I really hesitate in bringing this up so as not to bore anybody, be redundant, or a little basic for the sophisticates on the list, but I used to get a lot of stuff anodized and have pretty much shifted into what I consider a far more durable, versatile and inert process (which I
have taken in house), powdercoating.
Anodizing is a corrosive process converting the surface of aluminum to aluminum oxide. The piece is essentially attached to the anode in solution and allowed to oxidize. this is the opposite of electroplating
where the piece is attached to the cathode and attracts ions forming a coating. Anodizing 'pits' the surface of the aluminum in a bunch of parallel 'micro tubes', where dye (clothes dyes, vegetable dyes, etc- many fugitive) is later applied through capillary action and sealed with
a water bath or nickel solution
anyway, another way to aggressively corrode aluminum is with an base solution. Drano (an American drain opener chemical product) is one of the pinnacle substances for chemical corroding aluminum. I used this as a soak to prepare a part before mechanical finishing- anodized surfaces are not an asset to sandpaper (also aluminum oxide), steel wool
and the other finishing tools. mild detergents are also bases and may make surface issues with a long soak.
1. It could be removal of the thin anodizing (one can get 'hard' anodizing for certain applications) and staining, 2. absorption into the capillarys in the anodized surface of the 'dirty'
soap/dissolved lubricant solution of the soaking bath. It should have been sealed, but the cleaning could have opened them up. and/or 3. a discoloration of the copper content in the aluminum (predominately in 6061 and 2024 alloys)forming an uneven gray or goldish
'smut' (this is the technical term btw). These alloys are commonly prepared in a de-smut solution containing ferric and sulphuric acid pre anodizing so subsequent dyeing is taken evenly.
I don't know if any of this helps or if anyone made it this far, But it is not too difficult for me to set up a small system if there is no one doing small scale stuff and it needs doing out there.
If anyone else out there is interested in a small scale anodizing system, the latest issue of Machinist's Workshop has a pretty comprehensive instructions for setting one up.
-- gabriel l romeu in chesterfield where we are trying to reduce the uses of battery acids,
new jersey, usa
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