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Zinc Phosphate coating problems & issues; Q&A's

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A discussion started in 1998 but continuing through 2018


Q. I am having a problem. I am in a pipe threading company. All this while, we are running the zinc phosphating process on pipe thread. The pipes came from L80 grade, K55 grade, P110 grade, and Q125 grade. Our zinc phosphate process are as follows;

Use kerosene to clean up the pipe thread (having grease on it)

Tank 1: Alkaline cleaner
Tank 2: Rinsing Water
Tank 3: Rinsing Water
Tank 4: Zinc Phosphate
Tank 5: Hot water rinsing
Tank 6: Oiling

The last process is to apply grease and install protector to protect the thread.

However, we always have problems of thin layer of zinc phosphate especially on the P110 and Q125 grade. After the process, you can see that some portion is not fully coated with zinc phosphate. We have to do rework.

I would like to know what are the causes? Why the L80 grade does not give us such problem as compared to P110 and Q125 grade? It has been many years that we have no answer on that and hope someone can let me know the major causes and how to solve this problem? Thank you.

James Chong
Pipe Threading - Malaysia

Phosphating of Metals
by Rausch
from Abe Books


Preparation of Metals for Painting

Black spots and non-uniform coating

February 5, 2010

Q. After Drawing of High Carbon Steel (SAE 52100) Wire rod in the desired size, say from 12 mm to 10.40 mm or 9.80 mm, Black spot are generated on the wire surface i.e. non uniform coating is observed on the wire surface. While forging (Producing Roller) at customer's end they face problem of Muck Formation due to poor Phosphate coating. Our standard Phosphate dip time is of about 20 min. and Pointage is 18 - 20

J R Chaurasiya
- Navi Mumbai, Maharashtra, India

June 11, 2012

temperature 75-80 °C
Wire drawing lubricant must contain high amount of extreme pressure in it.
Never use single or self-accelerated phosphating solution; they cause hollow spots on drawn wire.
Sulfuric acid 15% pickle -- force rinsing -- deep rinsing -- phosphating -- borax solution dip at 90 °C.

- Tehran, Iran

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Black patches and pits

June 20, 2013

Q. Hello sir, we're doing a phosphate coating process for various dia. coils. In this we are frequently facing black batches and pitting defects in this process. I need your suggestion for sorting out this problem.

Vigneshwar Maruthapillai
- Tamilnadu, Karur, India

July 2, 2013

A. Hi Vigneshwar,

You are going to have to give us a lot more detail before anyone can help you.

Firstly you need to explain the process you use (pre-cleaners, phosphate system, times, temperatures, immersion or spray process).

Then you need to tell us what you have already investigated and eliminated as potential failure sources. Have you done a full chemical analysis of all the treatment baths involved? Are they all within control limits?

Finally, and I seem to say this a lot, go and walk your process, follow every step from on-receipt of material to final sign off. This will give you an idea of where you need to concentrate your efforts for improvement.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

July 2, 2013

thumbs up signPlease never tire of your suggestion that people walk the process, Brian.

As General George Patton said:"No good decision was ever made in a swivel chair".


Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

August 2, 2014

Q. Hi,

I came across this forum today and found some very educating and helpful information related to Chemical treatment processes.

We are into the processing and Heat treatment of wire rod and we are facing the following problems:

1. Time taken for pickling is too much (It takes 2 hours on average to finish pickling)
2. Improper Phosphate coating
3. Customer complaints of pitting in drawn wire
4. Dark spots appear and material begins to rust again if not used for drawing within a day.

Our process is following:

We are using HCl for Pickling Wire rod coils (we maintain strength of Pickling bath at around 5-7%).
After Pickling we Wash the pickled wire coil with water jet.
Then place it in Phosphate solution for 25-30 minutes. Our Zinc phosphate solution has TA of 22-24 and temperature of 50 °C.
After Phosphate coating we place it in Lime tank, the temperature of which is maintained at 40-45 degrees, then we dry the bundle using high speed fans and use the dried bundle for drawing.

Please assist me, as to what changes should be made in the overall process to ensure better quality of material coming out from Chemical Processing.

Steel wire processing and heat treatment - Ludhiana, Punjab, India

July 2014

A. Hi Anshul. My initial thoughts, subject to clarification, are that 2 hours in pickling is at least 10X what I'd expect as reasonable. Although I think that the HCl should be about 2-3X stronger, these coils seem to either have extraordinary scaling, or you are pickling them as a bundle such that the acid doesn't get into the middle and work right. Is there any feasibility of obtaining pickled and oiled rod from the mill? It sounds like you may be doing primary pickling without the proper setup for it (scale breaking, continuous coil processing).

A "lime tank" is not within my personal experience. Does it contain highly alkaline water of lime? I believe the more traditional approach is a passivating rinse and/or a D.I. rinse after phosphatizing. Thanks for any clarification.


Ted Mooney, amadeya42.com
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

October 21, 2014

Q. Currently my company is conducting it's own Zinc Phosphating process with Sterate Soap to act as a lubricant for deep drawing from coils of steel sheet stock. The process involves a coining step. This is creating issues with pitting. During coining something gets caught as the metal is flowing and is incompressible and creates pits in the parts.

What I am wondering is, do you know of what qualities of the coating (crystal size, microstructure density, layer thickness, etc) I should be monitoring. Also, what kind of microstructure/layer thickness should I be trying to achieve? Is there an efficient and accurate way to measure layer thickness besides gravimetrics?

Thank you.

Joe Burns

Joseph Burns
- Charlotte, North Carolina

May 13, 2016

Q. Zinc Phosphate coating failing test panels
Process is (sorry I'm not allowed to give too many specifics due to ITAR and policy restrictions):
1) Alkaline clean (140 °F) with a common commercial cleaning solution
2) DI Rinse (125 °F)
3) Pickle with a common commercial solution
4) DI rinse (ambient)
5) Phosphatize (165 °F) common commercial solution
6) DI rinse (ambient)
7) Seal (130 °F) common commercial sealant
8) Air dry

We have weekly test panels that are examined by SEMS. Occasionally they fail to have sufficient crystal growth. A longer immersion time will usually give just barely acceptable crystal growth. Titration analysis shows all solutions to be well within targets.

What could be causing this lack of crystal growth?

Mark Musselman
Aircraft Component Manufacturing - Dayton, Ohio, USA

May 16, 2016

A. Hi Mark,

Pity you can't give us much more detail, but understand how difficult it is to work with ITAR...

Anyway, phosphate coatings are normally controlled by coating weight, so that you have a quantitative measure rather than a qualitative one, which can be open to interpretation. MIL-DTL-16232 is a good place to start on a coating weight control method.

Other than that, what else can effect coating crystal growth? The temperature of the phosphate solution, insufficient accelerator, poor pre-cleaning quality and insufficient processing time in the phosphate bath, just to give you a start.

Brian Terry
Aerospace - Yeovil, Somerset, UK

simultaneous May 17, 2016

thumbs up signI was told the company uses crystal structure because a part can sometimes have an acceptable coating weight yet have a very uneven coating (areas with little-to-no phos) or a relatively smooth surface (no crystals) so it doesn't end up helping adhesive or paint bonding as much as expected.

We think the phos solution may have been contaminated. We are going to try a replacement lot.

Mark Musselman [returning]
Aircraft Component Manufacturing - Dayton, Ohio, USA

May 18, 2016

A. 1. The "pickle" may be too harsh. Try getting the rust off mechanically and skip the acid for a finer grain structure.

2. Also use a grain refiner to nucleate more spots for crystals to start forming. If you are using a reputable supplier he has a grain refiner. If you are using a government 3-bid low cost "product" -- well, good luck.

I have serviced phosphate at three government shops and they did not even have a bottle of brom phenol blue to check the free acid :-(

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

November 30, 2017

Q. Hi,
My company performs zinc phosphating acc to MIL-DTL-16232G Type Z, Class 1,2 and 3. (Using product Oakite Gardobond Z 3190)

Currently we have a chromic acid rinse/passivation as described in the MIL-standard para 3.2.5.

In order to comply with cutomers demand for non hex-chrome passivation we want to establish such a process.

We have tried the Chemetall product Gardolene D 6800/6 with little success, i.e. we fail in our internal 2 hour salt spray test.

Does anyone have a solution as to finding a hex chrome free passivation which will make it possible to pass the 2 hour salt spray test?

Best regards,

Hans Helgesen, site manager
Bandak NCT AS - Notodden, Norway

December 13, 2017

A. Hi Hans,

First, you should see if there is any imperfection in your coating process. All hex chrome-free sealers have less resistance than hex chrome sealers, and they are more expensive and less resistant to contamination.

You should try to increase coating weight and try again, or try with another sealer. Chemetall [a amadeya42.com supporting advertiser] has two other sealers that you can try, all more expensive than the first one.

My experience with non chromic sealers is not so good, so I would test those alternatives one by one in lab and try the best one.

Best of luck to you!

Regards :)

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

December 4, 2017

Q. We are doing Zinc Phosphate and Stearate coating on Carbon steel wire products. This steel is meant for further drawing. Our present systems include acid pickling, rinsing, Zinc-Phosphate coating, rinsing again and stearate coating.

Sometimes, we find rust spots on the surface of steel wire (red or dark brown colour).
1. Please tell me the root cause of the rust spot.
2. How to prevent this defects.

Note: zinc-phosphating is done at 85 °C and Stearate coating is done at a temperature of 75 °C.

Thank you for your time!

Jonathan Seo
- Busan, South Korea

December 13, 2017

A. Hi Jonathan!

Usually these red patches are cleaning issues. See if your pickling process is effective, try to improve it with concentration, temperature or agitation, add wetting agents and rinse with a lot of clean water.

If nothing is effective, try to load less weight in your racks, because it is difficult to clean between spires.

Best of luck to you, too!

Regards :)

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

January 29, 2018

Q. We are zinc phosphating and e-coating a zinc plated part (CRS substrate) and we are not getting a uniform zinc phosphate coating. What pretreatment process characteristics are key to getting a uniform zinc phosphate coating over Zn?

Thank You!

Tim Stitt
- Grand Rapids, Michigan

February 2018

thumbs up sign Hi Tim. I don't know the answer but am glad to see your posting! I have long felt that if zinc plating is going to be painted, it ought to be phosphatized rather than chromated because the adhesion will be better.

But on the other hand, I have never seen a plating shop that didn't chromate their zinc plating. So in this age of trivalent chromates plus topcoats, I'm wondering whether you truly are attempting to phosphatize a plain zinc plating, or if it already has a chromate & topcoat that are making phosphatizing difficult :-(


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

February 7, 2018

A. Hi Tim,

As Ted mentioned, if the plated part has a topcoat, it will be difficult to deposit a uniform coat of zinc phosphate. If it isn't, you need a accelerated zinc phosphate bath, because free iron-based phosphate baths are not suitable (in my experience) for that type of work.

We have an accelerated (nitrite) phosphate bath, with some nickel content (it makes the reaction a little faster and improves coating weight), originally formulated as a wire phosphatizing product. We use it on fasteners over steel or over zinc (chromated or not). Coating thickness is generally uniform, so it can be done and it can be controlled with the right parameters :)

So, if you may, tell us a little more about the zinc plated parts and about your phosphate bath.

Best of luck!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

February 10, 2018

A. Hi Tim,
Why are you Zn phosphating the job which are already Zn plated? Do CED directly on Zn plated job but before that remove any passivation present on Zn plating.

Avinash Vidhate
- Nashik, Maharashtra, INDIA

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