Stainless Steel Tubing

[HW Mueller]

Bruce, ok, reading your link more carefully I saw this NaOH wash, heating HNO3, and citric acid passivation. Especially the use of citric acid (or is it citrate?) is surprising to me, as I thought of citric to be rather closer to a reducing agent, but these authors mention that the oxidation by air after machining is passivation. Thus it appears that it is all a matter of what sort of passivation one needs. For HPLC purposes I will stick with the high concentration HNO3 at room temp., suggested in ther LC.GC refs above.

[Bruce Hamilton]

Bruce, I skimmed over the link you gave, except for the dichromate (which they obviate themselves by suggesting 50% HNO3) it seems to contain about the same info as the other refs. above. It is interesting that right in the beginning they make a clear distinction between cleaning and passivation (I think it was Mark who gave a good explanation of passivation, no need to repeat it).

I'm somewhat confused - which happens often... The cleaning process the article refers to is general metal finishing degreasing to remove oils, swarf, etc. - a similar role to the solvents I suggested earlier.

I thought you wanted confirmation that citric acid treatment is considered a passivating agent, rather than a cleaning agent?
" Do you have unequivocal evidence for something like citric passivating? Or high temp. nitric?".

The article supported my recollections that citric acid treatment is considered passivation, not cleaning, and warm nitric acid is preferred over ambient temperature. I accept that metal finishing semantics may cause confusion, but "cleaning" and "passivating" bath titles are used as per the article.

I also thought that 50% Nitric Acid is not preferred for free machining grades of SS, and the article appears to confirm that ( Table 2 ).

I'll try to get to the library sometime this week and confirm details, so it would be nice to understand what information would help addres issues that you may have.

With regard to your earlier query, my understanding of the reason for alkaline rinses is simply because any residual ions are not as aggressive as acidic residues in crevices/joints etc.

Please keep having fun,

Bruce Hamilton

[HW Mueller]

Again: After reading all this on passivation I am confident that the articles in LC.GC are best for HPLC. There is no further "thirst" for information unless some high grade studies prove the LC.GC articles, etc., are misleading.

[Bruce Hamilton]

Again: After reading all this on passivation I am confident that the articles in LC.GC are best for HPLC. There is no further "thirst" for information unless some high grade studies prove the LC.GC articles, etc., are misleading.

OK... Your prior reply coincided with my last post, so I apologise for any confusion.

As suspected, the change in the Federal Standard for passivation to ASTM A967 ( which allows citric acid passivation, whereas the earlier federal standard only permitted nitric ) has generated several recent comparisons of the two techniques.

The conclusion ( as seen in freely-available research abstracts ) seems to be that the citric acid treatment will produce a film with higher chromium content, and thus offers superior performance to nitric in several important applications.

Advocates claim that citric acid use and disposal creates less environmental harm, and also confers useful economic advantages.
Opponents suggest that the passivation quality is more sensitive to surface preparation and contact conditions.

Whether citric acid passivation is relevant to HPLC, I can't yet say, but I'll investigate it further sometime with an on-site metal corrosion specialist, as it seems more user friendly, and no less effective than nitric acid. Brown nitric skin stains just aren't fashionable anymore.
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Bruce Hamilton