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"Potassium Dichromate Sealer vs. sodium dichromate"


I have a specification callout requiring potassium dichromate sealer after sulphuric acid anodize of an aluminum part. We do not plate internally. Our suppliers are using sodium dichromate as a sealer. This part is aircraft related so I need to get supporting data to justify the substitution (if it can be done). My question is - Is there published data that compares these 2 sealers and could be used to support the substitution?

Ed Brandstetter
- Rockwall, Texas

1st of two simultaneous responses-- +++++

Most anodizing specifications, including Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil] and AMS2471 [affil. link to spec at Techstreet], permit either sodium or potassium dichromate sealing solutions.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

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2nd of two simultaneous responses-- 2005

MIL-A-8675 F, says "5% aqueous solution of sodium or potassium dichromate"

Alcoa, Aluminite, 1956, only mentions Potassium Dichromate.

ASM, Old Vol 2, Page 625, only mentions Potassium.

AESF Light Metals, Page 27 shows Potassium on the slide, but uses Sodium in the text ?!?!?

Electroplating Engineering Handbook, 1955, Graham, only mentions Sodium

Brace, 3rd Edition only mentions Potassium.

In the 1970's I specified and helped set up a new line in Kennesaw Georgia. We used Potassium Dichromate. We got approved by Lockheed, Goodyear, Rockwell, and two others. Then along came a green school boy engineer from an airline based in California and made us change to Sodium ?!?!?!? Go Figure !

robert probert

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

2005 Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil]Type II Class 1 allows the use of either sodium or potassium dichromate, as does DEF STAN 03-25 (the UK equivalent specification).

As far as I am aware there is little difference in performance of the sodium or potassium dichromate seals, both will easily pass the corrosion resistance requirements of the MIL spec.

Brian Terry
Aerospace - Yeovil, Somerset, UK


Lets throw another curve ball into the mix. I have recently received test results that show a trivalent chromate meeting or exceeding the test results on sealing the anodize surface. I'm looking at it closely and if anyone is interested I can email the test results to you or I should have it on my web site very soon.

drew nosti
Drew Nosti, CEF
Anodize USA
supporting advertiser
Ladson, South Carolina


Drew, I would be interested in that test data. Thank You. That is the sort of thing I am looking for. For me to support a substitution process I need data that supports that substitution. The fact that several specs allow both (or several) processes won't support a stance of "as good as or better". For that I need published data.

Ed Brandstetter
- Rockwall, Texas

April 13, 2012

Q. I had an issue before with specifying the clear anodize on aluminum parts. Depending on the sealing process parts may come yellowish. How to specify the "clear anodize" that is really clear w/o any hue?

Leonid Shatskin
- Aurora, Illinois, USA

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