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topic 20475

Manganese Phosphate pits question

A discussion started in 2003 & continuing through 2017


Q. Hello,

I'm an engineering student studying the effects of pits caused by phosphate manganese coating on case hardened steel.

I have some problems getting information about those pits, especially what is their size scale, distribution, what are they due to (acid bath during the coating process, but...?).

How can I remove this coating without creating new pits or machining surface defects?(cause I'd like to count them to get the distribution).

Do you think there is a way to create similar defects on sample I wanna test in fatigue and how do you suggest me to create those false pits.

Thanks a lot.

Falandry Xavier
- Göteborg, Sweden


A. Hi,

Are you talking about pits being created in the base metal as a result of manganese phosphate being deposited? Or are you examining a manganese phosphated surface and have discovered voids in the coating?

I don't believe that the phosphating process alone will cause pits to form in the base metal. Did you apply the manganese phosphate to the steel yourself?

The phosphate coating can be removed by dipping the phosphated article in a 20% solution of CrO3 at 180 F for 5 minutes.

George Gorecki
- Naperville, Illinois


Q. Thank you for the removal tip, concerning the pits, they are in the metal, not in the coating. I have some paper here saying this is due to the phosphoric acid bath during the coating process. And if you or someone else have further info, they are very welcome.

Just another question, in what do I have to make my 20% CrO3 solution? In acid, or something else?

Falandry Xavier [returning]
- Goteborg, Sweden


A. Hi,

Phosphoric acid causes pits in the base metal? Well, the phosphoric acid in the bath dissolves a little bit of the metal surface in order for phosphate coating to be formed. This is an essential aspect of the phosphating mechanism. The claim sounds dubious to me.

The stripping bath is made by dissolving 200 g CrO3 in 1000 mL of water.

George Gorecki
- Naperville, Illinois


A. Pits are caused by high free acid in the solution. To limit pitting, start heating the solution. When it gets to 185 °F, then right at the tank, right now, titrate the total and free acid and neutralize the excess free acid with Manganese Carbonate. By the time you complete the analysis and neutralization the solution will be up to 195 °F. Now immediately process the parts, then turn off the heat and cool the solution to prevent re-forming more excess 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

Uneven surface after phosphating

November 14, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello sir,
Our process for manganese phosphating is as follows:
1. Degreasing
2. Derusting
3. Water rinse
4. Phosphating for 50-55 minutes at 85-90 °C
5. Water rinse
Coating weight 12 gms/ meter square and above.
I am facing problem of pitting like surface after phosphating. Please suggest some solution.

Manish Das
- Mumbai, India

November 22, 2017

A. Hi Manish, from what I can tell without the specifics to your process it sounds like you may be etching the underlying steel in the process of phosphating. The current bath my company runs rarely allows for longer than 15 minutes in the bath in order to ensure we do not cause undue etching. However, we use EPI (Electrochemical Products Inc.) [a supporting advertiser] E Phos 630 which though proprietary I must assume has accelerator(s) present in order to achieve the coating weight necessary in that length of time. I might suggest posting more information on the bath type and / or parameters at which you are running in order to allow the community to further diagnose the issues you are running into. Specifically free acid content and iron content. I have read excess free acid is often the culprit of etching the base layer. Hope this at least gives you some sort of direction.


Jordan Egnatoff
Tri-State Plating - Huntington, West Virginia USA

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