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Effect on surface finish of zinc phosphate coating

(-----) 2006

Q. I would like to know the reason for change in surface finish after zinc phosphate coating. Does it always increase or may it decrease?

Rajib Kumar Dutta
bearings - Jamshedpur, India


A. The finish is bound to change once zinc phosphate is formed on the MS Surface; strip away the phosphate to get back the base steel finish. What do you mean by increase/decrease?

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India
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A. Dear Mr. Dutta,

Zinc phosphate coating is a conversion coating. The surface finish and roughness will vary after phosphating.

The acid pickling done before phosphating might also induce some change in roughness of the substrate.

If you want to achieve a low Ra value then you have to use some grain refiners in the phosphating bath.

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)


A. Dear Mr. Dutta,

Variations in conditions are solely responsible for phosphate surface finish. Only thing you have to determined the suitability of various changes like acid pickling, caustic or non-caustic degreasing, pre-dip activation etc.


Navin Maheshwari
- New Delhi, India


A. If you mean the increase and decrease in the phosphate layer in steel surface from time to time there are many reasons for that:
you should keep the concentration of your phosphate tank all the time in the standard values,
the degree of temperature must be limited,
the exposure time in the tank must be the same all the time,
you must analyze your tank two times during one shift; keep all points in standard limits,
your basic steel metal must be standard and from one suppler not change from time to time.

Ali Gomaa
- Cairo, Egypt

Phosphating make hydraulic block rough and damages O-rings

April 8, 2014

Q. Generally we make hydraulic valves, blocks, filters in company.

We have now face some coating issue in the blocks. Our requirement of bore area is Ra value 1.6. but do not achieve that Ra value after phosphate coating. And this damages O-ring areas when hydraulic valve is assembled in the bore area.
Anybody please tell how to rectify that problem?

Can you please tell me if Ra value achievable in the blackening process is better than in Phosphate?

J.Arunagiri joshi
Design Engineer - coimbatore, tamil nadu, India

April 1, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

we are carrying out zinc phosphating process for our valve manifold blocks. The material of these blocks is SG Iron made by centrifugal casting. We carry out grinding of these blocks and achieve Ra value between 0.4 to 0.8 microns. But, after phosphating, this value increases to more than 1.0 micron, leading to oil leakage from these blocks.

I want to know if I am doing something wrong with my zinc phosphating process which is leading to change in Ra value.

- Ahmedabad, Gujarat, India

A. Hi Joseph. TSN tells us, above, that a grain refiner will minimize this problem. Are you using one? Is this a proprietary zinc phosphate process you purchase, or something cobbled together in-house?


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