Letter 0012

Electrical Resistance Testing of MIL-C-5541 Chromate

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I am looking for information on chemical conversion coatings on aluminum and aluminum alloys. The issue that is in question is the Mil-C-5541 [link is to spec at TechStreet] E Military specification requirement to verify class 3 chemical conversion coatings by running an electrical resistance test per Mil-C-81706 [link is to spec at TechStreet]. This procedure involves applying an electrode pressure of 200 psi to the coating and measuring for less than 5000 micro ohms per square inch. Our company wants to find a test method that can verify compliance of our stock of parts without elaborate (costly) test setups. If you can help, it would be greatly appreciated.

I would recommend the approach of at least one major aerospace supplier:

In their process specification, they call out the same mil-specs that you do, except that they exclude clear coatings, this makes the identification of a chromated part very easy. If you must use a clear coating, at least specify a blue, slightly iridescent color to make identification easier. The Mil-C-81706 [link is to spec at TechStreet] is a qualified product list and I would mention it in the specification, and pick one brand to write your specification around, allowing for alternates.

I have never seen anyone use the resistance measurement for chromate conversion coatings. I have seen a simple apparatus using a light bulb and some low voltage source for testing for the presence of an anodic coating on aluminum, since the clear coatings are hard to distinguish visually. But an anodic coating is much harder than a conversion coating and the test is easy to do with a simple method.

I think you should specify the appearance and color for all parts; and corrosion resistance (ASTM B-117 [link is to spec at TechStreet] ), and coating weight (MIL-C-55410) on a monthly basis for a sample of parts or a test panel, depending on how critical the corrosion resistance is for the parts.

Tom Pullizzi
platronica.com
Falls Township, PA

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What is the difference between MIL spec MIL-C-5541 and MIL-C-5541E?

Ken Daly
- Greenville, South Carolina

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The revision level, Ken. Rev. D was released in 1989. Rev. E came sometime after that, probably the mid 1990's. These days people should see Mil-DTL-5541 [link is to spec at TechStreet]

Ted Mooney, P.E.
finishing.com
Brick, New Jersey