Home /
Search 🔍
the Site
pub Where the world gathers for
plating, anodizing, & finishing Q&As since 1989


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


A. Most anodizing specifications, including MIL-A-8625 / MIL-PRF-8625 [affil link or DLA] and AMS2471 [affil link], permit either sodium or potassium dichromate sealing solutions.

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

A. 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

A. MIL-A-8625 / MIL-PRF-8625 [affil link or 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
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

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

finishing.com is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages

(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Chemicals &
Consult'g, Train'g
& Software

About/Contact    -    Privacy Policy    -    ©1995-2023 finishing.com, Pine Beach, New Jersey, USA