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topic 57881

Inconsistent colour change in anodising

A discussion started in 2011 and continuing through 2020 so far.
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October 19, 2011

Q. How to avoid colour change in anodising (some areas dark shade, some areas light shade)? What is the reason for this colour change?

Franklin Dominic
Industrialist - Bangalore, India

October 20, 2011

A. Hi, Franklin.

Aluminum alloys contain other materials besides aluminum (such as silicon, copper, etc.), and none of these "anodize" -- they just turn into black discolorations and even smuts. Besides changing the alloy or getting better quality aluminum, the single best thing you can do is probably to minimize or eliminate the etching step. (My knowledge of this is mainly book knowledge, and hopefully some real anodizers will chime in).


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

October 20, 2011

A. Pretty vague question, so I'll assume you're talking about an inconsistent color on one part, not different loads, or different alloys.

Possible reasons for varying shades of color on one part:

Inconsistent coating thickness.

Larger anodic coating pore size on one area, as opposed to another due to localized overheating during the anodizing process (insufficient agitation, too much surface area for the anodizing rack to adequately handle, etc, etc)

Poor rinsing.

Hot/cold areas in dye tank due to lack of solution agitation.

If you're asking about a color change from load to load..part to part...all of the above would apply, as would many other factors.

The first place I'd start would be checking the coating thickness from the light area, to dark area.

Marc Green
Marc Green
anodizer - Boise, Idaho

July 21, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello, I am new to anodizing aluminum. I am trying to anodize two of the same parts in an anodizing process but they have different result. Can you guys help me what is going on in my anodizing process?
*I am using same parameter/treatment to both of them

Faisal Aprialdi
- Salatiga, Indonesia

July 2020

affil. link
probert book
Aluminum How-To

"The Chromating - Anodizing - Hardcoating Handbook"
by Robert Probert

A. Hi Faisal. If we are to believe in the law of cause and effect, you don't do the exact same thing to two identical parts and get different results :-)

So what you're actually asking readers to do is to guess what is different despite your claim that nothing is different. Hopefully, you understand that this is difficult and will require a lot of data from you :-)

What alloy are you anodizing? What are these two parts which you claim are identical? What is the actual thickness of the anodizing in both cases. This is non-dyed, natural color aluminum? Are you sure that both parts were thoroughly clean to waterbreak-free standards? Are you etching? Are you desmutting? What is the composition of your anodizing bath and your anodizing conditions? Send good pictures of both parts to for posting here please.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 21, 2020

Q. Sorry not telling my detail process. Here is my processing method :
-. clean with degreasing, then rinse
-. etch in caustic soda for 1 minute, then rinse
-. anodize in sulfuric acid bath for 52 minutes with 12 V 3 A power supply, then rinse
-. put the part to dye bath for 20 minutes in room temperature, then rinse
-. sealing with cool sealing for 10 minutes
I am doing that method for both of my parts but one of them is perfectly dyed but the other one is like bare aluminum, is there any problem with my method.


Both of them are same aluminum alloy 6063. Thanks

Faisal Aprialdi [returning]
- Salatiga, Indonesia

July 2020

A. Hi Faisal. It seems to me that there are two likely explanations:
1. For some reason the aluminum-colored part did not anodize, and therefore could not accept the dye.
2. For some reason the aluminum-colored part was previously anodized and sealed, and having been sealed it could not accept the dye.

I remember once, many years ago, on a new aluminum wheel plating line startup I was called in to troubleshoot a plating rectifier that would not put out any current. They had put one wheel on the rack to check the rectifier, and the chemical people, equipment people, rectifier serviceman, and I were scratching our heads for an hour. Turned out the wheel had somehow somewhere, even if only accidentally over the months, become anodized. So #2 does happen :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 22, 2020

A. Everything Ted Mooney said above. In more detail, one minute in the etch, at any reasonable concentration, would not strip old anodizing and would not strip weathered oxide; then you did not deoxidize. Not related to this problem, but if you do any serious anodizing, you need higher voltage, up to 15 for most alloys and up to 21 for the 2000's alloys. Also, electrical contact reliability could have been your problem.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

July 25, 2020

Q. I am using 1.5 A/square dm current density and use aluminum wire for my hanger. I shape my aluminum wire like letter "U" and hang my part at the end of wire (in one of the holes in my part). Does it cause the problem from my anodizing process?


Faisal Aprialdi [returning]
- Salatiga, Indonesia

July 2020

A. Hi Faisal. "Gravity" hooks are often a bad idea for anodizing, because the hooks (except for a tiny sliver where the part is resting) will become anodized and non-conductive. Then a small amount of jostling from handling or agitation will move the part a tiny bit on the hook, to a spot which is anodized and non-conductive.

If you used the same hook for the first and second part, the hook is completely anodized and non-conductive, and that's the problem.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 27, 2020

Q. Now I am using that wire to hang just one part of my aluminum. So I am anodizing my part in 12 V 3 A/square dm current density for 45 minutes, the result is the absorbed the dye perfectly but it seems like the part is getting burning in some spots. Is the problem the agitation system (cause I am not using agitation yet), too much current density, or too much time of anodizing?


Faisal Aprialdi [returning]
- Salatiga, Indonesia

July 2020

affil. link
Trouble in Your Tank: Handbook for Solving Plating Problems
by Larry Durney
from Abe Books
info on Amazon

A. Hi Faisal.
The answers are: Yes, yes, and yes :-)
There is nothing wrong with experimenting and trying to learn. Enjoy it! And a scattergun can be a useful tool when experimenting.

But when your situation changes from experimentation to asking people to help you troubleshoot, you can't say: "I'm supposed to do these three things for proper anodizing -- which one of them can I do and ignore the other two, and still get proper anodizing? And by the way, I doubled my current density" :-)

As Larry Durney reminds us in "Trouble in Your Tank", there is no point even trying to troubleshoot until you decide to "Obey the letter of the law". You need agitation, you don't anodize at 3A/dm^2, and 45 to 52 minutes sounds like quite a long time to me -- what anodizing thickness are you looking for?  :-)

1.5 A/dm2 was working and was in the right ballpark -- what was your reason for simply doubling it?

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 28, 2020

Q. Sorry for causing so much trouble for you sir, I am running my anodizing process at home so I don't have so much tool to optimize my process. Cause I am using my wire to hang 2 part seems don't work well I am deciding to just hang 1 part in wire (so the wire attach in 2 hole of part). The result is perfect (in this process I use 30 minutes) but I need more deep color so I increase my time to 45 minutes and that problem occurred. Based on your statement, the problem is the agitation system so I will try to add my agitation system to my setup.

Thanks for your help sir

Faisal Aprialdi [returning]
- Salatiga, Indonesia

July 2020

A. Hi Faisal. Your questions are no problem, and you are correct that for dark, saturated, colors you do need thicker anodizing than for light pastel colors because the greater the anodizing thickness, the more dye that can be absorbed.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 29, 2020

Q. Can using an aerator fix that problem?
I am still searching for a pump that doesn't have metal parts, apply in water and acid resistant.

Thanks for your advice.

Faisal Aprialdi [returning]
- Salatiga, Indonesia

July 31, 2020

A. Hi Faisal,

Yes, a strong enough aerator that gives good agitation all around the tank is in fact a common way to keep the tanks mixed. A CPVC pipe structure drilled with lots of holes and secured to the tank edge is common.

And that brings me to my next advice, relating back to your first picture. I looked at it and thought, "lost contact!" And then I saw your racking.

The advice of these gentlemen is excellent as always and I'll add that once you have good mixing in your tank, a strong, secure rack is even more important to hold the part with good electrical connection all the way from the part back to the rectifier! The bubbles will put pressure on it, and anything less than a rigid rack, clamped securely in position relative to the cathodes and sides of the tank, will allow flopping around, lost contact, uneven current, ... etc.

I would try racking that part using a stiff yet springy v-shaped bent piece of narrow aluminum or titanium plate with the ends bent outwards and cut to a point that will fit in the holes, to contact the bore of the hole where 'rack marks' have the least aesthetic impact on the finished part since I assume a bolt goes through there when installed. This type of pressure racking component is called a "clip", and mounted to a current-carrying frame or "spine" that is in turn secured to something rigid (there are multiple approaches to this) so there is no possibility of movement.

You can look at some of the rack manufacturers' catalogues to get an idea of what I mean. Google "anodizing racks" and you'll find all sorts of images of approaches to racking securely to minimize both movement and rack marks, while maximizing good electrical contact, with examples of various racks for different part configurations.

Don't forget that you have to strip your racks in between uses! The anodic layer is an insulator. You can etch it off in hot caustic soda and then de-smut it. And check the resistance all the way from the end of the rack assembly to the part surface before starting to anodize. Sometimes the bolts get degraded or don't strip right and you end up with a dead rack.. just take it apart and use fresh bolts and grind the contact surfaces to bare substrate.

Best of luck!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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