citric acid passivation of stainless steel

A. It has been stated repeatedly in the literature, so it's probably true, that electropolishing is a superior passivation technique to passivation itself. It's more expensive, and may not be practical for all ware, but it would eliminate the hex chrome issue.

Ted,

Since the company I work for does bulk passivating of small 303 stainless connector subcomponents (as well as other finishing processes like gold plating) I think the tradeoff to losing the hex chrome would not be worth it (huge rectifiers, very concentrated rinses, etc.). It is probably OK on bigger parts that people do on racks, but basketed stuff like ours would be a nightmare.

A. The QQ-P-35C is in the process of being replaced, and the replacement will allow citric acid Passivation. Before you jump into this process consider the effect of the Citric on your waste treatment facility. citric acid is a chelator in the purist sense and should not be added to a waste stream containing metals you wish to remove. There are distributors for a citric acid Passivation process popping up all over. Good luck, mike.

A. I am just going to have to visit www.finishing.com/letters/ on a regular frequency. I keep seeing these interesting discussion in MF journal and am probably too late to contribute on a meaningful basis.

In years past, citric acid , among other complexants, has been used in the nuclear industry to decontaminate (chemically clean) piping, vessels, etc. The procedure for stainless is to go in with nitric acid to oxidize the chromium and then strip out the Fe/Ni with the citric acid . Normally it was recognized that after the procedure, the surface would be very reactive and susceptible to recontamination if not allowed to "heal." It was not used on carbon steel just because too much would be needed.

Thus the use of citric acid to "passivate" is interesting. Is something else added, hydrogen peroxide or some such? To passivate a surface, you need to raise the electrochemical potential up to the passive region. I find it difficult to believe that citric alone will do it.

As Ted pointed out, electropolishing gives you the best surface. But if you use phosphoric acid, I suspect there is residual trace of phosphate. As a point of interest, if you ever get a chance, look at new stainless and electropolished stainless under a scanning electron microscope. A world of difference.

A. I have to concur with Dr Divine. citric acid on its own is not an oxidising agent. Passivation tests after citric acid "passivation" will prove positive but I would prefer to believe that the this is because this acid has played its role as a chelating solution and thus the stainless steel has effectively auto passivated.

A. I also agree with Dr Devine and Joel Levinsohn.

Whilst citric acid acts as a chelating agent to bind free iron contaminants, the subsequent (auto) passivation process with oxygen is the oxidising reaction which generates a passive film.

Use of nitric acid provides meaningful passivation, especially in high corrosion environments where auto passivation is interfered with by contaminants such as iron oxides, iron chlorides, chlorides, bromides, etc.

Metalurgical tests performed for us on stainless steel (304 and 316 grades) also indicate that auto-passivation in a clean room laboratory environment can take 1 to 2 days after grinding, and 6 to 24 hours following pickling (nitric acid/hydrofluoric acid).

Whilst the auto-passivation process commences immediately following chromium depletion, it does take time to regenerate a meaningful corrosion resistant passive film in fabrication environments and in installations subject to corrosion.

Whilst I agree citric acid cleaning can be useful following fabrication due to its ability to bind (chelate) free iron contamination, I still lean towards use of a nitric acid solution or nitric acid gel to chemically oxidize a chrome oxide passive film.

I hope I don't upset the citric acid devotees, however, it is only one of a number of weapons in the armoury and should not be seen as a means to an end. Yours in passivation,

A. I believe that the function of dichromate in the passivation solutions is as an oxidizer. The oxidation acceleration due to the dichromate produces a more stable oxide passive layer more quickly and with less metal dissolution than a plain nitric acid solution.

A. I would doubt it, Mark. See our FAQs on SST Passivation

A. Kevin:

Sounds like something is wrong with your citric acid solution, whatever it is. We have run extensive studies and not found this at all with our customers in the semiconductor industry. In fact, this is one of the advantages that we have found over nitric acid, which definitely can etch the grain boundaries.

Electropolishing "etches" away the entire surface of the part, of course. The energy is concentrated on the high points on the surface mostly.

I am very interested in this- to find out what is happening.

Q. This is Kevin again. Actually we have discovered that the annealing process is what is etching grain boundaries into the stainless steel. Is there anyway to anneal and avoid the etched grain boundaries? Or perhaps there is another method to removing the etched grain boundaries that is cheaper and easier than electropolishing. Any help would be greatly appreciated!

Thanks,

A. This has us confused. Normally saying that you etched grain boundaries means you used acid to make the surface grain boundaries visible for optical microscopy. Only one other similar thing I can think of, when you solidify an alloy through a eutectic region on the phase diagram, the alpha phase solidifies first forming the grains, and when the eutectic temperature is reached the remaining liquid solidifies as an alpha+beta phase solid in between the alpha grains. Depending on the annealing temperature, it would be possible to remelt that alpha+beta phase. It all depends on the procedure used and what the phase diagram for the 316SS used looks like.

A. Kevin:

To help shed some light on this could you give us the SS grade, such as 316, 316L, 316LS, etc.. and maybe list the elemental analysis as shown on the material certs? In particular, the Carbon and the Sulfur conc's, and roll/annealed or cast stock? And also your process cycle? Depending on temperature cycling, Carbon can sometimes introduce these effects, as can Sulfur. Since you indicate now that the effect becomes visible after anneal, my question is: are you annealing in some unusual atmosphere? or could it just be a bad rinse & dry cycle prior to anneal? My most recent problem with EP of 316L for SEMI was pitting on surfaces normal to the grain axis of orientation while the other surfaces polished perfectly.

A. Lee, it's quite reasonable for the annealing to cause grain boundary relief: surface diffusion along grain boundaries occurs at different speeds than along the rest of the surface, so the material essentially "balls up". The grains try to become more spherical, and atoms move out of the boundaries onto the rest of the grain. Yeah, I'm waving my hands here, and giving a simplistic explanation: but it happens, and can be explained far more rigorously. You'll never notice it until you anneal a flat, polished surface. Is that what you started with, Kevin?

Can you try annealing at a lower temperature?

A. If by "anneal" you mean heat then slow-cool (as applied to low alloy steels) then you're creating your own problems by generating grain boundary decoration by carbides and/or nitrides and/or sigma phase.

In the context of austenitic stainless steels --- which is what you're about --- "anneal" means heat then FAST-cool. Correctly, the process is called "solution-treat" or at worst "solution-anneal". Any slow-cooling operation with these materials creates problems. And any solution temperature less that 1050 °C (about 1900 °F) will also create problems. Either don't heat the stuff at all, or heat it to at least that temperature and fast-cool it.