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Coloring stainless steel (anodizing)

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Q. Can someone please direct me into achieving coloring stainless steel (anodizing). I need to know what equipment to use and how to go about processing it. Thank you in advance.

Yours truly,

MUI THEN
- Delta, BC, Canada


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A. Stainless steel does not anodize like Ti or niobium, it is called inco-colouring and is usually done with heat and moisture. Oven heating at a certain temperature with a certain amount of humidity will colour stainless steel.

Jeff Swayze
- Kelowna, B.C.


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Q. Jeff,

Regarding Inco-colouring of stainless steel, how colorfast is the application for exterior applications? Thanks, Mike

Michael Liu Taylor
   specialty stainless steel distributor
Dallas, Texas



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A. Well, the layer of colouring on the stainless steel is approximately 20-30 angstroms thick (the usual thickness of chromium oxide on most well passivated stainless), so if the parts are not abraded or in contact with corrosive environments, the colouring should remain intact. Perhaps if you used a clear aftercoat, the colouring will be protected from environmental damage. Which grade of stainless are you referring to, and what is the application? Will the parts be ornamental , or will they be handled regularly? Your results will be dependent on your application, as the colouring can easily be abraded, or etched away in relatively corrosive atmospheres.

Good luck!

Jeff Swayze
- Kelowna, B.C., Canada



A. The coloring is completely lightfast because no pigments are involved; it's a diffraction (interference) coloration caused by the light reflecting off the surface of the clear chrome oxide coating and off the surface of the part below the coating. The coating is a partial wavelength thick, which causes the interference.

For that reason, though, I doubt that you can clearcoat it without losing most of the color, because that adds a second transparent layer, probably too thick for partial-wavelength interference. Good luck.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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A. Actually, there is a proprietary method sold by one of the advertisers at this site. Look for Prismatic. Color is jet black.

James Watts
- Navarre, Florida


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A. A coating on stainless steel may be achieved by an electro-chemical process, which creates a super-thin layer of chrome oxide. However, coloring 17-4 stainless steel is somewhat unpredictable, and better results may appear on 304 and 316 stainless steels. In theory, this procedure works best when the metal has been electropolished and demagnetized. In my experience, I have gotten different results, varying from drastic variation of tonalities from sample to sample (from brown to fainted black), to unevenness hues on one piece.

Instead, would black-oxide work for your application?

Clinton Tharp
- Avilla, Indiana


June 13, 2011

Q. Clinton Tharp, would you share your process and chemicals with me. I just want to color code 302 stainless steel pins, 22 gauge, 21 gauge and 20 gauge that are 3/4 inch long. So my pot or volume required is minimal and since my volumes are very small, it is not worth going outside for the process.

Mike Dalton
- Skokie, Illinois, USA

May 13, 2008appended

Q. Please let me know what process is used to colour 304 stainless. Can I do it myself?

Theunis Pretorius
knives - Kempton Park, Gauteng, South Africa


January 24, 2012

Q. I WANT TO GIVE BROWN COLOR ON STAINLESS STEEL 304 BY DIPPING INTO A CHEMICAL (BY CURRENT OR WITHOUT CURRENT). PLEASE ALSO TELL ME , WHAT OTHER COLORS CAN BE OBTAINED ON STAINLESS STEEL BY DIPPING INTO SOME CHEMICALS.

REGARDS

Shahzad Amin
- Gujranwala, Pakistan

April 27, 2012

Q. Most people are talking about using only 18SWG thick SS 304 or 316 material. Can this be done on 202 grade material which is 24SWG thick?

Visweswaran Manickam
- Trichy, India



April 14, 2014appended

Q. Hello, I have a slight problem and would appreciate some help
I have been asked as a degree project to color some steel accessories (small pieces used for clothes).
After some research I have found that the steel in question is most likely stainless steel although I don't know what family exactly. I saw that it could be colored by adding a layer of chromium oxide by passivating it.
Now i used different baths and this is what I got so far :
. nitric acid 20% --> stainless steel goes yellow then dissolves if exposed any longer
. nitric acid 20% + sodium dichromate (3oz/gal) --> same result as above
. citric acid 10oz/w --> stainless steel goes tin pink but doesn't dissolve

So I want to ask, am I going in the right direction? And what am I doing wrong because I want to have other colors?

Thank you for your time

Salem Mohamed Medhat
student - Korba, Tunisia


April 2014

A. Hi Medhat. The first thing wrong for a science student is trying to develop a treatment process for an unknown material. That's not good science because it makes it inherently not replicable :-)

Austenitic stainless steel is much more suitable for coloration than martensitic, so start by verifying that your sample is non-magnetic if you have no other clues. I'm not familiar with chemical coloration, and have read that you must do an anodic treatment to get the chromic oxide for good coloring.

An excellent paper on all sorts of stainless coloration is "Colouring Stainless Steel" by the European Stainless Steel Development Association, available at www.euro-inox.org/pdf/map/ColouringStainlessSteel_EN.pdf

Good luck and Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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