51045

Sodium Dichromate passivation question

January 23, 2009

My name is Rob. I am running a Sodium Dichromate passivation tank and have some questions about specific gravity. By the ASTM A967 [link is to spec at TechStreet] the tank should be 20-25% acid by volume. We use 42-degree baume 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 degrees Celsius 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

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

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

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

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 baume 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
- South Bend, IN

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

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

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 AMS 2700 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

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     plating chemist Bangalore, Karnataka, India