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-----:MIL-C-5541 Type 1 & 2, and Class 1A & 3 -- understand the differences
Quickstart:
Aluminum is often 'chromate conversion coated' as a corrosion resistant final finish or as a pretreatment for painting or powder coating. The process actually involves many steps, usually including alkaline cleaning, etching, deox/desmut, the chromate conversion step per se, and water rinses between each step.
MIL-C-5541 / MIL-DTL-5541 / MIL PRF-5541 is the most commonly cited spec. "Alodine" and "Iridite" are proprietary trade names of the suppliers of the process chemistry. In the USA "Chem film" is an often used slang term for the process.
The process has long had two general classes: Class 1A for maximum corrosion resistance, and Class 3 when lower electrical resistance is required.
Until this century, most such coatings were based on hexavalent chromium. But growing concern over its toxicity (RoHS -- "Reduction of Hazardous Substances"), plus new technology made for processes free of hexavalent chromium. So the original hexavalent formulations are called Type 1 and the new hexchrome-free formulations are called Type 2.
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Q. Trying to determine whether or not a litmus test per MIL-STD-186F (sec. 5.5.3.2.1) is appropriate for an aluminum surface that has been conversion coated IAW MIL-DTL-5541F Type 2 Class 1A. Does a Trivalent Chromium conversion coated surface render this surface chromated for the purpose of MIL-STD-186, or is that designation reserved for hexavalent chromium?
Martha KrebsbachMetallurgical Engineer - Texas
February 25, 2025
A. Hi Martha,
This is tricky because trivalent solutions are not in fact actually "chromates"; per chemical terminology chromATE means the higher oxidation state, i.e., hexavalent 🙂
I'd say your question can't be definitively answered until a revision to MIL-STD-186 clarifies whether an object coated per Type 2 is considered chromated or not.
Hopefully you can simply put a moistened red litmus paper on the surface and it won't turn blue, and you'll be good to go 🙂
Luck & Regards,
Ted Mooney, P.E. RET
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Multiple threads merged: please forgive chronology errors and repetition 🙂
RoHS = Reduction of Hazardous Substances, a European Parliament directive
Q. What is the difference between these two mil standards? Is one for a clear RoHS compliant and the other for a yellow RoHS compliant? Or is the Mil-C-5541 for clear RoHS and MIL-DTL-5541 [⇦ this spec on DLA] for yellow hex. Please let me know. Thank you.
Mike Loganplating shop employee - Minneapolis, Minnesota
January 28, 2008
A. Mike, Hi.
Type I - Compositions containing Hex Chrome
Type II - Compositions containing no Hex Chrome
Class 1A - For maximum protection against corrosion, painted or unpainted.
Mil-C-5541 did not mention Type I or II whereas MIL-DTL-5541
[⇦ this spec on DLA] does. Class 1A is the same for both standards. For Mil-DTL-5541, it is very clear when Type II is mentioned, the conversion coating shall not contain Hexavalent Chrome, hence, RoHS compliant. When Type I is mentioned, the conversion coat contains Hexavalent Chrome but it may not necessarily be yellow. There are clear conversion coatings out there which are Hexavalent chromium based. As far as I am concerned, there is no yellow conversion coat (yellow chromate if you prefer) that is Type II. If you have time check out http://assist.daps.dla.mil ^ quicksearch.dla.mil and check out some excellent trivalent chromium base conversion coats (Henkel, Surtec, Macdermid, Luster On ) which can, incidentally be classified as Type II.
- Penang, Malaysia
Ed. note: The address for the mil spec service was updated as noted since Cheah's posting
A. Mike,
Further readings on earlier posts at finishing.com, apparently there are yellow conversion coating that is Type II - clear conversion coating with yellow dye.
MST - Penang Malaysia
QPL = qualified Products List
Hi Cheah. A point of possible confusion is that there may be RoHS compliant trivalent conversion coatings which are yellow (either through use of dye or thick-film types which are naturally yellow) but none which are on the QPL yet -- which means you're not compliant with MIL-DTL-5541 if you use them.
So such coatings can be called "yellow, RoHS-compliant" but probably shouldn't be called Type 2 because they aren't MIL compliant, so what would a naked "Type 2" even mean 🙂
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Ted Mooney, P.E.
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Q. Can anyone tell me what are the differences in the new standards (MIL-DTL-5541 [⇦ this spec on DLA] F) as compared to the old.
Thomas JosephPrecision Metal Fabricator - Singapore
August 27, 2009
Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert
You'll love this book. Finishing.com has sold a thousand copies without a return request 🙂
A. Hi Thomas. We appended your question to a thread where SK Cheah has answered the question: RoHS-compliant (Type II) coatings were not part of the old Mil-C-5541 standards and now they are. Good luck.
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Ted Mooney, P.E.
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Q. Is Iridite per MIL-C-5541E, class 1A compliant with RoHS?
Senthil Kumar- Bangalore, India
December 6, 2013
A. Hi Senthil. Iridite is a Macdermid trade name for a wide range of chromate conversion coating processes, some of which are RoHS compliant, whereas others are based on non-compliant hexavalent chrome. But it is important to remember that a coating is not a MIL-C-5541 coating unless it's on on the QPL. Good luck.
Regards,
Ted Mooney, P.E.
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Q. Is the part processed per type I hazardous in any way? Or just the chemical solution used in the process? We have parts processed per type I versus required type II. The customer is giving us a hard time. Thanks.
david ho- Milford, Connecticut, usa
September 26, 2014
A. Hi David. RoHS refers to the constitution of the parts, not the processes used to manufacture the parts. The situation is that parts processed in Type II solutions do not put hexavalent chromium on the parts, so the parts are very likely to be RoHS complaint. Processing parts per Type I does put hexavalent chromate on the parts, and they do not pass a test for RoHS compliance.
Many domestic and non-European customers, including all car manufacturers, require RoHS compliance as well, so your customer may be giving you a hard time because he can't use the parts 🙂
"...hazardous in any way?" is relative and qualitative, so I don't think anyone can really answer. But a former governor of NJ wrote the memorable line: " 'toxic' is a matter of statute, not opinion", and I think we can look at the word 'hazardous' in the same way: if you want to send something to Europe that RoHS says is hazardous, it's "hazardous". Good luck.
Regards,
Ted Mooney, P.E.
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QPD = Qualified Products Database
Q. Is MIL-DTL-5541
[⇦ this spec on DLA] Class 1A Gold Alodine acceptable for the Spacex callout:
Chemical conversion coating Class 1A using Alodine 1200 or equivalent? Thanks, gs
Quality Inspector - Mtn. View, California USA
June 8, 2016
A. Hi Gary,
Without being a Spacex employee I cannot guarantee my answer is correct, only they can do that, but it does look like it is meant to relate to MIL-DTL-5541 Class 1A, of which Alodine 1200 is on the QPD for MIL-DTL-81706 "Chemical Conversion Materials For Coating Aluminum And Aluminum Alloys".
Aerospace - Yeovil, Somerset, UK
Q. Is there a MIL-DTL-5541, Type II Class 1A Chem film that is doesn't contain hex chrome?
Gary Smith- Everett, Massachusetts USA
October 13, 2016
A. Hi Gary_. Yes, there are several -- MIL-DTL-81706 [⇦ this spec on DLA] lists them. Type II in actual effect essentially means "doesn't contain hex chrome". Good luck.
Regards,
Ted Mooney, P.E. RET
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Q. Is that possible that MIL-DTL-5541 [⇦ this spec on DLA] type II class 1a or type I class 3 coating processes on Aluminum alloys?
ender toprak- Ankara, Turkey
December 13, 2020
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A. Hi Ender. Yes, there are two types and two classes. If I am understanding your question correctly, the answer is that all four combinations are listed in MIL-DTL-81706 and its Qualified Products List Database (QPL/ QPD).
Luck & Regards,
Ted Mooney, P.E. RET
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Q. Is there any evidence of incompatibility between a Class I and a Class II chem film if these are in the same electrical bonding interface?
Ted LaurvikEMC engineer - Centennial
June 19, 2021
A. Hi Ted. I think you may have a typo because to my knowledge there hasn't been a 'Class II' for decades. There is Class 1A and Class 3. But there is a Type I (hex chrome) and a Type II (non-hex chrome) though.
Sometimes the same chemical is used for the Class 1A and Class 3 needs, and I believe the specs tell you to ignore the chromate in calculating galvanic compatibility, so I wouldn't foresee any incompatibility between the various combinations.
Luck & Regards,
Ted Mooney, P.E. RET
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Bravo! I read a lot of quite valid answers to good questions here. The only thing that puzzled me was what RoHS was. I have been around the MIL-DTL 5541 and MIL-DTL-81706 world for a few decades. I was happy to see the 81706 answer for Ender. Typically I see people confusing the process spec (5541) with the material spec (81706).
- Centennial Colorado
Ed. note: Exactly Ted. You can apply chemicals in the manner described in "-5541", but if those chemicals are not in the "-81706" database, you're not compliant.
Q. a customer drawing : AMSC5541 1A YELLOW (RoHS) 168H sst.
But my supplier told me: yellow conversion coating needing to pass 168 hours salt spray must be Hexavalent chromium, and cannot pass RoHS. Non chromium treatment cannot pass 168H sst -- is it right ?
- NYC NY [fictitious]
August 5, 2021
A. Hi Kasen.
What you heard might be correct but not quite for the reason you think ...
In the old days trivalents weren't as corrosion resistant as hexavalents, but these days they are -- so a 168 hour salt spray test should not be a challenge for a good RoHS coating. But what does appear to be true for the present is that there is no RoHS-compliant chromate (Type II) on the qualified products list which is yellow -- they are all colorless.
Ted Mooney, P.E. RET
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Hi.
How are you, my name is DAVE.L; I am from/in China Now I have problems.
Alodine Type 2 Class 1A, MIL-C-5541: Will It Work On Copper? What is the maximum temperature that the surface coating can withstand with no damage to coating characteristics?
Look Forward To Your Reply.
R & D Engineer - Suzhou, China
April 13, 2022
A. Hi Dave. You can apply Alodine or similar chromate conversion coating chemicals to copper and it will offer some tarnish resistance and corrosion resistance. However, it can't be called MIL-C-5541 because that is a specification for the surface treatment of aluminum, not copper.
The temperature resistance of chromates has traditionally thought to be 140 °F., but there is a lot of evidence that Type II coatings can actually withstand more heat than that.
Luck & Regards,
Ted Mooney, P.E. RET
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Q. When it states in a DFAR Clause on a Contract:
252.223-7008 Prohibition of Hexavalent Chromium.
Prohibition.
(1) Unless otherwise specified by the Contracting Officer, the Contractor shall not provide any deliverable or construction material under this contract that:
(i) Contains hexavalent chromium in a concentration greater than 0.1 percent by weight in any homogenous material.
The spec on the drawing notes to Conversion Coat per MIL-DTL-5561 ^ 5541 Cl 1A.
I read in the spec that Type I applies if nothing is noted but I don't know how to figure out if the coating weight would exceed the 0.1 percent by weight of the part.
Quality Assurance / Contract Review - Belmont, NH
October 9, 2023
Ed. note: "5561" was a typo; it should be "5541".
A. Hi Mary,
In MIL-DTL-5541
[⇦ this spec on DLA] conversion coating, "Class" has to do with electrical conductivity (which in turn involves coating weight), but in addition to "Classes" there are also "Types" which have to do with the presence or absence of hexavalent chromium.
Type II are "Compositions containing no hexavalent chromium". I think that once that DFAR requirement for minimal hexavalent coating weight has been made, something very important has been "noted" and the situation of "if nothing is noted" no longer applies. You should specify and do 'Type II' conversion coating to comply with the requirement you speak of.
Luck & Regards,
Ted Mooney, P.E. RET
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Q. Hi Ted,
Since the Drawing only states CL 1A and the Spec states Type I would apply if no type is specified I still don't know if I would be within the .1% by part weight. I read somewhere that a part would rarely exceed .07% by part weight which would be within the limits of the 0.1%.
- Belmont, New Hampshire
A. Hi again.
I can't intelligently critique that article, which I haven't read, that claims that hexavalent chromium rarely exceeds 0.07% of the part weight. But I do warn you that the part and the coating are not a 'homogeneous' material.
I am 99.44% confident that DFAR wants you to use Type 2, non-chromate conversion coating and I fear that even if you tested and proved that your hex chrome was less than 0.1% of the part weight you still won't be safe, but would be a target.
Luck & Regards,
Ted Mooney, P.E. RET
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