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topic 0960

Black zinc phosphate


Q. One of our clients is asking us to zinc phosphate some steel parts. We currently have one line in operation and the color we obtain is dark gray. This customer is asking for a very intense black color and none of our suppliers have been able to tell us how to obtain it. They only say that it might be manganese phosphate instead of the zinc we are asked for.

Reading the MF directory I found that phosphate can be colored using a special coloring agent. Does anyone know what these agents are, or how to obtain a black zinc phosphate?

Thanks in advance

Guillermo Luna
- Mexico City, Mexico

Phosphating of Metals
by Rausch
from Abe Books



A. Mr. Luna,
There are ways to chemically blacken zinc phosphate coatings. Several companies sell proprietary products to do that. Lots of fasteners receive a blackened zinc phosphate plus a rust inhibiting oil or wax. I'm not an expert in this process but you should be able find one below. Hope this helps!

Parker-Amchem (Henkel Surface Technologies)
Oakite Products, Inc. (Chemetall [a supporting advertiser])

Jeff Watson
Jeff Watson
- Pearland, Texas


A. To answer your question, the missing link is Antimony Trichloride, a blackener that is used after zinc phosphate to darken the grey color to black.

Jeff is right though, you will need to seal the zinc phosphate and blackener with a rust preventative oil or coating of some sort in order to retain the black color.

Craig Burkart

Craig Burkart signature
Craig Burkart
- Naperville, Illinois


A. Zinc phosphate coatings are very porous coatings. The blackening you require can be accomplished in standard zinc phosphate process with the addition of an another tank to provide the "black coating you desire. Rather than standard sealing with chrome or a suitable non chrome alternative, an oil emulsion is usually used.

Most proprietary zinc phosphate manufactures have the products you require.

Good luck

Dan Zinman


A. I read on another website that if you give your parts a black oxide treatment before the phosphate dip, it looks like a manganese phosphate appearance, though I do not know firsthand.

Mike VanOuse


A. The customer asked for the intense black color phosphate deposit coating. That may imply that the phosphate coating itself should be black, chemically active, natural, and available to paint or wax. If so, you may solve the puzzle with a particular formulation of phosphatizer.

Jun Q. Zhang
- Cincinnati, Ohio

All messages here are informational comments--not professional opinions--and do not represent the opinion of the author's employer.


Q. Dear sir
I am student of M .PHIL CHEMISTRY my research is on black zinc phosphating sir I want to get some literature about it and particularly about its chemicals sir if you are in this position kindly send me the required information and oblige.

Ahsan Amin Bhatti
student - Pakistan


A. Dear Ahsan Amin Bhatti,

The following article might be of some help to your work.

Guangyu Li, Liyuan Niu, Jianshe Lian and Zhonghao Jiang,
A black phosphate coating for C1008 steel
Surface and Coatings Technology
Volume 176, Issue 2 , 1 January 2004, Pages 215-221

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at

Is black phosphate coating environment friendly?


Q. Is black phosphate coating environment friendly? Does it contain prohibited materials such as lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, or polybrominated diphenyl ethers?

Yosinori Isii
electronics mfgr - Plymouth, Michigan


A. Yosinori Isii,

Black phosphate coating is obtained by post-treatment of Zinc phosphate coating in solutions containing As salts, which is toxic^Antimony.

The zinc phosphating process normally involves a chromic acid sealing stage to seal the pores of the phosphate coating. Cr(VI) is toxic and is being phased out.

Phosphating process is normally free from polybrominated biphenyls, or polybrominated diphenyl ethers

The chances for Pb, Cd and Hg depends on the purity of chemicals used in the various stages of the phosphating process.

Correction: Correction: It is not As salt.

Black phosphate coating is obtained by post-treatment of Zinc phosphate coating in solutions containing Antimony salt (Antimony trichloride), a blackening agent that to darken the grey color to black.

Toxicology of Antimony trichloride

Harmful by ingestion, inhalation or through skin contact. Corrosive - causes serious burns. Mutagenic effects have been observed with bacteria. Reproductive effects have been observed in tests with laboratory animals.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India

July 4, 2014

Q. Mr Shankar Narayan. I read your review paper on phosphating... Impressive. You mentioned that the post chromic acid rinse creates predominantly a mix of Cr(III) oxide and hydroxide which is about 88% and probably about 12 % of Cr(VI)oxide. Can you please quantify what is the equivalent weight or thickness percentage of Cr(VI) oxide present in the phosphated layer.

Debashis Dutta
aerospace systems - Bangalore, Karnataka, India

July 4, 2014

A. Hi Dutta,

I don't know how to quantify the Cr(VI) in phosphating post treatment layer.

But some proprietary zinc phosphating post treatment chemical (i.e., Engineering (Cr VI free) Passivation) are available nowadays which are eco-friendly.


Surya Narayana
Process Engineer - Tumkur , Karnataka, INDIA.

July 4, 2014

A. The thickness of the chromate sealing should be of the order of nanometers. The effectiveness of corrosion protection by chromate treatment stems from the sealing of the pores and the nature of the chemical compounds.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India

August 16, 2017

Q. God bless you all there,

I understand after reading the upper discussion, that if we want black phosphate coating, then we'll follow this process line,
Zinc phosphate coating
Blacking agent may be antimony trichloride

Now please tell me how to use antimony trichloride solution, that's parameters, solution making etc,
Please give response, because I'm waiting for this information,

Mustanser Hashmi
Five star e'plating - Lahore Pakistan

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