What is Clear Alodine? / Clear Iridite?

"Surface properties of Conversion Coated Aluminum"
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A. Hi Angel. Alodine is a Henkel trade name for their line of chromate conversion chemistry, and Enthone/Macdermid uses the name Iridite for theirs. So when you see those names we are speaking generically of chromate conversion chemistry ... and in your case it's used as a top coat on cadmium plating to deter tarnish and white rust.

The process required for the "leaching" is immersion in a warm water rinse tank for perhaps 20 seconds to 3 minutes depending on how much of the color you want to leach out. But as Jeffrey Holmes observes in this thread, the more color you leach out, the more corrosion protection you leach out.

Luck & Regards,

A. Years ago, the process from yellow chromate leached to clear was a conventional yellow chromate (an olive drab would work as well) followed by a dip in an alkaline solution of dilute caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] (sodium hydroxide) and soda ash (sodium carbonate). This was sometimes a step in a dyed article because the conversion coating that remained would accept some dyes. If one merely wants to reduce the color, a far better process is to use a 'leachant-sealant' which not only eliminates the color but gets the neutral salt spray up above 500 hours to red. Many suppliers still have products with Operating Data Sheets to meet these needs.

A. Hi Angel!

To go to the true "colorless" class of hexavalent conversion coatings and pass salt spray, I've had best results soaking/leaching in DI water at about 85 °F (bathwater) for as long as it takes to hit the colorless spec. I aim for the exact shade of a dingy old grey sweatshirt;) It can be up to an hour! And if your soak water is very clean and just comfortably warm, that is okay. Best not to rush it. Remember the iridite bath itself is only about 83 °F.

I may play a chemist at work, but I'm not ready to explain why this particular reaction does best at mild temps:)

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

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

A. Alodine is a trade name for a brand of chromating solution used for MIL-C-5541 conversion coating of aluminum. Iridite is another trade name.

The conversion coating can be done in clear, yellow, or yellow-and-leach (leach back to clear). Normally, the conversion coating is either for stand-alone use or for use as a base coat before painting.

I could be in error on this, and we have more knowledgable readers who will hopefully correct me, but I would not expect conversion coating under a clearcoat to maintain a nice polished shine.

I don't know the manufacturing process for the polished bikes in question, but would expect that they are either anodized or clear-coated without chromate.

A. Hi again Martin. You don't cook a dish for two hundred people by just following the cookbook ... you do a small trial run and taste it yourself first. Don't risk your bony, polished knuckles to someone's opinion. Pick some small piece, item, or a small inconspicuous area from your extensive effort, and try your best guess on that small item. Book knowledge and other people's ideas can be a shortcut along the way, but they're no substitute for hands-on experience & practice :-)

Alodine is not going to maintain the full polish, but a clear coating like Protectaclear from Everbrite [a finishing.com supporting advertiser] should.

Luck & Regards,

"The Surface
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A. Hi Prabhakaran. Alodine is a Henkel trade name for the process chemistry for the chromate conversion coating of aluminum, such as per MIL-C-5541 rev. E. This conversion coating can be colorless (essentially clear) or it can be yellow.

A. A good quality Class three clear chromate on aluminum will be clear. If properly applied it should not exhibit any rainbow coloration. Iridite is a registered trade name and only one of many proprietary products that can be used to apply a clear chromate on aluminum.

Testing for the presence of a clear chromate film can be accomplished by spot testing or salt spray testing. Spot test procedure may be found in the Metal Finishing Guide Book.

A. "Clear" chromate on aluminum is a myth. If there is no color whatsoever, then there is no coating whatsoever. MIL-C-5541 is misleading on this subject, and some years ago, I spoke to the author, although I can't now recall his name. He admitted that so-called "clear" chromate must exhibit at least a slight yellow color, but he did not write that into the spec for "political" reasons. This is often a point of misunderstanding between job shops and customers who ask for "clear", and many shops have simply given up, and either just clean and desmut and apply no coating at all, or they apply a yellow coating and leach it in alkaline solution until no coating remains.

The original intent of the "clear" coating is for a very thin coating which has low electrical resistance, but which does add some measure of corrosion resistance.

A. Or, they install a QPL certified Clear chromate tank. Its color will be faint and not completely clear. Coatings done on different shifts will have a slight but noticeable color difference when placed side by side, and both will be legal coatings.

A. Hello Kurt. At the time of this writing, no it is not, per my understanding. The reason is that MIL-C-5541E does not yet allow conversion coatings that are not based on hexavalent chromium, and hexavalent chromium is one of the RoHS-proscribed substances.

A new generation of hexavalent-chromium-free conversion coatings is now available, known as the Navair TCP coatings, and Henkel (the maker of Alodine) is one of the licensees. It is expected that Mil-C-5541 rev. F will be released very shortly and that it will permit these TCP coatings. You may find letter no. 40022 helpful. Good luck.

A. There was a typo in your spec, crystal -- it should be Mil-C-5541, not 55441, so we corrected the title. Iridite is a MacDermid trade name for a brand of conversion coating chemistry that can be used to apply the coating. Good luck.

A. Addendum:

Iridite is said to be a slang name or trade name for chromium conversion coating (plating) on a surface such as aluminum. The chrome is very likely a hex-chrome films.
There are therefore two kinds (MIL-C-5541E class:- 1a and 3. Alternatively there also exist the revised version, MIL-DTL-5541 [on DLA] F:- type 1 and 2.
Using MIL-C-5541E class 3 clear Iridite, requires that the chromium conversion coating (plating) have lighter clear coating unlike the one made for class 1A.
There seem to be a controversy in the use of the clause "thin thickness, lighter or clear color" of clear chromium conversion coating ( i.e. plating or deposition) on a substrate. As an Aerospace Engineer of Materials I have read and noticed in aerospace industries, that the chromic films have been removed have been physically brushed off the substrate to allow the required electrical contact required. Alternatively it has been witnessed that the chromium conversion thin coating being leached out to reveal the needed clear class 3 ultra thin film that enhances conduction. It is to be noted that high temperature has the potential of degrading the hex-chromium film to a possible tri-chromium type film. A clear hex-chrome does exist but it normally produces thicker films than tri-chrome, unless the coating time was varied. The use of clear tri-chrome would be more preferable than the leaching or mechanical brushing it off afterward.

The revised specification version (July 2006) of Iridite MIL-DTL-5541 [on DLA]F type 1 or 2 gives better direction on chromium conversion coating (deposition) from a non electrolytic bath that contains Hex-chrome or tri-chrome ions. I hope the light I have shed in this matter has been beneficial to the surface finishing reading audience.

A. Iridite class 111 is basically the same as class 1 with the exception that the appearance of the colour has been removed by placing the object into hot water.

A. Hi Jan. If you are involved in writing the spec., yes, there are RoHS compliant conversion coatings, and they may be good candidates for your consideration.

Otherwise, nothing is considered safe for aircraft parts except the exact processes and procedures specified by the design authority. Please see whether those specs call for MIL-DTL-5541 [on DLA] treatment and, if they do, whether the specific Alodine process you have in mind is on the Qualified Products List MIL-DTL-81706 [on DLA] .

Good luck.

Regards,

A. Hi William. The aluminum should stay reasonably clear and shiny after a clear Iridite, but will not maintain a full polish.

A clearcoat like Everbrite [a finishing.com supporting advertiser] or automotive clearcoat would probably offer more shine but less corrosion protection. The best route, but not the cheapest nor the logistically easiest, is probably to have the railing anodized instead of Iridited.

Luck & Regards,

A. Hello William,
ASTM B449 Cl3, 'yellow' chromate with a clear finish, may still have warm, even faintly rainbow, hues to it in the sun despite being 'clear', and the spec only calls for 120 hour salt spray corrosion resistance.

MIL-DTL-5541 [on DLA] TyII, RoHS/trivalent chromate is always clear with a cold, almost bluish, shine in the sun, and has a better longevity in salt spray than hexavalent chromium conversion applied as clear.

The quality of finish you will get on either type depends heavily on the quality of the aluminum extrusion. 6061 from a reputable mill will give the best results; impurities or irregularities during cooling/drawing/extruding will reduce both the aesthetic appeal and the salt spray resistance over time. A poor quality substrate is prone to coming out of the process dulled, and this is allowable since these are functional rather than aesthetic specifications.

If I were to choose a coating for a bright rail, that combines shine and corrosion protection should it get scratched, clearcoat over RoHS/Tri conversion would be high on my list- but only if I had run a test piece of the same stock first to be sure the material will come out nice.

Anodizing for long-term corrosion protection in a marine environment is the other excellent option, but you will have a more satiny finish. Most of the parts on Hobie Cats are anodized aluminum, and while they aren't exactly mirror-shiny, they do stand up VERY well to long term salt water use. The corner castings aren't pretty (castings never are) but the mast (older models before carbon fiber), transom, and rails where the trampoline attaches are a good example of anodized aluminum extrusions (both clear and black dyed) for marine use. And of course they are everywhere so it should be no trouble to check one out next time you see one!