Interference coloring and two-step (electrolytic) coloring of anodized aluminum

A. Are you possibly referring to "two step anodizing" of aluminum?
What is the question?
James Watts
- Navarre, Florida

A. If you are talking about anodizing aluminum, then most probably you want to know about multicoloring -- a 2-step process of treating freshly anodized parts. We are presently installing an automated line, and one of the processes here is this multicoloring. The process is capable of producing (and reproducing) greys, greens, blues, violets.
Max Stein
captive metal finisher - Montreal, Québec, Canada

Regards to Mr. Max Stein. I'm sure he was my manager when I worked in Montreal, Quebec. Dado Macapagal - Ont., Canada October 27, 2012

A. Both processes exist, a one step and a two step electrocoloring on aluminum. Both process produce "interference colors". Contact Pinakin Patel at Clariant, he has large experience with this kind of procedures. ALCAN also has a patented process and they might also help.
Guillermo Luna
- Mexico City, Mexico

A. Interference coloring of anodized aluminum has been introduced to the market since 1984. It is a process which is different from the conventional 2-step electrolytic coloring process. The color effect is due to the interference of visible wavelengths rather than light scattering. Interference colouring is by either a 2-step or 3-step process. There are basically two methods for interference coloring :

1) "2-Step Interference Process": This is a new process.The conventional anodizing bath and Sn-electrocoloring bath are used. But, a different power supply and close control of anodizing parameters is required. The function of the power supply is to widen the pores of the anodic film (This is called modification of the film). Anodizing solution temperature is maintained between 17.2 and 17.8 °C., sulfuric acid concentration between 175 and 200 gpl. The power supply for anodizing is a DC-and-AC device with current density controller. Normal anodizing is done to get the required film thickness and then the interference (modification) step is performed in the same tank. Thus anodized load is then colored in a tin-based electrolytic process.

2)"3-Step Interference Process": This process requires a second anodizing process in a phosphoric acid solution to widen the pores of the normal anodic film obtained in the first sulfuric acid anodizing bath. The first anodizing bath is conventional sulfuric acid solution, temperature controlled at +/- 0.5 °C, concentration controlled at +/- 5 gpl. In the second anodizing tank, a special power supply and phosphoric acid based solution is used to modify the pores of the anodic film. Thus anodized load is then coloured in tin-based coloring electrolyte to have bluish grey, blue and green colors.
Timur Ulucak
- Istanbul, Turkey
1997

A. S. Kawai & M. Yamamuro's article, entitled "Interference Coloring of Dual-Anodizied Films on Aluminum Containing Electrolytically Deposited Thin Metal Layers" and published in the May 1997 issue of Plating & Surface Finishing, may interest you. Ling Hao

A. Architectural anodising is a little out of my field, but I would be surprised if you could replace tin sulphate with nickel sulphate. Why do you want to substitute one with the other? Is there a tin sulphate supply issue?
Brian Terry
Aerospace - Yeovil, Somerset, UK

A. Hello Mr. Manoj ... hope you are fine.
I read your question and comments.
I can help you: yes, you are right, you can add nickel sulphate into tin-based electro-coloring solution.
By adding nickel sulphate you can earn two major benefits:
1. low cost of nickel sulphate because when nickel sulphate is added, then you can do color process with 8-10 g/l of tin sulphate instead of 12-15 g/l
2. you can get more red and shine in coloring of aluminium profile.
I'll come to India after Eid; so if you say, then I'll feel happiness.
Best regards.
Azeem khan
anodizer - Karachi Pakistan

----
Ed. note: Apologies, but we can't put readers into personal contact. Thanks for your understanding.

A. Hi Hassan,
Counter electrodes are usually 316 stainless steel. The useful area should be a large as possible and a least the area of the work load. By "useful", I mean that the general good practice of keeping counter electrodes (and anodes) slightly shorter and less wide than the work load should be followed. Failure to follow this rule, while increasing the area, has an adverse effect current distribution.

A. Hi, the stabilizer performs two roles.
1) To lessen the rate of oxidation of tin II to tin VI. in the bath.
2) To act as an oxygen scavenger at the work piece. This increases significantly the rate and depth of coloring.

Harry Parkes
- Birmingham, UK

A. Hi, Mitesh; hi, Manu. Per Wernick, Pinner and Sheasby the contents can be materials aromatic carboxylic acids, cresol sulphonic acid, thiomalic acid, pyrogallol, resourcinol, and ferrous salts. In view of the many possibilities and the fact that they tend to be proprietary, your options seem to be to either buy them or do a lot of detailed readings of the literature followed by extensive experimentation. Best of luck.
Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

180 g/l sulfuric acid
8 g/l aluminium
Temp range of 16 to 23 °C
With a Munk IF30 controller for interference

Stannous sulphate 22.2 g/l
sulfuric acid 42.4 g/l
Stabiliser 35 g/l

A. Hello Alaattin. We appended your question to a thread where Timur Ulucak explained the basic difference. Hopefully other readers will flesh out the answer in the way that you request.
Regards,

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

A. Hi Navnit,
In addition to carefully reviewing this thread, which contains a lot of information for you, please try a literature search at surfacequery.com, and also see if Werner, Pinnick, & Sheasby helps you.
Regards,

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