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Spots on sulfuric black anodized extrusions


Our finishing shop has been experiencing blotchy discoloration problems when applying sulfuric black anodizing to extruded 6061-T6 alloy. The extrusion has thin and thick sections, and the irregular discoloration seem to appear only on the thin sections. We have tried variations in the acid etch time, and raw extruded sections as well as machined and tumbled parts--all seem to exhibit the same areas of blotchiness in the finish. What could be the cause?

Matthew Kosh
- Seattle, Washington


I believe these "spots" are being caused by the extruding process itself.. If I'm not mistaken, they are referred to as "Hot Spots"..where the aluminum gets overheated during the extrusion process...and, perhaps, cools too quickly (which, in your case would make perfect sense, as the THIN areas are the ones being affected). Not being an extruder, I can't say for sure what actually causes the problem. I'm not aware of anything that an anodizer can do to make the part look consistent, as its a flaw in the material. My company doesn't do alot of extruded work, so there may be others in this forum that may be able to give you advice.

Marc Green
anodizer - Boise, Idaho


Thank you for the tip--it sounds logical. I have taken the matter up with the extruders, who do not yet understand the cause of the problem themselves; it may originate in the extruding process, but it only shows up after anodizing. They will do some testing of the metal and compare the properties of the thick and thin zones to the spotted areas when anodized. I appreciate the assistance.

Matthew Kosh
- Seattle, Washington


Did you ever get to the bottom of the blotchy anodizing problem? We are having a similar problem with 1/2" thick disc shaped parts, the outer 1" ring around the 4" diameter discs has this blotchy appearance. These parts are also made from 6061 T6 Aluminum.

Bill Sieff
automotive - Guilford, Connecticut, USA

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