Metal finishing Q&As since 1989

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Anodising after chromic acid stripping.

On occasion we have to strip off a black dyed, anodised layer,clean the component and anodise and dye again.

We strip off the layer with chromic acid. After this treatment and water washing, the new anodised layer is never as thick or as black as before.

Why is this?

Is it to do with using chromic acid, insufficient washing or to do with the aluminum surface.

Any suggestions gratefully received.

Nick Winstone-Cooper
- Bridgend, South Wales, UK
2000

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To answer this question properly, it would help to know the process parameters you are currently employing.

1) Are these wrought products or castings? What alloys?

2) What do you mean when you say the oxide is never as thick as before? Do you mean that for a given voltage (I am assuming you are voltage anodizing these instead of current density anodizing) and process time you do not get the same oxide thickness? Or do you mean that you cannot achieve a given oxide thickness no matter how long you run or how high a voltage you use?

3) What are your anodizing process parameters (times, voltages,temperatures, etc.) What dye do you use and what process parameters do you follow?

4) How thick an oxide do you put on during the initial coating?How thick is the oxide after the second attempt?

Sjon Westre
- Minden, Nevada
2000