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topic 34052p4

Black and White Spots after Mn Phosphating

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A discussion started in 2005 but continuing through 2020

March 17, 2020

Q. Hi,

I am working as Senior Process Engineer.

We are doing Mn Phosphating of Vibro-Finished steel parts and facing issue of black and white spots after phosphating.

Phosphating process:

Degreasing->Water Rinsing->Activation->Phosphating->Water Rinsing->Passivation->Dewatering->RP Oiling

Earlier we were preparing parts with shot blasting instead of Vibro Finishing and parts were okay by visual.

Please help.


Keyur Solani
- Ahmedabad, Gujarat, India

March 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

A. Hi Keyur. Before you spend a lot of time trying to analyze the difference between the surfaces produced by vibro-finishing vs. sandblasting, I think you probably should make sure that you're definitely talking about a causal relationship rather than a coincidental one :-)

So please sandblast a few parts and run them through the line now, rather than relying on what happened before, and make sure that sandblasting makes the problem go away :-)

Only if it does is it worthwhile to undertake the bigger job of figuring out exactly what is wrong with your vibro-finishing process because it seems more likely to me that something went out of whack in your phosphating process in the interim than that vibro-finishing is incompatible with manganese phosphate for your parts.

Your posting is very similar to a posting from Abhishek Patel back in 2012, from Dhanjeet Singh who posted about black and white spots back in 2017. And I can see that you're with the same shop as Jeet Umre who described the fixturing of your parts back in 2016. If you're all from the same shop and this is one on-going problem since 2012, I can rearrange the stuff scattered across a half-dozen threads; surely a co-ordinated discussion will be more productive than a scattergun:

Good luck and Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

March 26, 2020

Q. Thank you sir for your reply.

Yes, all names you mentioned are from same organization but some are not working with organization now ... [remainder of posting placed below the sidebar by editor]


Keyur Solani
Harsha Engineers Limited - Ahmedabad, Gujarat, India

April 2020

A. Hi again. Okay, before we continue then, I've moved all the related previous inquiries that I could easily find into the sidebar below so readers can see the whole picture:


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading




August 17, 2012

Q. Using S110 steel shots to do blasting on steel parts. Recently have been facing issues with regards to small pitting/denting on parts.

Have installed a drive on the impeller motor and am thinking about decreasing the shot velocity. On the contrary though I do not want to compromise on the blast cycle time and am on the line of increasing the shot flow rate (controlled by a damper). This way I believe I would be able to run at the prescribed amperage (90% of full load motor current)

1. When the amperes (measuring wheel current) are decreased, am finding that the flow rate is increasing? Would it not be other way around? I was under the impression that the amperes are a direct reflection of my blasting intensity (which is a product of mass flow and velocity as in Kinetic Energy)

Any recommendations on my thought process? Any assistance in form of technical input would be highly appreciated. Cannot figure out how to resolve this denting issue.

Thank You

Abhishek Patel
- Ahmedabad, Gujarat, India

November 19, 2013

Q. Hello,

Currently we are using Shot blasting as a surface preparation prior to Manganese Phosphating. Would the phosphating coating characteristics and adherence properties change if we change the preceding process to Vibratory finishing?

We have issues with dimensional distortion and surface roughness so we would like to ideally switch over to a milder form of deburring/pre treatment.

Any issues been foreseen with the change?

Abhishek Patel
Ahmedabad, Gujarat, India

November 19, 2013

A. Hi Abhishek,

I must admit that I don't have experience with using vibratory methods of preparation before manganese phosphate, I have always used an abrasive blasting technique.

Before you go and start looking at new ways of finishing can you describe the exact blasting process you are using (blast media type, mesh size, blast pressure, stand off distance)? It may be with some modification of your technique blasting may still be a viable option for you.

For example I use 180-220 mesh alumina grit at a pressure of 40 psi nominal (some jobs have higher pressure needs, some lower) and a stand off distance of about 6-9". This works for about 80% of jobs I do and I suffer little to no distortion.

Brian Terry
Aerospace - Yeovil, Somerset, UK

November 21, 2013

Q. Hi Brian

Being a mass production process, we are confined to use wheel blasting & not air....besides the issue is rather the geometry of the parts.

We use cut wire steel shots for blasting with tumblast wheel blasting machines.

Am of the understanding that phosphating happens at a molecular level so only surface cleanliness & uniformity along with proper surface conditioning should do the job.
Though, yes, higher roughness caused by blasting would give more surface area to the phosphate for better coating & adherence.

Just wanted to confirm from all the experts here.

Abhishek Patel [returning]
- Ahmedabad, Gujarat, India

November 29, 2013

A. Hi Abhishek,

Probably going to show a bit of my ignorance of wheel blasting machines now, can you not reduce the rate of feed of blasting media or change the speed of the wheel blasters so that you reduce impact speed? Alternatively can you change your media without modifications to your cabinet, you may be able to use a less aggressive cutting media?

You are perfectly correct that the reaction happens at the molecular level and that the increase in surface area by blasting increases the number of reaction sites.

If you wanted to go to a vibratory method then you need to talk to your local supplier to see what they have to offer in terms of media that will cut sufficiently to give your increase in reaction sites and still avoid distortion. I get a feeling you are going to have to try a bit of trial and error to see if the process is a viable method of manufacture.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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

February 24, 2016

Q. Hi guys,
I do Mn and Zn phosphate coating and use rotating tumbler for holding parts during the process to avoid touch marks of jigs/fixtures. For a recent new part I am facing problem of parts getting stuck to each other due to its cup like shape leading to light uncovered portion in 30% parts. Tried many options but couldn't succeed.
Can anyone please help me out with some unique fixture options or concepts or designs to avoid both touch marks and sticking of parts with each other during the process.


Jeet Umre
- Ahmedabad, India

December 22, 2017

Q. Hello Sir
We are doing Mn phosphating on AISI1208. After some components phosphate white and black spots observed on phosphate material. Due to this we do lots of quantity rework.

Dhanjeet singh
- Ahmedabad Gujrat

December 26, 2017

A. Hi Dhanjeet,

Please tell us how you clean your parts (alkaline degreasing? acid pickling? how many rinses?), parameters in phosphate coating (temperature, free and total acid) and sludge removal system you have so we can help you with some recommendation to improve your situation.

Best regards :)

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

March 26, 2020

Q. .... For Now we have already completed with shot blasting as you have said.

But now we want to eliminate shot blasting process and replace it with vibro finishing process.

So prior to phosphating we will have Vibro finishing plus dryer process and afterwards directly to Mn Phosphating.

With same process we are facing issue of black spot on our parts which is not 100% some parts are also found visually ok. Almost 50% is rejection.

Please give your guidance to proceed further.


Keyur Solani
Harsha Engineers Limited - Ahmedabad, Gujarat, India

April 27, 2020

Q. Thank You for the reply.

Yes as per your suggestion we have completed with shot blasting but now onward we want to replace shot blasting with Vibro finishing to improve surface roughness which is customer requirement.

We are achieving surface roughness within spec with vibro finishing but we are facing below issue with same:

1. Black spots on some of the parts
2. Higher coating weight of 15 - 25 gram/m2

Please guide for the same.

Thanks in advance

Keyur Solani

Keyur Solani [returning]
Harsha Engineers Ltd - Ahmedabad, Gujarat, India

April 2020

A. Hi Keyur. If you have run sufficient shot blasted parts this week to be confident that they all come out well, and your phosphatizing line is currently in fine tune, you can move on to examine the relationship between vibratory finishing and the decline in quality of phosphating.

Jeet Umre reported difficulty in processing the parts because they "nest" due to their cup-like shape. Do you have good reason to be confident that they are not nesting in your vibratory finisher, so all surfaces are being properly finished?

Although bead blasting is good prep for phosphating, it is not necessary for proper phosphating; in fact, probably only a small percentage of phosphated parts are blasted. But in addition to increasing the surface area, it serves to both clean and activate. When you skip it, a question becomes whether your cleaning and pickling is adequate without it.

I am personally not familiar with vibratory finishing before phosphating. My question then is why are you doing it? My impression is that vibratory finishing before phosphating would be for the purpose of deburring, not for cleaning or activating. What happens if you manually deburr a few parts as needed, then just clean, pickle and phosphate without any vibratory finishing? A good next step is to try to determine whether vibratory finishing is merely not helping or is actually interfering with the phosphating.

Satisfactory phosphating is complex, and everything is interrelated; sorry that it's not proving easy :-(


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

April 28, 2020

Q. Good Morning Sir,

Why are we doing Vibro Finishing?

Answer is our customer is having requirement of surface roughness of 0.5 Ra, to achieve same we shifted to vibro finishing instead of shot blasting where we were not able to achieve surface roughness as per customer requirement.

Addition to that, deburring and cleaning is other objective.
But as earlier said we are facing issue of:

1. Black spots on some of the parts
2. Higher coating weight of 15 - 25 gram/m2 with vibro finishing prior to phosphating.

Keyur Solani [returning]
Harsha Engineers Limited - Ahmedabad, Gujarat, India

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