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

Current comments:

Q. In an experiment, what are the normal voltage range & current that should be supplied to an electrolytic zinc phosphating bath?

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"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber
from Abe Books
or

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A. Hi Mihiri. According to Biestek & Weber, alternating current at 60 Hz and 3.5 A/dm2. The voltage would depend on electrode to component spacing but my guess is somewhere between 6 and 18 volts.

Please introduce yourself and your situation; it can be very helpful towards preventing miscommunication. Thanks.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Hello All,
I am working in a paint shop in automotive industry. Applying zinc phosphate mpt before ED paint. Recently suffering harsh mark after ZnPO4 tank.
Phosphate Parameters:
FA: 0,9 ml
TA: 22,5 ml
ACC: 3,2 g/l

We tried to increase activation concentration, reduce temp. and FA but it does not help.
I would be very happy if I could get your suggestions.

? Hello, Did you get the reply for the above thread? Because we are also having the same problem. Kishore Krishna Reddy - Wooster, Ohio A. Hi Siva, An image would really help to know what's happening. Without it, I'm blindfolded, so I would check: - Cleaning process - Sludge in phosphate - Rinsing process - Neutralization? Passivation? Chromating? So, would help you share how you see the problem in your workshop. Best regards, Daniel Alejandro Montañes Quality Manager - Cañuelas, Buenos Aires, Argentina

A. Hi Kishore. Yes, your problem may be similar, but I'm sure you don't have the identical FA, TA, ACC, activator concentration, and temperature :-)

Please try to include your parameters and a pic if possible, and replies to Daniel's possibilities. Thanks!

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Free Acid- 1.10
Total Acid-12.90
pH- 3.05
Fluoride- 449
Zinc- 2087
Nickel- 490
Manganese- 940
Phosphate- 4244
Nitrate- 3682

This is the data after acid pickling the zinc phosphating bath. It is a fresh bath. It is 7-stage zinc phosphating (pre wash, cleaner, zinc phosphating, city water rinse, non-chrome seal, Di wash, DI rinse )

We are also experiencing mapping issue. It is not in all the products, mostly in Aluminum substrate.

Q. Hi Daniel,

The cleaning process consists pre-wash (not a bath, just two risers on either side), next is alkaline cleaner bath spray and then a alkaline rinse bath spray along with the rinse conditioner spray with two risers on either side (grain refiner stage), which is about 6 seconds and then phosphating bath spray.
What are potential solutions to change in the current set-up?
Should we go with acid cleaners? The problem with that it is a mixed metal line, where we have galvanized steel as part of the process, in case should we implement a manual acid cleaning process for aluminum substrates before the pre treatment. Right now we are applying the highlighting fluid for all the parts.
Another question, we don't measure any of the parameters in stage 1 and 2, should we measure the Ti concentration and pH of the stage? Is there any range for Ti concentration and pH range? Should we measure the coating weight, SEM for these stages and look over the performance?
With all this said, if were are able to correct the cleaning process, can we reduce the mapping issue, because we do lot of metal finishing on the substrate.
Do you think anything to focus on the AL substrate itself?

Thanks,
Kishore

Previous closely related Q&A's, oldest first:

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.

Several threads were merged; please forgive repetitiveness, chronology errors, or perceived disrespect towards earlier responses -- they probably weren't there then :-)

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.

Several threads were merged; please forgive repetitiveness, chronology errors, or perceived disrespect towards earlier responses -- they probably weren't there then :-)

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.

• - 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

Regards,

---

2000

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

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2002

The 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.

Regards,

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,

---

2003

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
YB Plating Engineering and Quality - Haifa Israel

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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,

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.

Black spots and non-uniform coating

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

Black patches and pits

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.

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July 2, 2013

Please never tire of your suggestion that people walk the process, Brian.

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

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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.

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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.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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

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?

May 17, 2016 I 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

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 customers' 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,

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!

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February 2018

 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 :-(

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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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 Montañés
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina

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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

Q. Hello, why are phosphate crystals coarse and how can we have a soft zinc phosphate?

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September 2018

A. Hi Ahmad. Are you using a grain refining pre-dip as explained on this page which we've added your inquiry to? If you search the site for "coarse zinc phosphate" you'll find several more threads on the subject if the answer isn't on this page. Good luck.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Follow up question-re: 1998 post by-"Ray D.-- says "Chrysler spec says 'no acid pickle' "--what is the alternative?

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November 2018

A. Hi Jay. Abrasive blasting is a general alternative to pickling which leaves the surface active and ready for a next step. Alkaline derusting might be an alternative to pickling for some processes. Good luck.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Does zinc phosphate coating affect hardness testing values?

Zinc phosphated surface is rough for first few days of each new bath

Q. We run a 7 tank zn phos line for OEM part. We have a roughness issue at the start of a new bath. The roughness settles down only after 4-5 days on the components. Any idea why this happening and how to reduce them when we set up a bath?

Q. Hi sir I am Rajkumar zinc phosphating 13-tank process alkaline,rustban, oxalic, plxgi, phosphating.
Phosphating temperature 75 °C to 85 °C. Chastain ring will process two-pass ip 18 mettiriyal output black colour, but ring is brown colour.

Ip 18 time alkaline 60 sec temperature 85 °C
Rustban 150 sec
Oxalic 120 sec
Phosphating 60 sec
Hot water passive a 30 sec
Hot air 200 sec
Rustban. 100 sec
Plxgi 30 sec
Phosphating 50 sec
Hot water 30 sec
Hot air 200 sec

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May 2019

  Good luck, Raj, but I don't have much idea what you're talking about except that your phosphating is brown and I think you wish it was black :-)
Sorry, but "Chastain", "two pass", "ip", "18", "mettiriyal", "Plxgi" mean nothing to me :-(

Hopefully another reader will be more familiar with them.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Hi, my name is Anuj. We coated our couplings with Zinc phosphate coating. But the coating seems to peel off during shipping. Coating gets adhered to packaging. There is non-uniformity in coating too. Please could you suggest solution to this situation.

Can we use zinc phosphate coating to give anti corrosive coating to high alloy steel? Which coating is better zinc phosphate or manganese phosphate in terms of corrosion resistance and wear resistance ease and cost efficiency.

Q. Thank you Issac,

Is high alloy steel suitable for zinc phosphating?

We wanted to give anti corrosive coating for C5M (H) category according to ISO 12944. would zinc phosphating coating alone be sufficient to endure this condition or should we adapt any another coating option. please suggest

Q. I'm getting coarse Crystals on the underside of my parts repeatedly.
Process is as follows:
Cleaner
Rinse
Pickling acid
Rinse
Grain refiner
Phosphate
Rinse

TA 52.2
FA 11.2
Iron 4.3
Temp in phosphate is 165 °F
Titrations are in mLs and based on 10 mL sample.

I recently had to make a new phosphate bath.
The previous bath was running identical to what we have now and producing beautiful parts.

----
Ed. note: Eric also posted rather similarly on topic 14221 for a deeper understanding of his situation.

Black line on the wire after zinc phosphating

Q. hi
Our company works in the field of drawing and phosphate and annealed wire.
Our problem is the existence of a black line along the wire after phosphating.
The wire is annealed before phosphate.
This black line does not disappear after washing with acid.
In addition, our coils are closed when phosphating.
Our method of phosphating is immersion.
tanks

Q. We are doing zinc phosphating. We are not getting uniform coating: white patches are there.
Phosphating pointage - 29
FA -1.8

Q. My question is more of a theoretical one, and it deals with the fact that zinc phosphate (hopetite/phosphophyllite) attaches to the metal substrate.

Why it does that?

What can we do to make sure it does it evenly and uniformly across the surface and with proper weighting?

What can we do to ensure that it's not easily scratched off?

Any tips would be highly helpful.

---

"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber
from Abe Books
or

Affiliate Link
(your purchases make finishing.com possible)

February 2021

A. Hi Waqar. According to Biestek & Weber the short answer to why phosphates attach/adhere well is that they are formed "at the very reaction site". In other words, there is not a reaction out in the bulk of the solution which causes the precipitation, but a conversion reaction right at micro-sites on the surface of the metal.

I don't have personal experience to be able to tell you what makes for evenness of deposit and scratch resistance, but if you can get access to the book, it's very good at the theoretical level. Or you can read this thread and thread 13366 for a lot of readers' solutions to zinc phosphating issues.

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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)?

Q. We have application of zinc phosphating process in S.G. Iron material. We are facing issue of patch mark (uneven while / dark layer) on the surface. Can you please suggest the probable root cause and precautions to prevent.

! Hi: This is how foam formation in phosphate bath will effect the coating.

I found like dry marks after my CED coating. Then we verified that wherever foam forms over the vertical substrate, in those locations we identified those marks after CED.

We found the root cause, and how the foam can affect the coating ... for discussion and knowledge.

adv.
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