Cleaning chromate conversion coated aluminum

A. Hi Dennis. IPA, acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! , and ultrasonic agitation are all widely used methods of cleaning chromate conversion coated parts. But as Milt Stevenson warns, that doesn't mean they are utterly harmless in any and all situations. Relatively short immersions will not "remove" it, and should not be a problem.

However, when you speak of a porous part it raises the question of whether the Alodine finish is satisfactory with or without ultrasonic cleaning :-)

Luck & Regards,

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

on eBay or

AbeBooks

or Amazon

(affil links)

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. Prevention is the best way. Most surface finishers require their workers to wear cotton gloves, and to handle the parts by the edges.

A. Amazingly enough, fingerprints can be extremely difficult to clean -- some people's sweat is very acidic. The saying "An ounce of prevention is worth a pound of cure" comes to mind. It is S.O.P. in my plant that finished parts be handled only with gloved hands. I would first try a very mild alkaline cleaner; that should do the trick. This is, of course, assuming that the prints are happening AFTER your coating process, as I'm assuming that you are removing any prints that may be on the parts, during your pretreatment process prior to chem film.

A. Hi Greg. This question has been asked before for similar finishes like chromated zinc plating (see letter 18213 "Fingerprint corrosion, and cleaning of zinc plated steel surfaces") and the right answer seems to be: wear gloves and make sure it doesn't happen because they can be impossible to remove :-(
Good luck.

A. Finger Prints are not ON chromate conversion, they are embossed INTO the gel coating itself, hence these fingerprints cannot be "removed". However, fingerprints can be PREVENTED by applying a top coat of 10% water dip acrylic. Scientific-Atlanta has for 30 years had an in-house specification that calls for 10% water dip acrylic to be applied on top of all chassis' clear chromate. It works as well over yellow chromate.

If a single fingerprint is the case, then sandpaper off, and reapply chromate with a swab using a solution of about 4 oz/gal.

A. Unfortunately, some people's sweat can be very corrosive to aluminum, leaving prints that are very hard to remove. The way I would approach this issue is to have your assemblers wear powder free latex gloves when handling your parts. An ounce of prevention is worth a pound of cure.

A. As the fellow above said, there is no removing of a etched in finger prints and he gave you one method of prevention. One company I used to service prevented fingerprinting on aluminum electronic chassis parts by dipping in 10%/volume of the water dip acrylic lacquer that service vendors offer.

A. I've already answered one of your other questions so you're getting "two for none" answers out of me this evening! Anyway, here goes - ultrasonic cleaning, even in lack of detergent, etc. is likely to mechanically degrade the chromate layer. In essence, it will be shaken off if left too long - I'm guessing you'd be OK for 1/2 to 1 minute but I'd limit it. I'd also question the overall corrosion protection following such a treatment - it's like peening or sanding the surface.... it may still look OK but the properties otherwise compromised.

A. The chem film is a very thin coating, so abrasion isn't an option. I would try 2 things, a mild detergent first, secondly, perhaps either IPA, or acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! . The problem with fingerprints is that some folks' sweat is corrosive, and can actually damage a thin film coating. Have you explored anodizing as an alternative?

Q. Thanks for the post. We'll try those cleaning ideas.

As for your question about anodizing: The parts need to be electrically conductive for EMI/EMC/RFI shielding. We tried powder coating and had "leaks" at all the seams where the parts are assembled together, so anodizing is unfortunately not an option either. Sorry I didn't mention that in the initial inquiry.

A. Marc is right on about the post fingerprint cleaning, however, from what I've seen in handling trivalent chrome converted aluminum (namely the US Navy's TCP processes), there doesn't seem to be fingerprinting issue any more so than conventional, hex chrome chemical film. Keep in mind that while everyone is calling these TCP films "chrome" conversion coatings, one only needs to look at their MSDS to see that they're more zirconium-based than chrome! In as much, they're harder as deposited and less porous (why do you think we can't dye them yellow or gold?).

A. Hi Mary Ann. Eventually everyone will come to realize that they need to wear gloves when handling parts. 30 years ago 90% of production managers scoffed at the idea; 15 years ago we were probably down to about half; I don't think that many people still think it's okay to handle parts without gloves today, although there are apparently some ... they talk of it limiting dexterity whereas dentists wear gloves for root canals. Getting rid of the fingerprints would render the question moot.

But I don't believe for a minute that a fingerprint is a more powerful anti-corrosion treatment for aluminum than chromate conversion coating, nor that a chromate conversion coating which is contaminated with fingerprints is more corrosion resistant than one which is uncontaminated. Good luck.

Regards,

A. Hi Mary Ann
Finger prints are caused by sweat. This is basically salt, organic acids, etc., and can etch metal surfaces. The etched pores filled with reaction products can and do interfere with surface treatments.
But not all sweat is equal. The corrosiveness of ladies' sweat is affected by hormone levels and follows a monthly cycle and can become quite aggressive (the sweat that is).
There is another and more pernicious problem. Hand creams often masquerading as 'barrier creams' frequently contain silicones which are extremely difficult to remove. They should be banished from any metal finishing facility.
So your auditor was quite right to flag this up -- if only as evidence of careless handling techniques.
One more point. If you do not agree with or understand an auditor's comment - ask. Contrary to some opinion, they are there to help; and while an auditor cannot advise on processes (that would be consultation) they should certainly be willing to explain any problems.

A. After all these very thorough and well written responses, all I really have to add is this:

Fingerprints and the salts and oils they carry with them provide sites for uncontrolled chemical reactions. This is the LAST thing you want when dealing with conversion coatings, or any type of plating for that matter. Sure, if you're dropping in a bunch of hardcoat that is going to get aggressively etched first, the prints and the pits that come with them will be long gone, but good handling habits should be the goal of everyone in the shop.

If you have fingerprints, start over with fresh panels. This is a cause for receiving inspection to reject panels upon opening, and I have sent boxes of test coupons back to the supplier for this exact reason on a number of occasions.

And in the response to the poster who states that the QA team processes the salt spray coupons... Remember that for a salt spray sample to be considered valid, it MUST be run in the exact same manner as the parts it is meant to validate, and must be included as part of a load. So unless your QA team runs ALL parts, you are invalidating your test, and that will be an audit finding when it is discovered.

----
Ed. note: Thanks for your participation, Rachel; your response is very helpful.