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Chromate Conversion vs Anodizing



A discussion started in 2006 and continuing through 2017 . . .

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

milt stevenson jr.
Milt Stevenson, Jr.
Syracuse, New York


(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)

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 resistant 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.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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