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topic 14550

ANODIZING vs. CHROMATING of Aluminum


A discussion started in 2002 but continuing through 2019

2002

Q1. What is the difference between anodizing and Chromate plating in terms of electrical conductivity?

Q2. Is there any visual way to differentiate between the two?

Juraine Go Pimentel
- Thailand


2002

HI Juraine,

Anodizing is an electrolytically formed coating of aluminum oxide and it is totally non-conductive. Chromate is an immersion process which affords some corrosion protection but less than anodizing. Chromate by itself is almost totally non-conductive, however, when applied according to our Mil-C-5541 [link is to free spec at Defense Logistics Agency, dla.mil], Class III is conductive. There is a lot of controversy regarding "contact resistance". With any chromate coating if you push the probe hard enough it will cut through the soft chromate film and make contact.

Chromate is yellow or clear, the clear being better conductivity. Anodizing is hard and clear (unless dyed).

Chromate will scratch with your fingernail. You cannot scratch anodizing with your fingernail.

Get back to me if this is not enough information.

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



January 10, 2019

Q. What is typical electrical contact resistance values in micro-ohms for chromatized (Type I class 3) Al alloy. Why does it increase upon corrosion tests?

Deepashri Nage
Larsen & Toubro - Mumbai, Maharashtra, India


January 2019

A. Hi Deepashri. Per MIL-DTL-5541 rev. F:

"6.1.2.1 Electrical resistance testing. When under a nominal electrode pressure of 200 psi, class 3 coatings are qualified under MIL-DTL-81706 to have a resistance not greater than 5,000 microhms per square inch as supplied and 10,000 microhms per square inch after 168 hours of salt spray exposure. In addition to the coating or coating thickness (see 6.1.2), other variables heavily influence resistance values when using the test method specified in MIL-DTL-81706 or other similar methods. The following two variables (see 6.1.2.1.1 and 6.1.2.1.2) may have a greater effect on electrical resistance values than the conversion coating thickness. 6.1.2.1.1 Surface roughness of the specimen panel. Test specimens having rough surfaces will yield lower resistance values when subjected to a contact electrode pressure due to coating fracture. This reasoning can also be applied to the contact electrode. 6.1.2.1.2 Flatness of the contact electrode. If an electrode with a given surface area is not flat, the actual contact area will be lower than the theoretical value. A smaller contact area results in a higher resistance value. The same reasoning can be applied to the test specimen."

Presumably the corrosion test is partially successful in corroding the finish so that its surface conductance is not as reliably low. Good luck.

Regards,

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


January 11, 2019

Q. Thanks Ted, for your reply. However, I was expecting typical values based on the inherent film formed.
Irrespective of the measuring device, method etc. Does the measurement area under consideration affect the values?

Dr. Deepashri Nage [returning]
- Mumbai, Maharashtra, India


January 2019

A. Hi again Dr. Nage. I personally have never seen the test done and know little about it, but we already have many threads about it on the site ...

Please search the site with the term "chromate conductivity test" (google prohibits us from pre-loading search terms into the custom search engine) ...

Or post the method you are using, the type of parts you are testing, and the results you have been getting. After 24 years of postings, with 60,000 questions and a quarter million replies here already, abstract questions no longer draw much reader response. Good luck.

Regards,

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

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