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Black streaking on clear anodized aluminum

 

This original post dates back to 86 but we are currently experiencing exactly the same problem - can any one shed some light on this? "We make many parts that are clear anodized. The material most used is 6061 or 6063 extruded. We have an ongoing problem of black streaks that appear after anodizing. The streaks are in the direction of extruding. We suspect that it is a problem with the raw material but it can not be seen until after plating. We tumble deburr all parts to remove any machining marks with a mild media prior to anodizing. We instruct our anodizer to etch just to remove the tumbling marks. Since the parts are very cosmetic, the black streaks are unacceptable. Any ideas".

We are really having problems with this black streaking and still no definitive answer to the problem -

I found finishing .com because of a  a previous post with exactly the same problem - hopefully it can be resolved

Thank you,

Giles Newcombe
- Sydney, NSW, Australia

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Are you sure that if you turn the parts 90 degrees in the tank, you still get streaking in the same direction?

  
Tom Pullizzi
Falls Township, Pennsylvania
 

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Black streaks along a part, especially an extruded part, are the result of impurities embedded in the aluminum. The fact that the streaks run in the same direction as the extruding is especially telling. We have found that it is possible to solve the problem in two ways: have the manufacturer or the wire-draw house clean their equipment to prevent dirt or oil becoming embedded in the wire, or anodize the part twice. The first anodize will draw the impurity to the surface and a second anodize should remove it.

Jim Gorsich
Compton, California, USA
 

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November 26, 2008

the black streaks formation is due to the presence of copper and zinc in the raw material.normally AA 6061 and 7075 material behaves ( since these materials are alloyed with zinc and copper ) like this only this type of features can be seen after anodisation, it is better to change the raw material not process

Manjunath,
India

Manjunath.L
- Coimbatore, India