MIL-C-5541 Type 1 & 2, and Class 1A & 3 -- understand the differences

A. Hi Martha,

I don't claim expertise on this, but it sounds tricky. Although we often use the slang "trivalent chromate", trivalent solutions are in fact not "chromates" because chromATE means hexavalent. The reinstatement of a spec which preceded both the more modern trivalent technology and the callouts changing from MIL-C-5541 to MIL-DTL-5541, plus the back & forth of wording between "chromate" and "chemical coating", and the fact that 5541 uses the phrase chemical conversion coatings rather than chromates, I don't think a definitive answer will come from the semantics of this spec :-(

The spec says that surfaces which are not chromated should be made neutral or slightly acidic with chromic acid if not already so. Hopefully someone will tell us that trivalents are slightly acidic and will not turn the litmus paper blue, or why the pH is inconsequential for trivalent surfaces, thus clarifying that litmus testing is not required  🙂 Luck & Regards,


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

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.

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.

Hi Cheah. I think a big point of possible confusion is that there may be RoHS compliant trivalent conversion coatings which are yellow, either through the addition of dye or because they are thick-film and naturally yellow, but which are not on the QPL yet -- which means you're not compliant with MIL-DTL-5541 if you use them. So it probably isn't correct to call such coatings Type II ... just call them yellow and RoHS-compliant :-)

Regards,

A. Hi Thomas. We appended your question to a thread where SK Cheah had answered the question: RoHS-compliant (Type II) coatings were not part of the old Mil-C-5541 standards. Good luck.

Regards,

A. Hi Senthil. Iridite is a Macdermid trade name for a range of chromate conversion coating processes, some of which are surely RoHS compliant, whereas others were (and may still be) based on non-compliant hexavalent chrome. Good luck.

Regards,

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 are unlikely to 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 is not able to use the parts :-)

"...hazardous in any way?" is relative and qualitative, so I don't think anyone can answer. But a former governor of NJ once declared that " '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 them to Europe but RoHS says they're hazardous, they're hazardous. Good luck.

Regards,

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".

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,

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).

Luck & Regards,

A. Hi Ted. I think you have a typo because to my knowledge there is no 'Class II'. There is Class 1A and Class 3. There is also 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 spec tells you to ignore the chromate in calculating galvanic compatibility, so I wouldn't foresee any between the various combinations.

Luck & Regards,

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).

A. Hi Kasen.

What you heard is incorrect but has a seed of truth. 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. But it might be true that there is no chromate which is acceptable to AMSC5541 1A and yellow and RoHS compliant.

Postings are welcome from anyone anywhere, but this site is a place of camaraderie since 1989, and tiny untruths are a kiss of death to aloha. When a reader needs help understanding NYC environmental regs, we hope you don't reply that you can't help because you're from Guangzhou :-)

A. Hi Dave. You can apply Alodine and similar chemicals to copper and it will offer some tarnish resistance and corrosion resistance. However, it cannot be called MIL-C-5541 because that is a specification for surface treatment of aluminum, not copper. The temperature resistance is generally thought to be 140 °F.

Luck & Regards,

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,

A. Hi again.
I can't intelligently critique an article that I haven't read, that says hexavalent chromium rarely exceeds 0.07% of the part weight. But I can warn you that 'homogeneous' material doesn't mean what people expect it to mean -- it means what the European Parliament wants it to mean, and they want it to mean "don't use hexavalent chromium" :-)

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 the 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
Striving to live Aloha
finishing.com - Pine Beach, New Jersey