From: "John R. Johnson" <john@siu.edu>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Need help with Aluminum priming methods
Date: Mon, 14 Jul 1997 15:51:25 -0500

On 12 Jul 1997, DMoreho699 wrote:

> Hi!
>
> Just got back from the Boeing Surplus Store and the Arlington Fly-in.
> Finally nice weather.
>
> I am building a Murphy Rebel, whose metal components are mostly alclad
> 2024 T? and maybe some 6061. A man told me he used alodyne to treat his
> metal. I have used a non-self-etching zinc chromating primer on the mating
> surfaces. I'd like to know the pros and cons of different priming methods.
> What is alodyne? Can etching primer be used? What exactly is alclad?
>

Alodyne --  a chromic acid conversion process that leaves a corrosion
           resistant film on aluminum surfaces.

Etch Primers -- They normally recommend a "wash primer" which is a self
         etching primer.  It makes a little tooth and bonds well with the
         aluminum surface.  If it was alodyned, the wash primer should be
         applied as soon as it has dried after the water rinse.  Do not
         touch the surface with your fingers before priming it.

Alclad --  Alclad is a trademarkes process.  Berylium copper aluminum
         alloys provide very high strength.  These are the 2000 series
         alloys like 2024 which are usually used for aircraft.  These
         alloys have a problem because of the copper in suspension in
         the aluminum there are granular boundaries between copper and
         aluminum that are rather far apart on the electro scale.  In
         the presense of an electrolyte, like dirty water, they make a
         small battery and one of the poles becomes sacrificial and
         corrodes away.  This intergranular corrosion is a serious
         problem with these alloys.  However, pure aluminum will corrode,
         but when it does it forms an aluminum oxide coating that serves
         as an excellent corrosion barrier.
	 Someone got the bright idea of coating both sides of the sheet
         of aluminum with a thin layer of pure aluminum.  There is a
         layer about .001 inches thick of pure aluminum on each side of
         the sheet.

John

Date: Thu, 12 Jun 1997 06:26:13 -0700
From: Johnny <johnny@johnny-enterprises.com>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Aluminium coatings

F.L. Whiteley wrote:

>Clarence Snyder <clsnyder@Ibm.net> wrote in article
><339e0865.643251437@news2.ibm.net>...
>> On 10 Jun 1997 08:50:38 -0700, Leon (L.Pikros@univ-mulhouse.fr) wrote:
>>
>>>I am a chemist but I have never manege to protect correctly Aluminium parts.
>>>None of my mixtures have maneged to have the results offerd bu commercial
>>>alodisation products. I have herd about something like "chromic treatment"
>>>Dose anybody knows what it is?
>>>Dose anybody knows what an alodisation process exactly is? (chemicaly?)
>>
>> I believe you are speaking of anodization. This is like a plating
>> process, electricity and acid being used, but the plating is aluminum
>> oxide. It is very hard, and being an oxide layer, it is more or less
>> self limiting as far as corrosion is concerned.
> 
> I had always called the process anodiziation, however, I recently treated
> an aluminum piece to a chemical process call "alodizing" by the A&P
> overseeing our project.  Following bead blasting the part initially, I
> bathed the piece in Part 1 of the treatment(sorry I'm miles from the shop).
> This was a weak acid solution about the pH and strength of vinegar.  A
> quick rise and then Part 2(unknown substance) for 2-5 minutes to allow
> penetration.  Rinse and dry.  The part appeared a light green/gold color to
> me.
> 
> A second reference to alodizing was made by a local Kitfox builder the
> other day.  He'd 'alodized' his wing spar tubes.  While I had used a spray
> bottle to keep my part wet, he'd immersed the spare tubes in rain butters
> filled with the solution.  He wasn't overly pleased with the end result,
> meaning runs in the color.  Perhaps this is what you are referring to your
> mixed results, Leon?  The edges of my part were noticeably lighter than the
> other surfaces.
> 
> I could find no reference to alodization on the web, but perhaps this is an
> alternative term for chemcial(rather than eletrical) anodization?
> 
> F. Whiteley
> Colorado Soaring Association


The stuff I use, Iridite 14-2, is commonly refered to as "Alodyne".
After mixing the Iridite powder with water, you get that familiar
looking gold/red/brown liguid. The Material Safety Sheet says it
contains Chromic Acid, which may explain the term "chromic treatment".
At any rate, you won't be able to get that "factory" look unless you
submerge the parts completely (at least I can't). IWO, brushing,
rolling, sponging, spraying, etc. will leave it looking streaked or
uneven, but this is only cosmetic, and the part is still protected. The
plastic rain gutter idea for long tubing is one of the best I have
heard. Alodyne will not come out even looking if there is still some
oils or unetched areas on the metal. The process I use is to wipe with
MEK first to get any oils off, then dip in etch until the part has that
"fresh white look" (you'll know it when you see it), maybe 20 minutes or
more, rinse well, then dip in the Iridite solution until it has that
golden brown look (about 20 minutes, sometimes more), rinse again, let
dry. And then, because I live where it is wet 300 days out of the year,
I paint everything with self etching 2 part Zinc Chromate primer.

Here in Boeing land, Deft epoxy primer has become fairly popular for
coating aluminum with after alodyning. It's a water thinned, 2 part (3
if you include the water), epoxy. Personally, I have never been able to
get it to shoot any better than cottage cheese, but under perfect
conditions, it looks pretty good, but a little thick I think. Tough as
nails once it is all the way cured. Another one getting more popular
around the airport is US Paints epoxy primer for aluminum. This uses
reducer as reducer instead of water as reducer. I found it very nice to
shoot, easier to apply thin, and it's still tough as, well, epoxy.

Personally, I still think that the old time 2 part self etching Zinc
Chromate (most are enamel, some are laquer) goes on better than any of
the epoxys, but it's hard to deny the longevity and resistance to
chemicals of the epoxy.

note: most of the above mentioned products are for industrial use only
and not intended for sale or use by the general public, so don't be
surprised if you have to look a little harder to find a source for them.

-j-