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topic 56202

Stainless steel passivation color inconsistent, changes

A discussion started in 2003 but continuing through 2017


Q. Dear Sirs,

We are a precision tooling manufacturer, I have came across this problem with passivation. Recently,we have our tools sent for passivation process.The material is 440C,the different between this tool is one of them had heat-treated to HRC 56-58,the result we got after the passivation process is the one without heat-treat have a darker colour "black",the ohter which heat-treated came out with brighter colour "silver".

We were wondering what have happen in the passivation process.We hope that you have the answer for us.

Thanking you in advance.

Tai Mai
- Georgetown, Penang, Malaysia


A. What you are seeing is the carbon on the surface of what is affectionately called a "burned" product. This is not untypical with 440C and some other grades of high carbon, high sulfur stainless steels. it can be caused by a number of conditions in the bath. With using the correct pretreatment to remove some of the carbon and sulfur with alkaline (high pH) products you usually can eliminate this problem. If you are using nitric acid you need to have sodium dichromate or equivalent in the bath (caution: hexavalent chromium). If you use the correct citric acid based products that are on the market you will not have this problem and get consistent high quality parts.

Let us know if we can help.

lee kremer
kremer signature
Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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November 23, 2010

Q. I am the Quality Control Supervisor for a FAA Repair Station. We do some passivation in house, however, the results are not very consistent. Sometimes while performing this process on Stainless Steel parts the result is a clear and shiny color while in some other cases the outcome is a dull and darker color. Are the different results due to the dipping time, contaminants, temperature and/or humidity? or simply because there is a range like in any other coating or plating process. Thanks in advance for your cooperation

Gino Faranda
Quality Control Supervisor - Miami, Florida, USA

simultaneous November 26, 2010

A. Check the chemical makeup of each lot of SS.
Check the heat treatment state of each part.
Check the surface smoothness of each part.
Try to find out how each part was actually cleaned. What is on the process router sheet is not what always happens.
Since water is added to many EP tanks, the solution needs to be truly homogeneous to get the proper results.
Temperature is another factor that is overlooked when it is supposed to be automatic controlled.
How often is the tank analyzed?
Any possibility that the tank was stirred and sludge settled on the part

James Watts
- Navarre, Florida

November 29, 2010

A. Gino,

You are going to have to give a few more details:

What alloys are we talking about here?
What process are you using for the passivation?

If you are processing 440C or 416 type alloys I can understand where you may be having problems. If it is one of the other alloys it may be a little harder to explain.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom

December 16, 2010

A. This does depend on factors such as the grade of stainless, type of passivation bath used, temperature and immersion time.

Generally, when you see a "color change" in a metal surface, it is due to the surface finish. A smooth, shiny surface that is chemically etched will appear darker to the naked eye.

If you are having etching problems with some of your parts, you may need to reduce the immersion time or find a more appropriate bath. The passivation standards have charts that describe what nitric baths are best for which grades, or switch to citric acid which if done right will not etch the parts at all.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

August 25, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Currently I'm facing a problem with my supplier related to the finishing of the color surface after the passivation process. Some parts looks shiny and other ones look like gray color, not shiny color. So, during this process, is it normal to have this difference between parts? Just to clarify, these parts were not processed at the same time.



Ruben Alvarado
L3 comunications - Chihuahua.Chih. Mexico

simultaneous August 28, 2017

A. Ruben could you share the alloy of the parts and the type of passivation used?


Yohands Rey
KAT aerospace - Chihuahua, Mexico

August 28, 2017

A. Ruben,
It depends a lot on the grade of stainless and what is being done to them. Nitric passivation has a much higher chance of etching the surface then citric acid passivation. High chromium 300 series grades has a much lower chance of getting etched than low chromium 400 series grades.

In theory passivation should not change the surface finish, but there are still some people around that associate the etched/pickled look of the nitric acid treatment with successful passivation.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

August 31, 2017

Q. Sure Yohands the alloy is AMS5517.


Thanks Ray I appreciate your valuable help and comments

Ruben Alvarado [returning]
L3 comunications - Chihuahua.Chih. Mexico

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