<VV> Aluminum Base - Removing anodizing?

[Tony Underwood]

At 02:14 PM 2/13/2007, mhicks130 at cox.net wrote:
>------------------------------
>  <snip>
>  Also, I'd be curious if the DIY anodizing kits can do a clear finish.
>  ------------------------------
>
>It's not clear anodizing on the trim, it's bright silver.  Try as 
>you might, you can't get the bare aluminum as shiny bright as the 
>anodized stuff.
>  _______________________________________________



Actually you can.   Remember that anodizing is nothing but a very 
thin hard coating on the surface of the work.     The final finish of 
the anodized part is strictly dependent upon how brightly the polish 
was to  begin with.    And aluminum will take a very bright 
polish.   Depending on the alloy, an anodized piece of polished 
aluminum can be almost mirror-bright.


Anodizing is like a "reverse plating" process.  There's no plating 
actually happening... the polarity of the electrodes is reversed from 
what would normally be seen for actually plating the work (which is 
why it's called anodizing) since the anode (positive) lead is 
connected to the work and the cathode (negative) electrode connects 
to the conductive plate(s) or tank in which the anodizing solution is 
contained (generally a solution of H2SO4 and water).   Most anodizing 
is done in plastic tanks using cathode plates on the sides and bottom 
to attract the aluminum ions which are leached off the surface of the 
work during anodizing.

It will go without saying that the tank MUST be inert and not react 
with sulfuric acid.   Likewise the cathode plates in the solution AND 
the wires connected to them.   Lead works well.   Anything that won't 
react to the acid bath will work.

The surface of the work has metallic aluminum material removed by the 
electrolysis process (coats the cathode plates in the tank) which 
leaves a very thin layer of oxide on the work which is very hard (and 
brittle) which is why anodized aluminum doesn't like to be 
bent.    Oddly enough, this coating is NOT waterproof at first, and 
will dull if exposed to weather etc.    The work must be boiled in 
hot water for 10-20 minutes to seal the oxide coating.  Some shops 
will boil the work in nickel acetate to seal it rather than just 
boiling it in water, evidently it seems to give a better seal 
especially if you dye the work after anodizing (but before 
sealing).    The sealing process converts the coating to aluminum 
hydrate which will resist oxidation and abrasion.

It goes without saying that the work MUST be clean and bare, no oil, 
grease, fingerprints, or previous anodized coating.    What you see 
is what you're gonna get.

This process also is a bit dangerous, since you work with H2SO4 and 
during the anodizing process the aluminum ions leached off the work 
collect on the cathode plate, combining with water and acid to form 
hydrogen gas which bubbles to the surface.    NO sparks or flame are 
allowed close to the process, *Ever*.    Ventilation and safety 
procedures are paramount when you do this sort of work.

It's also a bit of an art form in knowing what to do and how to do 
it.   You likely won't get good results the first time (DAMHIK) so 
expect to experiment a bit.  You should *read up* on the process if 
you want to give it a try BEFORE you buy the kit.   Lots of web 
resources available.   I'm fortunate in that I have an 
*exceptionally* good plating/anodizing engineer's hand book that 
covers just about everything.

Again, the freshly anodized and washed work can be dyed any number of 
colors (some alloys take color better than others) before it's 
sealed... and the anodizing and washing process MUST be kept cool or 
uneven results will occur.    Likewise, washing the work afterwards 
has to be done in cold water.   Hot water will hydrate the oxide and 
seal it after which the work won't accept any dye.   If you want a 
clear coating, don't dye it.   ;)


If I missed anything, somebody correct me.



tony..