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"Chemical Conversion Coatings on Aluminum"

(to provide context, hopefully helping readers more quickly understand the Q&A's)

Chromate conversion coatings, sometimes called "chemical conversion coatings" or "chem-film" are a corrosion resistant coating applied to aluminum parts. The most common specification for the coatings is MIL-C-5541 / MIL-DTL-5541 / MIL-PRF-5541.

Alodine is a Henkel trade name for their popular line of chromate conversion coatings for aluminum; Iridite is MacDermid's trade name for theirs.

Current question:

December 12, 2021

Q. Hello,

Is it possible to perform Chemical Conversion on parts with blind female 0-80 UNF Threads, without making the thread unusable? (due to fluid locking & hardening inside the thread drill / or due to the thickness of plating - 0-80 is very small)
or should I mask the thread? (but then the area will be exposed for corrosion).
The aluminum is 6061.
The Chemical Conversion is type I (Alodine 1200s)

Thank you.

Alex Shuster
- Haifa, Israel

December 2021

A. Hi Alex. I wouldn't expect the thickness to be a problem, but getting the processing liquids in and out probably will be. A vibratory plating barrel (they're not really 'barrels', they look more like a sieve) might work. If volumes are very small you might see if an ultrasonic jewelry cleaner can help with getting the solutions in and out.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

December 16, 2021

A. Alex

Yes, you can conversion coat 0-80 threads. A while back, we were processing microwave parts with up to ~96 of them. They would take a considerable amount of time to process/rinse/dry. Price the job accordingly.

Willie Alexander
- Green Mountain Falls, Colorado

December 16, 2021

A. In addition to the two above, in all chemical metal finishing processes with blind holes, it is important to go up/down/in/out three times in all tanks: cleaners and rinses. Fill the holes, drain them, fill them again, drain them, three times completely in and out.

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

December 17, 2021

A. Alex,

I agree with Ted, the process wouldn't make the threads unusable, but the challenge is getting the solution in and out to blind thread hole, so you will need some agitation or rotation to make sure the solution can get in and out.

mark battles
Mark Battles
Best Technology Inc.
supporting advertiser
Plymouth, Minnesota
Best Technology Inc. banner

Closely related Q&A's, oldest first:


Q. What is the nominal thickness (and potential range of thicknesses) for Iridite and or Alodine type conversion coatings? Are there processing / material problems which may lead to adhesion / cohesion issues with conversion coatings?

Ralph J. LeBoeuf Jr.
aerospace mfgr - New Orleans, Louisiana

"Electrolytic and Chemical Conversion Coatings"
by Biestek & Weber
from Abe Books

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(commissions from your purchases make finishing.com possible)


A. Since chromate conversion coatings are gels it is difficult to measure thickness directly. It is usually reported in milligrams per square foot. clear coatings will have form 12-20 mg/sq ft. Yellow chromates from 30-50 mg/sq ft. Above 40 mg/sq.ft the coatings are not as good. 40 is optimum for yellow; this equates to about .0001" or less. Generally thickness is not of interest because these coatings are too thin to have an effect on dimensions. Chromate coatings on Aluminum works well because if scratched the gel will migrate over and into the scratch and continue to protect. (168 or hrs 5% salt spray is required by most specs. They usually provide more if the solution is maintained, Impurities kept out and the finished coating is not exposed to temperatures above 150 degrees F for more than a few seconds. (The gel dehydrates) Chromates are excellent for promoting paint adhesion to Aluminum. Paint over chromate can be baked at the normal curing temperatures of high bake paints because the paint seals the chromate preventing dehydration. This may be more than you wanted to know; or if incomplete contact me.

don baudrand
Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [affil link to the book on: Amazon or AbeBooks ])


A. Ralph,

Don is for the most part correct, and as far as thickness of the Alodine, it is in fact measured in milligrams per square foot. If you would like to check the coating of any color chemtreat (ex Alodine 1200) then use three test panels so you can test the best time to use all in one shot. Run the three panels at approximately 30 seconds, 1 min and 1 1/2 minutes. Then weigh your panels and strip them in a 50% 42-baumé Nitric strip for approx. 30-60 seconds or until the panels are stripped of the coating. Then weigh the panels again, get the difference of before and after, then multiply that number by 8000 and that will give you milligrams per square foot of the coating. Now, as with any conversion coating, precleaning is huge, as is temperature. Make sure that your cleaning is thorough and that your temperature is at the low end of the range, better adhesion at low temps. I have done thorough experimentation on this specific issue, if you want anymore info, post another response to this letter and I will help you out.

Have Fun Experimenting

Tony Schmaltz
- Kent, Washington


A. Tony,

That is the method I normally use, and while it works well I am looking for a method that does not have a time limit on it (the coating must be stripped within about 1 hour for full coating removal). As most small shops send the panels out for CW evaluation, there must be a method that can accurately be performed ex post facto.


Benjamin J. Curto
- Ponderay, Idaho, USA

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