<VV> milky stain - confusion

[Tony Underwood]

At 04:18 AM 6/25/2009, Chris & Bill Strickland wrote:
>Okay, we have one camp saying nasty base to remove anodizing, and the
>other saying nasty acid to remove anodizing.


Alkali removes the anodizing.   It will also remove some of the 
aluminum under the anodizing.   I'm not a fan.   The nitric acid will 
not remove the anodizing.   It removes corrosion.   That's where the 
milky coloring comes from.




>Just doesn't seem like both would be right -- is there more than one
>type of anodizing?


It's an acid bath "reverse plating" process with no reactive cathode 
to transfer metal to the work.   An anode is formed from the aluminum 
work piece itself after the positive lead of the electrodes is 
attached to it and causes O2 to be drawn into the aluminum to form a 
thin coating of pure aluminum oxide (not to be confused with 
corrosion which is an entirely different sort of oxidation).   The 
process leaves behind a very hard but thin coating which is pretty 
much going to look like the metal underneath, meaning that if the 
work was dull the anodized coating is dull... if the work was highly 
polished, the coating will be bright and shiny.

By the way:


After the anodizing process is done, the work piece needs washing to 
remove any acid, then it requires boiling in fresh water for several 
minutes to close the pores in the anodized coating which is initially 
porous and will absorb moisture, thus starting up the corrosion 
again.    Incidentally, this is how anodized parts are dyed, before 
they're boiled, since the porous coating absorbs dyes quite 
nicely.    Boil the work and the pores close up and the dye becomes 
permanent.   ...or, is supposed to.


>If so, which does the Corvair use?


That one.


>Anybody try clear powder or ceramic coatings after they polish their
>parts?  Does it work well enough for the bother?



Yes.  And yes.   You MUST protect the polished aluminum with 
something, a sprayed coating or anodizing unless you want it to 
corrode again.



tony..