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Patchiness on black anodized parts

2003

We have been test running 536 casting, in our Sulfuric Anodizing line. We have been able to get the black we want but have found it hard to eliminate a patchiness look on the finish product.

These parts have been heat treated and then glass bead before anodizing, for smoothness.

If anyone could give us some ideas on how to eliminate the patchy look, it would be appreciated.

Allan Basaraba
- Kingston, Ontario, Canada

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You anodize aluminum metal, you do not anodize magnesium which is at high percentage in the 536 casting. Several magnesium casting are floating around from the West Coast to the East Coast and no one can do them. The Magnesium in the surface is the "patches". If you have any doubt, then get an EDAX done on the patchy spot. You will find higher Mag in the patches than in the uniform anodized "aluminum" spots.

Be careful with the bead blasting. The media must have been used ONLY on that alloy of aluminum or you will be pounding foreign metal bits into the soft aluminum and where the wand is held slightly longer in one place versus another place - there will be a "patch". Also the work hardening by the blasting will change the influence of all chemicals that follow in the process.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003