Conference unifix::sailing

2258.0. "Underwater corrosion of stainless steel" by ESPO01::NEALE (Who can, do - who can't, consult) Tue Mar 26 1996 08:50

During the annual refit this spring, I decided to inspect the stainless bolts
which hold the lower rudder fittings on my boat. The boat is a gaff cutter, 16'
on deck, 22' overall, with a long straight keel. The keel itself is hollow in
way of the lower rudding hangings, and the rudder itself is a hollow GRP
moulding with hardwood blocks in way of the bolts. The reason for inspecting the
bolts was that there was a small amount of surface corrosion on the heads and
nuts, and I wanted to check that it was nothing serious. The two bolts that went
through the rudder were fine, and I put the original bolts back with a dollop of
Sikaflex under the heads and the nuts. The first bolt I withdrew from the hollow
keel area was badly corroded on the threads under the nut, so that about one
third of the thread area of the bolt had completely disappeared, leaving a grey
powdery-looking surface. The second bolt had no thread damage, but the under
surface of the bolt head had obviously started to corrode badly, and about one
third of the bolt had been eaten away in the "corner" between the head and the
shank of the bolt. It looked almost like a very rough hacksaw cut into the bolt.
There was proably still a good level of residual strength in the bolt (it was
10mm diameter to start with) but the corrosion could very soon have eaten away
sufficient of the bolt head to start a leak well below the waterline. The old
bolts I replaced (I had already bought replacements against this eventuality!)
were marked "A2-40" on the heads. The new ones were marked "A4-80", and were
specifically marked as suitable for underwater use. "A2" bolts in the chandlers
were marked as "above the waterline use". The boat, incicentally, is two years
old.

I suspect that the corrosion was due to the known problem with at least certain
grades of "stainless" steel, that they can corrode very quickly in the presence
of water _if air is excluded_. In this case, the area under a nut was corroded,
along with an area which was almost but not completely isolated from the small
quantity of fresh water (rainwater, etc) which collects in this boat in the
"sump" formed at the bottom of the keel, and below the point at which my bilge
pump strainer can reach (keel space too narrow, and out of reach of a hand with
a sponge, being below the propshaft tube).

I hope I have now fixed the problem in several ways. Firstly, the new bolts are
at least supposedly a more suitable grade of stainless. Secondly, the bolts are
bedded on to a good layer of Sikaflex to exclude seawater. Thirdly, I have
modified a portable hand bilge pump to take a long thin tube to empty the sump
area on a regular basis. Previously, I had not worried too much about the
half-gallon or so swilling around down there. I had already fixed the leak which
I believe had allowed most of the water to get there in the first place (badly
sealed inspection hatch in cockpit floor).

Has anyone had experience of this kind of thing in their own boat or have
knowledge of the grades of stainless available and where and how they may be
used?

- Brian

re .0:

Why use stainless steel bolts at all? Why not use bronze, which are less
prone to the kind of corrosion you describe and are nearly as strong?
Stainless steel bolts are certainly more readily available (chandlers
have them, bronze are usually mail-ordered from specialty suppliers, at
least in the USA), which may be why they were used in the first place. 

I'd also consider filling the hollow in the bottom of the keel so that 
the bolts through the keel won't be exposed to bilge water. I'd use 
epoxy with a low-density filler. 

Alan