Nitric vs. Citric Acid for Passivation of Stainless
My work is considering building its own passivation plant for in house use. We are still doing research on whether to use Nitric or Citric Acid bath. Your opinions will be be appreciated.
- Edison, New Jersey
Nitric acid is far superior to citric acid in relation to introducing a passive film on the surface of stainless steel parts. Nitric acid is so effective because of it's primary mode of action, that is, oxidation of the surface thereby removing iron radicals, etc as present in for example heat generated oxides. This effectively removes risk of corrosion of the part as well as introducing the passive layer. Citric acid works more likely by super-cleaning the surface of the steel allowing spontaneous generation of the passive layer. This however will not remove radicals from the surface and therefore the steel is prone to corrosion further down the line. I do not know of any job-shops that condone the use of citric acid in passivation. Chances are you are looking to use citric because it is more environmentally friendly than nitric. However, the quality of the finished part will be far inferior.John Martyn
While I agree with you in many circumstances pertaining to nitric acid corrosion results over citric, I have found in my humidity cabinet several cases where certain alloy's resist corrosion is longer with citric passivate than same parts with nitric. Also in special tests requiring confirmation of sulfide removal (303 SS) hands down to citric. While poor at removing heat scales (better addressed in ASTM A380 [link is to the practice at TechStreet] ) citric passivate has won the merits of an ASTM designation...ASTM A967 [link by ed. to spec at TechStreet] .Jon Quirt
- Fridley, Minnesota
With all due respect to John, he is very wrong. Citric acid is being used by MANY job shops and thousands of in house operations due to the INCREASED corrosion resistance obtained. Citric acid, when formulated correctly produces higher chrome oxide ratios than nitric acid, and has many other advantages in safety, cost and environmental friendliness. Many customers have data showing the improvement in performance. It is MUCH more than just a cleaner. It removes more iron from the surface than nitric acid. Please contact us or go to our web page if you need further information.
Stellar Solutions, Inc.
I am only knowledgeable of the job-shops in my neck of the woods, i.e. Britain and Ireland. Chemically, I do not see how citric acid could be better than nitric acid. Maybe you could explain the technicalities of the citric passivation process to me, what chemical processes are involved and what real data you have to back up your results. As a chemical engineer, I am very interested in obtaining environmentally friendly chemicals to use in my passivation process so obviously this would be an interesting change.John Martyn
Thank you for sharing your views. I am nearly convinced that Citric acid would do as good a job as nitric acid. I did some inquiries from other passivation plant and they mentioned something about it not being as easily disposed of as claimed (eg. cannot mixed with regular waste stream). Thanks again for your help.Anthony Samaniego
- Edison, New Jersey
Anthony & John:
Citric acid formulations can be very easily disposed of in the waste stream as opposed to nitric acid systems. However, it cannot be put carelessly into a heavy metal precipitation system. There are easy and economical ways to dispose of rinse water by putting it to drain, since it will not normally contain any significant heavy metals.
If you put the waste into a precipitation system it must be treated first. There are simple ways to do this.
Citric acid works much better than nitric acid for passivation because it prefers the iron over chromium and nickel. Therefore, it reaches a steady state at a much higher chromium content on the surface, producing an excellent chromium oxide layer.
There is data from hundreds of companies in the U.S. and around the world (in addition to our data) that proves the superiority of citric formulations. If you contact me directly I will share some of this data with you. It obviously cannot be done in this forum. Some of the data can be found on our web page.
Stellar Solutions, Inc.
October 18, 2009
The Reduction Potential between CH3CO2H
(from the citrical acid) and the Cr3+ is far more powerful when compared to NO3-. Even the dissociation capacity from HNO3 being greater than Citrical Acid, this property of the solution has less effect on the range of the passive layer.
That's my theory...
- Florianópolis, SC, Brazil
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