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Electrical resistance of Class 3 chromate conversion coating


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July 21, 2021 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I would like to ask if someone has a value table with the results of measurement of electrical results of alodine 1200S class 3? I would like to see the behavior of the alodine class 3 when measured according to MIL-DTL-81706.

Leandro reis
- Lorena SP, Brazil

July 2021

A. Hi Leandro. We appended your inquiry to an existing thread on the subject, and highlighted Ilya Ostrovsky's earlier response to a similar question.

Luck & Regards,

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

July 29, 2021

Q. Regarding the conductivity of Chemical Conversion coatings on Aluminum (MIL-DTL-5541) what does it mean that so many coatings are double qualified on the QPL to both 1A and 3?

For example we use T5900 and have a electrical contact resistance requirement. Our typical values post treatment pre salt spray are in the 1-2000 µohm range. Per the spec we also test to 168 hours salt spray where we still have contact resistance within spec, typically about double the original 1-2000 µohm range. To me, that seems to indicate these coatings are conforming to both class 1A and 3.

I think the typical thinking and a quick reading of MIL-DTL-5541 says that if you want low contact resistance use class 3 and if you want the best protection against corrosion use 1A. However, seems to me that you actually get both maximum corrosion protection and low contact resistance with these new double qualified chem conversion coatings.


Stephen Johnson
- Salt Lake City, Utah

July 31, 2021

A. Is it just a semantic expression to say "double qualified"?
The product used for application may be QPL listed and capable of Classes 1A AND 3, but each "coatings" are different because the concentration, time and pH are different in each of two tanks containing the same product.

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

August 2, 2021

tthumbs up sign The phrase "Double Qualified" is of my own making. I'm just saying it seems to be qualified to both. From my research it seems that there are two major differences between 1A and 3: 1) Contact resistance and 2) qualification corrosion resistance of 336 hours for class 1A (per Table 1 in MIL-DTL-81706). Conformance for both class 1A and 3 is still 168 hours per both 81706 and 5541.

I guess I'm trying to wrap my head around the fact that if a coating both meets the 168 hours salt spray and the contact resistance testing, how is it not both a class 1A and 3 coating?

Stephen Johnson [returning]
- Salt Lake City, Utah

Closely related historical postings, oldest first:


Q. Has anyone built the required electrical resistance (for chemical coatings) apparatus to conform to MIL-C-81706? Any information on where to obtain the equipment or components would be greatly appreciated. Thanks.

Brian Persing
- Springdale, Arkansas, USA


Q. I would like to know the electrical properties of ALODINE 1200. We are using it as protection on our metallic housing of Door Control Unit. Is it necessary to protect the ground stud from ALODINE 1200 ?

Emmanuel SEREZAT
transport - FRANCE


A. Alodine 1200 is one of chromate chemical conversion coating that meets requirements of MIL-C-5541 (you can find a lot of other processes). Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil] Class 3 requires maximum 5000 µOhm per sq. inch electrical resistance. Actually, electrical resistance for properly performed Class 3 is usually 1000-2000 µOhm per sq. inch. MIL-C-5541 Class 1A does not require electrical conductivity. Most chemical conversion coatings from QPL (including Alodine 1200) can be performed as both Class 1A and Class 3. Therefore you should check which class is processed on your parts.

Ilya Ostrovsky


Q. Could you please explain about the electrical resistance on Alodine surface. Do I understand correctly that Alodine surface itself gives some resistance on aluminium. And what is the standard for the resistance on the Alodine surface and how to measure the resistance? From Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil], the resistance needs to be less than 5,000 micro-Ohms/sq. inch. But current supplier cannot achieve that figure, 1-4 Ohms is the best they can do. I did experiment as below.

Measuring 100 of bare aluminium. I also did chromating evaluation by dipping bare aluminium for 10 different times in chromating bath. (Alchrom 713:AM713, 10 each at 40, 60, 80, 120, 130, 140, 150, 160, 180 and 200 seconds.) Below are summarized my findings:

1. Base on my evaluation, I found that the Alodine surface would create some resistance to aluminium. The average resistance of bare aluminium are around 0.2-0.5 Ohm. But the resistance of the aluminium after chromating are around 1-4 Ohms.
2. According to my investigation, thicker Alodine surface will give the same resistance to Aluminium. (1-4 Ohms) I asked my supplier to do chromating by 10 different dipping times in chromate bath. (40, 60, 80, 120, 130, 140, 150, 160, 180 and 200 seconds (my assumption is longer dipping time gives thicker Alodine surface).
3. The current dipping time in Chromate bath is 130 seconds.

P.S. If I need lower resistance on Alodine surface, what should I do? Change the chemical? Or process?


Jeadsada N.
York - Bangkok, Thailand


A. First, 'Alchrom 713' is not listed in QPL-81706 [affil. link to spec at Techstreet]-16. Does the manufacturer indicate it will meet MIL-C-5541, Class 3 requirements?
Second, is the resistance measured as described in Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil]B?
"The average resistance of bare aluminium are around 0.2-0.5 Ohm" -- Should be much lower. Maybe on raw material, prior to any pretreatment (or pretreated material aged in air)? Rinse a sample after desmutting, dry with a lint-free cloth & measure resistance.

Unfortunately, your findings did not appear above. Does the yellow coloration & resistance increase with increasing immersion time and then level off? An even shorter time may be necessary for a Class 3 coating, which may be a pale, iridescent gold or yellow (or even colorless).

General info for low resistance: The starting material should be a clean wrought alloy, smoothly rolled or finely machined, free of corrosion. Clean in a non-etching detergent, alkaline etch the minimum time for uniform bubbles, desmut in 50 vol% nitric acid, and rinse in DI water prior to chem-film (additional rinsing not listed). The chem-film pH & conc. should be maintained per mfr. The surface must not be noticeably roughened by etching, nor allowed to dry between steps. Sometimes, the alkaline etch is omitted, although it is preferable to do the same procedure as for production.
Shorter chem-film times give lower electrical resistances, but a compromise is necessary to maintain the 168 hours salt spray corrosion resistance.

In summary, proper pretreatment and shorter chem-film immersion time will probably help.

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, and we at finishing.com, continue to benefit from.


Q. The aluminium my supplier is using is 6063T5 and they are performing the pre-treatment process before chromating as following.

Water Rinse 1
Water Rinse 2
Strong Base
Water Rinse 3
Water Rinse 4
Strong Acid
Water Rinse 5
Water Rinse 6
Alodine (Alchom 713)
Water Rinse 7
DI Water Rinse

I did measure the electrical resistance after Water Rinse 6 and found that the avg. resistance was exactly the same as bare aluminium before pre treatment process. (0.3 Ohms) Therefore I can conclude that the resistance came from chromating process. Does anyone here know about what kind of chemical I should use if I need the lower resistance to my aluminium and I still need to have the yellow color?


Jeadsada N. [returning]
York - Bangkok, Thailand


A. In the aircraft fabrication and maintenance industries process, Alodine 1000 is used for low resistance and better conductivity (e.g. used for connector assembly on aluminium parts that need electrical bonding or grounding). Compared to Alodine 1200s, the 1000 has better conductive properties and creates a thinner film coating (the yellow color will also look lighter and almost transparent). You should try testing the 1000 solution for a change. Both solutions are mentioned in MIL-C-5541.

Syvan Chartya
aerospace - st-hubert, Quebec, Canada


Q. I am developing an aluminum sheet metal part that will be chem filmed and must meet Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil] F, CLASS 3. Where can I buy the equipment to measure a resistance not greater than 5,000 microhms per square inch with a nominal electrode pressure of 200 psi?

Mark Stauder
Mechanical Design Engineer - Irvine, California

simultaneous 2007

A. Build it or buy it! We tried for years to locate such a piece of test equipment and finally we purchased a load cell to measure force, machined a copper electrode 1" x 1", and put the unit into a hydraulic controlled anvil press. Using a simple amp/volt meter between the copper electrode and the test piece we were able to measure the surface resistance. The alternative is to have an outside electrical testing house measure it for you which is often easier and cheaper when you factor in the cost of calibration. Lastly, once you've settled in on a chromate product and defined the process needed to stay below the maximum resistance, you'll find the result doesn't vary.

The spec you reference doesn't require that you measure the resistance (whereas AMS2473 [affil. link to spec at Techstreet] might), MIL-C-5541 Class 3 requires the use of a QPL-approved product which has demonstrated its ability to meet the resistance requirement when operated per the manufacturer's data sheet.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser
Syracuse, New York
Anoplate banner


A. Not sure whether these are commercially available.
The design is relatively simple. See Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil]B CHEMICAL CONVERSION MATERIALS FOR COATING ALUMINUM.

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, and we at finishing.com, continue to benefit from.

sidebar April 15, 2011

Q. Dears,
About Alchrom 713, does anybody know what is its composition?
Is it safe to use, knowing that Cr+6 is considered hazardous today?
Thank you

Francois LEJEUNE
- Semarang, Jawa tengah, Indonesia

April 18, 2011

A. Hi, Francois. Yes, Cr+6 is considered hazardous in most jurisdictions today. But Alchrom seems to be a trade name of Nihon Parkerizing*, just as Thunderbird is a trade name of Ford. A Thunderbird was a 2-seat sports car, a 4-seat luxury car, and a 6-seat family sedan depending on what Ford decided at a given time. Similarly, the composition of a chromate conversion product can be hexavalent, trivalent, or whatever the trade name owner decides at any given time. Please get an MSDS from Nihon, as they alone decide.

* -- Further complicating things is that there are several different but similar-sounding trade names for chromate conversion coatings. Alchrom is one trade name, Alchrome is another, and Alocrom is a third, whereas the words Alochrom, Alochrome, and a few others are very widely used according to Google ... but I don't have the energy to track them all down and determine whether they are actual trade names or just pervasive misspellings :-)


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

Several threads were merged; please forgive repetitiveness, chronology errors, or perceived disrespect towards earlier responses -- they probably weren't there then :-)

November 30, 2015

Q. Hi,
I'm interested to know how or where I can get the equipment for measuring electrical resistance chemical film (Class 3) in accordance with MIL-DTL-81706 section 4.5.5.

Thank you.

With Best Regards

Sim Boon Tiong
- Singapore

December 28, 2015

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.

Tim Hamlett
Tim Hamlett, CEF
- West Palm Beach, Florida, USA

March 25, 2016

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

Ravi Chandran, Ph.D
CHEMIONIC Labs & Consulting
New Brunswick, New Jersey

Ed. note April 2018: Unfortunately, Chemionic Consulting is no longer operating that laboratory -- but Dr. Chandran may still be able to advise on the subject.

Ed. note: Readers may also be interested in
letter 13808, "Achieving Class 3 Chromate Conversion Coatings" and
letter 38755, "Aluminum conductivity, chromate coating, oxidation"

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