"CaveLamb" wrote in message
m...
Ned Simmons wrote:
On Sat, 29 Jan 2011 20:59:31 -0600, CaveLamb
wrote:

I snapped a stanchion bolt the other day!
1/4" stainless steel machine screw actually.

Starboard stanchion by the cockpit. Forward inboard bolt.
The stanchions get abused sometimes, people grabbing them or the
lifelines
while docking. This one was leaning forward an inch off of vertical,
but seemed
sturdy enough.

I had the lifelines loose and was pulling on it (aft) - not all that
hard!
to straighten it up - And something went POP! Pretty loud too

Inspecting it, it was pretty obvious what had happened, so I
dug the bolt out and replaced it.

This bolt (machine screw actually) had some serious corrosion issues.
(See the attached photos)

There is quite a bit of old damage on the mating surfaces.
That's the dull gray part.
And quite a bit of sealant over the old damage!

The bright metal tab seems to be the place where it broke.
There was NOT A LOT of metal there!

I have recently learned that there is an aluminum plate buried in the
deck lay-up where the stanchions are attached.

What say ye? Was this 20 years of dissimilar metal corrosion?
(4 photos)
http://www.home.earthlink.net/~capri26/bolt.htm

You'd expect the aluminum to lose in a galvanic couple with SS. There
are several other corrosion mechanisms that affect SS in marine
environments. Google:

crevice corrosion
stress corrosion cracking
chloride stress corrosion cracking

for a few possibilities which are, in my opinion, more likely than
galvanic corrosion in this case.

Carbide precipitation is another possibility, but that would be a
result of faulty manufacturing of the fastener. On the other hand, I
have seen what I'm pretty sure was carbide precipitation in the tubing
in a couple bow pulpits, so it's not out of the question.
As soon as possible, ALL of the stanchions are going to be pulled and
re-bedded.




I've heard of crevice corrosion and stress corrosion cracking, but
the third one, chloride stress corrosion cracking, I had not heard
of before. Interesting.. but I doubt we have had that kind of environment
onboard the boat. At least I hope not!


My photos are the limit of my camera.
Aperture priority (about f 3.5) in macro mode - zoomed all the way out
(about 1/4" x 1/4" frame size).

Certainly not 500 X by any stretch of the imagination).

The suspect surfaces look more granular/pitted than fractured.

But if we could zoom in 300 X to 500 x would it look like these?

http://www.atclabs.com/Photos/300%20...ion.%20BSE.jpg
http://www.atclabs.com/Photos/300%20...20Area%202.jpg

Or a simple overload mechanism?
http://www.atclabs.com/Photos/300%20...20Overload.jpg

Well, whatever it was, that screw mess it up for the whole bunch.
They ALL are getting pulled and replaced!



--

Richard Lamb
email me:
web site: http://www.home.earthlink.net/~cavelamb


A company I worked for had a product made from 316 stainless steel. Often
the product was submerged in a marine enviromment continuously.

A customers returned some with some M3 threaded holes in the body
"stripped". The screws looked ok, just the holes were "stripped". Turned out
it was crevice corrosion. We learnt something new that week!