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topic 12315

Cleaning of 304L for potable water use


 

I am manufacturing a SS 304L canteen for the military. The first lot of parts were passivated using Type 1 (2% sodium dichromate followed by nitric bath) and then DI water rinsed 4 times with air agitation. Is there a potential that there is sodium dichromate remaining in the canteen? If so, what would be the recommended treatment to remove any residual? The process has since been changed to type 2 per the AMS spec.

Raymond Tervo
- DeLand, Florida, USA


 

If I poured a couple of ounces of water into these canteens, shook it around and poured it out just to make sure the previous rinsing had worked, I would personally have no qualms at all about drinking from them. So, I would think you should have operators manually rinse them.

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


 

It would be very easy to check a few samples for residual chromium. Any water testing lab could easily determine the chromium concentration in a rinse, and if you are truly concerned with this and it is a long contract, it would be fairly simple to acquire the lab equipment required to make this check yourself. For a couple hundred dollars you could obtain a field-type spectrophotometer (e.g., Hach Company) & check the final rinse water. You could test as many of the canteens as you like; each test takes about a minute & costs a buck or two.

It is refreshing to see a manufacturer who cares! Keep up the good work!

Dale Woika
- Bellefonte, Pennsylvania, USA


 

Raymond:

For 304L you do not need to use the sodium dichromate, so there is no need to introduce the hexavalent chromium into the system. 304 grade calls for nitric 2 ( ASTM A967 [link by ed. to spec at TechStreet] ) which uses only nitric acid.

It should rinse clean with the multiple washings that you are giving it, but it is easier to eliminate the sodium dichromate from the system.

There are safer and easier ways to passivate the unit with citric acid formulations. Let us know if we can help.

lee kremer
kremer signature
Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner


 

A simple easy to use spot test would be with diphenylcarbazide solution.

Prepare your test solution.

0.125g diphenylcarbazide
50 ml Acetone [linked by editor to product info at Amazon]

Dissolve the DPC in the acetone them make up to 100 ml with DI water. (this solution will keep for about 2 weeks if stored in the dark, you can tell when it it is going off as it colours up)

Take you canteen wash about 100ml of water round in it.

Pour this water into a clean beaker and add approx. 10 ml of 25% sulphuric acid then approx. 2ml of dpc solution allow to stand for 10 minutes (to allow any colour to form (this if it happens normally goes at once).

If you get any colour get worried and rinse the canteens more.

To see what you are looking for colour wise add one drop of the passivate to the mix.

The test can show up chrome down to very low levels (ppm).

Note: we used this to check if our chrome reduction in the waste treatment had worked.

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England



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