Passivating Stainless Steel with citric acid

A. Jeff,
Hydrochloric acid is bad, bad, bad for stainless steel. It's not something to use for pickling even if you wanted pickling. Thank goodness it was only 2 minutes.

I have no idea what Camm SUS CP118 is.

In the major industry passivation standards, every listed nitric acid passivation treatment requires a minimum of 20 or 30 minutes.

Not only did they not passivate your parts they way you asked, they used nothing resembling any known accepted pickling or passivation treatment and probably did the parts more harm than good. Find a different supplier, and as a parting shot you can ask them if they were TRYING to ruin your parts.

As to your questions:
All the acid bath is doing in passivation is removing iron. (Well, nitric passivation picks away at the other metals too, but more slowly.) Extra time in the passivation bath/additional treatments generally will just have no effect, since all the iron at the surface is gone already. In your case I would definitely passivate again (if the parts still look okay), since there is probably a lot of iron there after the abuse your parts went through. Since you already are locked into using citric acid , you could even do it yourself if you wanted to. Run them an extra long time just to make sure.

Note that "oxide layer" does not mean passivation is a coating. Iron is removed, forming a thin iron-depleted surface layer in the metal itself (mostly chromium at that point), and the oxide layer is formed from that via addition of oxygen atoms from the O2 gas in air. The chromium oxide that's there (there's always at least a little of it, even on an unpassivated part) is unaffected by the acid bath (totally unaffected by citric, mostly unaffected by nitric). The acid just picks away the iron from around the chromium oxide.

Let me know if I can help you further with this.

A. Hi Jeff,

I am with Ray on this one, the process used could potentially damage your parts such that they may fail in service.

17-4PH steels typically have Ultimate Tensile Strengths (UTS) above 140ksi, meaning that if they have been anywhere near hydrochloric acid they need de-embrittlement, a minimum of 6 hours at 375 °F within 4 hours of treatment (if lower end of the strength range), 24 hours at 375 °F within an hour of treatment (if upper end of strength range).

If you are using the 17-4PH steel in the H1150 state (UTS 135ksi) you may just about get away with it, otherwise I would be really concerned about these parts.

A. Ooi Cherng Wee,
You really shouldn't be dragging potassium hydroxide precleaning solution on your parts into the acid bath. All it does is neutralize the acid and waste good acid. Use a good clean water rinse between your preclean stage and your acid dip stage.

If you are dragging oil into your acid bath, you also need a better precleaning stage than you have currently.

A. Jemarie,
Yes, passivation uses either nitric acid or citric acid , not both together. Refer to the industry standards (e.g. ASTM A967) on this.

A. Jane,
Somehow I missed this question earlier.

My best guess is bad rinsing with rinse water that was not being refreshed enough to stay clean. We have seen that happen before. If the rinse bath is good, check the source and age of the material in the acid bath. Usually there is no shelf life for this sort of thing but odd things have been known to happen from years in storage in a hot warehouse or whatnot.

Depending on what the residue actually is, probably a warm water soak will clean them off, but failing that I would try some solvents (acetone, alcohol, etc.)

A. Clément,
As has been mentioned many times above, ASTM A967 is a good reference document for the passivation process.

A peroxide post-treatment isn't used very often. Usually letting the surface dry in air is enough to form the chromium oxide layer. Most people aren't in such a hurry that they have to use peroxide to speed up that process.

(adv. If you like, I can put you in touch with the distributor for my products in Germany.)

citric acid does not passivate (oxidize) stainless steel. It activates (reduces) the surface so that, then, the air can oxidize (passivate) it. The types that contain nitric acid actually oxidize (passivate) the surface for better protection.

---

Here we go again :-)

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 2016

A. Robert,
No claim is made as to the formation of chromium oxide within the citric acid passivation bath. It's certainly not reducing the surface, though. The acid (H+ ions) oxidizes (removes electrons from) the iron atoms, which converts them to ions and removes them from the surface. The chromium remains zero-valence (electron count matching the proton count) until oxidized (formation of metal oxide via giving up electrons to an oxygen atom resulting in a covalent bond) by oxygen gas, peroxide, or a similar oxygen source.

It is a long-standing belief by many in the industry that the formation of chromium oxide takes place within the nitric acid bath, but chromium oxide measurements comparing nitric-passivated and citric-passivated parts have called this into doubt. Certainly nitrate is an oxidizing agent in the sense that it can takes electrons away from other things, but that would result in chromium atoms removed from the metal surface and released into solution as ions (which indisputably does happen in nitric acid) rather than the formation of chromium oxide.

Regardless of when or how the chromium oxide layer is formed from nitric acid passivation, test data shows the results with citric acid to be equal or superior.

Ted,
;-P

? Hopefully I do not offend, but surely there must be some method for the objective measurement of oxide film thickness.

A. Dave,
In fact there are methods for measurement of oxide film thickness. They are just expensive and most people in the general industry wouldn't want to use them.

We do have some of this data, however, that was gathered a number of years ago by customers in the semiconductor industry. They do show the thickness from citric passivation to be slightly higher than that of nitric passivation.

Ray,
First, thank you for your answer.
I was at university for a month so I hadn't seen your post. I checked A967/A967M, it looks pretty interesting and useful for the rest of my research.
It looks like there are many things to learn about passivation of stainless steels; maybe I will come back to you for some advice! Thank you again for your help.

Dave,
With citric acid passivation, zero Ni and Cr is removed from the surface of the steel and these elements will not be found in the acid bath or the rinse water.

Correct, with nitric passivation you will find these elements in your waste stream.

The wastewater from a citric line can't be sent through a metals precipitation system due to the chelating nature of the citric, but then, you don't have to either since iron is the only metal in there and isn't considered hazarous.

(Some users must anyway because they perform other processes producing heavy metal waste and are mandated to treat all waste streams though the precipitation system, in which case the citric acid must be treated and removed from the solution first.)

A. Dave,
Citric passivation does not pick up any Cr, Ni, or other heavy metals from the stainless. Only iron, for which no waste treatment is required.

If the company's total waste stream must be treated for heavy metals because they are doing other things in addition to citric passivation, then they do need to pretreat the citric waste stream to remove the citric due to it being a chelating agent.

A. Hi Clément. It is true that peroxide is an oxidizer. I am familiar with its use for etching and bright dipping copper, where it is mixed with sulfuric acid and used as an alternative to nitric acid, persulphates, etc.

I am personally not familiar with attempts to use it for passivation of stainless steel and cannot say whether it would have any usefulness.

But it would be important to find out why it was suggested. If it's because a sister company manufactures it and you wish to support that effort, for example, that could be a justification. But if it's based on some general principles issue of wanting to use less toxic or dangerous materials, you should fully investigate the hazards of hydrogen peroxide before assuming it's safer than proven solutions like citric acid :-)

Luck and Regards,

A. Hi Clément,

opinion!  If your company works hard to make your product last longer, they'll reduce their impact on the environment a hundred times as effectively as they will by trying to substitute "less toxic" passivating agents. I don't think it's coincidence that worldwide pressure to reduce the use of phosphatizing before painting coincides with the world now drowning in truly crappy steel patio furniture, gardening equipment, and other objects that last 1-3 years when we know perfectly well how to finish them to last 15-20 years. We eliminated a pound of phosphate at the cost of mining, smelting, rolling, fabricating, painting, packaging, selling and landfilling a ton of steel every 3 years instead of every 20 -- with all the transportation waste between each of those steps as well :-(

Let's hope your company doesn't move in the same direction when finishing stainless steel. Our planet just cannot afford the assault on sustainability that today's pressure to reduce toxics in finishing seems to be wreaking. Proprietary citric acid formulations are a proven solution which eliminate chromic acid and nitric acid; you don't need to hunt for "less toxic" than that, although you should certainly endlessly hunt for "better at prolonging the life of stainless steel" :-)

Regards, and best of luck,

A. Jim,
A lot of the problems with stainless steel come down to what happens during the heating and cooling of alloys. The operative technical term is "eutectic temperature", but the key ideas are that during slow cooling from a fully liquid state, the different metals in the alloy have a tendency to self-segregate, producing local regions where the alloy composition is quite different from the overall alloy composition. This comes into play with castings. A second key idea is that you can produce this effect even when the temperature didn't quite reach the full melting point, which comes into play during heat treatments and the heat affected zone of welds.

So the surface of a stainless casting is theoretically "not quite right" to begin with, and then throw in the roughness and porosity (the amount of which does vary depending on the exact casting method, e.g., sand vs. wax & such) and things are even worse.

Some sort of mechanical descaling method, usually bead blasting, does seem to help in most cases.

A. Jim,
I'll just say that any disruption of the surface is better than the original as-cast surface. I've seen a lot variation in casted parts and some of then need more work than others to get to a decent surface.

A. Hi
Regarding the question of whether citric acid is as good as nitric I found this presentation.

Interesting reading.

A. Different versions of that United Space Alliance/NASA presentation have been floating around for a while. They reach some dubious conclusions about the process times necessary (as compared to those in industry standards on passivation) but overall they do agree that citric is a viable method.

I have more test data and studies from a variety of sources that we've collected over the years.

A. Charlotte,
The basic idea behind stainless steel passivation is that the acid soak is pulling iron off the surface, which exposes new chromium and nickel. Then the surface is rinsed and dried so that the new chromium and nickel can grab some oxygen from the air and form chromium oxide and nickel oxide.

If you skip the air contact time, you've still removed iron, but the ideal process isn't QUITE complete.

One thing you can do in lieu of air contact time is contact with a peroxide solution, which serves as an alternate donor of oxygen atoms.

A. Mel,
Just click on the link next to my photograph there, that will take you to a page with my contact info. Drop us a line and we'll be happy to sell product to you.

A. I got mine from Bulk Apothecary. They have large volumes and will provide a cert that includes source info.
www.bulkapothecary.com/raw-ingredients/other-ingredients-and-chemicals/citric-acid/

A. Mike,
Regulations like this can be tricky. I recall a magazine article years ago that described a shop's problem convincing their local environmental auditor that their new powder coat booth should not be regulated as a painting booth. The hazards and necessary measures are completely different, but powder coating wasn't a thing when the laws were written and paint was the closest thing to it from the layman's perspective, so that's what the auditor wanted to use.

The 40 CFR 433 doesn't really mention passivation or anything that technically describes passivation, but you can tell their aim there was to capture all metal finishing processes that may involve hazardous chemicals. They probably has just never heard of stainless passivation, or else they almost certainly would have included it. A lot of environmental guys still today hear "stainless passivation" and assume the use of nitric acid.

So what I'm getting at here is that by the strict letter of the law, 40 CFR 433 doesn't include passivation, and by the spirit of the law, 40 CFR 433 wouldn't need to include citric acid passivation, but I wouldn't be at all surprised if many auditors would say that citric acid passivation is indeed included under 40 CFR 433.