Aluminum: Alodine vs Anodize Properties
I want to compare the electrical and thermal conductivity of aluminum Alodine vs anodize, but can't find any hard data on this. Many web sources say that Alodine has higher conductivities ... but don't back this up with data. Do you have any data or sources for this? Thanks!Brent Egly
Engineer - Orem, UT, USA
If you check the actual specification for the chem-film process
(the "Alodine" you are looking for is a trade name of one of the many chemicals you are allowed to use - the specification is
Mil-DTL-5541 [link is to spec at dla.mil]) you'll find that there is a class 3 that specifies a conductivity test - that would give you some of your numbers. Class 1A is less conductive, but still useable. For more specific numbers you can refer to any of the reference books noted through the links on the homepage - especially "The Surface Treatment and Finishing of Aluminum and Its Alloys" by Wernick Pinner and Sheasby (I think the most current version is just by Pinner and Sheasby).
Anodize itself (specification Mil-A-8625 [link is to spec at dla.mil, Defense Logistics Agency]) is non-conductive... so if you are looking for a conductive coating you pretty much can't use anodize. It will - at some high level of voltage - eventually breakdown, but that will also destroy your coating and can basically be disregarded.
With regard to heat conductivity - I'm not sure... But, it basically doesn't matter. Heat conductivity is rarely an issue for any Room Temperature conditions, and anything over 140F will cause the chem-film (Alodine) to break down and loose all corrosion resistance (and adhesion - I believe).
So - which one are you looking for? Heat conductivity or electrical conductivity? If the first and not the second, and chem-film and anodize are your only choices - anodize is the only way to go. If the second but not the first, chem-film. If you need both, you need to look at different technologies - probably nickel plate being the first among them.
Accurate Anodizing Inc.
Compton, California, USA
There is electrical resistance information for Class 3 chem film
(Alodine® is a brand, with several varieties) in
Mil-DTL-81706 [link is to spec at TechStreet] MIL-DTL-81706B CHEMICAL CONVERSION MATERIALS FOR COATING ALUMINUM AND ALUMINUM ALLOYS (02-MAY-2006). The resistance of Class 1A coatings is higher but not specified. I'm not aware of thermal conductivity data for chem film. The conductivity probably decreases upon heating above ~100
°C due to dehydration.
Anodize is primarily aluminum oxide, a pretty good insulator. The behavior varies with anodizing conditions, thickness, aluminum alloy and humidity (if unsealed). The breakdown voltage is typically about 30 Volts per micron of anodize thickness, but significantly lower on Cu- and Si-containing Al alloys. Thermal conductivities are <10% that of bulk alumina. However, anodize has a high thermal emissivity, so aluminum heat sinks are typically given ~8 microns of anodize.
For much more information on anodic coatings, see Chapter 12 of The Surface Treatment and Finishing of Aluminum and Its Alloys.
- Goleta, California
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