Stripping the Anodic Coating

Article From: Products Finishing, from Anodizing Technologies

Posted on: 11/1/2001

Question: I have several thousand hose fittings that are a machined casting made of 356-T6 aluminum alloy.

Question:

I have several thousand hose fittings that are a machined casting made of 356-T6 aluminum alloy. The parts are anodized and dyed olive drab. The parts in inventory are no longer needed by our customer so I am looking for a way to remove the anodized color and return the parts to their original, un-anodized appearance. We can then return the parts to inventory to sell as an unanodized commercial grade part. The dimensional tolerance on machined diameters is ±0.005". What do you suggest? C.W.

Answer:

I would suggest making up a stripping bath to the following recipe. This bath will readily remove the oxide coating from the parts without any appreciable affect on the aluminum substrate. The size of the tank used (volume of the bath) may depend on the number of batches you are willing to process in order to get all the parts stripped. It may also depend on what kind of container (tank) you have available. I would suggest anything from 50-250 gal in size. I present the recipe two different ways. Both give the same results, so take your pick.

  • 20 g/l chromic acid anhydride (CrO4) + 35 ml/liter orthophosphoric acid, 85 mass %, density 1.69. Use RO or DI water to make up the bath. Heat to 200-212F.
  • 3.5 pints phosphoric acid (75% H3PO4) + 1.75 lb chromic acid per 10 gal of RO or DI water. Heat the bath to 180F.

    I would recommend placing the parts to be stripped on a rack or fixture made of aluminum or in a stainless steel basket arranged so that they can't touch each other. Immerse the parts for 1-5 min. Determine the stripping time by running a few test parts first. Be aware that as the bath loads up on aluminum oxide the activity of the bath will decrease. The addition of more phosphoric acid will reactivate the bath. Rinse the parts thoroughly after stripping.

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