Re: [PSUBS-MAILIST] protecting aluminum parts

[VBra676539@aol.com]

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