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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? 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 for privacy by Editor]

Ed. note: Sorry, this RFQ is outdated so private contact is no longer available, but public technical replies are still welcome! No public brand/source suggestions please ( huh? why?)


A. ACT test lab in detroit - Assoma has an x-ray machine for coating weight.

Jeff B [last name deleted for privacy by Editor]
- Windsor , Ontario , Canada

"Phosphating of Metals"
by Werner Rausch
from Abe Books
or Amazon
[affil links]

"Phosphating & Metal Pretreatment"
by D.B. Freeman
from Abe Books
or Amazon
[affil links]


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

Manish [last name deleted for privacy by Editor]
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.

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

similarly 2005

Q. 1) What exactly the definition of 'p'ratio in zinc phosphating process?
2) What should be the ratio of free acidity vs total acidity in zinc phosphating bath?

Neeraj P [last name deleted for privacy by Editor]
Generator manufacturer - Rudrapur, Uttranchal, India


A. Dear Neeraj,

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

The free acidity and total acidity of zinc phosphating bath depends on the formulation and operating conditions.

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 https://advancementinscience.wordpress.com)

December 9, 2011

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

Ed. note: We appended your inquiry to a thread which hopefully answers it. Please advise of remaining questions.

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

July 6, 2012

Q. I want to ask the method to determine Phosphate ion and zinc ion in Tri-cationic phosphating bath in order to monitor the coating pattern on mild steel.

Thanks and regards

neman ahmad
- lahore, Pakistan

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