Metal finishing Q&As since 1989

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Black Mark in Aluminum Electrochemical Polishing

 

Hi Everybody,

We are making Al product by electrochemical polishing with CrO3/H3PO4 bath. We are facing problem of black mark around edge. As our product is made of layers and holes, the gas stream during ECP seemed to be related with black mark. EDX analysis showed that black mark had high carbon and oxygen content. This is contradict with my previous prediction that black mark is Al2O3. I have tried acid dip to 'bleach' the black mark but it caused whitening for whole parts.

Could anyone help to provide me some advice or miracle ECP bath formula or additive before my head is cut by my boss. I am still keeping my fingers crossed.

Xelon Chan
- Hong Kong

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I know next to nothing about this but it seems nobody else wanted to answer as well. So I give you some questions to ask yourself and the process. Where did the carbon come from? Could it be oil drag in? Has the process always done this or has it started over time? If a sample on a small scale (if possible) with a new solution is done does it do the same? Sorry but I can help no more.

Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England
 

Your problem may be solved with more agitation different racking or more current. There are additives which could solve this problem.

Wayne Chandler
- Atlanta, GA, USA