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Potassium Dichromate Sealer vs. sodium dichromate

I have a specification callout requiring potassium dichromate sealer after sulfuric 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
2005

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A. Most anodizing specifications, including MIL-A-8625 / MIL-PRF-8625 [on DLA] and AMS2471, permit either sodium or potassium dichromate sealing solutions. Chris Jurey, Past-President IHAA Luke Engineering & Mfg. Co. Inc. Wadsworth, Ohio A. MIL-A-8675 F, 3.8.1.1 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 [on AbeBooks or eBay or Amazon affil links] , 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 H Probert Robert H Probert Technical Services Garner, North Carolina

A. MIL-A-8625 / MIL-PRF-8625 [on DLA]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

A. Let's 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, CEF
Anodize USA

Ladson, South Carolina

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! 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
2005

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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
April 13, 2012