RE: [PSUBS-MAILIST] protecting aluminum parts

["Alec Smyth" <Asmyth@changepoint.com>]

Vance, nice hearing from you again. I think your response is destined to
be the official PSUBS tome on rust prevention of Aluminum!

After reading it I think I'll first try what I have on hand and is
free... I have some left-over Gluvit and could try putting on a thin
layer of that, as it's transparent. Basically the same as one of your
recommendations (epoxy) minus the cloth. It might be tough enough
without cloth in this application, and it has the advantage of being
clear so that I can see any water that gets through.

The story of the "anode on fire" is spectacular -- in the absence of the
anodes it would get REALLY interesting. I trust I can avoid that one
since my batteries are inside, so I can cut them off no matter what. But
it's a heck of a good anecdote. It reminds me (on a much more impressive
scale) of something that happened to me once on a scuba dive. I had a
new stainless dive knife of dubious quality, which after less than one
hour in the water came out completely red with rust, like it had been
submerged for decades. The rust turned out to be superficial, and I came
off with wire wool. But the strange thing is that I then used the same
knife for many years, and it never rusted again. The dive in question,
by the way, was in utterly un-polluted waters in the south Atlantic (off
Argentina). To this day I have no clue why that happened.

Thanks,

Alec

-----Original Message-----
From: VBra676539@aol.com [mailto:VBra676539@aol.com] 
Sent: Tuesday, January 06, 2004 6:11 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] protecting aluminum parts


Alec,

It has been my experience that SOME form of protection is better than
none 
unless everything is aluminum. Anodizing is definitely worthwhile, but
as an 
alternative solution, you might consider epoxy resin. We had great
success 
protecting MilSpec grade steels (which is very sensitive to corrosion)
on the Pisces 
boats--with nothing more complicated than clear epoxy. The hulls were 
sandblasted to white metal conditon (you might want to bead blast your
ring) then 
covered in lightweight fiberglass cloth for abrasion resistance and to
help 
stabilize the epoxy--but the adjacent areas (viewport frames and
weldments, 
bracketry mounts, penetrator reinforcements, etc.) didn't allow much
room for the 
cloth and so were simply coated. Any saltwater intrusion is instantly
detectable 
(the metal turns dark) and you can see it clearly through the resin. You
can 
sand a problem down and patch it almost between dives, which is very
handy 
sometimes.

HBOI anodizes their main hull plugs for the acrylic spheres, but the
support 
frames, DLO, etc., are bare aluminum. Works just fine like that, or at
least 
it has for the last three thousand dives or so. DON'T paint the aluminum

without some serious cathodic protection. The opaque coating will allow
saltwater 
intrusion,osmotically and otherwise,  and you won't know about it until
the 
corrosion and salt build-ups lift the paint off the metal. When you chip
it off, 
there will be white powder, and a hole in your high priced hatch ring.
Yikes!!!

At Perry, we built our dive planes and rudders from aluminum, and
painted 
them to match the sub (yellow or white, depending) with a high grade
zinc 
undercoat. The control surfaces were all hollow, free-flooding wing
sections, riding 
on stainless shafts, bolted to steel hulls, driven by bronze and
stainless 
hydraulic cylinders, activated electrically .... well, you get the
picture. They 
were a corrosion problem waiting to happen, but we never had a moment's 
problem with them. To protect them, we used homemade MAGNESIUM anodes.
They were 
just cut on the band saw (carefully!) from flat plate, and were
something like 
1/2" to 3/4" thick X about 1 1/2 " wide X 6" or so long. We bolted them
with a 
couple of stainless steel quarter-twenty cap screws and used a layer of
white 
lead paste to help with the continuity It worked spectacularly well.
Zinc won't 
do squat, as it isn't that far from aluminum on the table.

And here's a tip: for any blind hole drilled and threaded into aluminum,
set 
stainless steel heli-coils into the base material for your bolts if
exposed to 
pressure--and grease the beJesus out of them (make sure your grease
isn't 
water soluble) or, better yet, seal them with Loctite (the lightest one
will do 
fine). This includes setting the heli-coil. Drill and tap, test, then
Loctite 
the OUTER threads on the heli-coil and in the blind hole before setting
the 
locking keys. Trust me--scrupulous attention to detail during assembly
will pay 
dividends in spades one day when you are taking things apart.

Advice? I have to tell you, five hundred bucks to anodize seems pretty
cheap 
in the overall scheme of things. Especially when you consider what that
chunk 
of al-you-minnie-yum must have cost out the machine shop door!!! Do the
green 
color, which is nearly transparent. Don't get fancy with black or gold, 
because it is harder to diagnose problems with them. And keep in mind
that anodizing 
isn't the be-all and end-all. The bolt holes, and any nasty hidden chips
or 
flaws in the coating will cause you problems unless you chase the little

buggers down with relentless and unflagging enthusiasm.

One other thing comes to mind, but I don't know much about it, so can't
do 
more than suggest you look into it. How about powdercoat? It is
basically a 
polymer, so in theory (that's SWAG theory, of course) ought to bond
very, very 
tightly to the base metal. Seems like it might work, and it's cheap.
Something to 
think about, anyway.

And I can't recommend the homemade magnesium anodes enough. They will
make 
all the difference on the day you get a high resistance short to ground
and are 
a quarter mile underwater and can't do anything about it. I have seen
the 
magnesium corrode VISIBLY when that happens. The anodes look like they
are on 
fire, but slowly. It's very weird.

It sounds like you have plenty going on. Congratulations, and Happy New
Year,

Vance