Metal finishing Q&As since 1989
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Color-coding Phosphate conversion coatings?
Q. I am working on a hypothetical design project for school, and I am curious about the variety of colors available with phosphate conversion coatings, as a coating on steel. I thought that I had read that there are a variety of colors available for the iron and zinc phosphate coatings, but after spending a lot of time looking at web sites for companies that supply these coatings, it seems like "color" isn't mentioned much, except to say it is clear, or grey. I am not interested in getting a "great" finish, only that there is enough permanent color change to be able to easily distinguish one small steel part from another similar looking part. To get a good variety of colors would a chromate coating need to be used? Thanks
Daniel Bromanstudent - Duluth, Minnesota, USA
2003
A. Phosphate coatings, with the possible exception of a heavy phosphate and oil for collectibles, are not final finishes. They are temporary pretreatments for paints, cold drawing, and internal machine parts. So color-coding would be uncommon, although I imagine that it would be possible to dye the phosphate for temporary internal parts tracking.
Chromating is dyeable and is used for color coding, but remember that chromating applies to aluminum parts and to zinc plated steel parts, but not to plain steel.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003
Q. Thanks for the reply! I see it says not to post website links here, so I won't, but at the website for Allmetal Screw Products corp, under Buying guide/Finishes and Coatings/, about halfway down the page you see a listing for **Colored phosphate coatings: For Steel** Superior to regular phosphated and oiled surfaces ** Chemically produced color coating will increase corrosion resistance. Available in green, red, purple, blue, black, etc. ** They sure make this sounds as if it is a final finish. But, I can't seem to find any vendor describing available phosphate coatings in these colors, but as you said, it sounds like it might be possible as a temporary finish.
Thanks,
Daniel Broman [returning]- Duluth, Minnesota
2003
A. The page you are referring to is http://www.allmetalcorp.com/htm/pg8_6_00.htm.
Yes, we do discourage posting links. They go bad and then our site becomes littered with broken links; plus the site is made possible advertisers, and they certainly don't want us spending their money optimizing the convenience with which the readers are sent to their competitor :-) But every rule has exceptions.
I would suggest that you try to contact someone at Allmetal and see if their chart is authoritative or rooted in a misunderstanding. I personally haven't heard of this, and based on the question being asked here several times previously, a lot of experienced finishers are unfamiliar with the idea too.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003
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Ed. note July 2018: The Allmetal website is still on the air, but the referenced page no longer exists.
A. There are a several formulations of phosphating with different accelerators and used for different applications.
All the parameters (pH, Accelerator, temp., … ) affect the color of phosphating; your question is so general but you will always find this question how to get a good blue color of iron phosphate ?
… or how to change the color of zinc phosphate from gray to black?
The color always give a good feeling about the good phosphating so it is very important question but try to ask more deeply.
Ahmed Fouad Serag Eldin
Ein Shams - Cairo - Egypt
2005
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