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topic 17774

Passivating 303 vs. 304 stainless steel

A discussion started in 2002 but continuing through 2018


Q. We utilize a lot of machined 304 SS parts which are passivated for appearance reasons only. We Are switching to the machining friendly 303 SS and I have been told this grade can't be passivated. Chemistry is so close between these two specs it seems untrue to me. Any help out there would be appreciated.

Lawrence Moore
- Menomonee Falls, Wis, USA


A. You are correct! 303 can and is passivated quite often. Certain precautions need to be taken to prevent "outgassing" or "burning" in the process. Most finishing shops are setup to handle this material.

Bill Grayson
- Santa Cruz, California, USA


A. Far too many machine shops "forget" to tell the metal finisher "the rest of the story". My experience was you would be told that it was 303 but they would leave out the suffix such as Se which is not the same as 303 to a metal finisher. They would also swear that it was plain brass and when you had problems that caused them delays, it magically became "free cutting". You can not get excellent adhesion on leaded metals without special processing. Therefore, it does make a huge difference if it is 303 or 303Xx.

James Watts
- Navarre, Florida


A. There are a lot of misunderstandings about 303 stainless steel. 303 ss has high sulfur or selenium to provide good machinability, etc. It is the sulfur or selenium that gives the apparent problem in passivation. In order to passivate 303 stainless steel you must first remove the sulfides and sulfates from the surface (or selenium in the case of 303Se).

I have not worked much with 303Se, so I can only speak for the bulk of the material which is 303 or 303S. What happens is that after passivation (or even DURING the passivation process in rinsing) the surface of the 303 turns yellow, brown or gets spots on the surface that look like rust. But, they are NOT rust, they are sulfides and sulfates. It is easy to test for this. Soak the parts overnight in some 5% NaOH. If the yellow, brown or "rust" spots disappear-- it was sulfur problems, not rust.

So, the answer to the problem is to remove the sulfur problem from the surface before passivation by a bath of hot alkaline solution (adv.: contact us if you need details) and then passivating it. This works for both citric acid formulations and nitric acid formulations. This treatment is a MUST for all 303 stainless steel, or you can get yellowing or "rusty" appearance in the field.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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January 18, 2016

Q. You have answered my first question about the rust-looking spots on 303. But my second question is that after passivate I am getting white spots. I believe this to be some grease or oil that I have not removed. But I am now having a very difficult time trying to remove it aside from hand washing the parts. What could you recommend? Any help is appreciated. Thank you.

Rick Flynn

January 19, 2016

A. Rick,
So the spots do come off when you hand wash the parts?

A high pH alkaline cleaner is probably the best bet. If not, maybe a solvent will do it. If you have an ultrasonic tank available, that will be helpful too.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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December 7, 2018

Q. Hi All,

My Company now trying to do passivation for stainless steel (316/316L) injection molding part. The part will undergo sand blasting (glass bead) before passivation process. We are using alkaline degreaser (Ph:10-11) at 60 °C. Then using citric acid (Ph: 4-5) at 70 °C. The problem that we face right now is there is white dot on the parts that cannot be wiped off after passivation. Can anyone help regarding this matter? Any opinion is highly appreciated.

Ahmad Amri
Kako Suntax - Johor, Malaysia

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