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What is 'P ratio' in zinc phosphating bath and how to determine it?

Q. Can someone tell me a method of determining the "P ratio" of a zinc phosphate conversion coating?
RFQ: If you know of a testing laboratory that can provide this analysis I would very much like to know of that.

Dave G [last name deleted due to age of posting]

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

A. ACT test lab in Detroit [is a testing lab that can provide that analysis]

Jeff B [last name deleted due to age of posting]
- Windsor , Ontario , Canada


"Optimization of Phosphating Process" by Vinod Prasad Sharma (2022)
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"Phosphating of Metals" by Werner Rausch (1991)
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"Phosphating & Metal Pretreatment" by D.B. Freeman (1986)
avail from eBay, AbeBooks, or Amazon

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Q. Can anyone throw light on P ratio in zinc phosphating bath.

Manish [last name deleted due to age of posting]
UL - India

A. The P ratio is a measure of the forms of phosphate obtained from zinc phosphating.

Miyawaki, Okita, Umehara and Okabe(1) proposed that P ratio = Phosphophyllite (Zn2Fe(PO4)2.4H2O)/[Phosphophyllite + Hopeite (Zn3(PO4)2.4H2O)].

They found that the P ratio was higher in dip applications with a low Zn:PO4 ratio.

The P ratio may be affected by a number of factors, including temperature, agitation and method of application.

(1) Miyawaki, T., Okita, H., Umehara, S., and Okabe, M., Proc. Interfinish 80, Kyoto, 1980

Brian Terry
Aerospace - Yeovil, Somerset, UK


Q1. What exactly is the definition of 'p'ratio in zinc phosphating process?
Q2. What should be the ratio of free acidity vs total acidity in zinc phosphating bath?

Neeraj P [last name deleted due to age of posting]
Generator manufacturer - Rudrapur, Uttranchal, India

Ed. note: To limit confusion let's continue to restrict this discussion to Q1;
Q2, the TA/FA ratio, is addressed in topic 47351 and several other threads.

A. Dear Neeraj,

'P ratio' expresses quantitatively the proportion of hopeite and phosphophyllite phases in phosphate coatings. It is defined as:
Phosphophyllite / (Phosphophyllite + Hopeite)

The free acidity and total acidity of zinc phosphating bath depends on the formulation and operating conditions [see topic 47351].

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. Can you give me the details for how to calculate p-Ratio of phosphate bath?
We have completed SEM (Scanning Electron Microscope)for phosphate panel.

Rahul Thorat
Sr. Engineer,AME - Satara, India
December 9, 2011

Ed. note: We appended your inquiry to a thread where the question has already been answered by Brian Terry and TSN Sankara. Please advise of remaining questions or anything which is unclear.

Q. I want to understand ... what are the Factors effecting P-Ratio?

Can anyone help me by giving reference of some good online study material?

Have to make an Official report (Fish-Bone diagram).
I am virtually Illiterate in this Subject.

Mayank Upadhyay
- Jamshedpur, Jharkhand, INDIA
May 14, 2012

Ed. note: Brian Terry has mentioned one paper on the subject. It looks like returns a pretty good list of papers on the subject if you try the search term "phosphophyllite hopeite coating". Good luck.

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