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Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Chromate conversion coating aluminum parts having inserts of steel or other metals
Quickstart:
Aluminum is often 'chromate conversion coated' as a corrosion resistant final finish or as a pretreatment for painting or powder coating. The process actually involves many steps, usually including alkaline cleaning, etching, deox/desmut, the chromate conversion step per se, and water rinses between each step.
MIL-C-5541 / MIL PRF-5541 / MIL-DTL-5541
[on DLA] is the most commonly cited spec. "Alodine" and "Iridite" are trade names. "Chem film" is a slang term for the process.
Q. I deal with Alodine 1200s and my question is: Is it safe to Alodine aluminum parts which have dissimilar materials installed in them such as steel helicoils, bushings, dowel pins, etc.?
Thank You,
- Central Point, Oregon
2002
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A. I've never had any problem with chromating parts with pins, helicoils, etc., although they have always been stainless inserts.  Marc Green anodizer - Boise, Idaho A. We have Alodined parts with hardware installed. Have found that we must use stainless steel hardware because the acid cleaner and chromate solutions will attack and discolor zinc or cad plated hardware. Also must give the parts a good final rinse and blow dry the areas around the hardware to prevent any bleedout of entrapped solution. Lee Blanton- Denton, Texas |
A. To my knowledge, racking of dissimilar alloys when anodizing is not advisable. I have not heard of the same for Alodining.
Dado Macapagal- Toronto, Ontario, Canada
Q. Will Chromate conversion coating to MIL-C-5541 damage stainless steel?
We have aluminum panels that already have stainless steel pem serts installed and had to machine the panels requiring the panels to be re-plated. My question is will there be damage to the stainless steel pem serts? If so, how severe?
- Madison, Alabama
August 28, 2012
A. Hi, Keith. We appended your question to a thread where it was already answered. Looks like stainless inserts will be okay.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Can Alodine be 'wiped on'?
Q. I've got some machined aluminum parts I'd like to keep looking like new. I'd probably have these anodized or simply Alodined, but steel, stainless and bronze press-in parts (bearings, bushings, dowel pins, etc.) have already been installed and would be a royal pain to remove. Can I simply use rags and Q-tips ⇦ this on eBay or Amazon [affil links] to 'wipe on' Alumiprep ⇦ this on eBay or Amazon [affil links] , rinse and then Alodine 1201 ⇦ this on eBay or Amazon [affil links] , being careful to avoid non-aluminum surfaces, or is this asking for trouble? The alternative would be some sort of clear coat or wax, and I'm open to suggestions in this area too. Many thanks.
Jim WoodInventor/Hobbyist - Brea, California, USA
August 17, 2018
A. Hi Jim. Yes, chromate conversion coating can be swabbed on. Some can even be allowed to "dry in place" so rinsing isn't required (see thread 10893) Further, per the thread you are reading, stainless inserts probably aren't a problem for immersion chromating, but steel and bronze probably are.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2018
Q. Dear Experts, I have one more doubt that we are able to yellow chromate the assembly having aluminum plate fixed with ss screws?
Whether there is any issue during the processing or coating?
Please provide your valuable comments on the above.
Freelancer - Chennai, India
October 9, 2019
A. The process you need is: (1) typically clean in an aluminum cleaner, alkaline etch is optional, here comes the difference: deoxidize in Ferric sulphate 8 oz/gal (nothing else), must be air agitated to keep it Ferric and not Ferrous, - it is safe on the stainless and will adequately prepare the aluminum for regular yellow chromate.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
Q. Can we install steel hardware before clear alodine?
Lam Ho- Fayetteville Georgia
October 4, 2019
A. Electric motor manufacturers typically and successfully yellow chromate aluminum in the presence of imbedded mild steel. Ferric sulphate is used for the deox and is also safe and harmless on the exposed mild steel. At least one proprietary supplier of commercial "deoxidizers" for aluminum sell plain ferric sulphate as a dry powder product for use on ALL aluminum deox conditions.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
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Ed. note: For second opinions and additional discussions please see these related threads:
00664 "Chromate conversion coating on screwed & glued aluminum"
56597 "Can Alodine/Iridite/Chromating be done with steel inserts already in place?"
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