Authoritative Answers, Fun, & Aloha -- no cost, no passwords, no popups
(as an eBay Partner & Amazon Affiliate we earn from qualifying purchases)

Home /
Site 🔍
pub     mobile?
Metal finishing Q&As since 1989


Aluminum Anodizing Black Color Matching Problem

none is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages

Q. Hello everyone
I mounted 2 racks of aluminum 110 pieces each to be colored black 40 minutes in machine. One rack on Monday and another is on Wednesday. Required thickness from customers is 15 µm. It came out very different in colors. One whole rack became black and another rack became blue. (Temperature was OK. pH was 6.14)


Now I'm very confused and nobody knows the right answer. I hope I get it here. I need your brain now

William Shatner [fictitious]
- Stockholm, Sweden
May 25, 2024

Ed. note: Welcome! Please feel free to visit anonymously! But this is a forum with a 35-year legacy of camaraderie & warm aloha incompatible with anonymity; please post only with your full real name and location.

Defelsko Positector 6000
combo magnetic & eddy-current coating thickness tester
on eBay or


(affil links)

A. Hi William. When black anodizing comes out blue the first step is to actually measure the thickness of the anodizing rather than assuming its thickness from the spec and supposed anodizing time. What is the actual anodizing thickness on the black and the blue pieces? Thanks.

Luck & Regards,

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

Q. The actual anodizing thickness found on the black pieces was 14 µm and on the blue pieces was 8 µm max. This was what I found. But does it make any difference if I found 15 µm on a blue piece ? Or what do you mean?

William Shatner [fictitious]
- Stockholm, Sweden

simultaneous replies
for Shops, Specifiers & Engineers

"Aluminum How-To" -- Chromating, Anodizing, Hardcoating by Robert Probert (2005)
avail from

"The Surface Treatment & Finishing of Aluminium and Its Alloys" by Wernick, Pinner & Sheasby (1959 - 2001)
avail from eBay, AbeBooks, or Amazon

"The Metallurgy of Anodizing Aluminum" by Jude Mary Runge (2018)
avail from eBay, AbeBooks, or Amazon

"Surface Treatment and Finishing of Aluminum" by Robert King (1988)
avail from eBay, AbeBooks, or Amazon

"Anodic Oxidation of Aluminum and Its Alloys" by V. F. Henley (1982)
avail from eBay, AbeBooks, or Amazon

"Aluminum Finishes Process Manual" by Reynolds Aluminum (1973)
avail from eBay, AbeBooks or Amazon

(as an Amazon Associate & eBay Partner, we earn from qualifying purchases)

A. Hi again. I don't know why the thickness is only 8 microns (maybe the rectifier power failed or maybe there was bad contact to the rack for part of the cycle) ... but the lack of anodizing thickness *IS* the problem which made the parts blue.

8 microns is not enough thickness to get black anodizing -- the pores aren't deep enough to absorb enough dye. Black is the absence of color, and it is obtained by completely blocking all other colors, and you can't completely block them with dye in a layer which isn't thick/deep enough to do it.

Depending on the type of dye which is used, you'll get either dark blue or dark maroon instead of black when the anodizing is not thick enough.

Luck & Regards,

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

Q. It happened like this. We dyed the same material with same chemicals on 2 different times and we get 2 different results. All pieces on one rack was black which is approved but pieces on another rack are all blue which is not approved and make a big question mark.
Now I have 2 questions.
(1)... What are all the factors that can a black anodizing blue? Kindly mention all those factors.
(2)... Do you think a bad pH can make them blue? Or how much a pH is crucial?

William Shatner [fictitious]
- Stockholm, Sweden

Q. Hello everyone.
It says that a pH of 6.00 is an ideal for anodizing. My questions are:
What are the effects of low pH (less than 6.00) and high pH (above 6.00) on aluminum black anodizing during the process of sealing the pores on material? Does low/high make shift the color or what will be the reactions of low and high pH on material?

John Deer [same person, different fictitious name :-)]
- Stockholm, Sweden
May 27, 2024

? What kind of sealing bath do you use? Most likely you use cold sealing. Up to me, source of problem is cold sealing. Please let me know your sealing

- TURKEY Sakarya
June 5, 2024

(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Chemicals &
& Software

About/Contact  -  Privacy Policy  -  ©1995-2024, Pine Beach, New Jersey, USA  -  about "affil links"