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topic 13366

Problem in zinc phosphate

A discussion started in 2002 & continuing through 2017 -- add your Q to bring it back to the Hot Topics page.


Q. Dear Sir,

Problem is that I have trouble in my zinc phosphate bath; process sequence as follows --

1 cleaner, temperature 50-75 °C
2 water rinse
3 water rinse
4 zinc phosphate with catalyst, temperature 55 °C
5 water rinse

After above process I get bad result: phosphating solution colour turns brownish, result like uneven layer of phosphate. What is your suggestion for how I can get good results? If we make up new one, after two loads the results turn bad. Please tell me about the problem, remedy, possible causes, thank you.

Shaikh Rashid Mehboob
- Karachi, Pakistan


A. Hi Shaikh,

You need to give more details about your operation before a precise answer can be given. Specifically, is this system spray or immersion? What are the contact times in the cleaner and phosphating stages? What exactly do you mean by a bad result?

From what you have written, I can do a little troubleshooting. You say that the phosphate coating comes out uneven. Are you getting adequate cleaning in Stage 1? Is the work coming out water-break-free? The metal surface must be free of all oils and contaminants before a homogenous phosphate coating can be deposited. You also said that the phosphating bath was turning brown. Is something being carried over from Stage 1?

Are you overflowing the rinses in Stages 2 and 3 sufficiently? Poor rinsing will make the problem of carry-over even worse. Remember, water is your cheapest chemical!

Is the phosphating bath made up properly? Are you breaking in a new bath by dissolving some iron in it? Are your total acid and free acid numbers in the proper ratio? Your chemical supplier will be able to tell you the proper ratio for your application. An out-of-balance phosphating bath will produce poor coatings.

You say that your phosphate solution has a catalyst; I assume this means accelerator. Is the accelerator present at the proper concentration? Not having the accelerator at the right concentration can also have a detrimental effect on the coating.

Would it be possible to convert the rinse in Stage 3 into an activation stage? Your chemical supplier should be able to provide you with a Ti activator, which will help in the formation of a uniform phosphate coating.

I have presented a lot of "what-ifs" here. Without knowing more about your system, it will not be possible to pinpoint the problem.

George Gorecki
- Naperville, Illinois


A. Hi,

From your scarce info I would say you are running a dip bath. The catalyst is probably sodium nitrite.

1. The brown colour is due to the build up of ferrous iron.
2. I would say that the temperature needs to be 60+ °C
3. You are either putting too much steel into the bath or you are not adding enough accelerator. If it is a liquid add 500 mls per load per 1000 liters tank capacity for the first few loads. Control with starch iodide test papers.
4. The low temp will favour the build up of ferrous iron hence higher additions of nitrite will be required. The new bath will work at first until ferrous iron builds up.

Roger Bridger
- Croydon, UK


A. Shaikh,

May I suggest after your 2 rinses from the cleaner, dip your parts into phosphoric acid. This will pickle the metal, remove any unwanted oxides from your metal, and make the part more susceptible for the phosphate (which is really a glorified form of phosphoric acid). After that, rinse first in cold water then hot water. Next, after the hot rinse use a titanium activator to get more uniform coverage on your part. Next would be the zinc phosphate (165 °F): use 1 pint of sodium nitrite for every 400 gal. to remove unwanted iron (this is important... you want this bath to be free of iron, where you may be picking up the brown stains).

Then rinse in cold water, and last, rinse in a solution of chromic and phosphoric acid (160 °F). This last rinse serves 3 functions:
1. dries part off quickly
2. prevents short term corrosion
3. leaves the surface slightly acidic which is the best condition for paint or any other organic coating to adhere to.

Joel Antolik
- Nanticoke, Pennsylvania


A. Well, how I can understand your process have a problem on the final stage. The most of times the final stage it's occupied a bath called "sealing" that have a chrome solution and other stage (rinsing) to eliminate the excess of chrome.

The first two degreasings steps are made it with NaOH to clean the surface and eliminate the scale and rusting of steels.

If you have a final stage with this compound you can have a detrimental phosphating coating because NaOH to undo the crystals and the paint not have a good adhesion.

Check your final stage.

López L. S.
- Santiago de Querétaro, Querétaro, Mèxico

Granules, streaks, and poor adhesion in zinc phosphating


Q. Dear everybody,

I am working for automaker company. We have spray line (5-stage) for pretreatment of cars:
2 stages of degreasing, 1 stage rinsing, 1 stage zinc phosphating, 1 stage passivation+rinsing.

Unfortunately our line is very compact and usually some liquid from each stage overflows to other stage (specially degreasing to rinsing and rinsing to phosphating).

At the final stage our contractor uses NaOH for passivation the surface. We have granules on our cars and a streaky surface.

We tested our painting on it (cross cut test for adhesion) and failed.

Could you please guide me about troubleshooting of zinc phosphating line and guide me about suitable source (book, article, document) to see good zinc phosphated surface?

Best regards

Nader Bayrami Tarouni
- Tehran, Iran

Phosphating Metal Pretreatment
by Freeman

Phosphating of Metals
by Rausch


A. Hello, Nader. You have already identified what the principal problem is (carryover from one tank to another), and you know that it must be fixed.

Just one person's opinion: Don't talk yourself into wasting time on further "investigation" rather than confronting this difficult problem :-)

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

November 25, 2012

A. If bath solution shows brown color, it means your cleaning and rinsing is not proper. To avoid this, improve the rinsing and cleaning processes.
Also check that if you are adding chemical in old bath then check the pointage of phosphate and iron contamination.

N K Inamdar
- Mumbai, Maharastra, India

November 17, 2013



Phosphated metal turns red in molding process

February 21, 2014

Q. Why does the phosphated metal gets a reddish colour on its surface when it is molded?

Thoufiq P A
- Mysore,India

February 24, 2014

A. Thougiq,

We had the same problem years ago and weren't able to find a solution. The only thing we could think of was there was excess water in the rubber and during the molding process, the water flashed into steam and rusted the metal. There weren't any blemishes or any indication of water in the rubber after molding, except the redness.

Dale Pittman
- Kansas City, Missouri USA

What is this Zinc Phosphate Coating

March 18, 2014

Q. Hello my name is Cody and I work for an automotive supplier. We have recently had some testing performed on a sample part and we cannot figure out what type of coating this part has. The testing method used was an X-ray energy dispersion Spectroscopy for samples in Scanning Electron Microscope. The test results showed that the main elements in the coating were oxygen, zinc, and phosphorus. The problem is the part doesn't look like it has a coating. It is a dull grey with a smooth finish and it does not have the typical zinc phosphate appearance. The part looks like it was not coated using a bulk/barrel operation. This may asking too much but we are looking for a Specification that matches the coating. We are looking for a Ford, GM, or Chrysler Specification. We have not found one yet that has the resultant surface finish.

Cody Belknap
- Grand Rapids

Acid won't reliably rinse out of stitch welded area of zinc phosphated parts

June 11, 2014

Q. Hi

We are having a problem with acid being trapped in the 7 stage zinc phosphate process. In some cases these steel plates are laser cut and stitch welded together; in the areas where the plates are pressed up against each other but not welded, the acid is not washed away completely (counter-flow rinse tanks with overflow is being used) ... and then there is some corrosion due to the trapped acid even as far down the line as complete powder-coated fabrications.

Adding another rinse bath is not really an option for us, is there some way to enhance or improve the rinsing? Is raising the rinse bath's temperature an option?

Eugene Bester
- Johannesburg, Gauteng, South Africa

January 22, 2015

Q. We have done Zinc Phosphating process but we are facing visual difference in color. Parts are looking like they are processed through Mn phosphating means black.
Process steps:
water rinsing
activation @ room temp.
phosphating @ 80 °C
water rinsing
RP oil

Please suggest what should we do?
what changes need to be done?

Keyur Solani
- Ahmedabad,Gujarat,India

October 7, 2015

Q. Hi,

I have many problems in my phosphating unit. I have immersion bath at temperature 70.
1- a lot of sludge formed very quickly,
2- sometimes a white color presented above wire immediately,
3- wire turned to brown collour after two days storing,
4- very low corrosion resistance,
5- sometimes a blue color observed after only one hour from phosphating.

Hany Mohammed
- cairo -Egypt

Rust prevention after leak testing at welding market area

August 12, 2017

Q. Dear friends, we are coating some leak-tested components but we are facing rust problem each time. We have dip PT process as follows
Emulsion cleaner
alkaline degreaser 2 stages
Water rinses 2 stages
Zinc Phosphate
Water rinse
Dm water
Uf rinses 2 stages
Dm spray

We want to remove rust and burnt oil residue from components. Is there any chemical need to add or any new stage to add?
Now we are manually pre cleaning and de-rusting components using acidic agents; problem is minimised but not completely solved.
Please help me.

Vishal Ghawate
I am engineer and working in paint shop. - Pune, Maharashtra, India.

August 2017

A. Hi Vishal. Is there any chance of abrasive blasting or tumbling? Chemically removing scale is difficult, and chemically removing burned-on oil can be next to impossible. But if you were able to blast these parts, it seems to me that the rest of your process is very robust.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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