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topic 10242

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

Current question:

Ed. note: Please, please, pretty please,
No abstract questions!

April 6, 2021

Q. Purple Top Coat Stain on a Zinc Phosphate Steel Part?

Will excessive residual phosphate coating (i.e/, TT-C-490 [link is to free spec at Defense Logistics Agency, dla.mil]) cause a staining to pass thru the dry paint film (purple)?

Joe Schilling
- Burlington Iowa
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Previous closely related Q&A's starting in:


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 [affil. link to info/product on 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 °C 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 search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

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








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 search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)

Zinc Phosphating Defects (Nubbing)


Q. Hello,

Tai and Myself have been following the postings here at Finishing.com 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.

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
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 & Tai Ho [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

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