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Residue on "Cleaned" Aluminum Part Prior to Anodizing

I am having a cast aluminum (#713, Tenzalloy) part anodized. The cleaning process prior to the anodizing consists of about 20 sec in NaOH at 150 °F, rinsing with water, then 10 sec in a nitric/hydrofluoric acid (13:1, v/v) then final rinsed. When the parts come out of the NaOH they're black. When they come out of the acid some have a tan residue on them that can be rinsed off easily with a hose if done so right away. Any ideas what the black and tan stuff are?

John C. Jacco
Kingston, NY USA
2000

The residues are smuts from alloying elements. You may need to adjust your concentrations of etching solutions to optimize etch and smut removal. -tom

Tom Pullizzi
Falls Township, Pennsylvania
2000

The black stains are oxides formed during NaOH etch. The tan residue indicates either severe oxidation products and/or insufficient deoxidation with the acid.

Try lower NaOH temp (140 deg F) and longer acid immersion time.

Dado Macapagal
anodizing shop - Toronto, Ont. Canada
2000

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