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Thermal conductivity of aluminum finishes

(to provide context, hopefully helping readers more quickly understand the Q&A's)

Thermal conductivity (λ) is a measurement of a particular material type's willingness to allow heat transfer across it. The crystalline structure and free electrons of metals tend to make them not only electrically conductive, but thermally conductive as well.

Thermal conductANCE, the actual amount of heat that will be transferred (per unit of contact area) is λ * ΔT / thickness. Because conversion coatings are very thin, and thermal conductance is inversely proportional to that thickness, it is possible, as claimed several times on this page, for conversion coatings to not have much effect on thermal conductance.

Current postings:

August 11, 2021

Q. Is there a reduction of thermal conductivity after applying a conversion coat to aluminum? We have some 5052 aluminum plates (.093 thick) with conversion coat per MIL-DTL-5541 Type II Class 3. The color is gold. We're doing some testing and noticing less thermal transfer using these plates as opposed to our prototype unfinished plates.

Susan Spencer
- Fredericksburg, Virginia

August 2021

A. Hi Susan. Aluminum is a metal with high thermal conductivity as you already know. Conversion coatings are gels and definitely have significantly lower conductivity, and we would be very interested in any numbers you can offer us from your testing.

But what usually matters practically is the thermal conductance (the heat that is transferred across the surfaces that are in contact), and this is inversely proportional to the thickness of the conversion coating, and they are usually very thin -- thus the claims on this page that conversion coating doesn't usually have a major effect.

Still, exceptions are always possible: if, for example. it is crucial to you that your plates not just transfer heat across them efficiently but remain virtually the same temperature for some reason (very low ΔT), the conversion coating could be a problem :-)

Luck & Regards,

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

Closely related Q&A's, oldest first:

(to provide context, hopefully helping readers more quickly understand the Q&A's)

"Iridite" is a Macdermid trade name for their line of chromate conversion coatings for Aluminum. "Alodine" is a Henkel trade name for theirs. These are probably the two best-known trade names, but other suppliers have theirs as well. In the USA, the phrase 'chem film' is sometimes used as a synonym as well.

'Anodizing' is a totally different aluminum finish, see our FAQ "Intro to Anodizing".

Q. Hello!

I would like to know of where I can find information on how both Iridite/Alodine and anodize finishes affect the thermal conductivity of aluminum. Specifically, in fabricating a heatsink for an electronic component, it is typical to mask the actual contact area when anodizing, but is this absolutely necessary for Iridite/Alodine? Since the latter preserves the electrical conductivity of the aluminum, does it also have a negligible affect on the thermal conductivity?

Thank you!

Richard Andelfinger
- Tempe, Arizona, USA


A. I wouldn't imagine that the conversion coating would decrease your thermal conductance much, but the corrosion resistant properties do break down over 150 °F.

According to a list that I keep for myself, you may want to check out letters 15733 and 11120. Also, letters 3207 and 9881 may be useful as well.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner

We merged some threads on this page. Please forgive what may look like disrespect towards earlier responses; they probably weren't there :-)

"Iridite thermal conductivity?"


Q. Anybody know the thermal conductivity of Iridite layer on aluminum? Is it as thermally conductive as aluminum?


Tim Lee
Engineer - Los Angeles, California, USA

simultaneous 2004

A. It is nowhere near as thermally conductive as aluminum, but it is so darn thin a layer that it of no concern to almost all applications.

James Watts
- Navarre, Florida


A. I am not certain, but I believe that the conversion coating is slightly less thermally conductive but is so thin that it doesn't provide much in the way of insulating properties. I do know that the corrosion resistance properties start to break down above 140 °F though - so if you are going to be using your part in a high temp environment a chemical conversion coating would probably not be your best choice regardless of its thermal conductivity.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner

We merged some threads on this page. Please forgive what may look like disrespect towards earlier responses; they probably weren't there :-)

Thermal conductivity of chromate conversion coating

November 19, 2008

Q. I am designing a mounting bracket for a rectifier, and looking to maximize heat transfer. I am looking for information on thermal conductivity of chromate conversion coatings as applied over zinc plating. I have found the thermal coefficient of zinc to be about 5x that of steel, but can not find any information on chromate coatings. Any information would be appreciated,

Darren Riley
Product Engineer - Charlotte, North Carolina

simultaneous November 21, 2008

A. The thickness of the chromate will vary from part to part and even some on the part.
For a mounting bracket, the decrease in thermal conductance should be negligible as the normal yellow is very thin.

James Watts
- Navarre, Florida

November 22, 2008

A. Darren,

Due to the nature of the chromate film, I do not see most films as having any effect. Sealed films are a different matter. Chromate thickness varies with the type of chromate use, hence, for your purposes you would need to specify some additional information regarding corrosion protection and/or other specifics.

Gene Packman
process supplier - Great Neck, New York

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