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Phosphate Coating Weight Specifications

A discussion started in 2002 and continuing through 2017 . . .


Q. Dear sirs: I read about phosphate coating. Is there any specification on the coating weights so that any paint will adhere well to that phosphate coated surface.

Please suggest.

Sivakumar Balasubramanian
- Hesston, Kansas

Phosphating Metal Pretreatment
by Freeman

Phosphating of Metals
by Rausch


A. I suggest you will look at Fed TT-C-490 [link by ed. to spec at TechStreet] military standard. Anyhow as a rule of tomb and as appears in the literature for Zinc Phosphate the coating weight are as follow: For Spray application coating weight range from 1.5 - 3.5 grams per square meter For dip application coating weight range from 2 - 5 grams per square meter.

yehuda blau
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel


1. CLASS D: extra light weight.IRON, AND ZINC PHOSPHATE COATING weight 03 to 1.1 g/sq. meter.
2. CLASS C (light weight) ZINC PHOSPHATE COATING FROM 1.1 to 4.3 g/ sq meter of treated surface.
4. CLASS A2. (Heavy Weight). processes giving coatings that consist essentially of zinc phosphates and have a weight not less than 7.5 g/ sq meter of treated surface. Normally used for similar purpose as A1. where a good degree of protection.
5. CLASS A1. (Heavy Weight) processes giving coatings that consist essentially of manganese and/or iron phosphate, and have a weight not less than 7.5 g/ sq meter of treated surface. This class is normally used where maximum protection is desired particularly under oil, or grease, or where resistance to wear is required.





A. It is tricky to answer the question based on ANY paint. In my experience the type of paint makes a good deal of difference. That being said I have found that a Zinc Phosphate coating of 8-12 g/square meter is very versatile.

David J Wolf
- Painesville, Ohio


A. As my old friend Joe Menke at the US Army's Rock Island Arsenal taught me, SEM photo graphs of 2 zinc phosphate coatings of equal coating weight can give dramatically different results. If a coarse, sporadic phosphate due to low temperature and calcium modifier, for instance, you get reasonable coating weight values but the paint adhesion or corrosion results will be terrible. For a phosphate coating weight to be good, you have to look at the surface morphology as well.

milt stevenson jr.
Milt Stevenson, Jr.
Syracuse, New York

Two coats/layers of phosphate?


Q. Hi all,

We have a new customer who is asking for two coats of zinc phosphate on his carbon steel parts, followed by an organic paint coating. He is telling us that additional coats of phosphate will increase salt spray resistance. I don't see how repeating the phosphating process twice will achieve additional protection. Has anyone else heard of this?

Thank you in advance for any information you can give me,

Kim Mayberry
fabricated metal manufacturer - Muskegon, Michigan

simultaneous (2007)

A. Gut feeling is that you will end up with a worse coating since it takes iron to kick off the phosphate process and going back into the tank will probably soften the existing phosphate before it can kick off. For better corrosion resistance, go to a finer crystal phosphate such as a calcium modified one.
The paint is far superior to any phosphate for corrosion resistance.
What spec would you certify to?
A thicker single coat is possible, but it will be more prone to crush and become a worse coating.
Tell him that he is the customer and they are always right and you will take his money if he can come up with a process spec, since there is no published one. Also tell him that as a professional metal finisher that he would be wasting his money.

James Watts
- Navarre, Florida


A. Hi Kim,

Does your customer have any evidence to back up this claim that an extra application of phosphate coating improves corrosion resistance? If so, I'd like to see it.

A phosphate coating is a conversion coating, meaning that the substrate surface is converted to form a coating. As a result, the phosphating reaction mechanism is self-limiting. As coating is deposited onto the metal surface, the surface has fewer and fewer fresh areas for coating to form. The ideal situation is that the entire metal is eventually covered with coating. Once that point has been reached, no more coating can form because there is no fresh metal to dissolve in the phosphating bath.

If you have a chance to look at DB Freeman's excellent book, "Phosphating & Metal Pretreatment" [link is to info about the book at Amazon], he presents a graph that charts coating weight as a function of time. The form of the curve is that coating weight increases rapidly at the start of the process and then eventually levels off.

The notion that "more is better" does not apply to phosphating. The aim is to get complete, uniform coverage of the phosphate coating.

George Gorecki
- Naperville, Illinois


A. If your customer was aware of how a phosphate coating forms there would be no need to ask this question.

Zinc phosphating is a precipitation process. Zinc phosphate has very low solubility in higher pH solutions. When the phosphoric acid that the zinc phosphate is dissolved in attacks the substrate the pH at the substrate liquid interface goes up and the zinc phosphate precipitates out to form crystals at the substate surface. This is a self limiting reaction, if you want a heavier coat you just raise the temperature of the bath a few degrees. A second trip in the bath would just result in stripping some of the first coat and re-depositing it in any areas where the substrate may have been exposed. Higher coating weights don't necessarily give better salt spray resistance. Automotive and appliance lines like 250mg/ft^2 and coil lines like 180mg/ft^2. The fineness of the crystals is the most important attribute, much more than the coating weight.

Ronald Zeeman
Coil coating - Brampton, On., Canada


thumbs up signThank you for your responses and for confirming my limited understanding of the phosphating process.

Kim Mayberry [returning]
fabricated metal manufacturer - Muskegon, Michigan


A. Dear Kim,
phosphate coating is a conversion coating reaction from chemical and steel, if the steel surface is fully covered by the phosphate coating, no more reaction. you have to find out how to uniform the phosphate coating on the steel part. Coating weight is not important in corrosion protection, fine crystal coating is a point in corrosion protection.

Jaff [last name deleted for privacy by Editor]
- Malaysia

How to increase zinc phosphate coating weight

February 27, 2017

Q. I m having issue with the coating weight. Spec is 2-5 gm/mm^2. Bt every time i m getting it on lower side. I m maintain ing TA at 55-60 ml and FA at 3-4 ml. Using a activation bath at 3-4 ml. Please suggest me possible ways to increase coating weight. At first we are doing degreasing process at 2 bath having a time of 8 min each. Followed by three rinse. Then pickling bath having time of 9 min. Followed by three rinse bath. Then activation of 1 min. And then phosphating for 8 min. Followed by 3 rinse bath.

I am getting these type of rusty part if my coating weight is at lower side. Rahul Jangid
Senior engineer at zinc phosphate shop - India

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