Letter 27034

MIL-C-5541 Class 1A vs Class 3  

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Problem: Mil-C-5541 [link is 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-C-5541 [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 imput and guidance with this issue.

Thank You,

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

We both share the same view that this could be an eng'g. specs 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 chemical available for this class, i.e., alodine 1200s could be used to produce clear coat also.

Asyong
- Toronto

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

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

The easiest answer is to leach the color from the side that is not painted. Use hot water. Iridite 14-2 "clear" is qualifed 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 Don Baudrand, Consultant   Poulsbo, Washington

I read once that if you want clear coat you have to request it. Which to me makes sense, so as to not do the salt spray test.

Thomas V. Hicklin
- Sacramento, Ca, USA

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