Electrical Resistance/Conductivity Testing of MIL DTL-5541 Chromate

A. 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 QPL-81706 [on DLA] 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 B117 ), and coating weight (MIL-C-5541) on a monthly basis for a sample of parts or a test panel, depending on how critical the corrosion resistance is for the parts.

A. The conductivity is a function of the thickness or "coating weight" of the chromate. Our MIL C-5541 specifies Class 3 for low contact resistance, however, in the real world there is a great amount of controversy as to the contact pressure control in actually measuring the contact resistance.

Other presenters will give you a number "value", but I claim there is no reliability in defining this number because of the difficulty in defining a method of controlling the contact resistance.

A. I agree with the previous gentleman's assessment of how the electrical resistance is controlled, it is a function of coating weight. We have had the need however to provide a recognition that this control is "number related". We worked with our customer to agree on this number by utilizing an instrument to measure electrical resistance. Although we are using a different instruments than our customer to measure electrical resistance, we have agreed that our measurement of what constitutes "low electrical resistance" for his application is acceptable. The instrument that we are using is a simple tool and is relatively inexpensive.

A. I disagree with the comments, and one can make consistent measurements using 2 surface resistance probes as outlined in ESD Association's Worksurface Measurement standard. There are also .5 inch diameter probes and miniature probes available for this purpose. When I spec this class of finish I use < than 2 ohms surface resistance and it is and can be consistently measured.

Thanks Donn. I am no expert in that topic but I trust that the ESD Association is, and that their tests are good ones. The problem I see, and maybe it's a misunderstanding based on my limited knowledge, is that although you feel that you can measure surface resistance in a manner which you consider accurate & repeatable, it does not seem to comply with the spec. Probably the Navy ought to work with the ESD Association to resolve this, but it appears that the current test requires 1 square inch probes, kelvin bridge measurements, less than 0.5 ohms surface resistance, etc.

Luck & Regards,

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

A. See MIL-C-5541 or MIL-DTL-81706 [on DLA] for test panel preparation and salt spray requirements. The conductivity testing apparatus and procedure are given in MIL-DTL-81706. Conductivity of Class 3 coatings is measured both before and after salt spray testing. Briefly, contact resistance is measured by pressing a flat test panel between polished copper blocks. Both specifications are free downloads at the ASSIST site: http://assist.daps.dla.mil/quicksearch/.

ASTM B449 is similar, but with coating Classes 1, 2, 3 & 4 rather than 1A and 3 as in the MIL specs. Corrosion resistance varies considerably with coating Class and alloy.

Another specification is ISO 10546 [link is to spec at Amazon]: 1993 CHEMICAL CONVERSION COATINGS - RINSED AND NON-RINSED CHROMATE CONVERSION COATINGS ON ALUMINIUM AND ALUMINIUM ALLOYS. The abstract mentions electrical resistance and corrosion testing, but I don't have details.

See MIL-DTL-81706 [on DLA] for the testing apparatus and procedure.

A. Hi Ian, hi Kim. Dr. Ravi Chandran ran Chemionic Labs & Consulting for years, and featured this test. But he has moved on to other employment unknown to me. If you know someone from the Newark NJ area who might know Ravi, maybe (Dave Wichern or Steve Rudy?), or if you find him on linked-in, etc., by including "Chemionic" in your search, I'm sure Ravi can either run the test for you, send you to someone who can do it, or tell you how to do it.

Luck & Regards,

A. Sim,
First: Usually this type of testing is only required when you are developing a chemistry for applying the chemical conversion coat. If that is your goal then you are correct that the testing needs to be done before it can be submitted for inclusion on QPL-81706.

Second: In my 32 years in metal finishing I've never seen this equipment offered for sale by anyone. I'm not saying it isn't, but I've never seen it. Companies usually build their own or they send samples out to an approved laboratory that already has the equipment. It would most likely be less expensive to send it out unless you plan on performing the test frequently. The one and only time I've ever been requested to test the coating, I sent samples to a lab in California that performs this test.

A. CHEMIONIC Labs & Consulting is ISO17025 accredited to perform this test. We are accredited by American Association for Laboratory Accreditation (A2LA).

A. ERPO, based in France, manufactures and sells the equipment to carry out the electrical resistance test as per MIL-DTL-81706. It is called AMES EVO 1.5.

I had the chance to use one because the company I worked for had to do periodic testing with it. It works well.