Chem-film colors: are gold and yellow the same?

A. Hi Anurag. Alodine & Iridite are brand names for chem-film processes for aluminum. MIL-DTL-5541 [on DLA] is a generic specification you can use.

I would expect gold colored hexavalent chrome chem-film to look generally as you see in the video (when applied to a buffed component), and to be quite robust and corrosion resistant. However, there are several provisos here ...

First, dry it at moderate temperature (140 °F is the maximum); then let it cure for at least 24 hours.

Second, most paint (probably including two-component clearcoat) is usually applied over very thin and transparent chem-film. As you get thicker, for a more saturated color, the propensity to be 'shearable' such that the clearcoat and half the chem-film peels off will increase. I would have one nameplate done immediately and see how it holds up as you proceed with the rest of the project.

Third, chem film is a 'functional' finish, not a 'decorative' one. The video shows a uniform finish, but over a small, geometrically simple area. Your nameplates might be larger, but hopefully don't have corners, welding or rivets, pockets, etc., working against that consistency.

What you wish to do is probably do-able but will require close & consistent attention, and sample boards. And whether the tone will satisfy a third party who is looking for a gold look is hard to predict :-)

Luck & Regards,

A. Hi again Anurag. I don't know the cost sensitivity of your project, but the ideal answer is nickel plating, followed by gold plating, followed by clear coating. It obviously would look the most like gold because it is, and with the nickel underlayer it will be shiny and will not fade :-)

I think anodizing plus gold-tone dye will be aesthetically more consistent than chem-filming because the purpose of dye is to impart a specific color, whereas the purpose of chem-filming is corrosion-resistance, and a truly consistent color is not part of the usual intent or requirement. And, yes, smart phones are often anodized.

But anodizing will not impart shine; the best that you can hope for is that it not detract much from shine. Buff a part to a good shine and then anodize and dye it ... but while I can offer some technical suggestions, I cannot help with logistics half a world away of shops that want higher volume than you have, shops which have one shade of yellow dye when you need another, etc.

Luck & Regards,

? Juline, can you please tell us your own actual situation so you can get more meaningful answers? Thanks! Your 9-word abstract question resulted in 3 skilled practitioners trying to answer your question, only to have you essentially repeat it, with us still not even knowing whether you are the supplier or the buyer, and thus being able to advise how to proceed.

A. If you are the supplier and provided a finish that the buyer now says is "too yellow", you are probably in a defensible position; the terms 'gold' and 'yellow' are more or less interchangeable. But if the brand name was specified and you did not use the proper product, or if the buyer provided color chips you didn't match, we're back to your company being in the wrong again (if it was your company that provided the coating service). If you are the buyer, and you implement a color chip system in your P.O., the supplier must comply; if not, they have a lot of latitude in color. But buyers generally should not issue such a loose requirement as 'Gold Alodine' with no spec number.

A. I know part of the answer, William. DTL is an abbreviation for "detail" and is meant to indicate that this is a detail spec (telling you how to process parts) as opposed to a PRF (performance) spec. I believe we went from MIL-C-5541 rev. E to MIL-DTL-5541 F.

A. For those who still are wondering the difference between MIL-C-5541 and MIL-DTL-5541 I found a link that may help. It answered my questions.
www.engineeringtalk.com/news/mzv/mzv109.html

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Ed. note, June 2021: Sorry, readers, that link has broken. Please try to include an article's title or author rather than a bare link so we can find things again and update links when possible. Finishing.com has been 'on the air' for 33 years, and 99% of links work for less than a tenth that long :-)

A. The military started changing specifications from the old MIL-C-5541 type nomenclature to either DTL (Detail) or PRF (Performance) specs. The move will eventually eliminate military specs all together as industry specification come up to speed. DTL specs provide exact instructions and requirements. PRF specs give a performance minimum and let the vendor/supplier/user figure out how to get there, e.g. paint must be blue and provide 5000 hrs salt fog protection. MIL-DTL-5541F (revision F) is the latest military spec for chromate conversion coatings on aluminum replacing MIL-C-5541E. The spec is free for download - search for "Assist Quick Search" to get the government site. (Actually it's not "Free". Taxpayers already paid the DoD to develop and maintain it!) Unless you need electrical conductivity through the coating, use class 1a which is thicker than class 3 . The yellow or gold color is caused by the Type I hexavalent chromate which is yellow. The thicker the coating, the more chromate the more yellow the coating. Thin coatings may seem more gold over aluminum - same stuff. Thicker the coating, better the corrosion protection. If you want more corrosion protection, paint it too. MIL-DTL-5541 is a good base for primer and paint. If you want to powder coat, probably should use class 3 to get good adhesion of the powder to the part.

A. Hi, Shawn. The gold or yellow color is the inherent color of hexavalent chromium so, traditionally, the yellow color implied more chrome and more corrosion protection. In these days of RoHS, most chromate conversion coating is actually trivalent, and any yellow color is just dye.

Regards,

A. Hi, Thomas. Some of the Alodines are supposed to be blue. Are you unsuccessfully trying to get a blue, or are you unsuccessfully trying to hold a clear or yellow or black color? Which Alodine are you using? Are you working with aluminum or with zinc plating?

Regards,

A. Yes, James. MIL-DTL-5541 [on DLA] is free, and is a short & rather readable spec, so get a copy!

Regards,

A. Hi Brad. That spec number looks pretty fractured, but double check if it isn't Mil-C-5541. There is no chemical distinction between "gold" and "yellow", so now you're down to appearance. The buyer certainly can "extend" the spec, adding additional requirements if they wish; but when it comes to appearance and subtle differences (if any) in color, sample boards /color chips (as briefly mentioned in section 6.8 of Mil-DTL-5541) would presumably become their obligation: "These are examples of acceptable 'gold' color / These are examples of unacceptable 'yellow' color". Good luck.

Regards,

A. Hello James. See section 6.8 of MIL-DTL-5541. It implies that you are correct that such variations are acceptable. But it also says that you as buyer can establish a color chip system if you wish -- which of course is actually obvious; a customer can buy or not buy whatever they want. But if you do establish a sample board (which many professionals pooh-pooh) you presumably will have to pay more because your supplier will be rejecting parts internally, and they will probably want to negotiate what to do with rejected parts. Best of luck.

Regards,

A. Although Chem Film can be extraordinarily beautiful on some substrates, it is not intended to be cosmetic, as outlined in the spec that Ted referenced. To certify, the spec requires that it pass certain tests (salt fog, electrical resistivity, adhesion). A uniform, pleasing appearance is not among them.

I can speak from the Vendor's side here, having gone through explaining this to customers in the past. If the customer PO requests parts certified to the spec, and the parts conform to the spec, the PO has been fulfilled. Any aesthetic requirements need to be spelled out clearly in the original RFQ because, as Ted points out, many FUNCTIONAL coaters will balk at the requirement.

Variables impacting color include, but are not limited to; bath pH vs. bath concentration, bath temperature, vigor of bath mixing (how fast does solution flow across the part- part geometry also comes into play here), substrate finish, and both temperature of, and immersion time in, the rinse after processing.

I think the question a customer should ask himself in advance when considering adding an aesthetic requirement to the RFQ is, "How much are we willing to pay for PRETTY?" In many cases, Chem Film is being applied as an adhesion enhancer and protective pre-coating for a later paint application, and if it works... well, it works. :)