Passivation alternatives to Nitric Acid?(1999)
I have been searching for an alternative to passivation of 3000 series stainless steel. Currently a company I am working with is using Nitric acid in a standard passivation bath. They are however looking for ways to cut down on their hazardous waste and materials. I have found two alternatives, citric acid and hot air, but I am wondering if anyone has found or heard of any others. The company has looked into the citric acid but deemed it too expensive to retrofit their equipment. Are there any other alternative methods available?James Hanley
Washington State Department of Ecology - Seattle, Washington
There is so much rumor, speculation, and opinion on passivation of stainless steel. Almost everybody will agree that the basic purpose of passivation is to remove iron that the parts were exposed to in the manufacturing process; but some feel that truly thorough cleaning can accomplish this, while others argue that only an acid can remove the iron.
So, when you ask what are the possible substitutes for nitric acid passivation, some will feel that a substitute is to just skip the step entirely, and just use really good cleaning. But that leaves the two remaining issues of complying with specs that may require passivation, and the liability about what happens if the parts rust and they weren't passivated.
ASM's Metals Handbook suggests that when the need for passivation is in doubt, that parts be tested in a humidity chamber and/or with a copper sulphate spot test.
Ted Mooney, P.E.
Pine Beach, New Jersey
Ed. note: There has been a lot of development on alternatives in the years since the above was written, and we now have advertisers offering alternatives to nitric acid, including proprietary citric acid and acid-free electropolishing -- Ted
James: I would like to suggest a possible alternative to nitric acid passivation on Stainless Steel. With the intent being to scour away available Fe++ sites leaving inert surface sites of primarily chrome and nickel to improve surface corrosion resistance; i.e., passivation, I have seen excellent results with a process utilizing controlled ORP (Oxidation Reduction Potential) at the solution substrate interface. The solution consisting of Oxalic Acid, ORP regulating and surface tension additives. At this point I can only report improved passivation results when using this chemistry in vibratory finishing units on Austenitic Stainless Steels for surgical implants and 17.4 PH Martensitic Stainless Steel for ground based steam turbines.
As Ted commented, debating the necessity and method of Stainless Steel passivation has been around along time. I agree, the true test is to put specimens in corrosive control chambers and compare one process to another.
Regards, BillBill Boatright
- Raleigh, North Carolina
Hi James ,
People may argue about the various merits & demerits of treating stainless steels to restore passivation films after those stainless steels have been worked, but there is really only one answer, stainless steels (especially the 300 series ) will "rust" if not treated , and the most effective treatment is to enrich the surface with chrome & Nickel and form their oxides so that corrosion by normal means is prevented .
The simplest way to do this "Enrichment" , and get it right every time is by electropolishing the part; subsequent treatments by chromates, nitrates & citrates may speed the process of forming that "Protective Oxide Film" , but in the opinion of most of those experienced in this part of metal finishing electropolishing by itself is good enuff .
Regards, John . C . Tenison - Woods
- Victoria Australia
I'm not 100% sure that surface enrichment is actually the principal mechanism by which electropolishing makes stainless passive, John, but I do agree that electropolishing is the surest passivation process.
Ted Mooney, P.E.
Pine Beach, New Jersey
Thanks for the info, I really appreciate it! JimJames Hanley
Wa. State Department of Ecology - Seattle Washington
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