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topic 3227

How to prevent blotchy unneven anodizing

A discussion started in 1999 and continuing through 2020 so far.
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Our company supplies high strength tension rod assemblies and custom hardware for the architectural industry. We commonly use an anodized finish on 6000 series aluminum, specifically 6061. The anodizing is done to various finishes; machine finish, brushed, etc. The problem is blotchy spots and uneven anodizing. From what I've read it may be the grain boundaries being brought out or dendrites. Can anyone give me more information on what is happening and if it can be prevented to provide a uniform finish.

John Rappa
structures - Westford, Massachusetts


The uneven/blotchy spots on your finish may be due to uneven current density across the surface of the part. The blotchy spots are seeing a higher current density than the surrounding area, which raises the temperature locally and degrades the finish.

You don't state this in your letter, but I'm willing to bet the coating thickness is lower at the problem area(s).

Gerald Janssen
- Chicago, Illinois, USA


"Surface in is surface out", The first question I would ask is the surface which is blotchy, the same finish and texture as the more smooth surface? Look at the surface prior to finishing, and also prior to anodize after the cleaning steps have been completed. Most likely you will see the surface issues you are concerned with prior to the actual anodize portion of the process. 6061 when surface finished evenly produces a very nice anodized surface. Remember, anodizing "magnifies" the appearance of the surface conditions.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


These guys are all correct. I can't tell you how many times I have had to explain to a customer who will bring in a lot of say 10-15 pieces, and one of them they sandblasted for who knows what reason and it produces a poor finish (I have only seen maybe 2-3 really good blasting jobs). I had one customer, lets call him customer X, take a 4" sq. panel and basically make it a 4 section area of approx. 1" each. He blasted one area, buffed another, polished another and left one area with nothing done to it, I black anodized it for him and noted that all 4 finishes come out differently on the same piece to show him that it does matter, you guys should all try this if you haven't, you'd be surprised how it can really sell someone on a choice of finishes to make a better anodize. Because as Ward stated, "surface in, surface out" We can sometimes alter the look to a certain degree as anodizers, but that statement sums it up 100%

Matthew Stiltner
plating company - Toledo, Ohio

July 29, 2020

Q. I extrude 6005A aluminum with exit temperature of 1020 degrees. The material then gets anodized and ends up with a blotchy/ grainy surface.

What are ALL the possibilities that can cause the blotchy/ grainy surface? What are best counter measures to put into place ?

Pe Se
- Wausau, Wisconsin

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