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topic 18494

Black anodizing problem - purple sheen

A discussion started in 2002 but continuing through 2018


Q. In our Black Anodizing process, lately our parts are coming out with a purplish sheen to them, instead of the dull black matte finish. Any suggestions on what the problem might be?

Christi Blackley
- South Fulton, Tennessee, USA


A. If nothing has changed elsewhere (and you are dyeing to saturation), the dyeing problem could be due to: weak solution, improper pH, impurities clogging pores (e.g., high sulfates), high temperature causing premature sealing, or imbalance in a multi-component dye. Send a sample to your dye supplier for analysis; I know of a case where a gold dye addition was recommended to cancel a blue-ish tinge in a black dye (and it worked!).

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.


A. This is a very common problem.

When the anodizing thickness is low the blue components of the black dye are absorbed preferentially. So just increase your thickness.

Also common, is the fact that people who use so-called computerized controls do not understand that you must have the correct surface area for the computer to apply enough amperage.

If you are doing a 2000 series alloy, you MUST have 21 volts (plus 1 more if titanium racks) to get 12 amps per square foot. So override the computer and raise the voltage to 22, then all the components of the black dye will absorb.

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. The answers you have already received are accurate. One thing I would add other than the causes mentioned is the pH of the seal after the anodize and dye process. A lot of proprietary sealers have specific pH ranges. If the seal is not maintained within these parameters, I have seen it cause the condition you describe. Typically, manufacturers of the sealants recommend checking and adjusting pH on a daily basis as a means of control.

Peter Cox
- Seabrook, New Hampshire USA

March 12, 2018

Q. Black Anodize Type II. We are seeing sporadic color variation at times (purple instead of black) on billet machined T6061 parts. Situation is not limited to a given customer or lot of parts. We primarily process 6061 material on our line. We will have some work bars which are completely uniform deep black and at times I will get a work bar which have some parts on rack of the work bar that appear to be shades of purples and not deep black. This can occur on the same rack (some parts on rack deep black, others not) while everything else on the work bar is fine. The initial thought was bad contact to where thickness may be inconsistent. However, after checking the lighter shade parts with a thickness gauge we are finding parts have adequate thickness (.007"^.0007") and are consistent with other parts with the same thickness that are deep black in color. There is no smut formation on the parts either. Our dye is air agitated, pH is verified and within spec. Not sure what else to look to understand what may be causing the issue.

Mike Jetter
- Whittier, California

March 2018

A. Hi Mike. The only times I've personally seen this it was the result of the anodizing being too thin, as mentioned by Robert Probert. But in your case, with heavy thickness verified, and pH of the seal tank verified, I wonder about Ken Vlach's idea that the dye tank itself is causing partial sealing before the dye has been fully absorbed? Have you verified that the temperature is not too high?

If you search this site for "black anodize" you'll find a number of threads which might offer further ideas. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

March 12, 2018

A. Varying shades on the same rack has to be (1) contact reliability or (2) air agitation displacing solution (which again is thickness).

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


Please forgive the typographical error, which Willie and then Mike called our attention to.
The heaviest anodizing, hardcoating, is about 0.002" thick. Anodizing thickness of 0.0007" is mentioned in some architectural specs and is about as thick as decorative anodizing gets. A lot of anodizing is significantly thinner, but thick anodizing means deeper pores for better absorption of dyes/full saturation of dark colors, so .0005" to .0007" is not unusual for black anodizing.

March 2018

? Hi Robert. But if the problem is lack of thickness how do we account for Mike measuring it as .007"^.0007"?


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
We need "Aloha" now more than ever

simultaneous March 13, 2018

A. I'm guessing Mike means .0007", not .007.

Are these parts identical, or is there variation in the product being anodized? If identical, I go with Robert and lost contact. If varying, I'll throw out there that it could alloy/temper related, particularly if the dye concentration is not up to par.

Willie Alexander
- Green Mountain Falls, Colorado

March 13, 2018

A. Hello Gents,
Just a thought, but has the dye concentration been checked?
from my own experience, when we have had purpling of our black dye, some of the time it has been the dye concentration drifting towards the lower end of the spec, due to high use, that has been the cause.
I hope this is of some use.
best regards

Mark Lees
- a sunlit rock in the Irish Sea

March 13, 2018

Sorry, thickness was .0007". Ted, one thing you noted was dye temp. Our range is 120-140 °F per TDS. We are at the higher end of the range at 140°F. We decided to take it down to 130°F to see if that helps.

Sent a sample to vendor for dye concentration about 3 weeks ago. TDS recommends 10g/l, our current concentration is 13 g/l. Dye is about 1.5 years old. We also have good air agitation. We will see what that does. Please let me know if there are any other recommendations. Thank you all for you help.

Mike Jetter [returning]
- Whittier, California

May 2, 2018

A. Hi Mike,
If that black you're running is the ubiquitous Reliant HBL (specs sound about right), 1.5 years is fine if you've got your concentration kept up, since is a single-component dye and won't age-shift like a mixed dye in my experience, and anyway if you've got a mix of shades on one rack it's not a tank shifting issue. And anyway that dye is DEAD STABLE. The last time I changed my bath was in October of 2014- it's still black as night. I also run it stronger than recommended, but that's neither here nor there in terms of my response to your issue, which is that I'm wondering whether you've got good, even rinsing coming out of the ano tanks, and whether you precondition parts for dye with a nitric dip (about 5-6% vv from 42 °Baumé, not too long or they'll take on a scorched appearance, immersion time depends on concentration and circulation)? Do you find yourself having to run elevated buffer in your dye (this is totally fine per the manufacturer) or really keep beating the pH of the dye tank up constantly with ammonia? The theory here is that Sulfuric is way more 'sticky' than Nitric, and the post-anodize nitric dip will ensure that the parts are squeaky clean to accept dye, and minimize Sulfuric drag-in. It's also a lifesaver if you have issues with blind holes bleeding out after seal. Of course you still have to rinse the Nitric off really well before going into the dye, but it rinses MUCH cleaner, and with MUCH less effort!
If you're already doing this, then yeah I'm gonna go with lost contact :)

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

May 15, 2018

A. Sorry, is that coloring bath electrolytic bath as tin bath or electroless coloring? Also if I were you I would share pics which describe the problem. I have some answers but let me know the electrolytic or electroless bath?


alaattin tuna
- turkey,sakarya

June 12, 2018

When you mention varying shades of purple rather than black, is each piece the same color purple throughout the part, or is it a gradient with the middle of the affected parts being lighter than the ends? I have seen both of these cases on my line in the past.

If you have a gradient within each part, then I would highly recommend checking your connections on the racks themselves; including taking off the bolts/connectors and giving them a good scrub to make sure you're working with clean surfaces. If that were the case you would probably see a variation in film thickness within the part. Also make sure the connection from the work bar to the anodizer blocks is very solid and clean. A gradient would mean you're not getting enough power to your parts.

If every part is the same shade of light through the part then I would still recommend to service the rack to eliminate that as a potential source. Personally every single time I see this light black (purple) condition the part is always loose on the rack- regardless of the film thickness measurement. Every time. It does not make sense to me why the thickness is adequate even on a loose rack, but it is what it is.

Also of note, is the position of the light pieces on the work bar consistent? Or do they seem random? I have had instances where a poor rinse (when sprays do not penetrate the work load toward the inside of the rack) affects the colorability of the inside portion of the work load. If this is the case I would increase the immersion time of your pre-Dye DI rinse to give your spray rinses a helping hand. If you don't have spray rinses at all make sure your agitation does not have any dead spots in which gunk could hide in the pores and prevent dyeing.

Best of luck.

Scott Oberski
Aluminum Anodizer - Grand Rapids, Michigan, USA

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