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What is Clear Alodine? / Clear Iridite?

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Q. Hello,
I would like to know what's the difference in the process to create Iridite yellow/gold color and Iridite yellow/gold color, leached to clear.
I mean is the same bath for the conversion example alodine, and after there is a different step, or the process shall be done in different chemical conditions?
I need this to plan a process qualification.

I worked around 6 years plating nickel and copper on aluminum and composites, but I have no experience with chromate conversion or cadmium plate.

I hope you can help me.

Angel Aldana
Process Engineer - Sinaloa, Mexico
August 1, 2022

"Surface Properties of Conversion Coated Aluminum"
by Raj & Ali
from Abe Books

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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,

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

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 [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.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
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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:)

Rachel Mackintosh
Lab Rat and some other things - Greenfield, Vermont
October 10, 2022

↓ Closely related postings, oldest first ↓

Q. I guess I am hoping for the best with this question. I have researched a lot of articles, and some were helpful, some were not... I am polishing an aluminum bike frame, and would like to know if it would be in my best interest to hit it with a coat of Alodine, followed by a urethane clearcoat, or similar protective coating. Would this be a complete waste of time, or would it help keep my frame in good condition, both physically and aesthetically in the long run?.. Also, does Alodine stay clear, or does it discolor the clear topcoat? Is this method similar to the system used by the companies that manufacture polished aluminum bicycle frames?... Thanks to all who can help!

Martin Cattani
- Huntington, New York

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

finishing.com has sold 800+ copies without a single return request :-)

A. Alodine is a trade name for a brand of chromating solution used for MIL-C-5541 [affil link] 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.

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

Q. I am trying to find the best, most economical and maintenance-free clear top-coat for a polished aluminum bike frame. Am I to understand that Alodine will etch the surface of polished aluminum so that it will not retain a polished look?... And if this is so, is it possible to get a polyurethane or similar automotive finish clear-coat to stick to polished aluminum directly, or should I try a powder coat as my clear? Can I just leave it bare, or would this simply be asking for trouble in the long run as far as maintenance and life of the metals' surface?... I have been told aluminum won't oxidize. Please help, because I polished my knuckles to the bone on this frame, and it really came out well. It would kill me to have it get pitted and ugly prematurely.

Martin Cattani [returning]
- Huntington, New York

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,

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

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

Q. Dear sir/madam,

Will you explain, what is clear Alodine? And also kindly post me the process, instruments, chemicals, etc.


- Bangalore, Karnataka, India

"Surface Treatment & Finishing of Aluminium and Its Alloys"
Wernick, Pinner & Sheasby
from Abe Books

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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 [affil link] rev. E. This conversion coating can be colorless (essentially clear) or it can be yellow.

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

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

What should I expect a good quality clear Iridite coating to look like?

Q. I am a mechanical engineer doing some electronic packaging. I'm working with some some raw aluminum electronics enclosures and am considering applying a clear Iridite per MIL-C-5541 [affil link], class 3 for appearance only (dont need environmental protection). I don't have a lot of experience with conversion coatings, and have previously used Iridite only on connectors, where I sometimes observed a rainbow effect of color gradations in the final result.

Fred White
- Columbia, Maryland, USA

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.

Dayton Dailey
- Lake Zurich, Illinois, USA

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.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

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.

James Watts
- Navarre, Florida

Q. Is clear Alodine MIL-C-5541 RoHS Compliant?

Kurt Usry
- San Jose, California, USA

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.

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

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

What is this finish: Clear IRIDITE MIL-C5541 Class 3?

RFQ: We are preparing for quotation for customer, but we find that there is no supplier offering the finished cost price.

Please kindly tell me what is that as the title?

Miss crystal Y [last name deleted for privacy by Editor]
buyer - Shenzhen, Guangdong, China

Ed. note: Sorry, this RFQ for private contact is now outdated, but public technical replies are still welcome. No public brand/source suggestions please ( huh? why?)

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.

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

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 [affil link or 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 [affil link or 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.

Bassey J. Udofot
Bassey J. Udofot
Musgegon, Michigan
December 24, 2008

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.

clifford rye
- rainham kent england
September 27, 2012

Q. Hi. Is the clear Alodine per RoHS good and safe for stressed aircraft parts?

Jan Safranek
- Monterey, California, USA
April 22, 2012

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 [affil link or DLA] treatment and, if they do, whether the specific Alodine process you have in mind is on the Qualified Products List MIL-DTL-81706 [affil link or DLA] .

Good luck.


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

Clear Iridite for Boat Railings

Q. How can I place a very clear Iridite on aluminum tubing which is to be used as railing on a boat? I would like to do it myself but if I cam't, could you provide a resource or some resources to which I can we turn in getting this done? The hard part may come in that I wanted to be very clear so the aluminum shines silvery instead of yellowish. Thank you very much.

William Probasco
- PLAINSBORO, New Jersey
September 8, 2020

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,

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

A. Hello William,
ASTM B449 [affil link] 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 [affil link or 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!

Rachel Mackintosh
Lab Rat and some other things - Greenfield, Vermont

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