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Black Anodised 2024 T3 fails nitric acid dye test

We are reworking a part which was electrophoretically coated black. Material is 2024 T3.
We are stripping it in caustic and sulfuric anodising at 11 volts for 40 mins. It is in the black dye for 20 minutes then hot seal for 20 minutes.
When tested with 30% nitric for 15 seconds it fails i.e the dye comes off.

We process various other alloys e.g 6082 with excellent results. Any help would be appreciated.

Raymond Callaghan
Plating Shop - Belfast, Northern Ireland
2007

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First of two simultaneous responses --

What is the free acid concentration and what is the dissolved aluminum? If you are at or near 165.3 grams per liter of free acid then you need 21 volts to tet 12 amps per square foot (Alcoa said it in 1956).

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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Second of two simultaneous responses -- 2007

The first problem is that 2024 has a large amount of alloy material, of which copper is a major part.
You do not mention your temperature.
A wild guess is that a large portion of your current is being used to dissolve the alloy ingredients and thus your anodize coating is much thinner.
Why are you using such a low voltage? A higher voltage would allow you to build the anodize layer at a much faster rate. I think that 11 volts is quite near the point where the rate of formation of the oxide layer is barely ahead of the dissolution rate of the layer from the acid--IE: your layer is very thin. ( too thin to seal properly)

James Watts
- Navarre, Florida

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