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Thickness measurement of phosphate coatings

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February 8, 2022 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello! I am working on a project where I need to find a method/product that can measure the zinc phosphate coating weight on different metal plates. We are at the moment using a Gardo®meter but we are getting wrong answers with it, giving results that are not correct. We know this because of verification from the lab. Any help is much appreciated!

Kind regards.

David Coriell
Student/apprentice - Göteborg

February 2022

A. Hi David. I haven't used a Gardo®meter, but understand that it functions by measuring the reflectance/absorption ratio. If this proprietary testing device is not working properly for you, you should talk to Chemetall about what might be wrong either in your operating procedures or in the device's operation/calibration (we don't praise or slam proprietary products in this forum [ huh? why?]).

Brian Terry suggests on this thread that MIL-DTL-16232 [link is to free spec spec at Defense Logistics Agency, dla.mil] offers instruction on how to properly determine actual coating weight, and you may be able to do this strip & weigh method in-house or have a third party do it as verification of whether the results from the lab are accurate. Labs aren't alway right :-)

Luck & Regards,

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

Closely related Q&A's, oldest first:

February 16, 2009

Q. Respected sir,

One of our customer needs Anti seizure phosphatizing to his components with the required coating thickness at around 0.005 to 0.007 mm. Sir, Is it possible to measure the coating thickness after phosphatizing ? If ,yes please tell me the measuring instrument required for measurement.

Please suggest the standard for the same. As per our customer requirement, Anti-scuffing phosphating with de-hydrogenation FF AG8-10, Durability 13 Hrs. as UNI 4239, Anti scuffing efficiency test 100% min. as UNI 4722. Min. thickness req. 5 to 7 microns. please suggest us that instrument also which is checked thickness of the phosphatizing layer.

Thanking you.

Deepak Patel
Product Designer - Pune., Maharashtra, India

February 18, 2009

A. Most people will use mechanical measurement of before and after, dividing by two if it includes 2 sides. Test panels are not as reliable as sections of scrap parts. Notice that your "before" measurement has to be after any grit blast or chemical attack. Notice also that for really reliable information, the measurement has to be taken in the same spot--exactly. Notice that with repeated measurement, some of the tops of the phosphate will break off or wear off or polish. For electronic measurement, it requires excellent standards, which most people do not have. I do not think that magnetic will work if this is iron phosphate, but you might be able to prove that it does with the right fudge factors, IE: as a go-no go gauge.
So, you come down to what is the required precision of the measurement. Most people would just use weight loss rather than direct measurement, but this requires a lab with a good balance and haz waste disposal. (chrome in the strip agent)

James Watts
- Navarre, Florida

October 13, 2010

Q. We are measuring manganese phosphate coating thickness on steel components.
We are using instruments based on magnetic pulls. The thickness range of coating is 5 to 10 microns. We are getting average coating thickness in the range of 13-14 microns. My instrument has calibration standard of 70 microns. So we calibrate between 0 and 70 micron.

My queries are as follows:

1. Is it okay to check coating thickness of 5-10 micron with an instrument calibrated with 70 microns standard sample.
2. Is it possible to check accurately the coating thickness, say within one micron, with some other method.

Vivek Mittal
In charge of a testing lab - Haridwar, Utterakhand, India

October 14, 2010

A. First, Manganese phosphate is typically a very rough finish so that it can hold oil. That said, it is nearly impossible to get valid numbers.
Second, your standard should always be near the range that you desire to measure. See if you can get another standard that is about 20. 70 is far too big for good results.

James Watts
- Navarre, Florida


Q. I am doing a project on the phosphate coatings as my final year project in B.Sc Engineering at University of Moratuwa, Sri Lanka. At the moment we do not have a proper method to measure the thickness of the phosphate coatings, which were developed during the project. Please be kind enough to give any information on the thickness measurement of the phosphate coatings. Any documents of researches carried out under the above topic are very much appreciated.

Bhatiya Dewagiri
- Colombo, Sri Lanka


A. Hi Bhatiya,

Do you mean the actual thickness of the coating, or the amount of phosphate coating that has been deposited?

The actual thickness is difficult to measure because there is no clearly defined boundary between the coating and the base metal. Remember that the top portion of the metal surface has been converted into the phosphate coating.

The amount of coating can be determined gravimetrically by immersing a phosphated panel into a solution of chromic acid.

A good discussion can be found in Werner Rausch's book, "Phosphating of Metals" [affil. link to book on Amazon, ... on AbeBooks -->].

George Gorecki
- Naperville, Illinois

June 14, 2012

Q. Hi,

I need more explanation related this topic. I have a Positest coating thickness meter which according to the manufacturer that my device has the capability to measurement of paint, galvanizing, and phosphate coating. You mentioned that the phosphate coating cannot be measured due to the top surface of base metal has been converted to phosphated condition. Can you kindly elaborate more to your opinion?

Thank you very much.

Michele [last name deleted for privacy by Editor]
- Jakarta, DKI, Indonesia

June 15, 2012

A. Good day Michele.
There is some info in letter 51028 "Controlling Manganese Phosphate Coating Thickness".

Eric Bogner
- Toronto, Ontario, Canada

June 18, 2012

A. Hi Michele,

Phosphate coatings are normally measured in coating weight, not thickness (MIL-DTL-16232 [link is to free spec spec at Defense Logistics Agency, dla.mil] is a good source for how to do the test).

The problem with phosphating is that it is highly crystalline, discontinuous and uneven. This means it is nigh on impossible to get accurate readings with either eddy current or magnetic type thickness testers. There all sorts of influences that makes the method inappropriate including inability to place the probe flat to the surface, crystal edge effects, surface discontinuities, etc.

Brian Terry
Aerospace - Yeovil, Somerset, UK

December 15, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi

We want to know about amount of phosphate layer thickness on steel for protection and having better performance. Actually for incoming inspection of our wires, we need to have a method to get some information for checking them. Consequently could you help us to find a kind of standard or method to inspect of phosphate layer thickness on steel wire?

Thank you in advance for your cooperation.


Javad Takhsha
- Saveh, Iran

December 18, 2012

A. Hi Javed. We appended your inquiry to a thread which may fully or partially answer it for you. You may also be interested in letter 55593 "Is it possible to measure phosphate coating thickness with SEM".


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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