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MIL-C-5541 Class 1A vs. Class 3


Q. Problem: Mil-C-5541 [link by ed. to spec at TechStreet] Class 1A vs Class 3. A certain customer has requested (i.e. engineering, specifications, etc.) that their aircraft parts be Chem-Filmed with two different processes and results. One side of the part is to be processed per "MIL-C-5541, Class 1A," then primed. Masking. No problem. The opposite side is to be processed per "MIL-C-5541, Class 1A, Clear Coating" with no primer applied. Possibly an aircraft appearance item. I am no expert in this field, but according to MIL-C-5541/AMS C5541 [withdrawn / link is to spec at TechStreet], there is no "Clear Coating" that can be applied to the requirements of Class 1A. As far as I can ascertain, "Clear Coating" can only be applied per Class 3. Unless someone knows something I missed, basically all Class 1A approved chemicals will exhibit color after application. My thinking may be, possible engineering error?

I have a motto that I have kept and lived by for almost 25 years. "Do Right By The Airplane" I can use some input and guidance with this issue.

Thank You,

Brad Huddleston
aerospace quality assurance engineer - Wichita, Kansas, USA


A. We both share the same view that this could be an engineering spec. error. However, I note that there is a grey area in the MIL spec class I esp. in the color. It specifies a range of color. And one of the most common chemicals available for this class, i.e., Alodine 1200S could be used to produce clear coat also.

Asyong [last name deleted for privacy by Editor]
- Toronto, Ontario, Canada


A. If you are talking about "bleaching" with hot water, forget it. Color wise, it may look good, but to the protective requirements of MIL-C-5541, it will more than likely fail.

When it comes to aircraft up to 5 miles above this ground, I do not like taking chances or cutting corners. There are Class 3 compatibles out there. Alodine 1100 and 1500 I believe.

Talk Later, Regards.

Brad Huddleston
- Haysville, Kansas, USA


A. You are correct. Class 1A is less electrically conductive than class 3 and has a gold color. Class 3 also may have a very light gold tint as they are both processed the same with immersion time in our Alchrome II bath being the only difference. We run into this error quite often and the disturbing thing is that most times the customer cannot answer as to which type they actually require.

Good Luck.

Tim Knox
metal finishing shop - Springfield, Vermont, USA


A. The easiest answer is to leach the color from the side that is not painted. Use hot water. Iridite 14-2 "clear" is qualified as class III when leached. It will pass salt spray requirements of the spec. Yellow Alodine may not pass SS, and it is not qualified class III whether leached or not.

Tutorial for Newbies trying to follow this dialog:

Alchrome, Alodine, and Iridite are trade names from three different companies for their lines of chromate conversion processes for aluminum.

You can only comply with Mil-C-5541 is you use "Qualified Products" such as these; if you use "home brew" you are not in compliance with Mil-C-5541.

Don Baudrand
Consultant - Poulsbo, Washington

October 5, 2011

Q. When coated on aluminium, do we call Mil C 5541 Class 3 "clear chromate" or "clear anodising"?

Jimmy koh
Engineer - Minneapolis, Minnesota

October 5, 2011

A. Hi, Jimmy.

You call it chromate conversion coating; and some people call it chem-film. Anodizing is a very different process; see our "Introduction to Anodizing of Aluminum" for a jumpstart.


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Ted Mooney, P.E. RET
Pine Beach, New Jersey

November 14, 2012

A. In my opinion Alodine 600 is applicable for type 3 coating.

Rajeev [last name deleted for privacy by Editor]

July 27, 2015

Q. Hi. I have a question. The company I work for wanted to know if a class 1 can be used as a substitute for class 3?

malinda culley
- ozark, Alabama

July 2015

A. Hi Malinda. Essentially, "no". The situation is that you can't comply with MIL-DTL-5541 without processing per its instructions and using the qualified products. Good luck.


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Ted Mooney, P.E. RET
Pine Beach, New Jersey

July 28, 2015

A. Mooney is legally correct, but I have many clients who do a short dip in type 1a, call it type 3, and pass salt spray,

Editor's note:    
   Mr. Probert is the
   author of
Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

July 2015

thumbsup2 Indeed Robert is correct too. If you are processing parts and have no obligation to conform to Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,], a satisfactory coating may well be applied via a short dip in a Class 1A solution -- and you might refer to "class 1a" and "class 3" as slang or shorthand to refer to the general type of finish you want to apply.

... Much as you might "xerox" a copy of the spec on a Brother copier and "scotch tape" it together with generic "cellophane" tape rather than Cellophane® :-)


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Ted Mooney, P.E. RET
Pine Beach, New Jersey

August 13, 2015

Q. What is the difference between Mil-DTL-5541 [link is to free spec at Defense Logistics Agency,] class 1a and MIL-DTL-5541 class 3?

Rene Ramirez
Inspection - Santa Ana California USA

September 2015

A. Hi Rene. From the front page of the spec.:


But you have to download the spec.; I don't think you can inspect without it. Good luck.


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Ted Mooney, P.E. RET
Pine Beach, New Jersey

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