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topic 0909

Passivation with citric acid

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Dear Sir. I work for a small shop in Ohio. We make a part out of chromecore that has to be passivated with citric acid.We are having a problem finding someone who can do this in production. Any input would be great. Thank you,

Kenneth W [last name deleted for privacy by Editor]
- Ohio


Ken there are suppliers out there that are selling Environmentally friendly citric acid based passivation solutions.

They are easily accessible and easy to use.

Mike H [last name deleted for privacy by Editor]
- Philadelphia, Pennsylvania



Be careful if you plan to use the Citric Acid product yourself. Even though it is being marketed as environmentally friendly, it is a strong chelate. If you can batch treat you are o.k. because theoretically all that's in the solution will be free iron, but if this stuff is added to your waste stream it will tie up any metal including copper that it comes in contact with, and will increase your treatment cost and time.

The marketing tool these guys should be using is the safety factor in eliminating nitric acid from your operation, not the environmental angle.
Good luck, Mike

Mike M [last name deleted for privacy by Editor]
- Philadelphia, Pennsylvania


There seems to some strong marketing in the USA of citric acid as a passivating agent.

However, since citric acid is not an oxidising agent, I fail to see how it can effectively passivate stainless steel.

The ASTM A380 [link is to the practice at TechStreet] a recommended chemistry is nitric acid.

Supposed environmental safety should not outweigh the technical issues.

We would recommend using nitric acid and then neutralise with soda ash or equivalent.

What are your thoughts?

John H [last name deleted for privacy by Editor]
Callington Haven


Dear Kenneth:

Citric acid passivation is a tried and true solution to a lot of the problems in the stainless steel industry.

First, it is both safe to use AND environmentally safe when used as directed. Products on the market using citric acid have been thoroughly tested and found to equal or exceed the passivation produced by nitric acid formulae. Because it is a chelant, you do not mix large quantities into the heavy metal precipitation process, but there are ways to handle that very effectively, and many can eliminate the NEED to treat for heavy metals by the use of citric products.

The specification to use for passivation of stainless steel is ASTM A967 [link by ed. to spec at TechStreet] , not ASTM A380 [link is to the practice at TechStreet] , which is recommendations, not a spec.

You do not need an oxidizing acid to passivate stainless steel. You need FIRST to remove the free iron from the surface and in inclusions, then let the oxygen in the air take care of the oxidation of the surface. Stainless steel is self oxidizing in air, as long as you are able to get rid of the iron corrosion from the surface.

The proof of whether it works is in the test data, and there is much test data to show how well it works while being safer and lower cost.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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I certainly agree with all of what you have said, Lee -- but where is this 'much test data' that you refer to? Can you reference a published article or two for us?

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Hey! I think I'm going to do a science experiment on the effect of citric acid on the passivation of stainless steel, or something like that. I'm only in ninth grade and I was wondering if I could conduct an experiment on this safely and correctly. It would also be helpful if you could give me some information about citric acid and its effects. Thanks!

Liz R [last name deleted for privacy by Editor]
- Columbia, Missouri


My department invests a small amount of money in pollution prevention projects. A potential proposer has described a project to test a citric-based passivation system to eliminate nitric acid and reduce acidity of sewage discharges.
Two questions: Is this really something that needs to be researched or is convincing info out there(where?); 2) If it does need demonstrating, how many (ballpark) different samples would need to be passivated this way to be meaningful?

Thanks for any views on this.

Tom K [last name deleted for privacy by Editor]
NY State Dept. of Economic Development - Albany, New York


hello sir,

i just have a quick question on passivation with citric acid. We plan to do passivation on a continuous line oven and we are not sure on the percentage, or molarity of the solution of citric acid we should use. the volume of the reservoir in the oven is 1000 gallon, and this volume will be transferred from zone to zone, 5 zones in all. normally in the states we use a 2% nitric acid solution by volume, and basically we are looking for the equivalent using citric acid. if you can help me that would be great. I'm sure you are busy with real customers but this would help me tremendously.

Thank you.

josh j [last name deleted for privacy by Editor]
- moscow russia


I am interested in your answer to the Liz Roberts question from Columbia, MO. I am also involved in a science project about the effect of Citric Acids on Metal. If you do not have an answer, could you please refer me to someone or some website that can help?

Thank you,

Patrick M [last name deleted for privacy by Editor]
- Fairfax Station, Virginia

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