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Sodium Dichromate Passivation Question

A discussion started in 2009 and continuing through 2017 . . .

January 23, 2009

Q. My name is Rob. I am running a Sodium Dichromate passivation tank and have some questions about specific gravity. By the ASTM A967 [link by ed. to spec at TechStreet] the tank should be 20-25% acid by volume. We use 42-degree baumé and 3 oz per gallon of Dichromate to mix the tank. Every month I send out a sample of the tank to a local lab. The lab results indicate that I am within the spec however my company has an SOP that requires us to stay within a specific gravity of 1.06 to 1.10. I recently started logging the specific gravity before I send it out to the lab. The last sample I sent out had a specific gravity of 1.116 and the lab said it was 20% acid. If you look at the density table out of "Chemical Engineers Handbook" [link is to info about book at Amazon] at 20% Nitric acid and 50 °C the specific gravity should be 1.0966. I understand the specific gravity should change due to adding Dichromate. My question is are the tables in the Perry's book for % acid concentration or % by volume. I need to prove that the tank is in spec on the 20-25% acid by volume and the specific gravity parameter of 1.06-1.10 is wrong or that I am wrong and have been going by the wrong numbers I am getting back from the lab. I hope this makes sense and someone can help.

Rob Smith
Sr. electropolish/passivation tech - South Bend, Indiana

simultaneous January 26, 2009

A. After the first parts go through and/or after the first day the specific gravity changes without regard to the analysis of the acid and the dichromate.

Iron dissolves.

Dichromate which is hex, breaks down to tri, you add more hex to satisfy the analysis, but the specific gravity keeps going up.

Oh, be sure to dump or bleed/feed when the iron reaches 2%/weight.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

January 26, 2009

A. Sorry, you do not run a chromate tank by specific gravity as SG includes the amount of water, acid, dissolved aluminum and any other material like chlorides and sulfates. In short, it is grossly unreliable as the tank ages.
It does not take much of a lab to be able to do your own testing for both chromate and acid. You just need to train a couple of reliable employees.

James Watts
- Navarre, Florida

January 27, 2009

A. Hello Rob,

Don't forget that specific gravity can also go up due to iron concentration.
At 2-3% is when I will change my tank out.

Micheline Forth
- Rincon, Georgia

January 29, 2009

Q. My next question is the ASTM standard just says 20 to 25% by volume. Is that volume by weight or how is that measured? I am obviously no chemist, however I am very inquisitive. I will not stop until I figure this out. If I am using 42 degree Nitric in a 15 gallon keg how do I get a tank that is 350 gallons to be a 20% mixture? Knowing that the 42 baumé nitric is 67.2% weight/volume I am not sure how to dilute it to the target mixture without using the same weight/volume. But is the ASTM spec calling for weight/volume or concentration/volume?

Rob Smith [returning]
- South Bend, Indiana

January 29, 2009

A. It means percent by volume of the commercial strength of the acid. Easy. Non chemist needed until you get over 2%/weight of dissolved iron, then you will need outside instrumental analysis.

Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

February 10, 2009

Q. Mr. Probert,

Just curious, but what kind of bath loading is required to exceed 2%/weight Fe in a passivation bath? We run relatively small quantities of steel, usually 15-5PH, through our bath for months and barely exceeds 20ppm Fe. As a matter of fact, we only monitor Fe because AMS2700 [link by ed. to spec at TechStreet] requires it.

We are not a job shop and only perform captive processing, so that may explain our low concentrations.

Terry Lycans
Aerospace - Dayton, Ohio

May 4, 2009

A. If you mean only HNO3 %volume,it is better to analyse HNO3 gms/litre by a simple titration with standard NaOH solution and calculating %Vol/Vol HNO3 by dividing the gms/litre HNO3 by the sp. gravity of the fresh solution of 25% by volume HNO3 solution and the dividing the product by 10 for percentage.Dissolved Fe may interfere with the observation of the end point.Color change may be seen only in supernatant clear solution.

Subramanian Ramajayam
Bangalore, Karnataka, India

December 15, 2010

A. Dear Mr. Rob:

Sir, I believe Perry's Handbook is using the "standard" method of expressing percent composition in weight solute/weight solution (w/w) though frequently, we also find it being expressed in volume (v/v) or weight over volume (w/v) such as prescription medication which render the standard (if I may say so) expression ambiguous. Nevertheless, determine the solute's weight (i.e. nitric acid) from the expression, calculate its corresponding volume by dividing it with its density, and divide it with the volume of the solution to get the percent volume expression. See if it tallies with the ASTM standard similar to what Sir Ramajayam is saying.

Moreso, following Sir Watts, you may want to use a total iron test first that is not being affected by chromium levels such as TPTZ (2,4,6-tri(2-pyridyl)-1,3,5-triazine) iron method of Hach company including a portable equipment - even neophytes can do the test with confidence. You have to determine the amount of iron contamination because it could interfere with your hexavalent or total chromium test.

Hope this helps in any way.

Kind regards.

barlow campano
Barlow Campano
galvanizing chemist - Jeddah, KSA

August 8, 2011

Q. My name is Doug. I'm setting up a Sodium Dichromate passivation tank and have some questions about correct amounts of nitric acid and sodium dichromate. The tank is 400 gal. To make a 20-25% acid solution, I add 100 gal. 42-degree baume nitric acid, but to get to 2.5% sodium dichromate how much do I add? Also any information on where to get titration procedures would be greatly appreciated.

Douglas Laukaitis
industrial finishing - Deer Park New York USA

August 10, 2011

A. Douglas

Please see letter 39558.

Willie Alexander
- Colorado Springs, Colorado

August 10, 2011

A. 2.5 %/wt = 3.28 av oz/gal = 24.6 grams per liter

Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

August 11, 2011

A. Doug-
1% sodium dichromate by weight is 1 gram sodium dichromate per 100 grams of solution (or 1 pound per 100 pounds of solution). So, you have to determine the density of the solution if you want to figure out how many grams to add to a liter or how many pounds to add to a gallon. Density of the solution varies depending on the nitric concentration, sodium dichromate concentration, and iron concentration, so you have to check it every time you analyze. That may not have been what the authors of the Mil spec intended for you to have to do, but that is the way it is written.

Jon Barrows, MSF, EHSSC
Springfield, Missouri

"Never-dump" chromates cost efficiency?

January 20, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. We are a captive shop and we do zinc plating on steel parts. For our black color, we use a trivalent "never-dump" bath. Each week we dump half the bath and remake it to prevent the zinc concentration from rising above 1,000 mg/l.

Is this more cost efficient than using a different solution and doing a full dump/makeup every two weeks? What is the reasoning behind the colloquial knowledge that "never-dump" baths are more cost efficient?

Thank you in advance. Let me know if more details are needed.

Steve Knape
employee - Grand Rapids, Michigan, USA
  ^- Privately contact this inquirer -^

January 2017

A. Hi Steve. I'm not sure but you might be using the phrase "never-dump" in a way that most metal finishers probably would not, and I think we should address that terminology before getting into confusion over it --

In the plating industry, baths or processes have traditionally been of one of two types:
1. Those baths that essentially operate in equilibrium, like most electroplating baths. You don't ever dump an electroplating bath unless something unusual has happened that has contaminated it beyond correction.
2. Those baths which are expected to become exhausted because they react in a way that exhausts them, such as most cleaning baths, acid dips, electropolishing solutions, and chromate conversion baths. We could probably call them disposable.

To me at least, the phrase "never dump" replies to newer chemical concepts which significantly or even infinitely extend the life of the traditionally disposable baths by addressing what exhausts them, for example: "Acid extenders" that precipitate iron out of pickling baths so dissolved iron doesn't spell the end of the bath's life; or achieving similar extended life on electropolishing baths through similar means. To my current knowledge there is no "never dump" technology currently being applied in chromate conversion coating or trivalent chromating, although I could be wrong.

You seem to be questioning the best strategy for keeping exhaustible chromate coating tanks under control. Dumping half instead of the whole thing is one such strategy, but lots of people take it further, to a more granular "bleed & feed" approach, trying to keep the bath consistent by dumping and replenishing smaller portions either on a tighter schedule or in accordance with indications generated from tighter monitoring. Good luck.


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Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

January 20, 2017

Q. Ted, thank you for the response. You are completely right. Our trivalent black bath is disposable. We monitor the bath multiple times per shift. We have dumped half the bath when the zinc concentration is around 900 mg/l. While "bleeding and feeding" more frequently in smaller amounts would certainly help, we are worried about labor time. In addition, we have found that our crew works better with daily checklists rather than reactively. We have also found that if we do a 50% bleed and feed on Thursday, the bath will be around 900 mg/l next Thursday. We have found that in this range of zinc (~400 - 900 mg/l) we have consistent finishes/results.

Do you think that it would be more or less expensive to dump the whole bath and makeup a new one every two weeks? It would certainly cut down on labor, but would it increase/decrease the amount of periodical adds we would have to make? Would we get better or more consistent finishes due to the fact that we could rinse out the empty tank before re-making the bath? Would the zinc build in the tank faster or slower or no differently in a fresh bath?

Again, thank you for the catching my error and let me know if any more information is needed.

Steve Knape [returning]
- Grand Rapids, Michigan, USA

January 2017
Chinese Whispers

Hi Steve. I'll have to leave that one to others. Although I designed and started up hundreds of plating lines, I've never actually operated or been responsible for the operation of a plating line. Offering advice based on what I think I've seen others do is a game of Chinese Whispers I'd better not play :-)


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Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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