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.


--
Ned Simmons