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topic 57140 p2

Analysis & replenishment of passivation solution for stainless steel: nitric acid, chromate, iron, p.2



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A discussion started in 2011 but continuing through 2019

(2007)

Q. Hi Everyone,

We recently started a Passivation line and I have been looking for a procedure for determining iron content of the 25% Nitric and 2% Sodium Dichromate. We must change the bath out when the concentration hits between 2-3% iron.
I found this procedure from letter 40281:

1- take a known weight from the bath W(15-25 gm).
2- add 25 conc. HCL, then boil in w.p for nearly one hour.
3- add 400 d.w., then 10ml. conc. H2So4.
4- titrate against KMn04 0.1N, the color changes from colorless to pink after Xml of titrant.
Calculation: Fe% = Xml. of titrant*0.558/Wgm.

The problem I have encountered is that when I titrate, the solution is not colorless but dark green and it is very difficult to see any color change.
I have not used the tank very much, so I do know that the iron is not that high but I would like to figure this out before too long.
Am I doing something wrong?

Thank-you in advance,

Micheline Forth
Finishing Company - Rincon, Georgia


(2007)

A. With 25% nitric acid and 2% sodium dichromate the dissolved iron is already in the +3 oxidation state. Permanganate oxidizes iron +2 (ferrous) to iron +3 (ferric). Your iron is already +3 so this titration has no chance to test for iron +2.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA


(2007)

Q. Hi,

Thanks so much for your response.

Any ideas for me on a procedure that will work for me?

Micheline Forth [returning]
- Rincon, Georgia, USA


(2007)

Q. Hi Everyone,

I found a titration in letter #36621:

Will this work? If so what would be the calculation.

Thank-you,

Micheline Forth [returning]
- Rincon, Georgia, USA

Ed. note: For readers' convenience we are now incorporating Letter 36621 directly into this thread:

Letter 36621

2005

Q. We have passivation tanks at 25% to 45% nitric acid concentration. We need to test for Iron concentration in passivating solution not to exceed 2 Weight % per AMS2700 [link by ed. to spec at TechStreet]B. How do we perform the test?

David Quintana
spring company - Tempe, Arizona, USA


2005

A. Add a few mls of sulfuric acid to a 25 ml sample. Heat it, in a fume hood until the formation of white fumes indicate that all the nitric acid has been removed. Then let cool, dilute it back up to 100 ml or so with DI H2O, add ferroin indicator, and titrate to a red endpoint with standard sodium dichromate solution.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


March 10, 2008

Q. Does anyone know of a commercial test kit that can be purchased to check for the 2% max iron content in the nitric acid passivation bath per AMS 2700?

Stephen M [last name deleted for privacy by Editor]
Aerospace Mfg - Jacksonville, Florida
outdated


(2007)

A. No it will not work because the iron +2 is already oxidized by the hex chrome in your solution.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

(2007)

A. It's much easier to first separate the iron from the nitric acid, nitrates, chromates, etc., via precipitation.

Dilute a 25 ml sample with DI water, raise the pH to 4 using NaOH solution, then collect the precipitated ferric oxyhydroxide (hydrated rust) by filtering through a Gooch crucible.* Rinse sparingly with DI water until the rinseate is colorless (chromate-free).

Textbook of Quantitative Inorganic Analysis
from Abe Books

or

Treat the precipitate by either 1) complete dehydration to Fe2O3 in a weighed beaker or crucible, or 2) redox titration with permanganate or dichromate following boiling to a clear green Fe+2 solution in a reducing acid.

For dehydration, 1 hour at a minimum of 300 °C in a Pyrex beaker is usually sufficient (250-275 °C may only result in FeOOH). Heating in a crucible over a gas burner will also work. Vogel's "Quantitative Inorganic Analysis" [link is to info about the book at Amazon] mentions heating to 850 °C (bright red) in air.
Weigh after cooling (in a desiccator if available) to room temperature.
Weight of Fe = 0.6994 x wt. of Fe2O3.
For a 25 ml sample, wt% Fe =4 x wt. Fe / SG of passivation solution).
Note: Weighing as Fe2O3 is the classical gravimetric analysis for Fe+3.

To verify the procedure, spike 2 passivation samples with soluble iron (e.g., ferric or ferrous chloride or nitrate) equivalent to 0.1 & 2 wt% Fe, respectively (as Dr. Cook mentioned, the passivation solution will convert any Fe+2 to Fe+3).

*If using filter paper instead of a Gooch crucible, to minimize attack of the paper by chromate, dilute before filtering: Let the precipitate settle, decant off the clear liquid, add DI water to the remaining sample, let settle, decant and filter. Rinse with DI water.

Ken Vlach
- Goleta, California
contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



(2007)

thumbs up signHi,

I just want to thank everyone who helped me out.
Thanks Ken Vlach, I'm looking forward to giving precipitation a try.

Micheline Forth [returning]
- Rincon, Georgia, USA


April 21, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello Sir ,

How to analyse (Titrate) the concentration of stainless steel passivation solution ?
1. Nitric Acid ( ml/lit )
2. Sodium Dichromate (g/l)
3. What are the reagents required for this, how is it prepared?

Regards

Surya Narayana
Process Engineer - Tumkur , Karnataka , INDIA.

----
Ed. note: Hi, Surya. We appended your inquiry to a thread which we think will answer your questions, but please re-post if anything is unclear. Good luck.



January 9, 2015

Q. When you precipitate out Fe(OH)3 with OH-, wouldn't Cr(OH)3 precipitate even faster, thus giving erroneous filtrate amount? The KsP of the Cr hydroxide is orders of mag smaller than that of the iron hydroxide. Dr Paul

Paul Adl
chemistry consultant - Bonita Springs, Florida, USA


January 2015

Hi Dr. Paul. I'm not a chemist and don't fully understand this dialog, but I do have some experience with chromate waste treatment; the chromium in chromate is Cr+6 and it will not be reduced to Cr+3 without treatment by a strong reducing agent. So I would say it will stay in solution as Cr+6 when you add the hydroxide.

Regards,

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



April 9, 2015

Q. Dear Friends,
After chemical analysis, in formula we need to put a factor for calculation. For example, in sulfuric acid calculation we multiply by 9.8 to get concentration of acid,
So my question is how we can find out that factor?

Aijazullah Tajir
- Abu dhabi, UAE


April 2015

A. Hi Aijazullah. Exactly what chemical analysis calculation do you have in mind? Those "factors" are not magic numbers but they are merely the product of all of the conversion involved, and may involve molecular weights, densities, conversions from one measuring system to another, etc.
"9.8" happens to be a useful conversion factor for determining free acid when titrating sulphuric acid with sodium hydroxide, but may have nothing to do with whatever you are trying to determine about hexavalent chromium. What are you trying to determine? Thanks.

Regards,

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


April 16, 2015

Q. Dear sir,
my question is in general, how to get that factor for calculation; I have seen in some cases they mention formula with 'x' factor
So I want to know how to get that x factor in such cases.

Aijazullah Tajir
- Abu Dhabi


April 2015

A. Sorry, Aijazullah, but we are apparently on different wavelengths.

It seems to me that without a specific quotation related to a particular "x" factor, or a description of the specific calculation you want to do, or the reagents involved in a titration, "x" could be anything from zero to infinity because there is no basis for guessing what you are trying to calculate & convert. Maybe another reader has a better insight into what you are asking.

Regards,

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



May 25, 2016

Q. For Type 2 passivation IAW AMS2700E the limits are as follows.
20% - 25% by volume HNO3
2%-3% by weight Na2Cr2O72H2O

We measured 25.5 and 3.09 during an audit. Will this cause and issue on product produced while at these concentration levels? Adjustments have since been made.

David Prescott
aerospace - Warren Michigan


simultaneous May 26, 2016

A. David,
Passivation has such ranges because this generally isn't a process where high precision is necessary. You were just a little bit over the "maximum" concentrations, as long as the physical appearance of the parts is acceptable, they should be fine. I would worry more about dropping below the minimum concentration than going over the maximum.

However, out of spec is out of spec, and different customers may react with varying amounts of panic regarding this, even though you and I know the parts are fine.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner


May 27, 2016

A. Hi David,
My sympathy at being pulled up for being just over a spec limit during an audit. I've been there, it can be a painful process, given that as a chemist one knows it will not make the slightest difference to the product that was processed at the time.
But auditors are not practical people; they just see a box to be ticked or not. Their world is black and white, quite dull for them I know, but we have to work within the system none the less.
If you need proof of the product's integrity you could submit an above spec part to salt spray as well as an in spec one, and point out the identical performance. But otherwise rest assured that the over spec product will have been fully and correctly passivated none the less.
Best regards
Mark

Mark Lees
Aerospace - Forsaken rock in the Irish Sea


May 27, 2016

A. Hi David,

It is very unlikely to affect your product, being the fractions above the top limits.

This will not appease an auditor though, posts from anonymous websites, evens ones as recognised as this, will not suffice. You need to work out how to prove that the parts have not been affected. Other than passing the batch testing you could look at a sample of parts that have been passivated and check they are not suffering from any end grain pitting or inter-granular attack (good signs something has gone wrong).

Brian Terry
Aerospace - Yeovil, Somerset, UK


May 28, 2016

A. Hi Mark
I think you misunderstand auditors and their purpose. Most are highly qualified and experienced. In this industry they are certainly not box tickers. Many of them have themselves queried specs which are illogical.
However this misses the point.
You have a contractual obligation to operate within the spec you accepted from the customer and the auditor is tasked to find evidence of this.
Firstly, neither you nor the auditor know precisely why the customer requested this spec., there may have a valid reason for the tight limits.
Secondly, the auditor has found evidence of a failure of process control. Could you show a control graph with evidence of long term compliance?
It matters not that you can satisfy yourself that the parts are OK. The only authority that can accept out of spec parts is the customer.
Why are aerospace customers so particular? Next time you are 35,000 feet up look out and ask the question.

geoff smith
Geoff Smith
Hampshire, England


May 2016

thumbs up signThanks Geoff!

After a career of specifying stuff as an engineer, then running this site for 20+ years, I'm going to tattoo your
"Neither you nor the auditor know precisely why the customer requested this spec."
on my forehead backwards so I read it when I look in a mirror.

We humans suffer constant temptation to believe the contrary :-)

Regards,

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


May 31, 2016

A. Good day David.

I can relate to your situation with auditors = NADCAP = BRUTAL!= shelf life/lot #'s, received dates, open dates, signature, % accuracy TD pipettes, etc.... of ALL chemicals=lab/process. He hunted, but went home hungry!
The small % variance will not impact product, but, as the auditor indicated, out of spec. This is a moot point.
I am surprised the iron concentration was not questioned. As you well know, the max.concentration was increased from 0.5% to 2.0% wt. My auditor dwelled on this, and I have 2 concentrations for iron in my tank matrix analysis.
The spec I am working to for iron analysis involves a quantitative analysis,NaOH precipitation, which precipitates ALL metals as hydroxide = HIGH (Fe) numbers. I also shoot a sample on the AA = 30% value from NaOH analysis.
I walked the auditor through both analysis procedures, and guess what?
He bought it.
Food for thought.

Regards,

Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada


June 3, 2016

Geoff,
Point well made, however, I've seen the other side of this too. A subcommittee in charge of a standard may come back at the five year review point, and with fresh eyes on the document say "Why the heck did we write it like this"?

I have a few recent experiences with some line in a standard needing to be rewritten because one or more company's auditors got hung up on something that was really beside the point, and had not been foreseen as an issue when the line was originally written. I witnessed an hour long discussion that amounted to "We know what we mean, but how do we write it in such a way that will inform the processor what they need to do without leaving it open for auditors to nitpick something that doesn't need to be scrutinized?"

Standards are a best effort from people who are experts trying to communicate the important things to people who are not experts, and there's a certain traditional language to standards that doesn't always exactly reflect the situation. Oftentimes the focus is more on giving a recipe than an explanation of overly technical details.

However, yes, the larger point is that regardless of how vital the concentration range is or isn't, falling outside the set bounds shows that your process controls didn't do what they were supposed to. Which only raises the question, what other details are not being followed?

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner



Confusing Nitric Acid "%" Spec

June 2, 2017

Q. I am reviewing procedures to make sure they meet customer's specs.

We have a customer that lists their spec on the Passivation tanks (they do not use AMS2700) as:
Type A---"Nitric Acid (as 100% HNO3): 16-34% by weight"
Type B---"Nitric Acid (as concentrated HNO3-I guess they mean 70%?): 20-45% by volume

The person who wrote out internal procedure has:
TYPE A --- "tank should be 20-25% Nitric acid by volume"
How did they get this number?

Type B --- "The concentration of Nitric Acid solution should be between 14.0% and 31.5% Nitric acid by volume (or 20-45% by volume of 70% Nitric acid)"
I don't understand. A % by volume doesn't matter if you start with 70% or 100% Nitric mixed with water....it is just a concentration % in solution right?

Thanks for any help.

Mic Davis
reviewing procedures - Cookeville, Tennessee USA


June 9, 2017

A. Mic,
Commercially available "concentrated" nitric acid is typically a solution of 61.0 to 68.2 weight percent of HNO3. (Source: Mil spec O-N-350) You may also see this referred to as 42 °Baumé.

The density values vary depending on what source you use, but for now let's say 1.4 for a 67 wt% solution. Therefore a liter weighs 1.4 kg. If you add 1.4 kg of water, that cuts the nitric concentration in half, to 33.5 wt%. So that's 1.4 liters of water (density 1) with 1 liter of nitric concentrate. The volume percent of nitric concentrate is 1/2.4 = 41.7%

So let's see how the 25 vol% number plays out. That's 3 kg of water and 1.4 kg of nitric concentrate. That 1.4 kg represents 1.4*0.67 = 0.938 kg of nitric acid in the final 4.4 kg of solution, or 21.3 wt% nitric acid.

So the "20-25 vol% nitric concentrate" does not match the "16-34 wt% nitric acid", but it falls inside the range.

As for your second question, making an equivalence between 14.0-31.5 vol% pure nitric acid and 20-45 vol% nitric concentrate is silly, because you don't want to go anywhere near pure (white fuming) nitric acid. Golly, I hope the "20-25 vol% nitric acid" reference in your type A wasn't meant to refer to white fuming nitric acid. I suppose you could look up the density of that and run the numbers.

Anyway, yes, this shows how formulas can be problematic if you aren't specific about what's being mixed, what the concentration within the stock solution is, and if you miss the distinction between weight percent and volume percent.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner



January 9, 2018

Q. I am working as an advisor in China, where the authorities and QA insist on having a national or association standard for analytical methods.
In the case of our passivation laboratory controls for nitric / sodium dichromate we cannot find such an established analytical method with this control.
I see many different methods but none associated to a standardized method.
Does anyone have any advise?
Thank you. Tim

Timothy Aish
Gameco, FAA MRO / Overhaul - Guangzhou, China


January 10, 2018

A. Timothy,
The industry standards for passivation that I'm familiar with say the bath concentration must be maintained within the assigned range, but offer no advice or directive on how to go about doing so.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner



March 8, 2019

Q. Hi all,
I'm looking for a standard lab analytical method for nitric acid passivation (ams 2700 type 2). An auditor has asked where we got our methods from and since our method was in place before I started working here I have no idea. ASTM, standard lab, or chemical manufacturer's procedure are his request. Any ideas?

Tony Miller
- lee Massachusetts USA


March 10, 2019

Hi Tony,
Here's my titration for the nitric in that tank:

Place a 5mL sample of Bath in a clean beaker with about 30mL DI. Add 10 drops BCG. (Bromocresol green)
Titrate with NaOH 1.0 N to green-blue endpoint. Record mL as "A".
1.333 A = % v/v Nitric Acid 42° Be (67% stock material) = % w/v HNO3

That being said, when I started here, my predecessor had been buying those Kokour pre-fab solutions and was testing with 'N71' which is basically just sodium hydroxide diluted so as to make an equivalence so that the mL used in titration is equivalent to the concentration of Nitric in the tank, and I back-titrated the N71 against a known concentration of monoprotic acid and just recalculated everything stoichiometrically.
I had a conversation with a Nadcap auditor last year about this exact subject- relating to the Citric test, but same deal- and was also frustrated that it has to come from an 'expert', even though you can set up a chemical equation and calculate it yourself. And his comment that 'online or journals do not count as a legitimate source' just annoyed me to no end.
Also considering that shops will be buying Nitric from a chemical distributor who doesn't necessarily provide support specific to our industry, getting the 'official' scoop is like looking for a needle in a haystack!
I'm not sure if you've ever checked out the Carpenter Steel website, which is a rabbit hole of technical goodies, including a recipe for a hard-to-passivate-steels solution that seems to have a cult following (I'm a believer lol). And, arguably, this can be considered a 'manufacturer' method coming from a specialty supplier of stainless steels.
Here's a link to their titration instructions. You can skip the pH meter step; even in other tanks WITHOUT the dichromate, switch the phenolpthalein out for Bromocresol Green and use the blue-green endpoint. Also if you've got a ton of iron buildup the results skew high.
https://www.cartech.com/en/alloy-techzone/technical-information/manufacturing-guides/how-to-passivate-stainless-steel-parts
The nice thing is that if you want to start with different sample volumes (like, if you don't have an accurate 1mL pipet) or reagent concentrations, you can simply recalculate, while still maintaining reference to this work instruction. I'd print it out, do your calcs right on the sheet, squirrel it away in your library, and be ready to defend your math :)
Happy titrating!

rachel_mackintosh
Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont



June 28, 2019

Q. The passivation nitric acid bath that I use turned blue. Does anyone know why? The pH is in tolerance.
Clinton

Clinton McCollum
- Houston, Texas USA


June 2019
wikipedia
Copper (II) Nitrate

A. Hi Clinton. Most salts of divalent copper, including copper nitrate, are a strong bright blue. Any reasonable way for a substantial amount of copper to have become dissolved in your nitric acid?

Regards,

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

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