[sdiy] Panel Finish

[Scott]

Well, it was that thin oxide coating I was referring to.  My panels are still relatively bright, even after a year or 2 of being bare.  I thought aluminum oxidized to a dull gray.  That still wouldn't' be all bad, but the shiny brushed look they've got now is better.
And as others have mentioned, I have read that nothing sticks to the oxidized aluminum, and envision a cracking, peeling finish in 10 years time..  I have no idea, though, how finishes will really hold up.

-----Original Message-----
From: David G. Dixon [mailto:dixon at interchange.ubc.ca] 
Sent: Wednesday, April 01, 2009 11:36 AM
To: Scott; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Panel Finish

Hello, Scott.

Here is the good news: Aluminum doesn't oxidize!

As you may know, aluminum is characterized as a "reactive" metal because it
is so far down on the electrochemical series.  If ground to a fine powder
and mixed with iron oxide, it will actually catch fire (these are the simple
ingredients of the famous "thermite" bomb).  However, aluminum has one
all-important redeeming feature which allows us to forget all that in
everyday use: It forms a very coherent passive layer in the presence of
oxygen.

Yes, aluminum does oxidize; very rapidly, in fact, according to the
following reaction:

2 Al(metal) + 3/2 O2(g) ==> Al2O3(s)

However, this reaction only proceeds to the extent that a mono-molecular
layer of aluminum oxide forms on the surface.  This oxide layer is so thin
that it is totally transparent, and the aluminum surface looks perfectly
pristine.  The only problem with aluminum is that it has absolutely no
resistance to acid attack.  In acid, because aluminum is so far down the
electrochemical series (far lower than hydrogen gas), the following
reactions will occur.  First, the oxide layer is dissolved away:

Al2O3(s) + 6 HX(aq) ==> 2 AlX3(aq) + 3 H2O(l)

Where X is any common acid anion (chloride, nitrate, sulfate, etc).  This
leaves the bare aluminum underneath unprotected, and it can then react
electrochemically with the acid (i.e., corrode) to form hydrogen gas:

2 Al(metal) + 6 HX(aq) ==> 2 AlX3(aq) + 3 H2(g)

You may have noticed that aluminum foil will develop holes if used to cover
acidic food like spaghetti sauce.  This tells you how un-resistant to acid
it is.

Indeed, some researchers (including two professors in my department) are
looking at aluminum as a virtual "gas tank" for hydrogen-powered cars.  Of
course, this is a totally stupid idea (sh!) because the manufacture of
metallic aluminum (by molten salt electrolysis from anhydrous alumina in the
famous "Hall-Heroult" cell) is one of the most energy-intensive and
environmentally unfriendly processes around (and is also, unfortunately, the
only industrially viable way to make aluminum).  Not only is the process
very inefficient with respect to electricity usage, but it also converts
vast quantities of carbon to carbon dioxide through the consumption of huge
graphite electrodes.  Hydrogen generated in this way would cost much more
energy to produce than that generated by simple electrohydrolysis or fuel
cells, which can run off of fuel made from agricultural waste.  Also, in
order to use aluminum in this way, one would have to carry around not only
huge chunks of aluminum in ones car, but also large quantities of
concentrated acid solution to dissolve it in (I'd really hate to be the cop
at the scene of THAT accident!) and all of the associated
corrosion-resistant (read: expensive) equipment with which to pump it
around.  Oy!

I am in the process of making Al synth module panels pretty much exactly
like you are: bare aluminum with DecalPRO dry transfer decals affixed
(www.pulsarprofx.com) and covered with acrylic clear-coat to protect it.  It
should last indefinitely!  Just keep acid away from the bare aluminum!

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Scott
> Sent: Wednesday, April 01, 2009 7:56 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Panel Finish
> 
> Is there any long term problem w/ spraying bare aluminum w/ lacquer or
> poly?  I've been making panels by transferring an image  to the bare
> (silver) aluminum, then coating.   Will the aluminum still oxidize under
> the finish and cause trouble 10 or 20 years down the road?
> 
> thanks!   - still trying to work out the best diy method for panels!  (I
> used to (about 10 years ago now) use some translucent paper, glued on,
> then sprayed w/ poly.  That looked great until a few years later when the
> paper started absorbing moisture and expanding/wrinkling!  So far this
> toner transfer is working well)  Unfortunately I like black graphics on
> the bare metal.  I've found several people have posted great explanations
> for their methods of making professional looking black panels w/ white
> text, but not black text on silver.  The toner transfer does look good,
> but I have concerns about the longevity.
> 
> thanks again
> 
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