To repair this I first cleaned the corroded areas. Then I used some "Naval
Jelly" (get it from a hardware store) to clean the o-ring groove and the
bolt threads. This contains phosphoric acid in a gel, and maybe some other
stuff. I rinsed it off with a hose outside (it's way too big for a sink)
and repeated the process a couple of times untill the pitted areas looked
free of corrosion. I then scrubbed the groove with a toothbrush and some
soapy water to get any residue out. If you feel bad about "dumping" the
phosphoric acid compound down the drain, ask yourself how much coke gets
dumped down the drain each day at McDonalds?
Now comes the hard part. Make a dowel a little smaller diameter than the
width of the groove (delrin works nice). Next mix some epoxy, I prefer
2-Ton to 5-Minute for this because the 5-min. sets too fast and the 2-ton
dries harder. Apply some of the epoxy mix to the end of the dowel and use
the dowel to press the epoxy into the pitted areas and smoothe the epoxy
out. The smoother the epoxy is, the less sanding you have to do. After the
epoxy has cured (2 days is better than one) you can sand the bottom surface
smoothe. Use some wet/dry sandpaper and plenty of water. Use the dowel to
hold the sandpaper flat in the groove. It doesn't have to be a highly
polished finish, but the "grooves" must be parallel to the circumfrence and
not across the o-ring. If the stainless flange is rough it can be sanded
also. Be sure to clean out the groove to inspect it. You may have to do
more than one epoxy coat to get everything.
Finally, when assembling the magnet, be sure to use all NEW o-rings. These
can be purchased from Oxford or Ben Hamilton (512-846-2784) of MRS. Older
o-rings become less flexible and some old Oxford magnet flange o-rings
were made with a butt-joint, instead of a molded o-ring. It's also easier
to sleep at night with new ones. Our magnet has been running for 2 1/2
years now without any problems. Be sure to do a helium leak test before
cooling the magnet. It is important to check the o-ring groove and the
flange weld for any leakage.
Roy Van Vliet
Engineer
Acorn NMR
roy@acornnmr
(510)683-8595
(510)683-6784 fax
> Hi everyone.
> In the process of bringing up a 300 Mhz magnet we found severe corrosion
>on the aluminum base plate were it mates to the stainless steel can. The
>large Oring groove is mostly clean but the rest of the mating surfaces are
>severely corroded. Also this corrosion has atleast transfered itself to
>the stainless steel from which it is difficult to remove. Has anyone dealt
>with this problem before without having to replace the complete bottom
>plate. Any magic cleaning formulas perhaps? Any help would be greatly
>appreciated. Paul Bruins UC Davis.