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"Electrical Resistance Testing of MIL DTL-5541 Chromate"

(to provide context, hopefully helping readers more quickly understand the Q&A's)

"Chromate Conversion Coating" is a process performed on aluminum for corrosion resistance. It can be a final finish or a pretreatment before painting or powder coating. In the USA it is sometimes called "Chem-Filming". The two best-known trade names for the process chemistry are Alodine (Henkel) and Iridite (MacDermid), so people sometimes use the phrases "Alodining" and "Iriditing" to describe it.

The best known spec for the process is Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil], formerly MIL C-5541. For the coating to be in accord with that spec, the process chemistry must be a "qualified product" per Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil] like some specific Alodine and Iridite products. There are two general types of such coatings, Type 1A for general purpose use and Type 3 for low contact resistance (for areas used for grounding screws for example).

This thread is basically about the testing requirements for the Type 3 coatings, and if you search the site you will find several other threads on this subject.

Current question:

August 21, 2021

Q. I am reaching out to see if anyone in this community may know where I may find the apparatus used to perform Contact Resistance Testing IAW Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil]. I have been unsuccessful in my search thus far. This is my last resort before attempting to build one.

Matthew Cunningham
- Crestview Florida

August 2021

A. Hi Matthew. This thread has been on-line since 1995, and we have a half-dozen more covering the same subject, viewed hundreds of times every month ... and no one has ever written to say that they sell such a device or know someone who does.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

Closely related Q&A's, oldest first:


Q. 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 free spec at Defense Logistics Agency, dla.mil] E Military specification requirement to verify class 3 chemical conversion coatings by running an electrical resistance test per QPL-81706 [affil. link 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.  

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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 [affil. link 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 B117 [affil. link to spec at Techstreet] ), and coating weight (Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil]) 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 monitor
tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania 

sidebar 2001

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

Ken Daly
- Greenville, South Carolina


A. Just 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 specify Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil]

There have been long debates here regarding whether you should process parts on old non-revised drawings per the original spec or the latest revision but I'm not aware of any guidance on this from the spec-writing bodies.

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


Q. We are in a hurry to know which are the values of: ELECTRICAL CONDUCTIVITY of aluminum 7075 T73 and aluminum 6061 T6 after yellow Iridite treatment.

Thank you very much.

Jorge Rueda Nuñez de Villavicencio
- Madrid , SPAIN


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.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina


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.

Joe Hillock
anodizing shop - Philadelphia, Pennsylvania


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

Donn G. Bellmore
instruments - Binghamton, New York

To minimize search efforts and to offer multiple viewpoints, we combined previously separate threads onto this page. Please forgive any resultant repetition, failures of chronological order, or what may look like readers disrespecting previous responses -- those other responses may not have been on the page at the time :-)


Q. Is there any specification and test procedures on the conductivity or resistivity of the coating from chemical conversion treatment (eg. Alodine)? Any lead is greatly appreciated.

Asyong Salungga
researcher - Toronto, Ontario, Canada


A. See Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil] or Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil] 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 B449-93 [affil. link to spec at Techstreet] 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.

Ken Vlach [dec]
- Goleta, California

contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



Q. Ken,

Do you know of any finishing.com supporting advertisers or independent labs that do conductivity testing?

Kim Price
- Springfield, Massachusetts

undated & updated

A. 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 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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

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