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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 I am no expert in this field, but according to MIL-C-5541/AMS C5541 [withdrawn / link is to spec at TechStreet], there is no "Clear Coating" that can be applied to the requirements of Class 1A.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.

- 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.

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; some people call it chem-film. Anodizing is a very different process.


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

November 14, 2012

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


Thanks, Rajeev ... but we have many Rajeev's posting on this site. Which one are you?

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