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"Achieving class 3 chromate conversion coatings"


Q. Achieving class 3 coatings using Iridite 14.2 / Alocrom (Alodine) 1200

We have been successfully using the above systems to Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil] class 1A for some time, up until recently there has been no requirement for class 3 coatings. This is set to change as some of our new customers are requesting coatings to class 3.

My understanding of the difference between 1A and 3 is the film thickness i.e. the class 3 coating is thinner and has reduced resistivity and reduced corrosion protection. The thinner coating being achieved via reducing the immersion times. However, the manufactures details I have give no indication of concentrations vs. immersion times to give the required film thickness. Is the application of a class 3 coating simply based on experimentation and testing for a given bath concentration, or are there any guide lines i.e. concentration vs. immersion times, out there which could be applied to reduce the trial and error factor?

In addition what are the test criteria i.e. resistivity values etc to use in order to comply with the specification, considering that the only values given are in para 6 or the notes section which sates it is for info only and non mandatory. Para 3.8 states that the values and the frequency of testing are to be defined by the procuring activity. Does this not cause conflicts or contradictions. Is it possible for the 6061 panels to pass their resistivity tests for a given customer and comply with the spec but not for a.n. other customer?

Any info or insights into this would be gratefully received

Richard Mosley
metalsmiths - UK


A. Hi Richard,

The one bit of help I can give is how to test for the coating weight.

Alocrom your panel rinse and blow or air dry (do not oven dry).

Then find the mass on a lab balance to 0.0001g.

Strip in 50% concentrated nitric 50% water (it will strip quick if within about 3 hours of coating) then rinse.

Swab with damp cotton wool if required to remove traces of coating.

Rinse and dry.

Then reweigh. From the area of the panel you can work out the coat weight.

Also stripping in nitric does not have a go at the base metal unlike other strips.

You will find the relationship between coating weight and time is non-linear, so you will have to test different dip times to find the best time to get the coating required.

Also you will have to put in place some testing as the time will drift a bit due to bath conc., pH, temperature etc.

As to the bit about the customer supplies the requirements you are right two customers could supply two different requirements, so go back and make sure they know what they want before you start supplying products to them. (If I was a betting man I would bet that the customer only wants what they had before of someone else and don't know really what they want)

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England


A. There are only two requirements in MIL-C-5541E that differ from Class 1a and 3.

1) The solution used must be on the Qualified Products List (QPL) for that Class. Many products, including Iridite 14 and Alodine 1200, are listed for both classes.

2) Class 3 must pass a Electrical Contact Resistance Test per paragraph 3.8, but only when specified. I have never seen this specified.

So in most cases the specification makes no required distinctions between Class 1a and 3 coatings.

The only discussion about immersion times, coating thickness, and/or coating weights occurs in the non-mandatory Notes section of the specification. It suggests that Class 3 coatings are thinner due to lower immersion times but gives no guidance.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

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A. Richard,

What Martin said is absolutely correct. I would like to add thought that while two classes are different, they don't have to be processed that way. Our customers only require an electrical resistance test (<5,000 micro-ohms before salt spray and <10,000 micro-ohms after) to certify to class 3, since our class 1A w/ a 3 minute immersion time satisfies this test, we can run our class 1A and 3 on the same rack. Our customers also accept one salt spray test for both classes as they are the same thickness/coating weight.

I don't believe coating weight/thickness is the difference between the classes, I believe that the electrical properties are the dividing factor as class 3 needs to be acceptable for EB (Electrical Bond) areas.

Hope this gives some more info.

Benjamin J. Curto
- Ponderay, Idaho, USA


Q. Thanks to all for the info it is much appreciated !


As to the interpretation of the spec. If a customer is requesting a class 3 coating yet not specifying values etc, how do you verify that a class 3 coating is achieved? Do you interpret the spec such that the tests default to those detailed in Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil] and described in para 6.1.2 of MIL-C-5541 i.e., <5000 & <10000 microhms under an electrode pressure of 200 psi?

Rich Mosley
- UK


A. Without an electrical resistance requirement, only four conditions must be met for a Class 3 coating.

1)The product must be on the QPL for Class 3.
2)The monthly corrosion and wet tape testing must be performed.
3)The bath solution must be analysed weekly.
4)The coating must be uniform in appearance and iridescent yellow to brown.

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

luke banner


Q. Chris,

Again, thanks for the response.

Forgive me if I am missing something fundamental but I want to be as clear as possible on this issue prior to offering class 3 coatings.

If a customer requests a class 3 coating but does not specify any resistance values, and the chemical process used is capable of producing both classes. Bearing in mind that class 3 coatings are qualified under Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil] to the 5000 and 10000 values. How is it possible to certify a component as having a class 3 coating without testing ? The actual coatings resistance values may be outside the required 5000 and 10000 values and as such a class 1A coating. If resistance values are not specified & testing carried out essentially class 1A and class 3 could be one in the same.

Another question is, are the 5000 and 10000 values seen as qualification values only, i.e., could a customer legitimately ask for lower values i.e., outside those that the QPL chemicals are qualified to?

Richard Mosley
- UK


A. You are correct. If electrical resistance testing is not specified, the exact same coating could meet the requirements of both Class 1a and 3. Testing per MIL-C-81706 is not the responsibility of the applicator but rather the chemical manufacturer. The applicator's responsibility to use only QPL products listed for the appropriate class and certified to Mil-DTL-81706 [link is to free spec at Defense Logistics Agency, dla.mil].

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

luke banner

August 25, 2010

Q. How about when a class 1a finish is required for maximum corrosion protection? If there is no electrical requirement called out (because a class 1a finish is desired, not a class 3), how do you know if you are getting the class 1a? Meaning, if a class 3 is equivalent to a class 1a when no electrical resistance test is called for, does that mean the thinner, less protective class 3 can be substituted for a class 1a?

Terry Bartick
- Portland Maine

August 26, 2010

A. Class 3 is NOT the equivalent of class 1a, regardless of what some MFGs say or passing a minimum salt spray.
You need to fill the customers PO / spec or get him to change it. Darker chem film is a lot easier to spot a problem than pale will ever be.

James Watts
- Navarre, Florida

September 16, 2010

Q. Good day everyone,

I too and struggling with this very topic. I understand the requirements and testing responsibilities but still struggle to understand why the electrical resistance test is not a requirement. It appears to me that the other tests within this specification were "grandfathered" as the acceptable testing criteria.

I would like to discuss this with someone that performs the electrical resistance test. Can any of you recommend a testing facility that could provide input on this subject?

I look forward to additional comments on this topic.


Scott Peek
- New Hope, Minnesota

November 9, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Can Iridite 14-2 from MacDermid meet the requirements of ASTM B449 Class 1, that is thickness and salt spray test?

Daris Combs
motorcycles - Commack, New York, USA

February 21, 2013

Q. An engineer requires 2 classes of coating on one AL 6061 tube, MIL-DTL-5541 TYPE 1 CLASS 1A on the middle external surface and class 3 on the both tube ends where fittings will install (to get a lower electrical resistance). It is difficult in operation. The customer does not require the electrical resistance testing. So how can I know I have got class 3 coating on tube end area? If we used Alodine 600 to get class 1A coating. Can the electrical resistance test meet the class 3 coating's requirements?

Hans Lin
- Shanghai, Pudong, China

Ed. note: Readers may also be interested in Letter 40382, "Electrical resistance of chromate conversion coating class 3".

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