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topic 42251

Chromate Conversion vs Anodizing


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

A. 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



2006

Q. I'm new to this business, and was hoping someone could give me a quick overview of why chromate converting is a recommended process as a paint primer? Why is anodizing not?

Thanks,

Patricia Browne
Process engineer - Tucson, Arizona


2006

A. Patricia,

I think it comes down to a simple commercial decision. Chromate conversion coating is a simple immersion process and, in relative terms, is quick to do.

Anodising will give good paint adhesion but is an electrolytic process, takes longer, sometimes involves special jigging and can involve increased hazards (chromic acid mists for Mil-A-8625 [link is to free spec at Defense Logistics Agency, dla.mil] type I anodising).

If cost is no object to you I would always go for type I anodising as a key for paint. If costs need to be considered then chromate conversion coating is the answer.

Brian Terry
Aerospace - Yeovil, Somerset, UK


2006

A. Chromating is the norm for paint base due to it being an immersion process as opposed to a longer, more expensive electrolytic process such as anodize. Anodize is also frowned upon in the aerospace industry, for example, as it lowers the fatigue strength of the aluminum. The thicker the anodic layer, the more degrading to the overall fatigue strength - this is why the thinnest, Type I, chromic anodize is widely used in the aerospace field.

adv.
If you have specific question, please E-mail me!

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

Anoplate banner


2006

A. The main difference between anodizing and chromate conversion coating is the electrical conductivity of the finishes. Anodizing is non-conductive while chromate conversion coating gives an electrically conductive coating.

Reha Aktas
- Tulsa, Oklahoma


2006

A. Reha,

I would be very careful with that statement. In accordance with Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil] only class 3 coatings should be used for electrical continuity.

As the thickness of chromate conversion increases so does the electrical resistance, so much so that a class 1A coating, to all intents and purposes, can be considered non-electrically conductive. The only way to get electrical conductivity with this coating is by breaching the coating.

Brian Terry
Aerospace - Yeovil, Somerset, UK


2006

A. Chromating process is losing popularity as it is detrimental for environment. There are non-chromate products tested and recommended to replace chromate products.

Timur Ulucak
aluminum extrusions & finishing - Istanbul, Turkey


May 1, 2016

A. Chromate Conversion and Anodisation Both are Passivation processes on aluminium.(both are corrosion resistant coatings, both increase paint adhesion for aluminium due to formation of pores.)
In Chromatisation the Workpiece is dipped in a Hot Bath so that a layer of conductive Cr2O3 is formed on the job.
Whereas in Anodisation a layer of non conductive Al2O3 is formed through Method of Electrolytic passivation... passage of current in a bath with aluminium as anode.
Chromatisation is generally much cheaper then anodisation.

The process you should follow should depend on the application of your product.
Say if you need a low bonding resistance between two mating parts so as to provide a better conductive path, then you should go for Chromatisation (preferably Trivalent).

If you need abrasion /wear resistance, or say you are designing a heat sink for a system, you should go for anodisation and black dye.

Prateek Sharma
avionics - Delhi, India



October 17, 2016

Q. How to get electrical conductivity on aluminium after anodizing or from other coating?

I have a sample from customer where his anodized aluminium is conductive with 10^3. But when I did anodizing on aluminium it's not conductive. its only ESD with 10^10.

Can anyone advise me how I can get electrical conductivity after anodizing on aluminium? What kind of anodized?

Does e-coating(what kind?), Chromate, Alodine and Iridite get the electrical conductivity? But it must not have any banned substances on the finishing.

Thanks in advance.

Andrew Chok
- Jakarta, Indonesia


October 2016

A. Hi Andrew. Aluminum can be chromate conversion coated with trivalent (no hexavalent chrome) per Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil], Type II, Class 3. Good luck.

It is also possible to do conductive anodizing (see thread 7297), but this is quite a bot more complicated and expensive.

Regards,

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



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. Apparently the authors of MIL-DTL-5541 do not believe that such values are "irrespective of the measuring device, method, etc." but are highly dependent upon it. I personally have never seen the test done and know little about it, but we have several 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, and you'll probably draw enthusiastic response. But after 30 years of postings, 60,000 questions and a quarter million replies, abstract questions seem to no longer draw much reader interest. Good luck.

Regards,

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



December 19, 2019

Which coating is best for aluminium, anodizing vs. chromating

Arun kumar
- Tamil nadu


December 2019

? Hi Arun. To save us from referring you to whole chapters in textbooks (there's a lot to it), please tell your complete situation and which way are you leaning and I'm sure we can help you in a paragraph or two.

Regards,

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

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