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

A discussion started in 1998 and continuing through 2017 . . .


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

The entire process is a batch process carried out in respective process tanks. We would like to know the following :

a. Method of getting a uniform phosphate coating.
b. Similarly, method of getting uniform stearate coating.
c. Sometimes we find that after the phosphate coating and stearate coating the material develops rust spots.How to prevent this?
d. Above Coated material at times gives sound while wire drawing to a smaller size. What could be the reason?

Note: Phosphating is done at 65 °C and Stearate coating is done at a temperature of 65/75 °C.

Sunil Nair
- Navi Mumbai, Maharashtra, India


A. Hi,

Let me answer a step at a time:

a. Method of getting a uniform phosphate coating. Presumably you are using HCl acid; ensure sufficient time for complete descaling; tease the coil out on the hook to open the coil up to the solutions; immersion rinse in water then power wash the coil with high pressure sprays. This will remove residual acid and smut. Failure to rinse properly will contaminate the phosphate and slow the process up. Give 10 min process time in the phosphate minimum.

b. Similarly, method of getting uniform stearate coating. Alkalinity of the soap bath must be controlled. More alkaline soap will give heavier soap weights but will also dissolve more phosphate. A rinse in Borax [linked by editor to product info at Amazon] solution before soaping will protect the soap bath from acidic carryover.

c. Sometimes we find that after the phosphate coating and stearate coating the material develops rust spots.How to prevent this? May be due to poor storage conditions; poor rinsing; poor soap coating; incomplete descaling.

d. Above Coated material at times gives sound while wire drawing to a smaller size.What could be the reason? Note : Phosphating is done at 65 deg Centigrade and Stearate coating is done at a temp of 65-75 °C. Noise will be failure of the lubricant layer and will cause premature wear of the dies.

Hope this helps,

Roger Bridger
- Croydon, UK

To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.

Can zinc phosphate replace zinc plating?


Q. I am looking for info on zinc phosphate. The application is a small set screw used to hold a lever handle on an interior door and the screws have been zinc dichromated for many years. Recently the screws have had a nasty hydrogen embrittlement problem - and it appears to be linked to the plating process. We have used three different suppliers and all have the same problem - perhaps the material is the problem.

Temporarily we have switched to zinc phosphate. I am investigating the possible change to zinc phosphate on a permanent basis.

I am interested in the corrosion resistance of zinc phosphate as compared to zinc dichromate. I have some info from our supplier, but the info on the phosphate seems a bit vague.

We have a salt spray chamber and I will begin testing some assemblies today.

I would appreciate any info or direction on this subject.

Chris Dawalt


A. If this condition appeared abruptly, my guess is that the screws are of a higher carbon content or are heat treated to 2-4 Rc harder.

Prebake is one help. Longer post plate bake is another. Probably the biggest is getting the screws into the oven faster after plating. All three together should make your problem less.

James Watts
- Navarre, Florida





CONTACT GORD ALLISON AT Macdermid Inc. [a supporting advertiser] IN WATERBURY CONN.




A. Chris,

Hydrogen embrittlement(HE) is associated with zinc plating. A number of processes have been developed to minimize and even eliminate hydrogen embrittlement. Acid chloride zinc plating process is able to reduce the HE problem to some degree. Most importantly, baking is strongly recommended to immediately follow the zinc plating process in order to eliminate the risk of HE. I suppose your suppliers should be able to solve the problem related to plating processes.

You mentioned that you are intending to replace zinc plating with zinc phosphating. Of course, zinc phosphating can provide the limited corrosion protection to steel substrates. However, zinc phosphating is generally used as a paint base instead of a major corrosion protection measure. This is because zinc phosphating coatings cannot provide the corrosion resistance comparable to that resulting from zinc plating with the chromate conversion coatings. Zinc plating has a sacrificial function in protecting steel substrates while zinc phosphating only acts as a protective barrier with some micro pores.


Ling Hao
- Grand Rapids, Michigan

To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.

Zinc Phosphating Defects (Nubbing)


Q. Hello,

Tai and Myself have been following the postings here at and have yet to post a question or response. We have come to appreciate this excellent resource of information and hope that we may be of assistance to the finishing community in the future. Tai is The Quality Control Supervisor, and I am his assistant. We work in an E-Coat Facility using Chemfil and PPG Chemistries.

Our Problem,

Excessive Crystal growth (nubbing) in the zinc phosphating stage. On our hoist line we process steel, zinc plated steel, Galvanneal and electro-Galvanized parts, and on rare occasions a bar of Aluminum. We only have a problem with nubbing on the zinc-plated parts.

For the processing of these parts we use mildly alkaline cleaners with both dip and spray rinses prior to the titanated phosphate pre-rinse and a zinc/nickel phosphate bath.

- This problem has occurred sporadically across a bar of racked parts with no preferred location.
- With low and high agitation in the phos tank
- With fresh and old: cleaners, rinses, and pre-rinses
- We have even replaced the entire phosphate solution.
- With high and low additive levels
- With high and low Fluoride levels in the phos bath.

One bar may have no nubs, the next every part will have them and the next will only have 20 pieces / 800.

We have tried to reproduce this problem in the lab with no success (most likely due to its sporadic nature).

We have worked on this costly problem along with our suppliers for some time with no resolution.

Any ideas would be greatly appreciated


Kevin Rowe & Tai Ho
metal finishers - Mississauga, Ontario, Canada


A. Chasing sporadic problems is a pain in the butt. My first thought is that maybe you're getting parts with some inclusions in plating, or stress cracks, or maybe just a soft spot in the plating. Any of these things will cause the coating to form differently in those areas.

Try flexing a part to from stress cracks, smack one with a wire brush to from dimples in the surface, and maybe abrade one to see if the abraded area looks like the "nubbing" problem.

Another thought is that your cleaning may not be up to snuff, but it sounds like you've addressed that, and chemfil products will do the job. Maybe check the location on the rack of the problem parts to see if it coincides with any apparent dripping or running of solution that could carry trash onto the parts. Good luck.

Jeff Watson
- Pearland, Texas


thumbs up signThe Problem has been eliminated. After changing jobs, to work in a zinc plating plant. Tai and myself have been better able to troubleshoot the problem. We hypothesized that there were catalysts trapped in the matrix of the zinc that caused the phosphating reaction to be accelerated. The highest concentrations of these "catalysts" would exhibit the nubbing effect.

We then took a look at a very long list of possible substitute chemicals which can act as a catalyst in the phosphating, and found that an extensive number of different organics can behave in such a manner. And as we were aware, zinc plating uses many organic components to achieve the desired plating properties.

We then decided to test the zinc phosphated part for phosphate coating weight and found them to be very high. Tai then tried to significantly reduce the immersion time in the phosphate bath, resulting an an acceptable coating weight with no nubbing. So, it seams that this reduced time is enough to achieve desired coating weights without promoting the localized excessive phosphate crystal growth.


Kevin Rowe [returning]
- Mississauga, ON, California

Zinc phosphate white stain


Q. Hi,

My company does defense contract work , mainly 60 mm mortar rounds, and they require a zinc phosphate pretreatment to paint. My phosphate line consists of an emulsifier cleaner(200 °F) hot, cold water rinse, phosphoric acid, 2 rinses, titanium activator, zinc phosphate(run w/sodium nitrite)(160 °F)rinse, and finally a phosphoric/chromic acid final rinse(160F). Problem is that I am getting a white stain on the outside of the part. I clean the line once a week and all my chemicals are in good standing (titrations good, and fresh chem.) I tried diff things such as adding neutralizer to rinse tanks, skipping certain tanks to see where the problem may lay. I think I narrowed it down to 3. the phosphoric acid (not rinsing good enough) the rinse tanks(where I believe 75% of plating problems occur) and finally the titanium activator, which is a salt, and has the power to stain parts. It may also be the phosphate bath, but I never had a problem w/this until recently.Ii didn't change anything on the line. I would like to know if anyone can help me.

Thank you,

Joel Antolik
- Nanticoke, Pennsylvania, USA


A. Let me try to help you :

Your process is not the optimum one :

a.First stage: Instead of hot emulsifying cleaner replace to Alkaline colder cleaner. There are a lot in the market .
b.Second stage: Phosphoric Acid, cannot be alone component in the bath, the bath must have surfactants, I suggest as in the previous stage, buy yourself one of the commercial products in the market.
C. The two next stages must be commercial products too, not self formula products .
D. The problem might be the final rinse stage, there is no need to heat this stage, if you heat it to such a high temperature the water must be Deionized.

Shortly : Try to use well proved commercial products I'm sure you have several suppliers in your area. Do not heat the last final rinse it is not necessary at all.

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

August 14, 2009

A. Because your phosphate bath too hot, I suggest 140 °F max
and don't add anything (soda ash) in rinse tank, just fresh water, because your's pre-treatment line has a post rinse (Cr rinse).

Andreas Nasran
- Jakarta, Indonesia

Must zinc phosphate be removed before heat treatment?


Q. Please can you tell me if Zinc phosphate can cause problems in any heat treatment processes, e.g., gas carburising. The problem we hope is from zinc phosphate left on cold drawn wire products and would like to know if this zinc phosphate should be removed prior to heat treatment.

Kind regards,

Mr Mark Bates
heat treatment - Brierley Hill, West Midlands, England


A. I have been told by the manufacturer that zinc phosphate will destroy the carbon probe and must be removed before heat treating. If you are not running a carbon probe in your furnace I am not aware of any other problems.

Art Burke
AMT Heat Treat - Fort Worth, Texas

Too Thin layer of zinc phosphate


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? Iyt 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

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

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,
Teds signature
Ted Mooney, P.E. RET
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,
Teds signature
Ted Mooney, P.E. RET
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 H Probert
Robert H Probert Technical Services

Garner, North Carolina

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

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