[s-cars] FW: Anodizing effects on Aluminum thermal heat dissipation Q

mlp mlped at qwest.net
Tue Jul 12 10:53:54 EDT 2005

-----Original Message-----
From: JIM NALEWICK [mailto:jimnalewick at lorin.com] 
Sent: Thursday, February 27, 2003 1:16 PM
To: 'mlped at qwest.net'
Subject: RE: Anodizing effects on Aluminum thermal heat dissipation Q

Thanks for the additional info.
When you look at it, the aluminum oxide layer (anodic coating) can act as an
If you look at the thermal conductivity of materials, you see values as
expressed in units of 
BTU's / (hr) (square foot) (deg F/ ft).  If you look at it, it is dependent
on surface area, thickness of the material, and temperature difference.  I
do not have data on Anodize Coatings, but the CRC Handbook of Chemistry and
Physics 58th Edition, 1977-1978 lists sapphire c-axis at 14.0 and sapphire
a-axis as 12.9.  Anodic coatings would be close to this or a bit less.
As the thickness term for anodized coatings is very small (0.000125" or
0.0000104') vs the large surface area, the heat transfer is fairly high.
When I have looked at it, the emisivity improvement out weights the
insulation effect.

-----Original Message-----
From: mlp qwest [mailto:mlped at qwest.net]
Sent: Thursday, February 27, 2003 10:34 AM
Subject: RE: Anodizing effects on Aluminum thermal heat dissipation Q

Jim -
Thank you very much for your detailed & thoughtful reply.  In a separate
email I have forwarded to you, I've detailed the results, and my limited
ability to make sense of them, that some further searching on the internet
produced back on the 13th of February on the matter.  FWIW, my current
understanding of the matter is I'm looking at two major heat transfer modes,
convective & emissive.  
The very general Rule of Thumb I came up with was that anodizing generally
greatly improves emissive heat transfer of aluminum, and (??) can slightly
impair convective transfer properties (see the quoted rtheta table of values
in the accompanying  February 13th email,)  
I've made the leap of faith that the "conductive values" quoted by
R-theta.com of between 171 to 193 for "raw" 6061/6063 aluminum vs. @ 7 for
anodized is more or less worth the (a) gain in emissive values and (b) some
additional surface protection and (c) cosmetic values in the very thin
coating we would be looking at applying.  That in any event, given the
limited thickness of the anodized surface, and that an exposed raw aluminum
surface will, over time, developed a "natural" aluminum oxide surface in any
event, anodizing the convective surface would not (hopefully) have any
significant adverse effect.
Mike Pederson

-----Original Message-----
From: JIM NALEWICK [mailto:jimnalewick at lorin.com]
Sent: Thursday, February 27, 2003 7:52 AM
To: 'mlped at qwest.net'
Subject: RE: Anodizing effects on Aluminum thermal heat dissipation Q

I am sorry that I have taken so long to answer your questions.  When I came
back from my trip, I had a project I was working on take up my time.

Anodizing air intercoolers should help with the heat transfer. 

When looking as how heat is transfered from materials, you look at it's Heat

The book "The Surface Treatment And Finishing Of Aluminum And Its Alloys" by
S. Wernick, R. Pinner, and P.G. Sheasby, Fifth Edition published by ASM
International and Finishing Publications LTD. lists the following

Radiation numbers of aluminum 

Condition of            Radiation number                        Heat
Aluminum                [K cal/m^2 x h (deg K)^4]       % 
Mirror polished 0.2-0.3                         4.3-4.6 
Etched          0.3-0.4                         6.4-8.5 
Bright Rolled   0.25-0.35                               5.3-7.4 
Matt Rolled             0.4-0.75                                8.5-16 
Aluminum Painted        0.8-1.5                         17-32 
As diecast              0.75-1.2                                16-26 
As sandcast             1.2-1.7                         26-36 
Anodized                1.8-4.3                         38-92 
Black Body              4.70                                    100 

Data given lists that this is a function of film thickness.  Thicker films
radiate heat better then thinner films. 
1 um film (0.039 mils or 0.000039")  30% Heat emissivity 
2 um film (0.078 mils or 0.000078")  60% Heat emissivity 
3 um film (0.118 mils or 0.000118")  70% Heat emissivity 

I think the following quote says it all.  "It should be noted that anodized
aluminum is an excellent radiating surface which approaches to within 80% of
a 'black boy' regardless of its optical colour and there is no advantage to
be derived from dyeing it black."

I have talked to some of our customers that used anodized aluminum for heat
sinks for chips.  They have found that when they use raw aluminum parts of
the same size the chips over heat and fail.  When they use anodized parts,
the chips stay cooler.

I have other comments. 
1)  The high temperature of 400 F should be OK for anodized aluminum.  I
would expect that you would see thermal crazing of the parts when they get
hot.  The anodic coating will hold up to very high temperatures.  What
happens is that when the parts are heated, it will expand.  The aluminum
expands at a greater rate than the anodic coating (aluminum oxide).  The
aluminum oxide is a hard crystaline layer on the surface and when the
aluminum expands, cracks or craze lines will form in the coating.  The
coating will still stay attached to the aluminum and should not flake off.

2) The anodic coating should hold up well to neutral salt spary.  Anodized
parts have been used in exterior automotive parts for years.  A standard
film thickness for automotive parts is 0.3 mils.  Thicker coatings will hold
up even better.  There can be problems if the parts are cleaned with caustic
or acid solutions.  I have seen were solutions used to wash trucks have high
enough pH to remove oils that they will dissolve the anodic coating.

3)  Coloring the parts black will not change the thermal properties, but may
give you a good look.  You will need to look at how the parts a colored.

There are two main was to color anodized aluminum black. 

First you can use an organic dye.  There are a number of different companies
that make black dye for use with anodized aluminum.  Each company make 3 or
4 different black dyes.  Some dyes will hold up well to exposure to UV light
and high heat.  Other dyes will fade quickly when exposed to UV light or
high heat.

A second method is to use a process called electrolytic two-step.  In this
process either tin or cobalt metal is plated in the pores of the anodic
coating before it is sealed.  These colors do not fade when the parts are
heated or exposed to UV light.  (We sell aluminum anodized with this process
for use in heating plates for Mr Coffee makers.)

It will be much better than a painted product. 

If you have any more questions, please free to Email me. 

Jim Nalewick 

-----Original Mess-ge----- 
From: mlp qwest [mailto:mlped at qwest.net] 
Sent: Wednesday, February 12, 2003 10:03 AM 
To: jimnalewick at lorin.com 
Subject: Anodizing effects on Aluminum thermal heat dissipation Q 

I am building aluminum air to air intercoolers (IC) for use in an automotive

application.  The ICs are used to cool the hot, compressed, incoming intake 
charge in a turbocharged automotive application.  Operating temperature can 
run as high as close to 400 F. 

The IC looks, works and is constructed similar to a car radiator (lots of 
thin internal & external aluminum fins.)  The hot, intake air under pressure

is run through the IC core.  Heat in the compressed air is supposed to 
transfer "conductively" to the walls of the IC, and then from there, then 
dissipated into the atmosphere, again principally via thermal conduction 
through the IC's external cooling fins. 

The ICs can be left bare, and installed as is.  But I would like to consider

anodizing, painting or otherwise coating the ICs at least externally, and 
possibly internally (anodizing my be the only alternative for coating 
internally) to: 

(1) Provide protection from external environmental factors (e.g. road salts,

grime etc.) & 

(2) Possibly protect or improve the internal transmsivity of the cores 
internal fins, if that's possible.  Internally, cores can be exposed to some

contamination from oil blow at turbocharger bearings; 

(3) Externally, if anodized or painted black, to provide some visual 
cosmetic concealment. 

I'm concerned about adversely affecting the IC's ability to transfer heat. 

Can someone tell me if anodizing aluminum reduces its ability to absorb then

transfer heat?  I believe this pertains more to "thermal conductivity" than 

Should the loss (or gain) in conductivity &/or emissivity from anodizing be 
regarded as significant where the goal is to remove heat?  Is there a 
particular anodizing recommendation for this kind (i.e. IC) application? 

My current (obviously limited) understanding is in some cases anodizing can 
be done to thermally insulate aluminum?  Is this the usual case, or does 
anodizing usually enhance thermal conductivity as I take the above response 
suggests?  What would be the recommendation for anodizing something that 
works like a cars radiator (i.e. thermal conductivity enhancement)? 

Thanks in advance for any information you can shed on this 

Mike Pederson 
Denver, Colorado 

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