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Burning damage during anodizing

Quickstart:
In the anodizing of aluminum the components are immersed into a tank of acidic solution and connected to the anodic (+) positive pole of a power supply. The current separates some of the water in the solution into positively charged hydrogen and negatively charged oxygen. The oxygen is attracted to the positively charged components and converts the skin of the aluminum to aluminum oxides.
Readers new to anodizing of aluminum may wish to view our "Intro to Aluminum Anodizing".

Q. I run a fabrication shop and I just had my anodizer send me a photo of what they are calling a burnt spot on one of our handrails.

61936-1i 61936-1aDtl

The photo is of one of the vertical members inside the perimeter of the railing. This is all 6061-t6 material. I'm pretty sure we will have to remanufacture this railing, but what in the world could cause this in this one area? Are there any ways to fix this and have it re-anodized? ⇦ Answer?

Jason Patterson
- Hampton, FL
March 17, 2026

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A. Hi Jason,

When an anodizer or plater uses the term "burning" ⇦ huh? s/he is referring to a situation where the coating is rough, frosty, porous, or otherwise unsatisfactory due to excessive current density.

I could be misreading the photograph (it's happened before), but this looks more like the component touched the anode, or something connected to the anode, causing an actual short circuit.

The damage looks a lot more than skin deep to me, like the part has a deep gouge in it. Short of maybe filling it with welding material and regrinding, then having it stripped and re-anodized it looks irreparable to me.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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A. To me it looks like a void in the weld where trapped chemical (acid or alkali) has shot out and interfered with the anodic coating.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

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A. Jason
I gotta agree with ted's assessment of the problem. More than likely the part came into contact with live current somehow, causing an arc resulting in the burn and gouge like appearance. I do need to take Ted's advice/suggestion a step further though...

Weld filling a hole is a nice option but it does not always deliver the intended results. Sometimes it will discolor during the anodize process and tell a tale of what happened. Case in point. Some 30 yrs ago a client made some front panels for a prime contractor but they stamped the part number on the wrong side. They did the grinding, used the weld as filler and then timesaved the surface making a nice uniform grained surface, but after anodize, there was a 3/16 x 1 inch black oblong mark on the panel, all 500 of them. It was not pretty.
My point is be careful with the weld material that you use to fill that gouge or you may end up in the same boat. I never had the details on what alloy they used to make the repair thus causing the visual issue. The anodizing process does not think; it is only doing what it does but the end result is automatic.
Good luck.

Philip J. Verzal
retired after 35 years in anodizing - Newport, Tennessee

Q. Thanks for the response Ted! That's what we were all thinking here as well, it looks like something was dragged across the material that was able to carry a good bit of power.

I was told by this anodizer that this is caused by acid being trapped inside and actually burning its way through the material. They said it would look the same on the inside, I haven't cut it open to check as of yet. That seemed very strange to me, but I don't have much experience with anodizing, that's why I posted here.

It's very much more than skin deep, it's nearly half way through the 1-1/2" sch 80 pipe.

Jason Patterson [returning]
- Hampton, FL
March 31, 2026

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A. I fully agree with Ted. Something touched profile (anode), which either can be aluminium cathode (-) damaged by time getting thinner and by blower air touching profile or a profile where inside of bath was dropped any time before touching the profile during anodizing. So this issue/touch each other as +/- causing short circuit as pictured.

alaattin tuna
- Sakarya, Turkye
April 22, 2026

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