivating , then that, less’ns the chance of saltwater corrosion , but it still happens , Ouch !
Could you believe because the difference in alloy composition at the weld and the base metal ? Thinking dis-similar alloys and metals tied together create electrolysis ! They do in any engineer’s guide book !

I have found that if I can specify using 625 SS filler rod when welding 316 SS , welders , using that , can actually “en-richen” the weld with excess Chrome and Nickle , and subsequently reduce the rust pitting and corrosion potential at the weld area , Phew , less maintaince for all of us, and less SS rusting onboard !

Inconel | Technology and more | Burns Stainless Articles blog this website has v good info on 625 welding filler rod .

Calliste’s SS lower bobstay C/P deterioration could have been caused by a number of factors , unknown to me , like the photos show a rust stain horizontal to the water line , which ment saltwater intrusion into the potted cavity , so how did that cavity get breached to allow the salt water in ? Was that C/P fitting hit at a docking manuver somewhere , which broke the seal between the F/G shoulders and SS from the factory installation ?

Being the purchaser of a used 10 year old boat , I can’t venture a guess .

I don’t expect my SS lower C/P was fabricated of 316 L SS , but instead of the then (1985) industry standard of 304 SS , and what was the filler rod used to weld it , definately not # 625 because that rod is expensive !

That is why I think my lower SS C/P rusted , cracked on the weld , and had severe pitting corrosion enough , to fail a structural evaluation .

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