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topic 27719

Clear anodize aluminum appearance flaw -- looks galvanized

A discussion started in 2004 but continuing through 2018


Q. I have a customer who purchased some extruded aluminum, machined it into pulley wheels and had it clear anodized. It finished with a galvanized appearance on the outer perimeter. The material is 6061 T6. The customer claims it is a material defect, however I have spoken to a couple different anodizing companies in the local area who have stated that it is not a material defect. They have stated it is a common occurrence with extruded aluminum (one anodizing company was physically shown a part). The customer continues to dispute this. Is this typical to extruded aluminum? How can it be avoided if possible?

Joe Vardon
QA Manager - Kent, Washington, USA

Ed. note: send us a picture of the issue, Joe


A. A so-called galvanized appearance can be created by an alkaline etch tank that is too high in dissolved aluminum. This usually starts around 50 gm/L of dissolved aluminum. This appearance can be corrected by dumping the alkali etch and making up new, or decanting down to about 15 gm/L and adding back, or in some cases adding a smidgen of Hex Chrome. The appearance is certainly a reason for rejection.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


A. Dear Joe,

If by 'Galvanised' you mean that large grains and/or patches appear in the anodising, then it sounds very similar to a problem we have experienced at different times. the issue we have experienced is peripheral grain growth. As the extruded bar is cooled, the outside of the bar will obviously cool faster than the core. This means that the grain structure will be larger, and if the temper is not correctly controlled, you get these very large grains right on the surface of the bar. When we have experienced this, we have found that the problem will completely disappear if you machine the diameter down by about 5mm.

Mark Harris
Fly Reel Manufacturers - Cornwall, UK


A. Mark Harris,

I can confirm this. We make fly reels too. I have sent parts for analysis. The reason is separation of the layers in the process of forming the billet through the die (extrusion if you like). The temperature differential creates an onion-layer type effect which is worst near the outside of the billet. That's why you machine the 5 mm off ... but I can only imagine the cost! What we try to do is cut slices from various parts of the billet prior to machining and then do a sample anodize. The longer in etch the worse the problem appears.

Return the material if you find a problem; it's no good for decorative use like on reels. No structural problem but just can't sell a part looking like that. How often have you experienced this problem? Is it in each batch or more on a particular batch?

Michael Georgiadis
fishing reel manufacturing - Durban, South Africa


A. We anodize and blue dye a number of 12" diameter parts that are machined from slices of bar stock that exhibit the galvanized appearance exactly as you describe. It is more evident on the machined faces of the part and is worst at the outer edges. We have always suspected it is related to the grain structure of the stock resulting from the "drawing" and heat treating of the material. Looks like others are having the same problem.

Joe Goodman
- Greensboro, North Carolina

January 10, 2018

Q. Dear readers,

I am also facing the same problem but my question is: can we hide this patch that has occurred because of peripheral grain growth in Anodizing? Is there any solution for this problem?
If there is, please let me know.

Thank you all for giving such beautiful data on surface finishing.

Arun Kumar B V
Shree Karan Metal technologies - Bangalore, Karnataka, India

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