Chemical conversion coating on aluminum: what is default color?

A. Hi.
First, be sure that there is air agitation in the chromate tank. A long dip in a hot water rinse, or a shorter one in a warm alkali, will leach out some of the color and some of the corrosion resistance.
Luck & Regards,


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

A. Standardize your pH meter with 2.0 buffer, and raise the pH 0.3.

A. Both Ted and Robert are correct, both will reduce the coloration.

However, I wouldn't recommend going above 1.95 in pH; even if Alodine 1200S is spec'ed for 1.7 - 2.0 in pH, you will end up having issues with corrosion resistance. Same with a hot water bath.

I would not suggest a mild alkaline, as it will very easily strip the Alodine completely, if you are not careful.

Also, ensure that your temperature is around 25 °C (20-30).

The expected color is to be a mild to medium gold, not "colorless", nor orange. If it gets too dark, you'll most likely also experience a powdery surface, which is a sign that you've given it far too much Alodine.

Also, keep in mind that Alodine 1200S is a Chrome 6+ based Conversion Coating. Make sure you have good ventilation away from the personnel doing the treatment, as well as ensuring they are wearing the appropriate PPE. If it is manually dipped by hand, long sleeved gloves, mask, eye protection and so forth. As Chrome 6+ can cause cancer.

Best of luck!

A. Hi Suriyan,
It is possible to lighten the color of hex conversion coatings in a warm bath of deionized water, about 80-100 °F, after processing. You can leach off the color gently until it is where you like.

This is also an approach to get a nice 'colorless' film that meets corrosion requirements of ASTM B449 Cl3. It does not necessarily apply to the coating mass requirement though, so get your lab work done if you try this :)

Gotta remember to process test specimens in the same manner to ensure that you can still meet destructive testing requirements!

A. Hello,

The Mil-Spec states that unless otherwise requested the default color is Gold. That pertains to Class 3 coatings as well.

A. Class 1A is a paint base coating. It can be gold or clear, but the coating should be gold unless a clear chromate is directly specified.

A. Gold. Alodine is one of many trade names for aluminum chromate meeting MIL-C-5541.

A. Alodine (or Iridite or whatever) is sort of an opinion, isn't it? Anyway, MIL-C-5541 (or AMSC5541, both are "active" if anyone wants to toss stones at a NADCAP auditor) has Class 1A and Class 3. Class 1A is the "corrosion protection" and could be described as the "dark" and Class 3 is the "electrically conductive" and is the "light". In any case, don't confuse "clear" and "colorless" ... it should be (depending on which "proprietary" chemical, prep and cleaning (mechanical and chemical, the immersion time, temperature, alloy type, et. al) faintly yellowish ... again, this is subjective.

My personal gripe is design engineers who spec "colorless" and want you to "bleach" (hot water) ... what are they thinking!

A. Color can vary from colorless to golden - iridescent - brown. Materials conform to MIL-DTL-81706 [on DLA] . Coatings shall be continuous free from powdery areas, breaks, scratches, etc. Sometimes called yellow Iridite. No dimensional change are effected with Iridite finishes. Class 1A is used as a corrosion preventative film (unpainted) or to improve adhesion of paint finish systems.

A. Hi Mukand. I think you may have yellow (gold) chromate conversion coating in mind. The main step in the process is immersing the aluminum into a vat of Alodine, Iridite, Chemion TCP, or other brand of chromate conversion coating chemistry for the formation of the conversion coating. These days, trivalent chromates should be specified.

This step is preceded by mechanical finishing, non-etch alkaline cleaning, etching (usually), desmutting (with the specific chemistry dependent on what aluminum alloy you are using). We have dozens, maybe hundreds of threads about chromate conversion coating of aluminum in this forum, but books are probably a faster, more tutorial, path to general learning. Good luck.

Regards,

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Hi Glen. It doesn't, although it did say more or less that it in the earlier days of MIL-C-5541 (copies of which are no longer on line at quicksearch.dla.mil).

MIL-C-5541E says in Section 3.5: "Clear (colorless) coating shall only be used when specifically authorized by the procurement authority,"
and in section 6.2: "Ordering data. Acquisition documents should specify the following: ... d. Clear coatings, if desired ..."

It seems that the color expectation went away with MIL-C-5541F, concurrent with inclusion of Class II (non-hexavalent) coatings.

Regards,

A. Glen,

There are a couple of trivalent chromium conversion coatings on the market that meet this MIL-DTL-5541-F spec and exceed the performance of hexavalent alternatives. Currently all trivalent conversion coatings are clear, producing an almost imperceptible iridescent look. It is a complaint with many customers that they cannot tell if the coating has been applied especially with untrained operators on-line.

In the next year there is a USA based company that is fast tracking a dyed trivalent conversion coating through the QPL process to conform to this spec. It is commercially available now though distributors as an unspecified product.

  Thanks for the update, Isaac!
opinion!  Is it too much to hope that the dye color will be spring green rather than the yellow/gold natural color of the toxic, carcinogenic, hexavalent chromium it will be replacing? Can we proactively lobby for green? I think consumers would soon learn why it's green and choose to shop for the environmentally responsible product.

Regards,

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