I had the same problem as the result of condesation from the liquid nitrogen
vents dripping down and collecting on the lip at the bottom of the dewar.
I cleaned the corroded surfaces with "Scotchbrite" 3M cleaning pad and
some 300 grit emory paper. I polished afterwords using a white rouge with a
cloth buffing wheel. The cleaned the surface with ethanol ,then acetone.
There were still some areas that had pitted into the surface of the metal. I
coated the rough surface with a fine layer of "Q Dope".
(this is a clear viscous styrene polymer used to coat inductors by radio
amatuers avaiable at most Ham suppliers).This can be used to fill voids and
pits to allow the "O"ring to seal.
I also recomend that you install a heatsink vent to prevent ice and
condensation. Also construct something on the dewar to divert any water away
from the lips on the plates. Automotive polish (original Simonize) applied
to the outer surfaces will also prevent further corosion.Do not use the auto
polish on mating vacuum seals.
I also recomend using only "Apiezon"vacuum grease.In my opinion it works
better than the "Dow Silcone Grease".
The Q Dope works well in the area of the "O" ring but may cause problems in
areas that go through extreme thermal cycles( Liquid Nitrogen to room
temperature).It may seperate and leak.
If the pitting is severe I would recommend milling the surface by a machine
shop.
Regards Scott
*
A.Scott Chesnick
Senior Scientist / Electrical Engineer
IN-VIVO Nuclear Magnetic Resonance Center
NATIONAL INSTITUTES OF HEALTH
NHLBI / Laboratory of Cardiac Energetics
BLD10 / Room B1D-166
9000 Rockville Pike
Bethesda, MD. 20892-1060
LAB 301 496-6564 /FAX 402-2389
RES 717 642-8888
chestnic@mail.cvn.net