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Hot vs. cold phosphating -- differences and advantages

Q. I would like to know the difference between hot phosphating and cold phosphating. Basically we are using hot 7 tank process for our pre treatment of steel, both HR and CR before powder coating. I would like to switch over to cold process. Kindly give me the advantages.

Kindly advice

Gangadharan Ruppa
- Chennai, Tamil Nadu, India

A. Dear Mr G Ruppa,

Cold phosphating has no advantage over hot phosphating apart from saving your heating costs.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India
saify logo

A. Dear Mr. Gangadharan Ruppa,

I do agree with Khozem Vahaanwala that cold phosphating offers savings in heating costs. The other advantages of cold phosphating are:

The problems due to overheating of the phosphating bath [Metal Finishing (U.S.A.), 93(1) (1995) 30-31] and difficulty in maintaining heating coils could be avoided with the use of cold phosphating. However, cold phosphating requires more processing time to achieve the desired amount of phosphate coating weight and in a continuous process line the processing time has to be adjusted accordingly.

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

Q. Hello everyone, my situation is that I want to change my Hot Phosphating system to Cold. Please can anyone help me out on this issue.

Adeel Ahmed
March 25, 2013

Cold phosphating

Q. Is 3-in-1 solution good for surface preparation. Some manufacturers supply 3-in-1 chemical (Degrease, Derust and phosphate combine) and recommend to use it by dipping as well as by simply wiping on the surface. How authentic is this process? Should we opt for it for painting heavy machinery as 7 tank is not possible and blasting also is not possible.

Ravinder Negi
Paint Shop employee - Bhiwadi,Rajasthan, India
October 24, 2017

A. Hi Ravinder. I'm not sure why the 7-tank process is not possible, but dropping from 7 hot stages all the way down to 1 cold stage is a pretty big fall. I think the only favorable thing we can say is that anything is better than nothing :-) Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
October 2017

Q. Dear sir
I am new in powder coating industry. So please tell me the details of 3 in 1 chemical. How to use and what are steps to follow.


Raman gresan
Powder coating - Chennai,tamil nadu, India
July 19, 2019

A. Hi Raman. Traditionally, the 3-step process, or 3 in 1, has meant iron phosphatizing, wherein a single process tanks contains the necessary ingredients (as Ravinder says above) to clean, de-rust, and phosphatize; the second tank is a rinse; the third is a dilute chromic acid passivating solution.

Although it is theoretically possible to formulate your own 3-in-1 processing solution, it's probably far more practical to find a supplier of the process. Because of concern over the toxicity of chromic acid, they will probably suggest a proprietary passivation chemical as well.

"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber
on AbeBooks

or Amazon

(affil links)

Biestek & Weber is virtually impenetrable as a quick reference, but if you take the time to tune your mind to their wavelength, pp. 212 to 224 are an extremely clear exposition of everything there is to know about iron phosphatizing :-)

You can also search this site with the term "3 in 1". Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
July 2019

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