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topic 15825

Black anodize turning bronze color

A discussion started in 2002 & continuing through 2017


Q. We have some aluminum parts that were black anodized. Alloys 6061 and 5052 were used in the same assembly. When the units were returned from the field, the anodize had changed to a bronze appearing color on both the 5052 and 6061 parts. The units are not exposed to sunlight or adverse atmosphere in installation so I wouldn't expect fading to be an issue. Does anyone have an explanation of what is happening here?

Lou Volka
- Farmingdale, New York

Ed. note: Fading of black anodized parts is a very common problem, addressed countless times on this site, so readers may also be interested in
- letter 2459, "Discoloration of Black Anodized Aluminum Parts"
- letter 22602, "Black Anodized parts fading to brown / bronze",
- and letter 31658, "Black anodized parts fading - Non-UV cause".


A. Heat can make black anodize parts fade to bronze or gold in a hurry. A poor seal will accelerate the process.

Victor Waldman
- Naugatuck, Connecticut


A. I suspect that the parts were poorly sealed...poorly sealed parts will fade even if exposed only to fluorescent light. Another, but less likely culprit, would be inadequate coating thickness, or perhaps the dye used had very poor lightfastness. You didn't say how long it took for these parts to fade, but I'd bet my lunch on the bad seal.

Marc Green
Marc Green
anodizer - Boise, Idaho


A. 1. The first thing that comes to mind is inadequate sealing.
2. The second thing is iron contamination in the rinse water and/or the seal.
3. The third thing is copper in the anodizing solution from previous 2000 series alloy work where the copper is so high in concentration that it occludes in the pores later to bleed due to, again, inadequate sealing.
4. Also, some very cheap black dyes are very fast to fade, ask the vendor what the light fastness number is on the black dye he is using, it should be 7 or 8.

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


thumbs up signMy thanks to all of the responders to my question of black anodize fading to a bronze color. You all seem to have a consensus that poor sealing is the culprit. I will be taking this up with the anodizer.

Thanks again for your help.

Lou Volka [returning]
- Farmingdale, New York


A. No problem, Lou.. its nice to get a "Thank You" every once in awhile! Send your anodizer a coupon of the same alloy as your parts, have him run it with his next load, then ask him to preform a dye-stain test. If he's worth his salt, he'll know how to preform the test, and it will pass.

Marc Green
Marc Green
anodizer - Boise, Idaho


A. As to the discoloration on your black anodized parts, what type of process did the parts go thru prior to anodizing? If you get any oxide impregnated into your part, that will cause surface rust to start to bleed thru the anodize. Maybe someone used some type of oxide sandpaper [linked by editor to product info at Rockler] to deburr the parts?

Ed Menchaca
- Whittier, California

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Black hard anodize is turning brown


I have a black hard anodized finish in accordance to Mil-A-8625 [link is to free spec at Defense Logistics Agency,] C, type III, Class 2, applied onto a aluminum plate 6061-T651/T652. These anodized parts are sitting in some facility in Asia. We have an initial report of rusting, on parts that have a Nitrite finish. This is still under investigation. Today I got some pictures of Black Anodized finish with brown discoloration. I have never seen this on anodize and I am wondering if anybody has seen this type of problem before. I have some pictures to share with you...


Any info would be great.

Gerry Piccone
- Montreal, Quebec, Canada


A. Yes I've seen it plenty! It's the lack of a quality seal (or possibly no seal at all!). The mil spec is ambiguous when it comes to Class 2, Type III dyed hardcoat. The spec mandates "unless otherwise stated, Type III coatings shall be unsealed." Now anodizing house supplies unsealed dyed anodic coatings, however, where is it otherwise stated to seal the dyed hardcoat. Our shops take on it is that sealing is implicitly stated by indicating the parts are to be dyed. ASTM ASTM B136-84 [link by ed. to spec at TechStreet] , anodic seal integrity test will quickly tell you if the parts are sealed or not - if you drop a little nitric on it and it turns instantly white, it's not sealed! The safest thing for an OEM or designer when calling out Type III, Class 2 coatings is to specify "nickel acetate or equivalent seal required."

P.S. Of course I might be wrong! If your nitrided parts are prematurely rusting, is it possible that your parts are in an overly aggressive corrosive environment that no coating can withstand. I recently had a relative spend a couple of weeks in China, Shanghai in specific, and he came home with a terrible cough he blamed on the air there. It took him a month for his throat to recover.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

Anoplate banner

(2005) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We have type II, class 2, black anodized aluminum parts that are fading to a bronze/brown color (in a matter of a few months) except where someone has a finger/hand print. Military spec is 7.1.2

Some of the parts were single and some were duplex sealed.

The thickness of the coating is .7 mil
The ADT is 609
The coating weight is acceptable >1000 mg

Not sure if it is the organic dye or a bad sealing process. We have used two different platers with the same results.

Need help,

Ken Loree
- Huntsville, Alabama

Black Anodising Turning Blotchy & Bronze

April 9, 2014

Q. Hi,

I'm puzzled as to why the new equipment I bought 2 months ago has started to show fading on the black anodised parts.

The only thing that may have come into contact with the anodising is possibly a cleaning product a trainee used. It's a foam cleaner called AFC antistatic foam cleaner.

I did a test on the underside of the same anodised parts over several days by spraying it on & leaving it but it hasn't had any effect.

Thanks in advance,

Kevin Foy
- United Kingdom

April 9, 2014

A. Suggestion. Do a seal test and re-post.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

April 10, 2014

Q. Thanks for the reply.

I'm the end consumer for this product. Do you suggest I go back to the manufacturer and request a seal test?

This equipment is track used on film sets so in my mind it's highly unlikely the anodising has come into contact with anything aggressive enough that could do that, Industry standard WD40 is the only thing regularly used to free run the wheels on the track.

The finger markings and strange blotchy appearance looks as though it has been in contact with a liquid. Although what look like splatter marks are in reverse i.e. dark & not faded.

I know it's very tricky, but I wanted to at least have some clues to the possibilities of how this could happen before taking this up with the manufacturer.


Kevin Foy [returning]
- United Kingdom

simultaneous April 10, 2014

A. The fading or turning purple/bronze is common with an organic based dye. You need a metallic based one. Note that I am not referring to architectural 2 step anodizing.

The cause is from UV light reacting with the dye. This can be from sunlight or fluorescent lights.

Some dye companies post a number for fade resistance of each dye. If I remember, a rating of 9 is very good.

James Watts
- Navarre, Florida

April 10, 2014

A. Kevin

If by "film set" you mean an area where there are bright lights, that would likely accelerate the rate of discoloration. To confirm, locate a light source as close as possible to the surface of the part without allowing the part to get too hot to touch. Cover an exposed area with a coin, electrical tape, etc. to prevent exposure and check after 4, 8, 16 hours. There is a cumulative effect, so the exposure need not be continuous.

Willie Alexander
- Colorado Springs, Colorado

April 10, 2014

A. Unsealed or partially sealed anodic pores will bleed and smear dye from humidity cycling of the air. WD-40 solvent will pick-up and smear dye from unsealed or partially sealed anodic pores. Do an ASTM B136-84 [link by ed. to spec at TechStreet] seal test and re-post.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

April 11, 2014

thumbs up signFirst off, thank you very much everyone for their valuable input, it's very much appreciated.

In reference to the ASTM B136-84 seal test. Unfortunately, as I understand it, this type of test is beyond my facilities and capabilities. However, the light test will be very easy for me to perform in a controlled manner and I will undertake this immediately. As soon as I have some results I'll post them up here.

Thanks again.


Kevin Foy [returning]
- United Kingdom

April 15, 2014

Q. Hi again,

I did a light test as suggested by Willie Alexander.

This came up with a positive result. The light source was a small focused circle of light about an inch across on the underside of the anodised sleeper part of the track. Part of the light falling onto the anodised surface was masked using gaffer tape.

I will post some before/after pictures to show how the anodising has faded where the surface was exposed to the light source, and remained darker where it was covered up.

15825-2  15825-3  15825-4

Any suggestions as to why this has happened are extremely welcome. Also, any advice on if or how I should approach the manufacturer would be much appreciated. Bearing in mind I am an end consumer/purchaser of the equipment and do not have anything to do with the commissioning or manufacturing processing of it.

Thanks in advance.

Kevin Foy [returning]
- United Kingdom

April 15, 2014

A. OK, so you cannot do the ASTM test with nitric acid, then at leaset dip a cotton ball in acetone or alcohol,and wipe it across the black dyed surface - if it picks up black color, then the pores are NOT sealed.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

April 17, 2014

Kevin, Robert is most likely correct in that the parts probably weren't sealed well. A couple of other causes could be that the black dye used in the anodizing process did not have good light-fastness properties, or that the coating was not thick enough to allow the dye to absorb deeply into the pores.

Keep in mind that dyed anodizing is not the best coating for continuous exposure to light, and it will eventually fade, even if it is properly applied, and sealed. But certainly you should expect much better results than what you're currently seeing.

Regardless, you are an unhappy customer whose purchase is not performing up to par. As with anything else you'd buy that didn't perform as expected, I'd return the product, and ask for a refund, or a replacement. It would be up to the manufacturer to stand behind their product. Obviously some do, and some don't. If they don't, I guess you're stuck, and chalk it up to a lesson learned not to buy equipment from that manufacturer again.

Marc Green
Marc Green
anodizer - Boise, Idaho

April 18, 2014

Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby

Q. Hi All,

thanks for the responses.

I tried wiping the surface with what I believe to be pure Acetone (Care+ Acetone Household Solvent) but it doesn't say on the bottle. I didn't get any residue left on the cotton wool.

So I have black anodised surface which isn't light fast but also doesn't leave residue.

Does this help at all to give a rough diagnosis or opinion of what's wrong? Is it not the sealing what is the problem but rather the lightfastness of the dye used?

I have written to the manufacturer and included the photos so will await a reply. It's easter weekend here in the UK so I won't expect to hear back until maybe mid next week.

Thanks again

Kevin Foy [returning]
- United Kingdom

A. Hi Kevin. Although your acetone rub test is not as conclusive an indicator of proper sealing as more formal tests, yes the sealing sounds okay, and the dye may be the problem. Can you determine the manufacturer, type of dye, and lightfastness rating?


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

October 16, 2016

Q. We recently face uneven black Colour on Al 6061, which go for anodizing at thickness more than 15 microns, black dye with Japanese dye maker and nickel acetate salt hot sealing at 80 °C, the uneven black coating was found at the upper right corner of the part. Can anyone to help to solve this problem?


Anthony Aw Teow kheng
engineering services - SINGAPORE

October 17, 2016

A. Anthony

The blemish looks like chemistry dried on it during a tank transfer, or the part was not completely submerged during the dye or seal step.

Willie Alexander
- Green Mountain Falls, Colorado

October 18, 2016

Hi Willie,

Thank you very much for your advice, we will reduce the transfer time between tanks and see it any improvement...

... October 20, 2016

thumbs up signHi! Willie, the problem already solved.

Thanks once again for your advice.
Best regards,

Anthony Aw Teow Kheng [returning]
- Singapore

February 9, 2017

Q. A lot of great information here, but I'm not completely positive that my case is a seal problem. I have a 5051 aluminum part that is black anodized. This part has 2 lasers that are placed on it. Each laser has an adapter plate that fits right into this shelf. The area under the adapter plate is still fine, you can even see the outline of the plate, but everywhere else is a different color. I originally suspected heat, which could still be the case, but after some investigation, it could also be UV light exposure (the lasers are in the UV spectrum, but should not be leaking like this). I appreciate your comments! Thanks.


Nick Anderson
- San Jose, California

February 11, 2017

A. I would suspect it's the UV exposure, which can lead back to a poor seal, inadequate anodizing thickness, or choosing a black dye that does not have good light fastness (albeit most of the black colors do). If it's extreme UV exposure, then dyed anodizing may not have been the appropriate choice in coatings.

Marc Green
Marc Green
anodizer - Boise, Idaho

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