Passivation of stainless steel: Analysis & replenishment of nitric acid, chromate, iron

A. Hi Jill,

Looking for the source book sounds like you might be trying to find a "not found on the internet" derivation, as auditors seem to require, no matter how trustworthy the source? It's definitely a dated requirement, and I seriously wish they'd just publish the details in an ASTM spec, so we can all use it!

I can give the titration for both nitric & dichromate from memory ... but the calculations? Ehh ... I can give a NADCAP accepted full correction once I'm back at my desk.

There is a source online that is related to an approved software supplier:
https://www.anoplex.com/Titration_Passivate_T2.html [a finishing.com supporting advertiser]
It's not quite what I use (even though I use that software... I heavily modified mine in the nuts n bolts ), or had approved. But if you need to prove a source, it's an option. In conversations with auditors, they accept vendors and consultants as approved sources.

In a pinch, I've actually run enough tests to develop an internal titration technique and calibration curve, shot the whole thing off to my chemical supplier for formal verification, and filed the approval letter away for posterity. You can do this, it is totally legit. The process is a pain in the toosh, but no one can argue with a documented repeatable self-derived titration. Scientific method and all that...

Opinion here: it's a bit funny that auditors will accept a whole mess of internal lab technique of dubious documented origin -- as long as it's documented, and they approve of it over internet community confirmed SOPs. I guess any of us with a decent set of glassware and a .001 scale can develop our own methods, if we document them properly. And I don't disagree with this at all!

One thing that has caught folks (myself included) up is that it's not a straight g/L for dichromate in nitric. The spec asks for g/kg.

What process specification are you adhering to? I'll keep an eye on this thread and if I can get you a good equation and a corroborating source, I'll be back. Even if only to drop an excel doc with some self-calibration prompts ;)

A. Hi D,
Those numbers don't sound familiar to me, but we appended your question to this thread because you may find that the procedures you follow are the same as something quoted here, such as the Anoplex Software [a finishing.com supporting advertiser] procedure linked to by Rachel.
Luck & Regards,


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

A. D,
Titration math is on the level of first-year chemistry. It's a sad day when an organization such as NADCAP will not allow a processor to possess a procedure unless it has the official stamp of approval of somebody or other.

Frankly, I don't think the actual problem is that your titration procedure isn't from ASTM or AMS or what have you. The actual problem is that all the math is hidden. The procedure provides a constant with no source or justification. It's fine for the guy on the shop floor to just know that the 6.45 in the nitric acid titration is a magic black box that gives the correct answer, but an auditor probably just wants to know that 6.45 wasn't picked randomly from a hat. It ought not need an ASTM committee to ensure the number is correct, it just needs to show the calculation written out completely. That way it hopefully justifies itself and you wouldn't need a source to point fingers at as being responsible for things.

I.e., for the nitric acid titration, something that goes from the mL of NaOH solution, to moles of NaOH, then to moles of acid based on the equivalence point relevant to the bromophenol blue ⇦ this on eBay or Amazon [affil links] indicator being used, and then to volume percent based on known properties of the nitric acid stock solution. All that math put together will be that 6.45.

As a side note, based on the informal nature of the documents, with no SDO or company name listed, and especially the handwritten signature next to "approved by", I think this is most likely an internal procedure developed at your own company, probably by somebody long retired who was presumably a chemist, or else copied it from some other trusted source. Yes, it would have been nice had it been clearly identified as such, with the original author's name noted as well.

A. "Powder Pillows" is a term used by Hach to describe some of their products.

A search result of 'Magnesium CDTA Powder Pillows' will lead you to a page describing the product. Halfway down the page is a drop-down list titled 'Methods/Procedures'. From that list, Method 8211 appears and may be the origin of QF-09-163. QF-09-163 appears to use some of the 'Optional reagents' to address interference. Very similar wording. Other differences exist.

Of interest, Method 8211 appears in Hach's Water Analysis Handbook. The Scope & Application for the test method is for 'closed system cooling towers', with a range of 20 - >400 mg/l. The typical sodium dichromate concentration range for a passivation solution is 20,000 - 30,000 mg/l (2 - 3% w/w).

Search 'Passivate T2 Titration' and you should find the Anoplex procedure mentioned in the thread. At the bottom of the procedure, reference is made to Boeing, Document D180-17994-1, procedure # B-156. If you do work for Boeing, maybe you can get a copy. I would hope the document would also have a procedure for nitric acid titration.

If you can't get a copy from Boeing, consider looking at Section 3.5.5 of MIL-M-3171, superseded by AMS M3171 (2018). While the process is pickling of magnesium with nitric acid and sodium dichromate, solution control should be the same. QF-09-162 produces the same result.

"Electroplating Engineering Handbook"
by Larry Durney

on AbeBooks

or eBay or

Amazon

(affil links)

A. 1 Ml soln 250 ml flask, add 100 ml D.I. water, 30 ml 50% phosphoric acid,10 ml conc. hydrochloric acid. Add 1.0 g potassium iodide ⇦ this on eBay or Amazon [affil links] , titrate with 0.1N sodium thiosulphate ⇦ this on eBay or Amazon [affil links] to straw colour, add 1ml starch indicator, continue titration colour change from dark blue to colourless, giving titre A.
Sodium dichromate oz/gal= 0.662 X A

Nitric acid= 1 ml sol'n 250 ml beaker [beakers on eBay or Amazon [affil links] , add 50 mls D.I. water, titrate with 1.0 N sodium hydroxide to ph 3.57(calibrate meter ph range 1-4) giving titre B.Nitric acid in %B.V.=6.45 X B. This works for me, Any new analysis procedure it's a good idea to make known standards to verify results. Good luck.

A. Pappu,

Do you mean sodium dichromate? It looks like a passivation formulation to me. The analysis method won't be any different, just the calculation will change.

Nitric acid can be done by a simple acid-base titration using an appropriate indicator; bromocresol green ⇦ this on eBay & Amazon [affil links] would work well I think.

The chrome can be done by an appropriate REDOX titration. Use ferrous ammonium sulphate with sodium diphenylamine sulphonate indicator, back titrate with potassium dichromate.

A couple of good reference books with lots of analysis techniques common to treatments are The Electroplater's Engineering Handbook .
and Arthur Vogel's Textbook of Quantitative and Semi-quantitative Analysis ⇨

A. See thread 15860

Ed. note: Based on Willie's good suggestion, we've merged thread 15860 directly into this one. Please forgive the consequent lack of chronology:

? Hi, Esppi.

Please see the above answers. If they are not the answer you need, can you please rephrase your question in terms of the answers already offered? Thanks!

Regards,